OnagerManual

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CatapultaSo you want to

Build an Onager

By

Brian Lapham

Also known as:

Lord Doughal Stewart

Copyright ©2003

Fowl Farms PublishingA division of Fowl Farms Inc.

Contents

Getting Started 1Background 1What do you want your catapult to do? 2How will you transport it? 2Where will you store it? 2What are those parts called, anyway? 3What will it look like? 4Calculating the dimensions 4Making plans 6Choosing materials 7Tools 8

Construction 9Main frame 9Subsidiary structure and buffer 11Throwing arm 12Torsion hardware 14Attaching the skein and arm 15The winch 16Sling 18Pouch shape 18Sling length 19Sling stability 19How to build a sling 19Projectile ramp 20Wheels 21Projectiles 21Finishing work 22

Firing 23Preparing for a live shoot 23Tensioning the skein 23Aiming and firing 23Tweaking 24

Conclusion 25

Book sources A

Internet sources A

Parts list B

Additional schematics C

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Getting Started

So how long have you wanted to fling things? I’ve had the urge for this since I was in elementary school. My interest was re-peaked when I saw a Saturday Night Live sketch which was a commercial spoof for a backyard catapult to launch your trash into your neighbor’s yard. But it wasn’t until 1994, after I’d been in the Society for Creative Anachronism for a couple of years, when I went to my first Estrella war and saw a catapult powered by garage door springs. That was when I decided to make catapults the focus of my SCA hobby. I’ve been hooked ever since.

I currently have two catapults in operation. Odin’s Pride is a 2’x 3’ catapult which can shoot a single tennis ball 50 yards. Gungnir is 4’x 6’ and can throw most one pound objects 100 yards, provided the object has some aerodynamic properties.

Odin’s PrideBackground

When researching ancient artillery, there are two main sources of information: Greek and Roman Artillery, by Eric Marsden and The Book of the Crossbow, by Ralph Payne-Gallwey. This is where the bulk of my information comes from. However, there are other good sources, which include: Latin siege Warfare in the Twelfth Century, by Randall Rogers and Medieval Siege Warfare, by Christopher Gravett. Plus there are many web sources two of which are Trebuchet.com and the Grey Company Artillery site.

Gungnir

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The catapult was one of the most continually used pieces of artillery until the adaptation of gunpowder. The Greeks called their version one-arm. The Romans referred to this device as the scorpio, catapulta or the onager. In the Medieval ages it was described as the catapult and mangonel. The catapult waxes and wanes throughout history until the 4th century AD when it becomes the primary form of artillery until the 12th century with the addition of the trebuchet to the medieval arsenal.

The catapult is a torsion device. This means that the throwing arm is powered by the elastic tendencies of a skein of twisted fibrous material, usually hemp rope, horsehair, womens’ hair, animal sinew, or a combination of any of these. The catapult is in the same family of artillery as the ballistae, but instead of two skeins powering two arms, a single larger skein powers a single arm. This makes it a somewhat easier machine to build and maintain. The ballistae is a much more sophisticated device due to its dual arm design requiring a complex balancing of the power in each arm in combination with the bowstring.

Exact details of catapult construction are sketchy at best. Period illustrations can be notoriously wrong in their representations on engines. This is attributed to two reasons. First, the artist was an artist and not an engineer. Therefore he drew what he thought he saw, sometimes omitting important components. The second is that these were most likely military state secrets. The pictures were purposely drawn wrong so as not to give away technologies that may make an opponent’s engines more effective than your own.

Marsden could only find three main sources covering catapult construction. Ammianus, Apollodorus of Damascus, and interestingly, Anonymus Byzantinus are Marsden’s only written sources concerning the catapult and of these, Amianus’ brief non-technical description is the best source, the other two describe some sort of torsion device added to the front of a battering ram. This apparent lack of source material is probably why there exist so many variations of the catapult. For the purposes of this booklet, I will guide you through conception and construction of a catapult similar to that in Marsden’s book, pointing out where applicable, variations which you may wish to choose.

What do you want your catapult to do?How will you transport it?Where will you store it?

These are probably the three most important questions you will have to answer before you begin building anything. The larger the object you wish to toss, the larger the catapult you will need to build, the larger the vehicle you will need to transport it, the larger the structure you will need to store it in. I highly recommend storage in a shed or garage or something, above storage outside or even under a tarp. Weathering leads to accelerated deterioration of the wood. Since the main scope of my endeavors is for SCA combat, then the bulk of this booklet will be in that context. However, the formulas and proportions I will give should work with larger scale engines. So if you have a 10,000 square foot warehouse, own a flatbed tractor-trailer, and want to chuck 250 pound rocks, go for it! But if your like me and have a small work shed, a pickup truck, and shoot at you friends, then this is the guide for you.

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What are those parts called, anyway?

A lot of people make up their own names for the parts of the catapult. That’s fine if you’re talking to your buddies who are helping you build the thing. But to keep us all on the same page I’ll be using the names the ancients gave them in most cases. We’ll start at the ground.

The bottom most framework, in the shape of a rectangle as seen from above, is called the main frame. The two longer sides, where the holes will be for the skein, are the main ground joists. The shorter sides are the front and rear crossbeams. Any framework above the main frame is called the subsidiary framework. The subsidiary framework supports the pad that the throwing arm strikes, called the buffer, and consists of front and rear legs.

The throwing arm is pretty self-explanatory. The ring around the throwing arm that the winch is hooked to is called, of course, the winch attachment. The two fixed points where the sling is attached are the primary sling attachment(s). And the rod sticking out the end is the nock point.

The hardware used to tension the skein is called the torsion hardware and is broken down into two sets of four parts each. Each skein hole has what Marsden calls a metal ring or flange. But this terminology is a bit confusing so I call it a bushing as this more accurately describes its actual purpose. What Marsden calls a washer, which is more akin to what a modern person would call a flange, is the modiolus. The next piece is a hollow or solid bar that rests in the notches across the open cylinder of the modiolus. Marsden calls this the tightening lever, but the ancient sources and I call this the epizigus. The final piece is the locking pin. This goes through the modiolus, holds tension in the skein, and keeps the whole thing from unraveling.

modiolus, epizigus, and locking pin

The winch consists of three main parts. A bar or pole is suspended about half the length of the throwing arm’s length, between the main ground joists. On the outside of the joists at either side on the bar is the ratchet and drum. The drum is where the winch lever is used to crank down the throwing arm. The ratchet keeps the arm at tension by the use of a prawl, or clicker, which

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falls into the notches of the ratchet. The winch attaches to the throwing arm by means of a rope with a quick release, or slip hook.

