Hi Pat.
Thanks for the clarifications. I did make the assumption that the boat would have some sort of speed control, rather than switched direct wiring between the batteries and motor. So for any practical purposes, the pilot will be chosing the desired speed with the throttle and the controller will load the batteries as needed to meet the demand.
Boats without a way to "easily adjust the current to achieve the desired result" i.e. throttle control, are dangerous to the user and the other boats around them.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "greenpjs04" <greenpjs@...> wrote:
>
> Hi Eric,
> Everything you say is true, but there is an implication that a boat user can easily adjust the current to achieve the desired result. For a given voltage, the main way to increase the current drawn by a motor is to increase the mechanical load. If you travelled from a normal lake to one filled with molasses, the current would certainly increase! To use examples similar to yours, if you have a motor that is designed to use 2400 watts of power at 48 volts, it will draw 50 amps. As you stated, a similar motor designed to 2400 watts of power at 24 volts will need to draw twice the amps, or 100. However, if you take that first motor and just lower the voltage from 48 to 24 volts, the current will drop in half (to 25 amps) - NOT automatically double to 100 amps. I know you already know that, but the original poster seems to be confused by that concept. (Of course, when using a PWM controller, you need to consider the average power, voltage, and current, but the theory is the same).
>
> I hope I am helping here. I am not trying to be difficult, but as an electrical engineer, I always get the urge to correct misconceptions.
>
> Pat
>
>
>
> --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> >
> > Here's a page that discusses how PWM controllers vary the voltage to a DC motor. http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html
> > You can jump to section 2 for the basic premise.
> >
> > Remember that the power of the motor is the product of the voltage and the amps. So a 24V system at 100A will produce about the same power as a 48V system at 50A. If your boat needs 2500W of power to motor at a given speed (my boat motors at about 5kts at 2.5kW) then it would take 100A at 24V or 67A at 36V or 50A at 48V.
> >
> > Since it takes power to drive the boat, halving the voltage will require doubling the current for the same speed. If your system can't handle twice the current, then your power and speed will be reduced.
> >
> > Boat speed is not linearly proportional to power. As far as boat speed is concerned, a very rough rule of thumb is that each knot of speed takes twice as much power. So halving your power should slow you down about 1 knot.
> >
> > Fair winds,
> > Eric
> > Marina del Rey, CA
> >
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