Thank you
This is what I am considering so far.I checked the drag calculations for your 6m hull. The boat requires 173W to do 6mph in calm water with the APC 16X16 prop. That will be a battery drain of say 200W allowing for motor losses - maybe a bit more.
If you use a 24V 100Ah lithium battery it will operate at 8A through most of the discharge to delivery that power. Hence it will last 12 hours to give a calm water range of 70 mile at 6mph. Dropping back to 5mph would take range to 100 mile.
So 8 3.2v 100ah Lifepo4 Cells
I have been getting cell prices from a Chinese supplier. They will do deals on cells and transport:
http://www.aliexpress.com/fm-store/314081/211478471-541006028/3-2v-100ah-Lifepo4-Cell.html
You need 8 cells for a 24V battery - it will average about 25V through the discharge with only 8A drain to give 200W. These cells weigh 3.5kg so total weight is 28kg. At such low drain even two 12V lead/acid batteries around 120Ah would work OK. They will be 30+kg each so more than twice the weight of lithium. Cost of the LiFePO4 will be around $1000 and the deep cycle lead/acid maybe $600. I would go for the 8 lithium cells.
If you want to do some fast and low cost electrics then you should look at the scooter stuff:
http://www.electricscooterparts.com/motors24volt.html
I can get these motors with gears included.
You should get an electronic controller because it will allow precise control with negligible losses:
http://www.electricscooterparts.com/speedcontrollers24volt.html
Then you buy a model aircraft prop:
http://www.hobbyking.com/hobbycity/store/%5F%5F7975%5F%5F%5FTGS%5FSport%5F18x12E%5FPrecision%5Fpropeller.html
This one will be a bit underpitched. Top speed about 6mph. The best may be an APC 16X16 - many use these for pedal boats:
http://www.apcprop.com/ProductDetails.asp?ProductCode=LP16016
This will give about 7mph and require something like 350W - would need to check what total weight is.
Use a 10mm diameter aluminium shaft (machinable rod - NOT weldable rod) that curves to get the prop perpendicular to flow so loading is even. If you set it up off-centre then you could lift the prop out of water for sailing.
That is a mouthfull. The motor seems alot less expensive than a 24 v trolling motor, and the battery seems expensive, but worth it. Probably buy a couple replacement props for the oyster beds that I will hit.
Any ideas are welcome.
Tom the rower
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