Golf Ball Trebuchet

I have no idea why these medieval siege engines hold such fascination; maybe it's the slinky way in which they operate; the rapidly accelerating arm, followed by the whip like sling. Maybe it's the desire to hurl objects just like our ancestors did... or maybe it's just because little boys never quite grow up. Whatever the motivation, they are very addictive. There are many factors in getting the best out of your trebuchet, irrespective of whether it is a 2 foot model or a 30 foot monster. I suspect the incessant tinkering is part of the attraction.

I looked around the internet for plans and ideas and settled on the very fine set you can find at Ingenium. I multiplied the dimensions by 1.25, as I wanted to build a slightly larger version.

I settled on 1" x 2" pine as the main building material. I have no idea why it is called 1" x 2" as it's actually 11/2" x 5/8"? Anyway, at 99 cents Canadian for eight feet of the stuff, who's complaining? My tools comprised: a hacksaw, electric drill, a screwdriver, a sanding block and a sharp knife. Cutting a square edge with a hacksaw can be quite a challenge, especially when you don't have a try square. Anyway, having a great set of tools isn't a necessity but a tenon saw would be nice.

Click on the thumbnails for a larger view.

The axle is made up of 5/16" steel rod with Canadian Dollars (Loonies) as bearings and end caps. I'm not totally sure about the legalities of drilling a hole in the Queen's face - but I guess her dentist gets away with it... I drilled 1/2" holes in the wood, which allows plenty of room for fine adjustment and ensures the axle rests on the bearings. The friction is negligible. The axle is epoxied to the beam in order to keep the whole thing square and prevent any side movement.

I used a 10 lb downrigger lead fishing weight as the Counterweight. I had already settled on golf balls as the projectiles, as they are easy to come by and designed to take a lot of damage. The magic ratio for counterweight to projectile in Hanging Counterweight Trebuchet is 100:1. The average weight of a golf ball is 1.6 ounces, which just so happens to be 1/100th of 10 lbs, so it's an obvious choice. I intend to make the counterweight look a bit more authentic but it will do for now and works very well.

The hangers for the Counterweight are just twisted pieces of coat hanger wire. I will also upgrade these when the mood takes me. The hangers are suspended from a piece of 5/16" steel rod, which passes through the short arm of the beam. I reinforced the short arm with some shimming metal, which is obtainable from most auto shops. The shimming metal is epoxied and screwed to the beam.

This is the view from the left side and shows the trough. I cheated a tad by buying a length of hardwood ceiling coving. It has a fairly gentle curve but manages to keep the sling and golf ball on the straight and narrow, so accuracy doesn't appear to suffer. The release pin is just a piece of bent coat hanger wire, set at 30 degrees. It is bound to the arm with cord. I chose this method just in case repeated bending, during tuning, led to it snapping. It probably adds some weight just where you don't want it but it is easy to replace.

Close up of the release pin. I used the chain from an old keychain for the loop as I didn't have any rings of the right size. The trebuchet has been fired about fifty times so far with no sign of failure, however, I would imagine a two ounce projectile is its limit.

View from the right side. The trigger is another piece of coat hanger wire (surprise, surprise) with a loop fashioned at one end  and a length of cord attached. The wire goes through two screw eyes on the trough, set either side of another screw eye on the beam. The wire was polished with very fine wet and dry sandpaper to remove any burrs on the metal. It works very well on a trebuchet of this size but a more sophisticated trigger would be needed for anything bigger.

Dimensions: Base = 19" long x 23" wide Height of Axle = 16" Length of beam = 25" Height when fired = 36" Ratio of beam = 4:1 Length of Sling = 19" Maximum range with a golf ball = 72 feet