T-Pain Is On A Boat: Re: [Electric Boats] Amp draw vs speed for 37ft sailboat with 10 ft beam, displacement 13,000 lbs

Wednesday, July 3, 2019

Re: [Electric Boats] Amp draw vs speed for 37ft sailboat with 10 ft beam, displacement 13,000 lbs

Jeff,

I have a 2:1 reduction, and my 15" prop has a pitch of 15". Having a "square" prop makes the transfer of energy from motor to water efficient, but at the cost of motor heat, and a higher propensity for cavitation. The motor provides adequate power for my boat but it tends to heat up if I stay above 5.5 kts for more than a few minutes. It was a deliberate design decision to pursue efficiency over power and heat, but I have no complaints. If I ever decide I want to be able to run faster than 5 kts I would start by adding some cells to make my battery 72V instead of 48. That would provide more power with less heat generating current. But then I'd be over the 48V nominal where additional electrical safety issues present themselves along with additional USCG requirements. If you have the Motenergy 1115 from TSM like I do, it is actually rated 12kW. Not sure why TSM doesn't advertise it as such. A 20kw motor would suck down more power but could do more work without overheating. But, it would be heavier, more expensive, require higher voltage, and would probably move you into an AC motor rather than the BLDC you probably have. It's a different animal with it's own strengths and weaknesses, and probably way more of everything than you really want.

Darin

On Wed, Jul 3, 2019, 2:18 PM Jeff LaCoursiere jeff@jeff.net [electricboats] <electricboats@yahoogroups.com> wrote:

I'm curious what motor/reduction you are using, and if that should even make a difference. I suspect your boat, like mine, is on the edge of 10KW versus 20KW. If these numbers are with a 10KW motor, would we expect them to be significantly different with a 20KW motor? Perhaps only at the top end?

Cheers,

j

On 7/2/19 8:14 AM, armyofpenguins@gmail.com [electricboats] wrote:

I completed my conversion a couple of months back and am loving it. It's time to start sharing what I've learned. ;)

I've collected data during a couple of outings under relatively calm conditions (65F, 5-10 kt winds), and I used excel to generate a trend line and a formula (using a power series if you're into math) that provides a pretty reliable estimate of how much speed I can expect for a given amp draw. This is really easy to do in excel and if anybody needs help I am happy to show how to do it. Here's the formula (keep in mind the formula would be different for a boat with different characteristics than those in the subject line of this post):

Speed (in kts) = 1.3728 X Draw (amps) ^ 0.2704

Some examples:

At 20A, my s peed would be about 3.1kts.

At 40A, my speed would be about 3.7kts.

At 50A, my speed would be about 4.0 kts..

And at 100A, my speed would be about 4..8 kts.