Due to the recent rain I decided not to ride my mountain bike today. I would have got myself and the bike covered in mud just before we go away so I chose to use my pedal powered boat instead. What I’ve noticed about pedalling the boat, which is done from a feet forwards recumbent position, is that it takes it out of the muscles like hill climbing on a bike but is far less aerobic. I figured that the reason for this must be that I’m using a lot of force to turn the pedals but not spinning them as quickly as I would on a bike. Today I’d do a little assessment whilst I was on the water.
30 revolutions per minute is far too slow when compared to cycling so it would be good to do something to address this. The most obvious thing would be to add a gear or chain and sprocket element into the drive so that the pedals could turn at twice the rate as the paddlewheel. This would add a weight and complexity that doesn’t interest me so what else could I do?
Another way to make pedals turn more quickly would be to allow the paddle wheel to slip more. I could make it narrower or smaller in diameter, perhaps. Intuitively the more slip I get, the lower my efficiency will be. I don’t want to reduce efficiency by turning pedalling energy into eddies in the water and other water movement if it doesn’t result in more force driving the boat forwards. So what about reducing the paddle wheel diameter?
The rule of paddlewheels is basically summed up as “the bigger, the better”. The reason for this is that you want the paddles to enter the water as near to upright as possible and remain upright as they rotate in the water. For this reason it’s best to have a big paddlewheel where only the tips of the paddles are in the water for as short an angle of rotation as you can manage, whilst still getting sufficient drive. So what would be the effect of reducing the diameter of my paddlewheel? Interestingly if I move the paddles, say, 5cm inwards, the tips will be in the water for a shorter time hence lower angle of rotation. Whilst they’re in the water they’ll be closer to upright so I may even improve my efficiency. The tip speed will be lower, so it may be that I can turn the pedals more quickly yet still push the boat forwards at the acceptable speed that I’m now achieving.
This is a problem with any unique piece of engineering. You have no sources of reference which can really instruct you. It’s worse in the field of boat building than many other disciplines because it’s hard to find formulae to help you work things out. Fluids are complex to describe mathematically and I feel no inclination to do all the learning I’d need to help. My best course of action is going to be trial and error and this will be something I enjoy. I’m going to reduce my paddlewheel diameter from 80 to 70 cm in a way that will be easily reversible and see where that takes me.