Human Powered - Two Props?

[Casey]

Hello...

I am still in the early design stages of an expedition HPB, for use on the great lakes. I have "changed courses" from a hull of a kayak, to a hull of a canoe, in order to increase stability and packing space. (I am still looking for a light Kevlar canoe hull that someone is willing to part with

In order to avoid the cutting of a hole in the center of the hull, and building a well, sealing it, etc., - I was thinking about having two prop setups, on each side of the hull, driven by the pedal drive unit. The mechanics can be dealt with later (I have a mechanic in mind who would love to tackle this), but I have a more pressing question:

IS THIS FEASIBLE? I know that a drive unit can be mounted on the side, like was done in the Microship (http://microship.com/microship/album...deployment.JPG). If I had two such setups, it would also make beaching easier, as they could be easily retracted.

Does anyone have any thoughts about having two props, one on each side of the boat? Is there too much drag? Do the props need to go in the same direction, or opposite? Spin towards the hull or away from it? Any insight would help - this is a bit outside of my realm of expertise.

[Submarine Tom]

Great question, great project.

Use the "search" window in the header, you'll find lots of info already posted on this forum.

Generally speaking, two props are less efficient than one larger one.

I would spin the tops away from the hull though if you do two.

-Tom

[gonzo]

Each gear will make you loose about 3% of your power. A single two blade propeller is the most efficient setup.

[Casey]

Okay, one is more efficient - could it be on the side? I am trying to avoid cutting a hole in the bottom of the hull - which do you think is "less efficient" - one prop on the side of the boat, such as in the above mentioned Microship, (where constant ruddering would be neccesary to keep it straight) or one on either side? I know one prop in the center of the boat is the most efficient, but which of the above less-invasive-to-the-hull plans would be preferable?

[Tiny Turnip]

Hi Casey

if you skim the pedal powered boat thread, you'll see that one prop over the side is probably the most popular format. The fact that the thrust from the prop is slightly eccentric to the centre line of the vessel is no big issue. (as long as its not miles away.) Its pretty straight forward to trim the steering to accommodate this. The prop over the side, particularly with flexible prop shafts and moveable or unstayed propshafts, also allows the prop to kick up in shallow water or at obstacles, and to be lifted out to remove any weed that gathers.

[Wavewacker]

A single prop is best. The prop should look more like a very large model airplane prop, about 14 inches long about 3 1/2 inches wide and a shallow pitch,,,best for your boat, I have no idea, but not at all like a motor prop. I have a friend in Ca who I use to train with, a racing cyclist. He won the Iron Man, but I was in the Army, LOL. He built HPB commercially, more like a kayak. Gearing does not make that much difference from what I recall. But he's a speed record freak, LOL. Anyway, I heard from a dealer that they were going to a catamaran design, can ride higher, better position to pedal, more stable. I can see pushing down with the left leg and having to learn to the right for a wake, rather ackward.

I would think you would do better moving the prop further behind the center of the boat and maybe using a flexible shaft to go over the stern to a shaft that might have an adjustable pitch.