I visited Cal today. His ammeter was hooked up somehow so it measured the current going to the motors, at least, but he was using a 200 amp shunt for a 100 amp gauge. which I now know isn't kosher. We moved the shunt to the common ground and replaced the shunt with a 100 amp one Cal had lying around. Maybe even the one that originally came with the gauge. <g> He should be getting more accurate and complete current readings now -- if the shunt's mV rating matches the gauge -- but we can't tell until we get the boat in the water, and Cal bunged up his knee so that'll be later, after he gets it rested up.

The dial ammeter Cal's been using isn't very precise. It has a range up to 100 amps, and the swing of the needle is only about 90 degrees. We really should keep trying to talk him into getting a digital gauge so he can keep better track of the current draw under different conditions. Like trying different propellers.

Cal's idea about switching off one motor to save juice is an interesting one to ponder while sitting in our armchairs. Since he started dreaming up Surprise I thought Cal only needed one of the 80 lb. motors for her. But now he's got two bolted to her bottom. Let's say he's cruising along with both motors at 3 mph. the switches one off and "throttles" up the other until he's going 3 mph again. Will he use less juice? The motors have a fairly efficient pulse width modulated speed control, so not much juice is wasted when running both motors at a reduced speed. If he switches off one motor it's still there under the boat with its propeller free wheeling causing drag. Two motors running slow, or one motor running harder and dragging a dead motor. Which will give longer range? If we can rig up a more precise ammeter in Surprise we can have fun doing some experimenting. :o)

I looked at the documentation that came with Cal's motors. Their instructions for installing them are probably OK, but I was looking for specifications. In vain. <sigh> But I did find something interesting. They said that the rule of thumb for current draw for their 12 volt motors is 1 amp per pound of thrust, but the rule of thumb for 24 volt motors is 3/4 amp per pound of thrust! I wonder whty it isn't just half the draw of the 12 volt jobs, as you'd expect... So Tuffy;s 50ish amp draw at full chat isn't above her rating. Cal's motors could draw 120 amps at full power,

--
This email has been checked for viruses by AVG.
https://www.avg.com