What will it look like?

The more common image of the catapult is one that has the buffer mounted to a subsidiary structure so that the rear legs are perpendicular to the main frame and parallel to the upright throwing arm. The skein is usually mounted at a point equidistant from either end of the main ground joists. I call this the “classical” frame. The only real problem with this is that the buffer has to span the entire width of the engine and therefore has to be a fairly stout piece of lumber. With the classical frame, the force of impact from the throwing arm is first diverted 90 to the dual front and rear legs, then down to the mainframe joists.

Thor’s hammer, classical design

The design we will be using is attributed to Roman sources and therefore I call “Roman” style. I prefer this style because it uses less lumber and all of the force of impact is directed straight forward through the buffer, the front leg, then through the front cross beam and directly to the ground. The skein is located 2/5, or 40%, from the front of the main joists. The rear legs run from the buffer to the main joists, just forward of the rear cross beam. By offsetting the skein forward and the rear legs backward, more of the catapult’s weight is shifted to the rear of the force of impact. This weight shift reduces recoil or “jump” that is indicative of the classic style catapults, and from where they get the name Onager.

Calculating the dimensions

Now that you know what everything is called you need to figure out how big all these parts will be. The first thing that needs to be calculated is the diameter of the skein. This is done in the same manner for either style catapult. Think of this like designing a car starting with the engine. Marsden bases all his further calculations from the diameter of the skein. The first step in determining the size of skein is to decide on the maximum weight that you want your catapult to throw. For SCA purposes the maximum allowable weight is one pound.

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Marsden’s formula for finding the diameter of the skein is:

D=1.1 ³(100M)D= the diameter of the skein in Dactyls

M= the weight of the projectile in Minea

Here he quotes the “Artillery Manual of Philon”:

First we must find the cube root of 2,000. Now 12 cubed is 1,728 and 13 cubed is 2,197. If we choose 13, we “endeavor to diminish proportionately the added tenth”. A tenth of 13 is 1.3, so that a Greek mechanic, making his rough reduction, would calculate the diameter of the hole as 13+1¼= 14¼ dactyls, or, more likely, 13+1=14 dactyls.

Marsden also says that the figures given in the “Artillery Manual of Philon” are inaccurate. They are. Marsden figured that there was an error in the text translation, so he ran the numbers again using logarithmic tables to find the cube roots and came up with more accurate figures. You can use a calculator to find cube roots. It may take some time, but is quite workable and is more accurate than using Philon’s table.

Taking the measurement conversions into account, the formula becomes:

D= ¾ (1.1 ³v(100w))D= diameter in inchesW=weight in pounds.

If you follow this formula, then the diameter of a skein for a one-pound projectile comes to approximately 3.8 inches.

Philon’s corrected table converted to pounds and inches is as follows:

10 pounds 8 ¼ inches15 pounds 9 7/16 inches20 pounds 10 3/8 inches30 pounds 11 7/16 inches50 pounds 14 1/8 inches60 pounds 15 inches

120 pounds 19 inches150 pounds 20 3/8 inches

Marsden also believed that in order to hurl a stone weighing 5 pounds he needed to calculate a skein diameter for a projectile of 10 pounds. His reasoning was that Philon’s formula and tables were for ballistea which use two skeins and that a single skein would have to do the work of two. However this seems to not be the case for two reasons. First, an onager can twist the skein through a full 90 of arc, where ballistea, will only twist through 47 ½ to 59 of arc. If it went further, the bowstring would come off. Second, the onager uses a sling on the throwing arm, which is a force multiplier, adding as much as one third more potential power.

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But what about range? Only Payne-Gallwey sites sources, which tell of ranges up to 450 yards. He also reasons that 250 to 300 yards was the maximum range for archery during Greek and Roman times. A catapult only needed to be 350 to 450 yards away from its target to be safe from enemy fire. So I expect that the figures in Philon’s table are for engines that will fire from 350 to 450 yards range. This does not take into account firing a smaller projectile from a catapult intended for a larger projectile. This probably wasn’t done except for rare circumstances, as the increased throwing arm velocity would destroy the sling. I found this out first hand when during some test firing I shot a single tennis ball from Gungnir. The projectile didn’t go as far as I had expected because the sling was damaged during the throw.

Now that you know the diameter of the skein, you will use its dimension as a proportion of all the other components.

Marsden suggests the following proportions:

Main Ground Joists 12 d x 2 dFront and Rear Cross Beams 7 ½ d longThrowing Arm 8-9 d long;

2/3 d at the base,1/3 d at the tip

Every thing else is dimensioned to fit these proportions.

For the catapult we will be making the dimensions are:

Main Ground Joists 4”x 10”x 5’Front and Rear Cross Beams 4”x 6”x 3’Throwing Arm 33 ¾”

3” at the base,1 5/8” at the tip

If we were to follow Marsden’s recommendations, our catapult would not be large enough to qualify as a siege engine for SCA combat, so the dimensions we will use are somewhat inflated.

Making plans

If you’ve never built something like a catapult before, or if you don’t have much woodworking skill, then I highly suggest that you make several plans and drawings before you cut one piece of wood. These will help you to see your catapult in three dimensions and greatly help in eliminating mistakes that might have occurred. You will be able to determine how close board “A” and board “B” will come to each other and if you need more or less space between them, making sure they don’t come in contact with board “C”.

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Making drawings will also help you to determine the amount of materials that you will need. Your drawings don’t need to be CAD quality, but should be rendered with some sort of drafting tools and drawn to scale. I prefer the old-fashioned way of using pencils, scale ruler, 45-90 and 30-60-90 triangles, compass, protractor, and of course, eraser! I like to draw in 1 ½” scale (1 ½” = 1 foot). These basic drafting tools have helped me to solve many problems before I made mistakes with expensive materials.

