Hi Dan,
"I also figured out amp hr ratings for batteries. 240 amps 6v + 6 batteries = 240 amp hrs at a 20 hr rate. 25 amp?" With six 6V batteries in series, you've got 240Ah at 36V. 240Ah / 20hrs = 12A @ 36V.
You can also directly see the Peukert efect in their 75A rating. 75A * 2.2hrs = 165Ah to 100% DoD.
Remember that my speed to amp conversions are estimates, your boat will probably be different, but these should be close enough for range-duration estimates. A boat your size should take about 4000W (5.3hp) to run at 5kts. 3kts should be around 1500W (2hp). Again, your results may vary, but without observed data for your boat, this will have to suffice. I'm also going to push your DoD to 60% in the calculations. I think that quality flooded batteries can deal with that level of service, as long as they are recharged reasonably quickly. And you can see that throttling down will almost always extend your range.
6 * 240Ah @ 6V = 240Ah @ 36V = 8.6kWh
8.6Kwh * 60% depth of discharge = 5.2kWh
110A @ 36V = 3960W. 5200Wh/3960W = 1.31 hours
With Peukert Effect for FLA = 0.75 hours @ 5kts
40A @ 36V = 1440W. 5200Wh/1440W = 3.61 hours
With Peukert Effect for FLA = 2.65 hours @ 3kts
Your prop needs to be matched to the boat speed that you expect and the RPM of the driveshaft. Running some sort of gear reduction allows for slower shaft speeds and more aggressive props. Once you know your target shaft speed, almost any prop vendor will be able to suggest a prop that should deliver your desired boat speed from your motor speed. Many diesels have gearboxes with 2:1 reduction, meaning that 3200 engine RPM is 1600 shaft RPM. Your smaller candidate motor might be able to spin the stock propellor to 1600 RPM, giving similar performance to your old engine. Going to a more aggressive prop may require some sort of gear reduction to match the new prop to the same motor.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Daniel" <nov32394@...> wrote:
>
> Eric, Can we go over this again it is late and my brain is not working as well as it should.
>
> I found what I think is the motor I will use. It is called the Beast by GE 5BC49JB 1134 or 32 or sometimes by this # D-394 or 5. The specs that I found are 36v 275a = 9 hp 1600 rpm or 48v = 400a = 15hp 2200 rpm. These numbers are very close to the Atomic 4 in fact quite a bit better. I could maybe re-pitch the prop or go with a 3 blade.
>
> I also figured out amp hr ratings for batteries. 240 amps 6v + 6 batteries = 240 amp hrs at a 20 hr rate. 25 amp?
>
> I hardly think I will be running this thing at 9.9 kwatts and the motor does not give hp ratings for less than. I am not sure if I can just cut it in half or fourths. On the Trojan site they list the T125 @ 75 amps for 132 min. I know I can run a golf cart into town 5 miles golf and come back. I think time on the cart would be close to this number. For a bolt right on almost I think it is worth a try.
>
> Dan
>
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