We have been asked many times - how did we get started with King Arthur?
Back in 1996, I was watching an episode of the television program Northern Exposure and they had a
trebuchet on this show. They threw a piano. It was launched by this huge machine and the piano
floated through the air like it was in slow motion. This is when I learned the word trebuchet. I then
searched through the Internet and found tons of information on these medieval siege machines, and
the best source was The Grey Company Trebuchet Page.
I thought this was fascinating, and I built my first machine the following year. It was a wooden “work of
art” called Prince Valiant. It had an axle that was 7 feet high and a fixed 50-gallon drum attached to the
throwing arm (TA). These wooden timbers were much too heavy to put in place to measure, and a lot
of them had compound angles to cut. I found the need to draw it out to find what the lengths and
angles should be. Rather than forge ahead, I learned how to run Quick CAD. This turned out to be a
savior. I tested the construction of different types of joints and settled on a combination of wooden pins
for alignment and metal “all-thread” with bolts to hold it together.
Prince Valiant was gorgeous. Its only down fall (excuse the pun) was that the best that it could throw a
gallon jug of water was only 120 feet. This was very depressing. It went into the barn and was never to
see the light of day, as far as I was concerned.
I heard about the event called the “World Championship Punkin Chunkin”. I told my tale of woe to Harry
Brown and talked him into going with me to see our first Chunk. Well, Harry and I were so hooked we
called the girls and informed them that we wouldn’t make it back home that night. On Sunday we got
into the pits and I had a chance to meet some of the folks and see their marvelous machines up close.
Two things happened: First, I ran into a great bunch of people. I knew that I wanted to be a part of this.
And second, Prince Valiant in it’s current state would not do. It stirred up my creative juices. I was
On the web, I found a number of good modeling tools for trebuchets. They were useful and I learned
about the physics of this medieval machine. To win this contest, I needed something more. I then
found a modeling tool (I don't think they make it any more), and I applied for their demo. It was a
reiterative and lengthy process. The demo version had one thing missing - I couldn’t save the design,
and the real version was too expensive. So each time I used it – I had to input the entire drawing. I
found a number of things I needed to know to understand the motion of a trebuchet.
It is a lot easier to build a model and make changes than to make changes to a real machine. So I
started with a 1-meter model. I was testing the length of the sling, the length of the short side of the
throwing arm, the angle of the release pin to verify the things I found on the computer model. After
many hours, I could throw a pebble 15 feet. This would not do.
When testing alternatives, I had it in the normal cocked position with the CW hanging down. As a
change, I raised the counter weight (“CW”) to a propped position. The CW arm was horizontal. When I
released it, the stone went further. Well, what would happen if I raised the CW even higher? It was
now sticking almost straight up. The stone went 35 feet - WOW. There is something interesting here!
I was using the action on the CW to build up a kinetic force before allowing the throwing arm to start. I
then went back to the modeling tool to verify what I had found. The tool, to my surprise, came up with
the same conclusion. I also found the right geometry, and this became the input to my design. I
wanted to test it out using a live prototype and using my old wooden base. As with the model, I
achieved much better distances.
During the tests with the small model, I also noticed that the throwing arm would actually move
backwards slightly when I released the CW arm. I thought I could use this action to act as a release for
the TA. When the TA went backwards, this would allow a holding bar to be released and move out of
the way, and the TA would then go through it’s normal swing.
I couldn’t finish King Arthur in time, so we took Prince Valiant to the big show (World Championships).
This was the first year for the Trebuchet Division. We were positioned next to a machine called “ The
Junk Yard Chunker “. It became our turn for our first throw. The knot in the pit of my stomach was hard
as a rock. We pulled the trigger and listened to the crowd cheering us on. Remember the action of
Prince Valiant is really two actions. First the CW starts falling and then the TA starts. On our first throw,
the crowd had no idea what to expect, and when the CW started, it looked like the machine just broke in
half. The crowd gave a sigh, then what appeared to be from nowhere, a pumpkin emerged. The crowd
let loose with a roar. Then they saw how far it went, and the cheering elevated again. The blood was
racing through my body.
It was Junk Yard Chunker’s turn and they went a little further. It looked like we were destined for
second place, but there was still another day in the competition. Dana Drew has a huge machine
called Fibonacci. When he saw the motion of my machine, and fell in love with it. It was innovative,
smooth and appeared to be quite simple. Unfortunately Dana broke the arm on his machine on
Saturday and was kind enough to give us our very first white pumpkin. These pumpkins are smaller
and denser for the same weight - and do they ever fly!
Well, on our last throw, we had a beauty. It was a personal best at 485 feet. The knot in my stomach
didn’t disappear yet because Junk Yard Chunker had added a lot of weight to his CW (which was a V8
engine block). Now he added jacks, lead weights and other engine parts and tied them to his V8. It
was hilarious. He is a retired judge and a lot of fun. The Judge pulled the trigger and off it went. My
heart stopped. It was out there pretty far. Now we wait for the measurers to do their thing. I’m pacing
around not feeling so good. The number came back - 37 feet less than ours.
The team became a single unit from the hugs!
At this point - I knew I was fully hooked on throwing pumpkins.