Choosing materials

Unless you’ve just won the lottery, try to use as many standardized materials as possible. This will keep the costs down and ensure reliable replacement later. Many modern materials can be made to look “Period”. Also, your home stores can cut your lumber to size. This eliminates having to manhandle large pieces of lumber on small consumer grade tools. Scrounging for materials is also a lost art that you may become proficient at. Basic guidelines for selecting your wood are: no warping, no cracks, straight narrow grains, minimal knots, and not too green. If you go to Home Depot and all they have is dripping wet wood either come back another day, or take it home and let it dry for a couple of months. Many small knots are better than a few big ones. Try to avoid center cut wood, or wood with a tight spiral grain, these will tend to warp and crack as they dry.

For our catapult we will use 2-4”x10”x5’ from 12’ stock for the main ground joists and 2-4”x 6”x3’ from 12’ stock for the crossbeams. We will use the remaining 4”x 6”x3’ lumber for the forward buffer leg and the buffer itself. For the rear legs we will use 2-3”x 4”x4’ redwood from 8’ stock. For the throwing arm we will use a 3”x 36” ash baseball bat blank.

Another point of concern is the torsion hardware. If you’re a real metal smith with a welder, you can probably make these yourself, or if you’re like me, will have a professional or a friend with the tools do it. The concern lies in the size of raw stock that will be used to make the parts, specifically the modiolus. The modiolus not only has to fit in a hole in the main joists, but the skein has to go through it. A compromise needs to be reached between the size of skein you want, the size of hole you can put in the main joists and the size of material for making the modiolus. I suggest making the modiolus out of some existing standard size of pipe rather than custom making a cylinder. Your costs at the weld shop will mostly be in labor, as there isn’t that much material in making these pieces. This is also true for the primary sling attachment and the winch attachment. Metal parts are better, but for our catapult I will show you how to make good working torsion hardware using mostly wood parts.

For the torsion skein, we will use parachute cord, as it is a reasonable modern substitute. I’ve thought of trying artificial sinew like you can get at some leather shops but rejected the idea because of the cost. There is a way of determining how much cord you will need. I’ve found that when tightened to a point where the skein is just able to support the arm against the buffer, the number of strands per inch of diameter of skein averages about 100. This increases by a factor of about 10% per inch for skeins larger than 5” in diameter.

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If we take this and apply it to our catapult it should go something like this:

3.8” rounded to 3 ¾” 33” per strand3.75x100= about 380 strands (catapult width including torsion hardware)380x33”= 12,540” 12,54012”= about 1,045’ of cord

So for our catapult you will need about 1,000 feet of cord for full strength. I was only able to buy 750’ of cord. My catapult can throw one 4 tennis ball shot well beyond the 80 yard limit and two 4 TB shots 50 yards. This is good enough performance for me. However, if you want to shoot farther, you will need the full 1,000’.

Tools

While being a master craftsman is not a requirement for building this catapult, having basic knowledge of some of the more common carpentry tools is. The most predominant tools you will need access to are as follows: circular saw, crosscut (hand) saw, and either a drill press or hand drill with a drill guide and 4” and 2” hole saws. Other tools that you could use to make the job easier would be a table saw, a band saw, and a lathe. Of course, you will need the usual hand tools: square, compass, hammer, wood chisel, wrenches, pliers, clamps, screwdrivers, etc.

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ConstructionWhile the majority of the construction is pretty straight forward, you may find it necessary to improvise or change things to suit your needs. Gungnir conforms to about 90% of these directions with small variances in the winch mechanism. Everything else is pretty consistent.

Main frame

Start with the main ground joists. These are the most important and the most time consuming pieces to mill. Start by marking the notches for the crossbeams at either end of each joist on the same edge so that the crossbeams will be placed vertically into them on the bottom side of the main joists. These should be 3 ½”x 3 ½”. Next mark the slopes on the upper edges of the joists as follows: start by making a mark 2” above the notch marks at the end of the joists. Along the upper edge of the joists, make a mark 2’ from one end. Using a straight edge or chalk line make a line from the marks at the ends of the joists to the mark on the upper edge. This should look somewhat triangular in nature with the point of the triangle up and off center to what will eventually be the front of the catapult.

Next, using a square, make a line from the upper point of the triangle down to the bottom edge. Take your compass and draw a circle or mark a drill point for your 4” hole saw, 4” from the top of the point on the joists. Now make a mark along the bottom edge of the joists 16”, or about halfway between the line above and the rear crossbeam. Draw a 2” circle tangent to the bottom edge of the joists or make a drill mark so that the hole cut by the 2” hole saw will be tangent to the same. These will be the holes for the winch rod. You may want to draw and cut one joist first, then use that as a template for the second.

not to scale, dotted lines show removed wood

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Now you can cut. This is how I did it, but you may do it however you please. I used a circular saw to do the majority of the cutting. I cut along the marks, but not over cutting the lengths. What I did then was to finish the cuts on my band saw. An example would be, I cut the line I drew for the joist slopes with my circular saw set a maximum depth. This did not cut all the way through the wood, so I re-cut the same line on my band saw. This made for very accurate cuts, without overburdening my band saw and without those annoying offset edges you get when you try to cut one side with the circular saw then flip the piece over and cut it from the other side and don’t get it lined up accurately enough. There is sometime a small edge, but one that can be sanded away. If you have strong arms, you could also use this method with the handsaw.

To cut the holes it is definitely best to use a drill press as the hole saws are not as deep as the wood is thick. You will need to make several height adjustments to cut all the way through the wood.

I start with the hole saw cutting just enough to make a mark in the wood with the teeth of the blade. Then switch to a spade bit, the largest you have, and drill all the way through. Replace the hole saw and cut as deep as it allows. Now I remove the drill from the wood and use a hammer and chisel to remove the doughnut shaped piece of wood left from the cutting. I do this about three times and if I’ve cut deep enough each time, I usually get through on the third time. This is a slow process, so take your time and don’t get too excited if it seems like your taking too long and not cutting anything.

