Analyzing the data from our last test and some motor experimentation has made it clear to me that both our batteries and our motor are toast.
First, the batteries are nine years old. From the test experience and the data, I was pretty sure they were toasted. I took them in to Batteries Plus for testing, and they smoked during the load test. So they're done. Luckily, the guy who donated the first set has a second set available for us, so that's not a show stopper.
However, the motor is dead. I suspected after we weren't able to get the speed we expected out of the boat during our last test that it had the wrong prop on it. Trolling motors are generally propped to drive a heavy boat slowly, so with the lighter and faster boat, it just was not up to the task. In order to properly size a new prop, I had to measure the motor constant.
Sadly, I don't have any picture of my setup -- I was using an IR diode with an IR remote receiver mounted in a breadboard and the prop as an interrupter. I drove the LED at the required 38 kHz with an Arduino. I measured the back EMF with a multi-meter and the pulse rate out of the IR receiver with my Saleae. I chucked the prop nut into a drill to drive it. I measured the Kv just fine -- 420 RPM and 2.5V for a Kv of 168 RPM/V, pretty much as expected from online discussions.
I then went to measure the DC resistance through the motor. Nominally, this motor draws 30A @ 12V when at full throttle. I expected a DC resistance commensurate with those numbers. However, I measured 10 ohms. This means the motor is most likely dead. This would explain why the boat slowed down dramatically in the last half of the test.
A new trolling motor would be about $100. However, we decided that it's long past time to make sail development our highest priority, because that's what's going to get us around the world. We filled and sanded the first wing this past weekend, so it's just about ready for paint and final electronics fit. The second one is waiting on its leading edges for final assembly. We'll go back to power if we really need it for something, but for now, we're a sailing operation from here on in.