I just got off the phone with a very nice fellow from Holland who felt compelled to phone me and let me know that he thinks that adding a hydrofoil to my human powered boat would really work for me. He actually contacted me for another human powered boat reason, but I thought it was funny that he had to throw in the hydrofoil tip – like I almost knew he would, because everyone does. 🙂
A day doesn’t go by without someone letting me in on this new idea. There are a number of YouTube videos of human powered hydrofoils and they look amazing, and fun and super fast and efficient. The pilots are flying effortlessly above the water, passing every other human powered boat in sight, without a care in the world. And always smiling. It’s the future, and it’s happening NOW!
Fact is, I think a human powered hydrofoil boat would be great fun to design and build. And… I just might do that. Someday. But… As to hydrofoils making a human powered boat MORE EFFICIENT FOR LONGER DISTANCES, it just doesn’t seem like it will work. And I’m not saying this because this is what I think based on watching some YouTube videos. I’m saying it because the calculations just don’t work out, as there is always more drag from the foils and struts than reduced displacement. It just doesn’t appear that it is worth the trade off. I’m also saying this because both Rick and I have actually MADE hydrofoils, and have TESTED them. IN THE WATER. ON REAL BOATS. I use an expensive SRM watts meter on my boat, so I am able to quantify the TINIEST change in design to see if it actually is resulting in more speed per watt of power put into the pedals.
So, my experimentation with foils started with the stabilizers. Over the years of human powered boating with both CP2, and Special K, I have noticed that there is some drag associated with the stabilizer floats (some call them outriggers) that drag through the water and keep the boat balanced. We need those stabilizers because any very narrow boat would simply just tip over if there wasn’t some way for the pilot to balance it. Narrow kayaks like surf skis, and rowing singles are balanced by the the paddle/row stroke. Narrow outrigger canoes are balanced by a single stabilizer float on one side, and the paddle stroke on the other. Narrow pedal powered boats are balanced by 2 stabilizer floats.
So the most obvious use for a foil would be to have a hydrofoil lift the weight of a stabilizer float up, off the water to eliminate the displacement drag of that float. This is a very small and simple foil, as the amount of lift it needs to generate is only about 5 pounds per side. And the potential gain is a lot, as not only would the foil contribute to reducing displacement in total, it also potentially lifts up two draggy fat things out of the water.
My first test was a single hydrofoil mounted under the right hand stabilizer with a control arm to adjust the foil angle of attack. The idea was that I could use the control arm to dynamically adjust the lift and ‘fly’ the right float. In testing there was plenty of lift from that foil, as I could pull the lever up and roll the boat from the right stab to the left, then back again by pushing the lever down. But balancing between the two stabilizers was impossible.
Then I added a second foil to the left hand stabilizer and did more tests. I could feel lift from both sides, but the balance was never equalized, and again, I was constantly flopping from one stabilizer to the other. The third test was to angle the foils which creates self equalizing lift. As one side lifts too high, it’s foil starts to lift up out of the water creating less lift which in turn will lower the stabilizer. This was Ricks test, and he found that even using foils with double the surface area (ie: PLENTY OF LIFT), he was still unable to get the stabilizers to balance. This is not to say that balancing the stabilizers is impossible – it’s just going to take a bit more experimentation. But there is no point because the third test that I did was to measure the resulting drag from the foils to see this pursuit was even worth it.
Here are the results:
150 watts of power input, water slightly ripply, calm wind
No foils speed = 11.1 kph
2 foils set at slight angle of attack = 10.7 kph
2 foils set with no angle of attack = 10.8 kph
Rick also measured more drag from his angled foils – even when the stabilizer floats were up and out of the water.