When you’ve finished cutting the main joists, you can now attach them to the crossbeams. Lay the parts out so that the crossbeams are sitting with the 3 ½” edges top and bottom. Place the joists so that they span between the crossbeams and rest 3” in from the end of each cross beam. Countersink holes in the joists and drill pilot holes in the beams. Screw the pieces together using ½”x 5” lag screws with washers, making sure the frame is square.

One more piece is needed, a stiffening brace between the main joists. I used the piece of leftover 4”x 10” for this. Measure the distance between the joists just forward of the 4” skein holes, leaving room for the larger end of the throwing arm to move in, and cut your lumber to the measurement. It can be slightly narrower that the joists as they will compress inward as pressure is applied to the skein. I bolted mine in place with a pair of ½”x 6” lag screws, one on either side. These are really only there to hold the brace in place when the skein is loose.

Main frame

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Subsidiary structure and buffer

Start by either taking the finished throwing arm, or if you don’t have the arm finished yet, another piece of scrap lumber to approximate it. Using more scrap wood, place the arm in a position that closely approximates where it will be when the arm is in its upright position. Place a 4”x 6” on end, next to, and forward of the throwing arm. This will be the buffer. Mark the wood where you want the finished buffer to be. This will normally be the top 1’ or so. You want to ensure that the buffer meets the arm over most of the distance between the winch attachment and sling attachment on the arm.

With the buffer stock standing in place (before being cut) take the remaining 4”x 6” and rest it on the forward cross beam and against one side of the buffer. This will be the forward leg. Mark a line adjacent to the front side of the buffer on the forward leg duplicating the angle where the forward leg and buffer will eventually meet. While still holding the leg in place, mark two lines at the top and rear of the forward cross beam creating a notch in the forward leg where it will rest on the cross beam.

Once these marks are made the two pieces can be cut. Using a piece of flat steel 1”x1/8”x 6”, make a strap to mount the forward leg to the center of the forward cross beam. Mount this with two lag screws to the front of both the leg and cross beam. The tip of the lag screw, which is in the forward crossbeam, may need to be cut as it might go all the way through the wood. Now mount the buffer to the top of the forward leg in the same position that it was in when you made the cut marks with a third lag screw.

front leg buffer

Place on of the 3”x 4” redwoods on top of the buffer and on top of the rear portion of the main ground joist just forward of the rear cross beam. This will be a rear leg. Mark the underside of the leg to duplicate the angle that the leg will eventually be cut and mounted at between the outside of the buffer and the inside of the joist. Make sure that the front leg and buffer is centered and square with the forward crossbeam. When the skein is finally in place and powered up, the arm will naturally go to the exact center between the joists. So it is very important that the buffer be accurately centered.

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Cut at the marks you just made on the first leg, and then use it as a template to mark and cut the second. Trim the 90 edges off the ends of the legs so that the legs can be adjusted to fit properly. Using a large clamp, place the legs in position on either side of the buffer so that a hole can be drilled and the ½”x 10” carriage bolt put in place. Make sure to leave space for the existing lag screw already in the buffer. I generally make this to be above the existing lag screw by about an inch or so. Now clamp and drill the holes for the ½”x 8”carriage bolts at the bottom of the rear legs. I use ½”x 2”square washers for all three bolts, on the nut sides as well as the bolt head sides.

Now is a good time to pad the buffer. You can do this by taking a piece of 4-6 oz. leather 16”x 8” and using upholstery tacks, tack it into place on the throwing arm side of the buffer. Make sure to only tack about the outer 1” or so of leather so that a pocket is formed toward the throwing arm. Tack the bottom and sides first. Then stuff foam padding, or I just used an old bed sheet, into the pocket until it is full. Then finish by tacking the top closed. Remember, don’t make the pad air tight! If you do, you run the risk of impaling someone with upholstery tacks when you fire the engine later.

completed frame assembly

Throwing arm

The throwing arm can be fabricated using three methods: lamination, lathing, or a combination of both. While the techniques may differ, the final dimensions are the same. Marsden tells us that a throwing arm should be 1d in diameter at the base and about ½ d at the nock end. The length of the arm should be 8-9 d. This would make our arm 4” in diameter at the base, 2” diameter at the nock and 30”-34” long.

I made two arms for Gungnir. The first I made using the combination lamination/lathe method. I started with an already existing arm from Thor’s Hammer and I lathed it down to fit the dimensions I needed. However, even from scratch, this is a sound way of building an arm. Start with three planks of 1”x 3” ash and glue them together with a marine grade resin glue. When you do this make sure the grains all run parallel to each other with the straightest grain in the center. Once the glue has cured you can then cut the arm to its proper dimensions with a table saw and be finished. Or instead, if you have a lathe, you can lathe the arm down to its intended dimensions. This looks a little classier, but is just as strong as the square cut arm. It may also depend on the

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hardware you are able to get for the arm. I was able to get winch and attachment hardware made from metal pipe stock. This and the fact that I had a lathe made it possible for me to make the round arm.

The last way to make an arm is to start with a baseball bat blank. Of course this method requires a lathe and is how I made my second arm. Already having a working arm, I decided to take a chance on making a narrower, lighter arm. The new arm is 3” in diameter at the base and 1 ½” at the nock. The arm is also 1 ½” for about half its length before tapering to 3”. This arm has performed better than the previous arm and is noticeably faster as well. This non-laminated arm must be wrapped to meet SCA safety standards, but for the casual flinger, this is unnecessary. I used parachute cord for the wrap.

I also recommend that no matter what method you use to make your arm that you make a notch or knuckle at the base of the arm for the skein to grip. I made mine about 1 ½ d wide and ½” deep, 1” from the base of the arm. This does a nice job of reducing wear and keeps the skein on the arm.

To complete the arm I slid my winch attachment onto the arm until it gripped. I tapped the piece firmly into position then used a short cut off nail to secure it in place. Don’t drill any holes through the entire diameter of the arm! This only weakens the arm. If you have to make your own winch attachment, a good way of doing this is to take a 1” wide piece of flat stock steel long enough to wrap around the arm, leaving enough for two holes at one end and one hole at the other. Two holes are used to clamp the piece in place with a rivet or bolt, the other to take the slip hook.

