Lets work up those battery numbers.
I'm assuming that you're estimating 3.25hp (2400W) at the propellor shaft. Assuming 80% combined efficiency of the motor, controller, drive leg, etc. (probably optimistic) then you're looking at 3000W from the batteries.
Flooded lead/acid batteries (FLA) have similar energy densities. Eight Trojan T-105s weigh about 500 pounds and have a 20 hour rating of 10.8kWh (225Ah x 48V) of energy storage. Assuming a 60% depth of discharge (DoD), this gives you a usable capacity of 6480Wh.
For a 48V system, 3000W is a 62.5A load. Using a Peukert's exponent of 1.25 for FLA batteries, the 500 pound battery bank will 60% dicharged after 84 minutes. Back off to a 50A load (2.5hp at the prop shaft) and the range will stretch to 111 minutes to 60% DoD.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "luv2bsailin" <luv2bsailin@...> wrote:
>
> I've been eyeballing that design too....
> As a design exercise, my suggestion would be to start out by forgetting the motor/reduction/prop details for a bit and start with the batteries, since they will likely be be the limiting factor. First how much weight can you carry, and then how much can you afford. If you can settle for about 3.25 HP at cruise, you can get a couple hours, with some reserve, out of a set of 8 golf car batteries. They'll weigh about 500 lb, and cost about $600 to $800 last I checked....
>
>
> --- In electricboats@yahoogroups.com, "b504now" <b504now@> wrote:
> >
> > Last year, we converted a small outboard to electric power. We ended up with a Etek clone (MARS), a AXE controller, 48V car battery system but it was on top of a B&S electric outboard base (the previous owner had stripped the motor out). The boat was a 18ft homebuilt garvey flats and to say it was inefficient is being kind. We can run about 1 hr all out and even that gets about 6mph. (105ah batteries). Deep cycle (thats what I had). My guess is that since the outboard was built for 5hp, pushing with 10 was ineffective (prop size)
> >
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