Third homemade boat test.

After my last test I thought I was on the right track with the boat. My problems had been that it wasn’t nearly fast enough and that it bobbed up and down as it went along. There was a danger that water could have spilled over the side but also that I want to move forwards, not up and down. I believed that with a few modifications I could overcome the issues and reach my target speed. A kayak only achieves 2 to 3 mph so it shouldn’t be impossible by using leg rather than arm power to match this with a tiny dinghy. I chose to try an oscillating fishtail for propulsion. It was easy enough to make and even on my first test I’d managed to get some forward motion. On test 2 things were little better, even with a much bigger fishtail, until a bracket bent accidentally. This allowed a change in the angle at which the tail met the water and provided some propulsion, though only my right leg seemed to be doing the work. If I adjusted the other bracket which controlled the angle when the tail moved in the other direction then surely I’d get much better performance.

It has a certain elegance to it.

My wife, Ali, had said she’d be present at my first launch but she’d been at work so this was her first opportunity. The fact is she only wanted to laugh when I fell in, as any good wife would. Fortunately for me the boat is really quite stable and easy to get in with no drama. Getting out again is similarly safe and it doesn’t leak a drop, which is some accomplishment at least. We walked down to the canal, Ali with our dog, Freddie, and me towing the boat. It must weigh around 30 kg so is no problem to pull around on wheels. I climbed on board and pushed the treadles. I got some forward motion but strangely it was little different to my previous test. All his after lengthening the stroke of the tail and adjusting the angle of attack to 32 degrees in each direction. It still bobbed up and down and it’s been enough to convince me that the fishtail system is not going to work. I did all the mathematical calculations to help me design a promising system but it was still taking little effort to push those treadles. A bigger, longer tail would simply be impractical. To reduce the bobbing it would need to be a long way behind the hull to reduce the leverage and for a mini boat this isn’t going to work. So where do I go from here?

The only flipper type of system which has found commercial success is the Hobie Mirage drive. It’s 2 counter rotating flippers under the hull, usually of a kayak. You may be able to reac( and hold 3 mph in a kayak with the Mirage but struggle to maintain this speed with a mini boat. What I’m now looking at is a way, still with leg power, to more actively push the boat along. If I can get some sort of paddle to propel me, rather than relying on the complexities of trying to generate vortices with the fishtail, the efficiency is not critical. Legs provide vastly more power than arms in a kayak or skull so I hope to get to a good speed. For the record Strava had me at an average of 1.6 mph but it felt desperately slow to me. I’m enjoying the experimentation and am formulating a plan for my next attempt.

1 Comment

  1. idlecyclist says:

    What about a wheel for propulsion similar to the Mississippi steamers? I’d imagine you could power that with your legs? Good luck 🤞


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