Should say in the buy me brochure. But PWM stands forr Pulse Width Modulation. It is a way of rapidly switching the current on and off several thousand times a second in a deliberate manner which appears to a DC motor to be DC of voltage proportional, sorta, to the average of the wave form which is generally a square wave. With modern MOSFet transistors, it can be a very efficient way of controlling power into a DC motor or other similar load. Cheap trolling motors use whaat amounts to a network of resistors to attenuate current into the motor. Obviously this is very wasteful, and drains a lot of power off as heat dissapated by the "controller". This is why those kind of motors are not so inefficient at full throttle but are horrible on a slow bell. And actually it is possible to feed such a motor, with the throttle permanently set to wide open, with an external PWM power supply, which does the actual controlling of the motor from all stop all the way to full ahead dam the torpedo speed, at typically 92% to 98% efficiency. More expensive motors typically have a self contained PWM controller. The throttle is either a hall effect control or a simple potentiometer that supplies usually a 0-5v signal to the controller that tells it how hard to run the motor. The cheapies are a total horror show, from an engineering standpoint. Unless like I said, you feed it with an external PWM controller and leave the unit's throttle full open.
Ken brings up a good point. One that I did not push because it is so hard to get guys to give up on their dreams of slicing through the water at hull speed or better, leaving a perfect vee wake behind. Low and slow will gitter done. When you go electric, typically you have to readjust your perception and expectations of speed. HALF of your theoretical hull speed is generally a good goal. That is 1.34 x square root of LWL. So with a WATERLINE length of say 25 feet, that would be 5 x 1.34 = 6.71 kts. Half of that, well, let's just go 3kt and done with it. A useful speed, and one that you can sustain for a reasonable length of time and a worthwhile range. This is because as speed is increased arithmetically, power needed to make that speed is increased exponentially. There are several reasons for this, but mainly it boils down to reduced efficiency of the displacement hull as it begins to climb its own bow wave, and internal resistance of the batteries, which disappate considerable energy as heat.
If you can discharge your battery bank at the 20 hour discharge rate, you can get nearly the rated amount of energy from the bank. For instance, my batteries, 8 220ah 6v golf cart batteries connected in series, can go for 10 hours at 11a discharge rate, and leave me with a 50% charge. This is as far as you should discharge FLA (Flooded cell Lead Acid) batteries for decent life expectancy. If I double the discharge rate, first of all I do NOT double the speed, not even close. I increase by about 1kt or a bit less. But I also do not merely reduce my runtime by half... I reduce it considerably more due to losses within the batteries. This is called the Peukert effect. It is less pronounced with certain other battery types such as LiFePO4, but it is still there, and it slaughters your efficiency with FLA batteries at high discharge rates. So you want a fairly large bank, and you want to go fairly slow, in order to have a propulsion system with a usable range and good bang for the buck. The stated capacity in amp hours is usually at the 20 hour discharge rate. IOW, the rate that would fully discharge your batteries in 20 hours. So if you want to run at a reasonable efficiency for up to 10 hours, you will have a bank approximately equal to 20 times the anticipated cruising discharge current. 5a? 100ah. 30a? 600ah. And it gets better as you reduce the discharge current (and therefore speed) even more. Okay with 1.5kts instead of 3? Your range has just doubled, more or less. Its like getting something for nothing.
In addition to battery losses, there is wire losses. This loss is directly proportional to resistance of the wire, which is really low but significant at high current. The lower the voltage, the higher the current needed, to perform a given amount of work. So you increase current when you decrease voltage! Plus, that voltage drop, which would be of little consequence at say 48v, is crippling at 12v. Lose/lose. So I cannot recommend a 12v setup. Please, go with 36v or 48v. The 6v golf cart batteries are a low tecgh but rather cost effective solution, but dont stop with just a pair of them. Get 6 of them, wire them in series. And get a motor rated for 36v, too.
Generally, a bigger motor ran at turns for a given speed, say 2kts, will not use up hardly any more power than a smaller motor, at the same boat speed. So go for a big motor. When you don't need the full power of the bigger motor, which is almost never, just don't use full power. When you need it, you got it, though.
Just want to go on a short electric boat ride, without spending any real money? Go with a 12v bank (Puh-LEEEEZE, at least get proper deep cycle batteries, though!) and borrow a trolling motor of whatever size or configuration. Take a small gasoline outboard and a couple gallons of gas with you, too. Motor on out. When your batteries are down to half capacity, start the outboard and motor home with it. Terribly wasteful but you can then say you went electro boating. But if you are serious, go 36v with a 100 lb or better motor. Better yet, a torqueedo or a proper electric outboard at 48v. Even better, a normal inboard installation.
Kevin brought up another good point... the prop. If you use a trolling motor, you will probably want to try some different props. The rule of thumb is use the biggest possible prop, with a pitch of at least 70% of the diameter. Normally you can adjust the reduction ratio to make it work, but you can't do that with a trolling motor, so just nearly double the diameter with a 2 blade prop, or put on a slightly bigger 3 blade prop, and start from there.
Personally, I would go for much heavier wire than 6ga. I would go at least #2ga for what you are doing.
So, whip out your wallet, get to work, and let us know what you ended up with and how well or how badly it worked.
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