[QUOTE=Ex-Calif;1766425]Glad you brought this up. I have heard that the 2KW (2.X hp) electric acts like a bigger hp petrol but I admittedly don't understand electric math. I do get the instantaneous torque stuff. So maybe it's a propping thing.
I presume the batteries get plugged into a
charger. And why I contend they are great for the case where you can
plug them in.
As a replacement for hydrocarbons many advantages disappear when you have to have a large battery bank and tons of solar to charge it.
We have two
solar panels that keep the batteries charged. Not a problem since there is a lot of solar time in between need for the torqeedo.
I too don't know the conversion between kw and hp but it was explained in one of our club's site threads and finally made a little sense. It's supposed to be equal to 6hp and seems to push the boat about the same as a 6hp however will not last as long as a small
fuel tank as you know.
"Newtons 2nd law says that force = mass times acceleration.
The drag, or friction, force on an object like a boat
hull moving through water increases with mass and speed, approximately as mass times speed squared.
The 2 things above mean that when you push on a boat with a constant force, it will accelerate until it reaches the speed where the drag force equals the pushing force and then the speed will be constant (zero acceleration). A heavier boat will accelerate slower (high mass) and will generally have a larger drag force at a given speed because it displaces more water.
In electrics, power (P, in Watts, W) equals Voltage (V in Volts, V) times
current (I in Amps, A), so for example a 12V motor drawing 10 Amps is using 120W of power. P=VI, I=P/V, V=I/P
Horsepower (HP) ratings are physically a power rating. For lots of calculations, it's easier to
work in Watts (W) or Killowatts (kW) (1kW = 1000W).
If you push against a resistive force (like drag) then pure physics tells us that the power you use is equal to the force times the velocity (P=vf)
Electric trolling motors have thrust ratings, in pounds (lb or strictly, pounds of force, lbf). This is a measure of the static thrust generated by the motor, ie if you attach the motor to the
dock (or something else that will not move) and run it, it is a measure of the force with which the motor will push on the
dock. Note that once the thing being pushed starts to move (like a boat will), the amount of force may change). For lots of physics calculations, it's best to
work in units of Newtons (N). 1 lbf = 4.4N.
You might see from the above that converting from a static-thrust rating (lbs) to a power rating (HP) cannot be done by pure physics or like a unit conversion - there are many assumptions about the
propeller in use and boat that is being pushed that need to be made. Different manufacturers will almost certainly make different assumptions, presumably depending on the boat they have in mind for their products. The main thing here is that it's very different to be driving a small dinghie like a walker bay, than it is to drive something like a Folkboat.
Gas engines
For the
record, gas engines are rated by peak power output, usually measured in HP.
[edit]Minn Kotas
These are marketed as trolling motors for small boats and dinghies. The 12V ones come in 20-50 lbf static force versions. Minn Kota make more powerful motors that come in 24V and 36V versions. They make "outboard" versions like the ones we have and also ones that are designed to be permanentely mounted to the
hull. Some of the fixed mount ones are actually double motor, double screw pods. This is because it's cheaper / easier to have 2 smaller motors than one larger one.
I read on a website that a good
rule of thumb is that a 1 HP gas engine is approximately equivalent to 75lbs.
The largest 12V Minn-Kotas are 55lb thrust. They make more powerful ones in 24V, 36V and 48V. You can get these voltages by connecting 2, 3, or 4 12V batteries in series.
They use higher voltages for higher power because the size of wire you need and the resistive losses in the wire all depend on the
current. Because power = current * voltage, you can keep the current from getting too large by using a higher voltage for higher power.
I think it's reasonable to say that the 55lb Minn-Kota is borderline for Cheers. Often it's adequate, but it would be nice to have something stronger.
[edit]Torqueedo
I thought some notes on the torqueedo would help. These are generally marketed towards bigger boats than the Minn Kota target audience.
Rather than the Minn-Kota lb rating, Torqueedo give their motors ratings that they claim are the gas engine equivalent sizes. They make a 6HP and 2HP version.
The 6HP one is actually a 2kW motor (physically 2.7HP). It's powered at 24V and draws about 83A at full power. It generates 120 lb of static thrust.
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