Next place the sling attachment on the nock end of the arm. These can be eyebolts, but I had an attachment made from pipe stock. My attachment is a ¾” ring cut from 1 ½” stock with two 3/8” studs welded on opposite sides and two washers welded to the ends of those. The resulting piece allows me to switch slings easily by simply “buttoning” them, so to speak, to the end of the arm.

The last piece is the nock. I used a piece of 3/8” steel rod about 8” long. I recommend it extends out 1 d from the end of the arm. Drill a hole in the center, top of the arm and insert the nock. Leave it straight for now.

finished throwing arm

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Torsion Hardware

The torsioning hardware can be made either from wood or metal parts. For Odin’s Pride, I had the hardware made at a weld shop, but this doubled the cost that I had put into the whole project. So I did some thinking as to what the actual job the individual pieces of hardware were doing and came up with low cost, wood alternatives. A common misconception is that the hardware needs to be geared winches. This is not the case, and wasn’t implemented until late period when milling techniques were perfected. If you really feel the need to make geared torsion hardware, go ahead! However, I find that it is unnecessary, extremely costly, and only serves to over tighten the skein.

What most people don’t realize is that the skein acts as a shock absorber as well as a power source. I’ve had the good fortune to be able to use slow motion video to watch the arm and skein while they do their job, When the arm hits the buffer, its direction of travel changes 90downward to the ground, dissipating a good portion of the force of impact. If the skein is too tight, the force is not dissipated, and is absorbed solely by the arm. Over time this force weakens and eventually breaks the arm prematurely.

One drawback to wood parts is that they are not as durable as metal. If you’re planning on using your catapult only once or twice a year, then the wood parts work well. However, if you plan on using your catapult more often, then metal parts are recommended. To tension our catapult we will use a large metal lever.

Regardless of which type of hardware you choose to make, the first thing you will need is a bushing. The bushing displaces the force applied by the modiolus to the main joist and is a smoother surface than the bare wood. This needs to be a fairly substantial piece of material, but not exceedingly stout. I used two 1/8” pieces of scrap flat stock steel large enough to cover the area around the skein hole and to reinforce the hole where the locking pin will be inserted.

Generally, the modiolus is a tube of metal welded to a flange about ¼ the distance from one end of the tube. The flange has equidistant holes drilled in it, and the short end of the tube is then notched to allow the epizigus to lie across it. Usually this piece is made from substantial metal stock. On close examination, you can plainly see that the force applied by tightening the skein only exerts pressure on the flange portion of the modiolus. The tube portion is there to keep the skein from “floating” around in the skein hole and to keep the locking pin holes lined up. Using this, my wood modiolus is in two parts, a tube portion and a flange portion. The tube portion is literally a tin can! It can also be in the form of pins or guides that will hold the flange portion in place over the hole. I found some old coffee cans that fit snugly into the skein holes, cut off thebottoms, bent over the cut off end, and tacked it into position from the inside of the joist with the stronger crimped can opening jutting out about ½” from the outside of the joist.

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For the flange portion I made six 8” “doughnuts” with 4” center holes for the skeins out of ½” birch plywood. Four of the doughnuts were cut into pairs of “C” shapes so that there would be a gap to accommodate the 1 ¼” diameter epizigus. The pieces of the flanges were each glued and screwed to the remaining uncut doughnuts and eight holes for the locking pins were drilled equidistant and parallel to the skein holes.

wooden modiolus

These wood parts worked well to get the engine off the ground and to its first couple of events, but time would show that sturdier parts were needed to handle the repeated torsioning of the skein. Fortunately I had a friend with metal working tools help me out with the new metal modiolii. Basically it’s a piece of ½” flat stock cut to the same shape as the wood modiolii but with small flanges welded to it to accommodate the epizigii.

The two epizigii are 8” pieces of standard 1 ¼” pipe and are the same for either wood or metal hardware. The locking pins are ½” steel rods about 6” long with one end slightly tapered for easier insertion.

metal modiolus

You’ll also need a tensioning lever. Mine is a 1” solid steel bar 3’ long. (a standard crowbar is too wimpy!)

Attaching the skein and arm

Now things start to come together! Hopefully when you purchased the parachute cord for the skein, the merchant wound it around something, or you just bought the whole spool. If they just wound it around their arm, it will take longer to coil the skein, but either way it will be much easier to use a jig. Don’t thread it through the catapult frame! This takes forever and is a

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cumbersome process. Making a jig is easy and pre-tensioning the cord as you thread it doesn’t add any significant power. All you have to do is figure out how wide the catapult is and then add the thickness of the torsioning hardware. This should be about 35”-36”. I made my jig out of ½” scrap plywood about 8” wide. You also need to simulate the thickness of the epizigii. I just nailed on a piece of 1”x3” across each end. Make marks on the jig as wide as the skein hole so as not to make the skein too wide to put through the joists. Remember to cut off about 4’ of cord for use later with the sling.

I clamped my jig to a worktable then leaving about 3” loose at the beginning, wound the cord around the jig’s length. You can make a longer jig if you choose. A longer jig will make for more turns of the skein when you tension it up. More turns will give you more power, but not to an extreme. I recommend trying to attain at least one full turn on the skein before significant power is being applied to the throwing arm. Also, when coiling your skein try to keep the individual strands under even tension and place them side by side as much as possible. This will help prevent fraying and provide even tensioning on all the strands.

When you finish coiling, tie the end of the skein to the loose end from the beginning. A simple square knot will do the trick. Then using wire ties or string, secure the skein in several places so that it won’t get unwound as you remove it from the jig and place it into the joists. Do one side first. Thread the skein through the skein hole from the inside of the catapult, through the modiolus, and then insert the epizigus through the loop in the skein. Pull it tight , then do the same through the other side. You may find that the skein is too tight to put the epizigus in all the way by hand. Use your torsioning lever to stretch the skein, with the epizigus partially in the loop. As you stretch the skein, use a hammer to tap the epizigus into position through the notch in the opposite side of the modiolus.

