Typical Wh/Mi for an electric kayak?

[blisspacket]

There's an event called the Wye River Electric boat marathon. It's 26 miles. With NiMH batteries in a lightweight 16' ply boat I required about 1440 W to cover the entire course; motor was a MinnKota.

 

[nechaus]

Found some kayaks at my local dump in their "market place"
im going to buy one, for $40 its quite cheap and they look like pretty decent kayaks..

im keen to see what kinda of speed ill get with a 86lbs trolling motors and a 24 volt lipo pack thats about 1000kwh..

they say the 86lbs trolling motor can reach 24kmh on a larger boat..
I could get dual trolling motors but I think id need a 2 person kayak and sit at the front perhaps

http://www.youtube.com/watch?v=SKOD6uf7pts petrol canoes that go fast

 

[DeaninMilwaukee]

I am thinking of doing a tiny outboard for my 10 ft kayak in the simplest possible way, ( r/c motor with long straight shaft ), but am missing the key part of the calculation, what rpm to design for.

I am planning on using a r/c airplane apc-brand prop in 5x3 size, and want to put about 75 watts into it, the goal is to maintain about 3 to 4 mph without paddling. I'm guessing I need about 2k rpm or so, but haven't been able to find a prop calc for something this small. Can anyone help?

 

[DeaninMilwaukee]

I believe a low cost solution for the big gear reduction ratio's needed to use small brushless motors could be rc helicopter parts.

A 450 class heli has a main gear reduction around 13 to 15:1, runs in good bearings, and uses decent quality rc motors. The motors typically are around 3000kv and run well in the 100 to 200 watt range. They can also be had quite cheaply from places like hobby king.


You could conceivably use the copters existing frame, strip off all the unneeded copter parts, and mount a boat friendly prop to the mainshaft in place of the original setup, possibly with an extension to keep all the copter parts out of the water.

Thoughts anyone?