The difference between bikes and boats.

Stupid title? Read on if you’d like to know how differently bikes and boats perform as the speed increases. I’ve ridden bikes the whole of my life but only now am I building a boat. I haven’t had a big project for a good few years and had always intended to build a sixth sports car but I don’t feel I can justify the expense at this time. I live a few hundred yards from the Leeds/Liverpool canal so thought it would be fun to build a boat for use along the 127 mile long waterway. As a cyclist it seems obvious to me that a boat, if it must be human powered, should use the legs rather than the upper body. After all, isn’t a hand bike considerably slower than a conventional bike? OK a boat with a sliding seat uses both upper and lower body muscles to power it but I fancied something more relaxed which I’d just pedal.

Performance always seems important to me. When I rode my mountain bike with a group of friends, before they let age and infirmity get in the way and left me to soldier on alone, I was the only one who timed myself. Sometimes I’d time a long ride but more often I’d time a short, usually downhill segment, then try to improve my time on subsequent rides. As soon as I started to study boats and boat design I began to wonder how quickly I’d be able to go. There’s a massive difference between a bike pushing it’s way through the air and a boat cutting through the water. In terms of wind resistance, which is the big thing which opposes a bike, the resistance rises with the square of speed. If you double your speed the wind resistance rises 4 times. To double your speed you need to put even more energy through the pedals. The situation is compounded by other forces so it’s generally accepted that you need even more than 4 times the energy to double your speed. So how is the situation on water different?

Your still pushing through a fluid, though water is far more viscous than air but there is another big difference. When a boat moves slowly through the water it creates waves from the bow at the front and also a stern wave behind the boat. The bow wave is by far the most important to consider. The wavelength of this wave will increase proportionally with speed. Below is a picture from Wikipedia’s entry concerning boats and you can see that the bow wave extends about half way down the boat’s hull.

The bow wave dips down then rises back up to the hull about half way along.

If the speed of the boat pictured above was to rise the length of this wave would increase, eventually reaching the whole length of the hull. At this point it would be as if the boat had fallen into a hole of it’s own creation. The boat cannot possibly increase in speed with it’s hull sitting in the water. The only way it can speed up is if the hull rises out of the water and begins to plane. This transition needs a large amount of extra energy but the reward is that the hull now skims across the surface unencumbered by the water it was displacing. This may not be a problem for a powerful motor boat but for a human powered boat it’s very hard and a situation that is unlikely to be sustainable for long. What this means is that every boat hull has a maximum speed which it can reach whilst still displacing water to support it. So if my human powered boat can’t plane and skim over the surface how fast can it go?

The way to go faster is obvious. If the wavelength of the bow wave increases with speed, eventually reaching the length of the hull. The longer the hull, the longer the wave and therefore the higher the hull speed will be. So if you want to go quickly you need a long hull.

A picture of a boat from Boat.com. It’s surpassed it’s hull speed and started to plane.

For my first ever boat build I’m not being ambitious. I’m building a hull from a single sheet of aluminium (outside America the word has an extra “i”, it’s not a spelling mistake!) which measures 2500 mm X 1250 mm. My boat length at the waterline will be a little over 6.5 feet. An equation has been constructed to allow me to calculate the hull speed from the length.

SQUARE ROOT OF LENGTH IN FEET x 1.34 = HULL SPEED IN KNOTS.

My 6.7′ hull will give me 3.47 knots or 4 mph.

This is slow compared to a bike where I can easily reach and hold 15 mph along the canal bank. If I keep pedalling my boat will try to climb up the front of it’s bow wave. The extra energy must go somewhere so perhaps the amplitude of the wave will increase? What won’t be happening is that my speed will not be going beyond 4 mph and I will not be planing. It will probably be better to put less effort in and sustain a good cruising speed, sailing for miles in either direction before I reach a lock. I’m excited by the prospect but that first journey is a long way off. I have a boat to build first and it will be quite unusual. My propulsion system will be something different which I’ve been researching. I’ve used large sheets of aluminium on previous projects and have already cut and shaped the metal. Next I’ll make a frame for the stern with a marine plywood transom. That’s the back of the hull and will provide a mounting for the drive mechanism. With some marine sealant the hull will be water tight at this point but will need more structure to give it some stiffness. I’ll keep you informed.

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