Once the skein is in place suspended in the torsioning hardware you may remove the wire ties. Now you can place the arm in between the halves of the skein, at the center of the catapult. The arm will hang limply until you put tension on the skein. I suggest only putting enough on to just hold the arm in the upright throwing position, but not so much that it is not easy to pull down by hand. This will keep the arm in place, but also put a reasonable load on it when you’re experimenting with the winch.

I don’t leave my skein stored under heavy tension. I feel this weakens the cord as causes a breakdown of the cords natural elastic tendencies over the long haul resulting in cord breakage (sort of like pulling an already stretched tendon again and again). I haven’t had the need to lubricate my skein, either. However, some people leave theirs under heavy tension all the time and/or lubricate them with vegetable oil.

The winch

Start by getting what I call a peeler pole, that turned lumber they sell in home and garden stores you use to prop up saplings with, which is 2” in diameter. This is a standard item and should be readily available. Cut this to a length that is about 2” longer on each end than the joists are wide. This shaft will fit into the two smaller holes in the main joists.

Next, square off the ends of the shaft so they can accept a drum and ratchet. Use two thin pieces of scrap material to hold the shaft in the holes in the joists. Make sure the sides of the squared off parts of the shaft are parallel to their counterparts on the other end of the shaft. This will make ranging adjustments easier.

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You’ll need to make two wheels. One will be the ratchet and the other the drum. These can be made from any thick stout hardwood. I used oak for the ratchet and birch plywood for the drum. I made wheels about 8” in diameter then marked them with radii 45 apart. This will give you 1/8 pie shaped sections.

Now for the drum, drill ½” holes in the edge of the wheel 1 ½” deep adjacent to these marks and finish by cutting a square in the center to fit one end of the shaft. For the ratchet, take your compass and draw a curved line about ¾” in from the edge on one mark to the edge where it meets the mark directly next to it. Draw a similar curved line between all the arks. This should give you a circular saw blade shape. Cut the wood along this shape and finish by cutting a square in the center to fit on the remaining end of the shaft. It may be necessary to reinforce the ratchet or drum by attaching metal strips to their outside faces. These should be placed across the grain of the wood.

modiolus and ratchet, Gungnir modiolus and ratchet, Odin’s Pride

I fastened the wheels to the shaft with 3/8”x 4” lag screws and two wooden discs left over from the baseball bat blank. I predrilled the end of the shaft and drilled through the center of the discs. I did not glue the ratchet and drum to the shaft as the square edges give ample friction to turn the winch, also this allows for easy replacement should one of them brake.

You will also need a prawl. I made mine from a scrap piece of oak, about ½”x 1”x 3”. Attach the prawl with a long wood screw above and to one side of the ratchet so that the prawl will fall into the notches of the ratchet by gravity alone. This makes it so the winch is always safe and locked in position regardless of how much tension is applied or where the arm is in its cycle.

A rope is then attached to the outer ends of the winch next to the inside surfaces of the main joists. The ends should line up with each other and held in place with a small nail or staple. Enough rope should be used to have a couple extra turns worth on the winch shaft. This will give you extra rope in case of a needed quick repair and reduce the amount of pressure on the nail holding the rope on the shaft. Thread your slip hook on to the rope before attaching both ends. Or, what I did was use a removable chain linkbetween the slip hook and the rope.

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speed winch, 12:1 ratio

The slip hook is one I had made for a previous catapult. It is a piece of 5/16” round stock about 8” long with a loop bent into one end. About ½” up from the loop is a 1” piece of the same material bent 90 and welded to the first piece. This is the actual “hook”. The rest of the handle is bent 90 to fit between the throwing arm and the inside of the rear leg, when the hook is placed in the winch attachment on the arm. You may wish to put a lanyard of some sort on the end of the slip hook handle.

slip hook

Lastly you will need to make a winching tool. It will need to have a ½” diameter steel prong about 1 ½” long on one end. I made mine about 3’ long, from the leftover piece of peeler pole and lathed it down to a similar shape as a shovel handle.

Sling

First off, you want a sling. While many period images show catapults with a spoon style arm, they are in large part much less effective than a sling. Both Marsden and Gallwey verify this. Slings can double an engines range with little cost in weight or power.

The sling you will make can be specific to the projectile you wish to throw, or it can be more general in variety. Either way it will have some traits that will make it a longer lasting, more accurate sling for your flinging pleasure.

Pouch shape

The shape of the sling is very important in two regards. First, having a sling shaped to fit closely to the projectile prevents it from moving around in the sling pouch and also prevents it from falling out of the pouch during the acceleration process. This ensures that when the projectile

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takes flight it does so at a 0 yaw (relative to the longitudinal axis of the throwing arm). If the projectile is not released at 0 yaw, the projectile will curve off in the direction in which it was angled. Second, because the close fitting pouch has more physical contact with the projectile, or grip, it puts a more powerful backspin on the projectile when it is released. This backspin notonly gives the projectile balance, but also provides aerodynamic lift.

The projectiles I fire from Gungnir are somewhat unique as they are what I call a “flat four”, or four tennis balls taped together in the shape of a square. They are fired with the narrow side toward the opponent, are balanced, have a fast even backspin, and don’t drift off to the sides as readily.

Sling length

For a catapult a good length to start with is 1/3 the throwing arm’s length, the sling measured while loaded and the arm measured from the fulcrum to the sling attachment. This is the length Marsden used. He goes on to say that a longer sling will fire at a lower angle with greater forward momentum and a shorter sling will fire with a higher angle, but a shorter range.

Sling stability

Even though centrifugal force would generally send the sling to its apogee of arc, this is not always the case. Sometimes the sling itself can yaw left or right during acceleration. To counter this I found that a sling with three attachment points prevents this yaw. Two primary or fixed attachments, preferably of some lightweight rigid material like leather, going to the sides of the arm and one loose attachment, a lanyard, being at the nock end. This triangular system gives excellent left/right stability during acceleration.

