Electric outboard for 20' x 6' flat bottom skiff?

I'd like to design an electric outboard for a flat bottom skiff that will be able to transport my entire family (two adults, two kids, and a dog-about 600lbs total) around local electric only lakes. The design of the boat will not be the most efficient, because the primary concern will be carrying the weight of the crew, batteries, etc... Basically I'm looking to bolt a 15HP golf car motor:

This one? http://www.evdrives.com/product_p/mot-me0709.htm

to the lower unit of a conventional outboard so that I can hang it off of the back of the boat. I thought about doing an inboard, but given the fact that I want to be able to beach the boat and operate in very shallow water, plus be able to mount a conventional outboard from time to time, I think the electric outboard idea is a better one. Mounting the motor is pretty straightforward-my biggest question is how much battery capacity do I need/can I realistically take with me?

The motor I referenced will operate between 24 and 72 volts. I figure at most I can carry eight batteries wired in series parallel, giving me 48 volts at however many amp hours each bank combined will bear. Will 48 volts be enough? What do I need to take into consideration as far as gear reduction in the outboard leg? How can I figure out how much runtime I'll get out of a given amount of amp hours?

 

First, 48 volts are enough to electrocute and kill a person. The battery capacity depends on how many hours you want to be able to run the motor. You can use about 50-65% of the rated capacity of the battery. A 15HP engine is about 11 KW. At 12 volts that is about 933 Amps. A size G2 battery has about 190Ah reserve at 6V. They weigh 77 lb. You will need about ten to run for an hour.

 

would you consider reducing the speed of your boat. At half speed you should get at least 8 times the run time and 4 times the range. For simple calculation purposes, using the grab numbers out of thin air method...

My 16 foot x 2 foot 300 pound kayak can go 4 knots with about 200 watts of human power. Assuming your boat has 4 times the wetted surface and twice the displacement, and somewhat worse shape for low speeds and better shape at high speeds...

Your 20 foot x 6 foot 600 pound skiff can go 4 knots with about 800 watts, and 2 knots with about 100 watts. Using the 2 knot scenario, 100 watts of drag x speed might require 200 watts from the motor. At a 20 hour discharge rate at this lower speed this might require 200 watts x 20 hours / 12 volts = 333 amp hours and this will give you a range of 20 hours x 2 nautical miles = 40 nautical miles. Operating same system at a 5 hour discharge rate at twice the speed you might require 1600 watts x 5 hours / 12 volts = 666 amp hours, but then accounting for the higher discharge rate increase that to say 800 amp hours of 12v batteries, connected however you wish depending on your motor.

The hardest thing to get right is probably the propeller pitch and speed and reduction ratio. Generally larger propeller diameters at slower rpm are more efficient. If you can make your own DC motor that is designed to run at lower rpm that would be a good start. You might do just as well with a larger motor run at lower voltage. Not sure. Outboard motor will add some complexity and losses in the gearing, but if you need gear reduction anyway it might not be so bad to work in that 90 degree turn. Again not sure. The 50% efficiency from motor to water might be optimistic, but is a good target to shoot for if you are able to do that part right. The hull drag and battery stuff is easier to calculate, reasonably ball park, but the propeller and gear reduction losses can be very high, and harder to estimate, and even harder to get right. Cheers.