My name is Bill Goldthorpe. I've been building small boats like these since 2006.
When I first got the idea to build a pedal powered kayak, I searched the internet for plans or information. What I found is that there was very little out there about the type of boat I had in mind. There are a few commercially produced pedal powered boats on the market, but I didn't want to spend thousands on a small boat. Besides that, I wanted to build my own boat. I had to start almost from scratch and learn by experimentation what works well and what does't. Here, I share what I have learned.
Design Evolution:
My first pedal powered kayak was sort of a successful failure. I used the basic hull dimensions (width and length) of a plastic 12 foot kayak that I owned as a starting point. The plastic kayak felt very stable but the first time I tried out my pedal powered kayak, I discovered something strange. It was terribly tippy; uncomfortably unstable even. What I learned is that I didn't realize how much you use the paddle to help you balance in a conventional oar powered kayak. I wanted a clean hull and so I was reluctant to put any kind of stabilizing outriggers on it. I was able to overcome the instability with an inverted vee rudder. The way it worked was that the inverted vee rudder induced a roll in the direction of the turn. If I felt like I was tipping to the right, some left rudder would act to keep the boat upright. The inverted vee rudder was surprisingly effective and quite natural. I ended up taking that boat out into lake Michigan several times and it handled waves of several feet and boat wakes with no problem. I was the only one who ever felt comfortable on that boat, however. Years later, I cut it up because I was starting to accumulate too many boats. I would never have felt comfortable selling that first one because of the instability issue. I ended up giving it the nickname "TinyTanic".
My first pedal powered kayak was sort of a successful failure. I used the basic hull dimensions (width and length) of a plastic 12 foot kayak that I owned as a starting point. The plastic kayak felt very stable but the first time I tried out my pedal powered kayak, I discovered something strange. It was terribly tippy; uncomfortably unstable even. What I learned is that I didn't realize how much you use the paddle to help you balance in a conventional oar powered kayak. I wanted a clean hull and so I was reluctant to put any kind of stabilizing outriggers on it. I was able to overcome the instability with an inverted vee rudder. The way it worked was that the inverted vee rudder induced a roll in the direction of the turn. If I felt like I was tipping to the right, some left rudder would act to keep the boat upright. The inverted vee rudder was surprisingly effective and quite natural. I ended up taking that boat out into lake Michigan several times and it handled waves of several feet and boat wakes with no problem. I was the only one who ever felt comfortable on that boat, however. Years later, I cut it up because I was starting to accumulate too many boats. I would never have felt comfortable selling that first one because of the instability issue. I ended up giving it the nickname "TinyTanic".
I knew my second prototype would need to be wider, but how much wider? I didn't want to sacrifice too much speed for stability so I decided to make it just a little wider; only about three and a half inches. I second guessed myself the whole time I was building that second boat. Would three and a half inches be enough? Was I wasting my time? Finally, when the moment of truth came, I found that it worked great. It feels very comfortable. I refined the design a little more and constructed a third prototype with a deck. The Roman 12 was born.
Props and gear ratios:
I experimented with a few different props and gear ratios. What I discovered in my quest for optimization is kind of interesting and might save you some time and trouble. What I found is that it really doesn't make much difference what kind of prop is on the boat. The smaller props needed to be geared to turn faster, but ultimately, the speed of the boat turns out to be more a function of the size and shape of the hull than the prop. I think that a leaner/longer hull would benefit from some tinkering in the area of props, but hulls in the 12 foot range are simply too inefficient for the extremely low power output of a person. It will never get on step, or plane with human power. In fact, if you look at the common formula for calculating the maximum speed of a displacement hull; 1.34 X the square root of the length at waterline, you see that the only variable is the length of the hull. Horsepower isn't even a factor with a displacement hull.
With every prop/gear ratio I tried, the boat would cruise comfortably with very little effort at about 3.5 MPH (measured with a GPS). it would go 4 with a moderate effort and 5-6 with a burst or sprint effort (incidentally, 3.5 MPH was the max sprint speed for me on my plastic oar powered kayak). People often ask me if the boat would or could go faster if it had the ability to change gears like a bicycle. The answer is no, without planing, or getting "up on step" it can only attain it's maximum hull speed which is about 4.7 knots, or 5.6 MPH. A human being does not have enough power, with any kind of prop, at any gear ratio to get a boat like this up to planing speed. Another interesting thing to consider is that bike riders change gears primarily when they go up and down hills. A boat in water will never go up or down a hill (except maybe a waterfall).
Drive legs:
Pedal powered boats like these have the limitation of having to operate in water that is deep enough to accommodate the diameter of the prop. They also don't like weeds very much. For these reasons, I think a removable compact drive makes more sense than a drive system which places the prop at the back of the boat like a conventional boat would have. If the prop bottoms out, or gets tangled up in some weeds, I simply pull it up and use an oar or clean the weeds off. Below are some pictures of various drives that I've tried. These all use a twisted #25 roller chain to get the power from the cranks down to the prop. I built the first three and the fourth one was made in China by a company called Seawind. I'm not sure if Seawind is around or if they are still making boat drives. If anyone has some contact info for them, please e-mail me. Another option that I would like to try someday is the Hobie Mirage drive which uses flapping "fins" instead of a prop.
I experimented with a few different props and gear ratios. What I discovered in my quest for optimization is kind of interesting and might save you some time and trouble. What I found is that it really doesn't make much difference what kind of prop is on the boat. The smaller props needed to be geared to turn faster, but ultimately, the speed of the boat turns out to be more a function of the size and shape of the hull than the prop. I think that a leaner/longer hull would benefit from some tinkering in the area of props, but hulls in the 12 foot range are simply too inefficient for the extremely low power output of a person. It will never get on step, or plane with human power. In fact, if you look at the common formula for calculating the maximum speed of a displacement hull; 1.34 X the square root of the length at waterline, you see that the only variable is the length of the hull. Horsepower isn't even a factor with a displacement hull.
With every prop/gear ratio I tried, the boat would cruise comfortably with very little effort at about 3.5 MPH (measured with a GPS). it would go 4 with a moderate effort and 5-6 with a burst or sprint effort (incidentally, 3.5 MPH was the max sprint speed for me on my plastic oar powered kayak). People often ask me if the boat would or could go faster if it had the ability to change gears like a bicycle. The answer is no, without planing, or getting "up on step" it can only attain it's maximum hull speed which is about 4.7 knots, or 5.6 MPH. A human being does not have enough power, with any kind of prop, at any gear ratio to get a boat like this up to planing speed. Another interesting thing to consider is that bike riders change gears primarily when they go up and down hills. A boat in water will never go up or down a hill (except maybe a waterfall).
Drive legs:
Pedal powered boats like these have the limitation of having to operate in water that is deep enough to accommodate the diameter of the prop. They also don't like weeds very much. For these reasons, I think a removable compact drive makes more sense than a drive system which places the prop at the back of the boat like a conventional boat would have. If the prop bottoms out, or gets tangled up in some weeds, I simply pull it up and use an oar or clean the weeds off. Below are some pictures of various drives that I've tried. These all use a twisted #25 roller chain to get the power from the cranks down to the prop. I built the first three and the fourth one was made in China by a company called Seawind. I'm not sure if Seawind is around or if they are still making boat drives. If anyone has some contact info for them, please e-mail me. Another option that I would like to try someday is the Hobie Mirage drive which uses flapping "fins" instead of a prop.