Something else to consider is where the sling is at rest when the catapult is ready to be fired. Our engine will have a projectile ramp to hold the sling/projectile parallel to the ground before firing. On most catapults this is not the case. The sling usually hangs behind the engine perpendicular to the ground. The sling parallel to the ground has more arc to apply more centrifugal force to the projectile before it leaves the sling. Ultimately what you want is a vehicle for uniform and consistent projectile acceleration, not just a floppy piece of leather or cloth.

How to build a sling

This sling is one that I use for more general purposes, but that is specifically designed for the sling attachments described earlier.

Start with a leather strap (6-8 oz) 2” wide about the same length as the circumference of the object you wish to throw. Now cut two leather (4-6 oz) “patches”, roughly football shaped to act as the sides of the pouch. With the patches in place along the edges of the strap, centered on it longitudinally, the pouch should be bowl shaped. Attach the patches with standard leather rivets.

Next get two 1” or 1 ¼” leather straps (6-8 oz) about half the throwing arm’s length. Attach one end of each to one of the ends of the 2” strap from the pouch, making a big “Y” shape with the loose ends. On the loose ends of the narrow straps, punch a keyhole shaped hole so that the sling can be buttoned onto the sling attachment on the throwing arm. Lastly punch two holes latitudinally at the end of the reaming 2” pouch strap. These are for the parachute cord lanyard

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which will be hung on the nock when a projectile is loaded. The cord should be about the length of the throwing arm. Loop the cord through and tie a simple loop knot so that the excess will stay outside the pouch. Don’t cut this off! You can use this later to adjust the range of your catapult. The excess will not foul the projectile as it leaves the pouch. Additional holes may also be punched in the loose ends of the narrow straps for range adjustment.

different slings for different jobs

Projectile ramp

You will probably want to make a ramp to hold the projectiles in place in the sling when the arm is pulled back for firing. This ramp will hold the sling up against the underside of the arm and is easily made with scrap materials. I used a 1”x 4” oak plank about 20” long, just long enough to reach to the back of the crossbeam. To support the ramp I used a 1”x 1” piece of oak to span the width of the joists and screwed it to the bottom of them just behind the winch. I then cut a notch in the center of the rear cross beam so that the ramp lays flush with the top of the cross beam and screwed it into place.

top view, projectile ramp and winch shaft

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Wheels

You may want to move your catapult around; then again, you may not. In which case you can ignore this section. But if you have a need to move your’s around, here’s how that’s done.

I used the KISS method for making my wheels! I started off with a set of four home store 18” round laminated tabletops. I cut 1” wide strips of sheet metal and nailed them to the tread of the wheel like an “iron rim”. For hubs I had the benefit of much experience. I used four 6”x ½” lag screws to mount the wheels to the cross beams. I use a double bushing system. A copper bushing, made from copper pipe, against the bolt and a PVC bushing between the copper bushing and the wood. That way, steel rubs copper, rubs plastic, which rubs the wood. I greased all the parts as well. These wheels have gone through four wars and two catapults! You can make your wheels look more “period” by taking some scrap wood and nailing reinforcing slats to the outside of the wheels.

wheel

Projectiles

Anything! Gungnir can shoot most one pound objects 100 yards, provided they have some form of aerodynamics (as opposed to none!). Odin’s Pride can shoot single tennis balls 50 yards. If you built a larger or smaller catapult then you’ve already determined the engine’s capacity. Novelty items are a must! Any objects from daily life can be catapult projectiles. For the record, INANIMATE OBJECTS ONLY!! Don’t get crazy on me!

SCA legal projectiles are:

1. One pound foam rocks; basically 12” diameter denim bags filled with one pound of foam rubber, sewed shut then duct taped.

2. Four tennis ball clusters; these can be any shape, but a pyramid is preferred. Tennis balls are duct taped together. Some people puncture them with holes, but I find this unnecessary.

3. Single tennis balls; these are duct taped, some people puncture them. These projectiles only count as an archery blow and are usually used as “grape shot”, not as a single munition.

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Finishing work

While I’m highly confident in the labor I’ve put into this project, there’s only one thing I know for certain. The better finishing work you do, the longer your engine will last. Obviously life throws us curve balls every now and again and we all can’t build at the same level. But the finishing work is where I never cut corners…but wait , I do!

I routered all the edges on all the lumber with a rounded edge bit. This removed any major splinters where moisture might penetrate or where they might crack and break off. Next, in lieu of sanding, I used a technique that I’m not ashamed to say that I got from a cheesy home decorating show.

I used a butane torch to lightly scorch all the wood surfaces except the arm, the winch, and the torsion hardware. This not only “tiger striped” the surface but also burned the roughness down to about the equivalent of 60 grit sand paper. Satisfied with this, I went on. On Odin’s Pride I sanded down to 150 grit on the framework, with four coats of polyurethane sealer, and down to 220 grit, with twelve coats of polyurethane, on the throwing arm. I also pinged all of the hardware on Odin’s pride and painted it black to make it look wrought. Gungnir has the same number of coats of urethane as Odin’s Pride.I would probably not recommend a lot of finishing work on a first or second project. But the more catapults you build, the nicer you want them to look and the longer you want to keep them around. My first two catapults only lasted a few months and one Estrella war.

The next catapult, CAT 2A, made it to three wars. My fourth catapult, Thor’s Hammer, fought at three Estrellas and some other smaller wars. I expect Odin’s Pride and Gungnir to last for at least five years of heavy use to include three SCA wars a year and numerous renaissance fairs and demos.

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FiringThis is the reason you’ve come this far. Its what your engine was built to do. But remember this, a catapult is a big machine that is trying to smash itself apart. Which brings us to our first safety tip: always choose a WIDE OPEN space for initial test firing of a new engine. Find someplace at least double the size that you expect from you catapult. Try to make it as level as possible. Lines on sports field can be handy, too. Also, make sure that there are as few people around as possible.

Preparing for a live shoot

Once you’ve picked your “wide open space”, don’t forget your tools! Like me, you’ll probably be a considerable distance from your home or workshop. Take things you know you’ll need like pliers, screwdrivers, vice grips, and any specialty parts for firing the engine. Extra materials are also something good to take the first few times. As your engine improves, you’ll find that you need to bring fewer items with you.

