Electric Trolling Motors (was: Good Girl For Sale)
A bit of work with the calculator...toggle quoted messageShow quoted text
Tuffy's motor (an 80 lb. thrust Minn Kota bolted to the side of the keel -- half of what Cal has on Surprise) eats 5 amps x 24 V = 120 watts at my usual, leisurely, rate of progress.
A Minn Kota 30 lb. thrust motor, at full chat, eats 30 amps x 12 V = 360 watts.
Both motors will produce a bit less power than they consume, but the calculations above are good for comparison.
Tuffy is supposed to weigh 924 lb. Add another hundred for the batteries. Good Girl is supposed to weigh 900 lb., without batteries. But who knows what they really mean by "displacement?" <sigh>
The 12 volt 30 lb. motor should have plenty of oomph for Good Girl. At full power the 30 will have about as much oomph as Tuffy's running at 15 amps, which is the highest I've run it at so far, except for when maneuvering.
The cheap Minn Kota 30 won't be as efficient at less than full speed as the much more expensive trolling motors with electronic speed control, but I think you'd be better off spending less on the motor and using some of the savings to get a bigger and/or better battery(ies). I'm probably gonna replace Tuffy's flooded lead/acid deep cycle batteries with AGMs someday...
On 8/8/2020 2:20 PM, I wrote:
The security of the Kingdom is increased by every man being more or less a sailor. (Capt. Marryat)
John. I just ordered a "cheap" 65 pound short shaft from the back alleys
of Sri Lanka or some such. But, of course, you brought me up short with
discussion of 24 volts...I still have an unused 24 of unknown thrust--about
65, I think, but gave up on the wiring to also have a full suite of
services on Walkabout. Anyhow, the latest lurch is to drop the gas backup
motor and use the electric as both a maneuvering (twisting around and
anchoring stern first is what we do alluhtime) and with the addition of a
roofmount solar panel I'm hoping to have a get home backup. I gave up on
the twin 4-stroke setup and am back to just the 9.9 Suzi. We lost about
1.5 mph and run at about 500 rpm higher. Mostly what I lost was delusions
of grandeur, with the loss of twins...Somewhere in my checkered past there
was mention of lower tron consumption with higher voltage...is that your
implication with the above "higher math?" Dan.
Displacement can be deceiving when thinking about moving a hull through the water. It's not so much the "weight" as it is hull shape which determines the size of the bow wake. The the bow wake is created by pushing the boat through the water and pushing it requires power. Complicating this, it's somewhat exponential so that as you increase speed the power required goes up a lot, especially as you approach hull speed (top end of displacement motion). The electric boat community has worked this out well, and as I recall the rule of thumb is doubling the power for an increase of a knot approaching hull speed. So moderate speeds, in the realm of half hull speed, can be fairly efficient, like your "leisurely, rate of progress".
Regarding 12 vs 24 volts. From your examples, 5 amps x 24 V = 120 watts & 30 amps x 12 V = 360 watts; lower watts = less losses to heating in all components. If you are using lead acid batteries, either flooded or AGM, higher amperage cause losses due to the peukert effect which can be quite significant at the higher amperage.
The trouble with "displacement" for comparison of boats is: what do they _mean_ by displacement. <sigh> Ideally, the displacement given by a boat manufacturer should be the weight of the boat with it's normal load --toggle quoted messageShow quoted text
the figure the designer used for calculation. Alas, what gets published as displacement can vary from the boat's weight when completely empty, to the weight when loaded to the max -- with no indication what. <sigh> But we have to work with what we can to get some _very_ rough idea about how different boats compare.
I think the crude spreadsheet I worked up a few years ago is useful, nonetheless.
Even though it doesn't take into account hull shape, which can have a big effect on wetted surface, which is the big part of resistance at low speeds. I've been noticing that Tuffy, because of her shapely hull and molded in keel, moves along under sail even in a barely perceptible breeze that would have left flat bottom, fin (technically) keeled Pearl dead in the water. But it would be hard to come up with a simple spreadsheet that included hull shape. <g>
You're right on the money about higher voltage having lower line losses, and lower losses from the Peukert effect.
On 8/9/2020 8:07 AM, johnacord wrote:
"My country, right or wrong," is a thing that no patriot would think of saying except in a desperate case. It is like saying, "My mother, drunk or sober." (Gilbert K. Chesterton)
I distrust high-thrust trolling motors that run on 12 volts. I spent a bunch of good money on a fancy-dan 50 lb. thrust Minn Kota -- a good brand -- 12 volt motor, and it doesn't push Pickle along noticeably better than the cheap 30 lb. thrust motor I had before. Maybe it really only produces 40% more oomph rather than the 67% boost you'd expect from the advertised thrust, since it only consumes 40% more juice...toggle quoted messageShow quoted text
When you start pulling the kind of current the high thrust 12 volt motors consume you end up with loss to heat in the wires, and the Peukert effect means that you run your batteries down a lot faster. If that cheapo 12 volt 65 lb. thrust motor really does make that much thrust, it'd have to draw around 65 amps. That's a lot. A 24 volt motor would draw half the current.
I'm curious what'd happen if you ran a 12 volt trolling motor on 24 volts... I may try it on my $5 garage sale 28 lb. thrust motor. I haven't done so before because I didn't have two matching 12 volt batteries, but now I could borrow Tuffy's. :o)
On 8/9/2020 7:01 AM, Dan from Almostcanada wrote:
John. I just ordered a "cheap" 65 pound short shaft from the back alleys--
History teaches that grave threats to liberty often come in times of urgency, when constitutional rights seem too extravagant to endure. (Thurgood Marshall)