T-Pain Is On A Boat: Re: [Electric Boats] Measured performance

Monday, March 14, 2011

Re: [Electric Boats] Measured performance

Intuitively (and admittedly, I don't know much), that seems like a lot of watts. The hull speed for that Bermuda is probably somewhere around 6 knots, give or take. A high rating golf cart controller maxes out at somewhere around 500 watts for carts that go perhaps in the high 20 MPH range. I would think that it takes less power to move a sailboat at hull speed than to push a golf cart around at 27 MPH...

But the numbers don't lie. What's wrong with my thinking. Can you translate this to torque and prop RPM?

----- Original Message -----

From: Eric

To: electricboats@yahoogroups.com

Sent: Monday, March 14, 2011 7:57 PM

Subject: [Electric Boats] Measured performance

So I finally got a chance to do some controlled performance trials over the weekend. I headed into the main channel of Marina del Rey at about 11:00 AM to measure system data while travelling a 400 yard course in opposite directions. Unfortunately, there appears to have been some residual tsunami surge that skewed some of the runs. From nearly no difference between the 2 runs to 1kt faster speed southbound, 1.4kt faster northbound 10 minutes later and 0.5kt faster southbound 15 minutes after that. So I'll have to repeat the tests sometime in the next few weeks.

The boat is a 1964 Bermuda 30 ketch, 8'9" beam, 24' LWL and 10,200# displacement. The conservatively rated 5.5kW electric drive was purchased from Propulsion Marine in Santa Barbara. I assembled my own battery pack from 16 Thundersky 160AH LiFePO4 3.2V lithium cells for a pack rating of 8kWh @ 50V nominal.

I measured pack voltage, motor RPM, southbound speed and average amperage, northbound speed and average amperage. Then I averaged the results for the 6 major speed settings. The pack voltage and current was measured with a E-xpert Pro HV battery monitor, and the speed was measured with a Garmin 60CSx GPS.

But here's the quick recap of my boat at 6 different speed settings, 4 runs per setting.

500W = 3kts
1000W = 3.8kts
1475W = 4.3kts
2460W = 5kts
3910W = 5.7kts
5170W = 6kts

The funny thing is that my old diesel maxed out around 5.5kts at full throttle. While I don't intend to motor places at 5kw, it's nice to know that I've got the extra power if I get into a tight spot.

The system worked great and now I get to re-work my range estimates for the better. This system exceeds my expectations, and I want to thank James for guiding me through the conversion. It has been a real partnership, again more than I expected. The only negative that I can see so far is that the new prop has more prop walk in reverse than the old one, so I've got to spend some time practicing backing down so that it becomes second nature again.

That's it for now....

Fair winds,
Eric
Marina del Rey, CA