Tensioning the skein

I ended up having to use a 1” solid steel bar three feet long to adequately tension Gungnir’s skein. At first I tried a crowbar, but bent two of them before I figured it out. You’ll probably need something similar.

The bar is placed in the epizigus and tension is applied from the rear of the catapult to the front. This is a common error in many period illustrations. They show the skein wound clockwise or counter-clockwise rather than back to front. To wind a skein the way they show would just clamp the arm either up or down and it would be unable to move in any direction. Maybe it was a scheme for fooling one’s enemies?

The skein moves slowly, even if the bar is removed before the locking pin is placed in the modiolus. If the skein frays, it will usually do so over a long period of time. The skein won’t “explode”. It will just slowly pull itself apart getting weaker as it goes.

Tighten the skein to a point where you think appropriate. This should be in the ballpark of 2 to 3 turns. This is measured by counting the turns on the skein, not how far you move the modiolus. Remember, you already have some tension on the skein just to hold the arm in place. Use the locking pins to hold the tension.

Aiming and firing

It is a good idea to dry fire the engine before taking it out to the range. This gives you an opportunity to see how it will operate and fix any potential problems that may pop up. I usually dry fire mine at least a dozen times before going to the range. When you dry fire don’t put the sling on. Because there is no weight being thrown the arm will move with greater speed and force and can damage the sling.

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Generally, a catapult will fire forward. However, during initial test firing, keep the rear area clear as well. Your sling may not take the strain and come apart. Place the slip hook into the winch attachment on the arm. Take up the slack in the winch rope by hand. Continue using your winch tool. When the arm is about ¼ cocked, place the projectile in the sling and continue winching until the arm is fully cocked. Once cocked, check downrange to make sure everyone is clear. When the urge hits you, grab the slip hook lanyard and YANK! This has to be a quick and powerful yank as the slip hook needs to release quickly and cleanly. Now sit back and watch it fly!

Tweaking

Chances are your catapult didn’t shoot as far or as well as you’d like. Since the nock point is straight up, it probably traveled in a fairly high parabolic trajectory. There are four ways to adjust the flight characteristics of the projectile: sling length, skein and arm power, nock angle, and projectile weight.

Since we’ve already touched on sling length and powering up the skein in earlier sections, I won’t repeat myself here. Arm power is simple, the lower you winch the arm, the farther you will throw. The projectile weight is also an obvious fix. The lighter the object, the farther you can throw it. But don’t go too light because you can damage the sling. Also try to keep your projectiles somewhat aerodynamic. It will fly farther if its not fighting the air.

The nock is ostensibly a timing device for releasing the sling. The more straight the nock, the higher the trajectory. Start with the nock straight up just to get an idea of how the engine will operate. Then as you do more test firing, gradually bend the nock to an angle that best suits your needs. I usually end up around 35 to 45. Some people like to have a nock that they can bend by hand to adjust the range. I don’t like this as the angle will change during repeated firing. I prefer to adjust range by changing slings and arm power. This seems to work best for me.

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Conclusion

Hopefully you’ve enjoyed this little romp through the world of ancient artillery. Be prepared for many setbacks and frustrations, this is not an exact science. The text and pictures I have given you should allow you to construct a reasonable replica of an ancient Greek/Roman/Medieval catapult. My last piece of advice would be to start off small. Make a couple tabletop versions of this or any other device to get to know how things are supposed to work together before you tackle something on this scale. Remember; safety above all else. While these are not even considered weapons by most law enforcement agencies, they can still hurt and even kill people. So use some common sense when operating your engine.

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A

Book SourcesGreek and Roman Artillery, Technical Treatises by E.W. MarsdenOxford University Press, 1971

The Book of the Crossbow by Ralph Payne-GallweyDover Publications, 1995

Medieval Siege Warfare by Christopher GravettOsprey Publishing, 1990

Latin Siege Warfare in the Twelfth Century by R. RogersOxford University Press, 1997

Internet Sourceswww.Trebuchet.com

www.KnightsArmory.com

Grey Company Trebuchet Page

Sling Effect by Alfred Evertwww.evert.de/eft293e.html

The Medieval Centerwww.middelaldercentret.dk/acta.html

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B

Parts list

Wood1-4”x 10”x 12’ pine1-4”x 6”x 8’ pine1-3”x 4”x8’ rough cut redwood2’x 2’x 3/8” birch plywood

1-36” ash baseball bat blank1-2”x 8’ peeler pole1-1”x 1”x 3’ oak/pine1-1”x 3”x 2’ oak/pine

wood scraps

Hardware7- ½”x 5” lag screws8- ½” round washers4- ½” x 6” lag screws1- ½” x 10” carriage bolt/nut2- ½” x 8” carriage bolt/nut2-3/8” x 4” lag screws2-3/8” washers6- ½” x 2” square washers2-1”x ¼” eye screws

wood screwsnails

Other (See text)1,000’ military grade parachute cord23’x 1” sheet metal strips2 square feet 6-8 oz. leather4 square feet 4-6 oz. leather1”x 6”x ½” steel8”x 16”x ½” steel8”x 20”x 1/8” steel3’x 1” bar steel8”x 3/8” bar steel12”x ½” bar steel12”x 5/16” bar steel16”x 1 ¼” steel pipe12”x ½” copper pipe12”x ¾” PVC pipe

upholstery tackspoly urethane sealerfoam rubber or old sheet

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C

Additional schematics

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About the Author

Brian Lapham, or as he is known in the Society for Creative Anachronism, Lord Doughal Stewart, hails from the Kingdom and Barony of Atenveldt or, Phoenix, Arizona; where he has been building catapults since 1994. He has been awarded numerous times by his colleagues for the performance and detail of his catapults. Brian’s catapults have fought at over a dozen SCA war events, including the Estrella and Great Western Wars. Doughal Stewart is his 3rd Crusade era Scottish persona.

Brian has worn many hats in recent years; as a resort audiovisual technician, a television news photographer, and most recently as an urban farmer. Brian is an ordained minister of the Universal Life Church.

Brian would like to hear from individuals who have read this document and tried recreating the Roman Onager. He can be reached at: [email protected]