After gaining experience with Prince Valiant, I found that I could improve on the design by controlling
when the TA was released. This would allow me to control the path of the counter weight. That’s what
generated the change to the second trigger that is used in King Arthur. When the CW arm gets to a
certain angle, this pulls a line that is attached to the second trigger. This line is adjustable. I could
then allow the CW arm and the TA arm to be at 90 degrees for a longer period of time generating
I used to be very thin – clocked in at 127 lbs. I loved the game of golf. In high school, I could hit a drive
significantly further than guys twice my size. This did not do much for their ego. I found out that brute
strength is not what is needed to whack the golf ball far out there. So how did this happen? It was with
timing and leverage. That is also the key to the King Arthur design.
After surveying the competition, it became clear that I had to finish the bigger machine. “Prince Valiant
“ used a 2-inch steel axle and during the competition, it bent, so I knew something had to be done to
improve that part of the design.
Back to the modeling tool. I talked to their director of sales and told him that I could throw a gallon jug
of water over 600 feet with only 400 lbs of CW. He had used a trebuchet in their modeling classes and
was fully versed on their designs - he didn’t believe me. I convinced him to send me a copy of the
software for student prices. This was terrific. My obligation was to give him a press release.
I modeled my design using a 10-foot arm. I wanted to tune the parameters and see the distances.
This was a reiterative process. I discovered the best release angle, the best sling length, and the best
time to let the main arm start it’s process. After that, I used the modeling tool to find the forces on the
stress points, and used this as part of the design process. I then spent most of the summer building
My first effort was to learn how to weld, so I attended a class at NOVA and became a certified welder.
This was to be a fairly heavy machine and I tried to find an appropriate trailer, but nothing really fit. So,
to test out my new-found skill, I built one.
There was another innovation designed into this machine. All the other machines pull the TA down to
cock it, which means that the TA must be strong enough to haul the CW up. This is like death to
trebuchets, because you then have all that additional mass to accelerate. So instead, in the King Arthur
design, we pulled the CW up using a home designed crane. This allowed the TA to be significantly
Back in 2000, no one paid much attention to us. “Prince Valiant” was a little wooden machine with a
metal arm. This contraption had a motion that no one understood, the throw distances were moderate
and the machine was plain and simple. All in all, it wasn’t very interesting.
When the first “King Arthur” was built in 2001 - people started to notice, but again there wasn’t enough
mystery for it to take center stage. The machine was too small, too smooth, and the throw was
interesting but too mystical. King Arthur won again with a throw of 643 feet. I was expecting someone
to copy my design because there were a lot of video cameras running.
2002 and 2003 at the Chunk
In 2002 I added two feet to everything. The arm was now 12 feet long, and the axle was 12 feet in the
air, and the counter weight arm was 12 feet. Even with this, King Arthur looked like a puny little device
next to these huge machines.
When I pulled the trigger, like the response with Prince Valiant, the crowd moaned because it looked
like my machine was broken. Then like a delayed reaction, the main arm started and much to everyone’
s surprise, it went 927 feet. All of this was in front of a video camera from the Discovery Channel. This
was pretty impressive considering we only used 400 lbs. of counter weight. My competitors were using
upwards of 2,500 lbs. King Arthur’s throws were much further, and we were getting a lot more
In 2003, we spent most of our time tuning King Arthur, and we added 100 pounds more to the CW.
When testing King Arthur, everything happened so fast that you could not see it. Using a video camera,
I could see what was happening -- I could step through a throw frame by frame, and compare the
results with what was happening in Working Model 2D. In most cases, I was using the modeling tool
to understand what I saw in the video. For each run, I would cock King Arthur, and tape the throw.
Using a measuring wheel, I would check the distance of the throw. Then I would take the video camera
up to the house and transfer it to a VCR, and step through the throw to see the results. Each throw
would take over 45 minutes to complete. We spent an enormous amount of time tuning - we went
through 30-40 pumpkins.
This was the year of breaking the 1, 000 foot barrier and two machines did it. King Arthur ruled again
with 1,150 feet, and Pumpkin Hammer came in second with 1,045 feet. Pumpkin Hammer is a huge
machine, and they upped their counter weight to 5,000 lbs, compared to King Arthur at 500 lbs.
I believe there are two main strategies, and both will work: One is to build a large enough machine
with enough weight that you can power a pumpkin out there. As we found out in the following years,
Yankee Siege blasted pumpkins out there with 11,000 lbs of counter weight. One of the problems with
this design is you need to build the machine strong enough to manage the tremendous forces. Most of
the crews using this design underestimate the forces and they break something. Yankee Siege, on the
other hand, is so big and well designed - that it would be hard to believe that anything would break.
The other design alternative is to build a machine that relies heavily on timing and leverage. This is
how King Arthur is built. To get this timing right a modeling tool can cut your tuning time dramatically,
because making a change to the parameters is a lot easier compared to changing the real machine.
The “Punkin Chunkin Contest “ is a tremendous outlet for me. I get to dream up a new design, test it
using a modeling tool, build it, and see the results. In addition, it is exciting to rein triumphant in the
battle of competition. There is something about throwing something enormous distances using a
unique design that gets to the heart of what makes me smile.
One question that comes up all the time is "How do I cock King Arthur"? Well, here is a video of the
steps. King Arthur - Cocking the machine
|Prince Valiant in the
build process - lifting the
left arm in place - that is
a tractor bucket holding
Building up energy
TA starts and is 90
degrees to the CW arm
as long as possible
|Doing a count down with
the crowd - hit 3 so far
Pumpkin Hammer -
ready to go
History of King Arthur