T-Pain Is On A Boat: Re: [Electric Boats] Trolling Motor Question

Monday, February 22, 2010

Re: [Electric Boats] Trolling Motor Question

Kevin is exactly right. Just check the amps at WOT with the boat tied to the dock. Call that X amps.

The best prop for your boat is the one that causes the motor to pull X amps running free and gives the highest speed. Staying under X amps will insure you are not overheating the motor.

Note that large power (thrust) differences are needed to make small speed changes at Speed Length ratios approaching one and above. A more sensitive measurement might be to compare props at the same cruising speed and record the amp draw. The one with the lowest draw wins.

The reason boat props look the way they do is the strength required to absorb lots of power, strength to withstand minor grounding and the desire to minimize draft. All three requirements point to short fat blades. Long skinny blades are more efficient. That's why gliders have long skinny wings.

The MK power is so low the nylon model airplane props are adequately strong. I doubt they will survive grounding or even weed entanglement nearly as well as the MK stock prop but they are cheap enough to carry several spares.

My experiments show fairing the round motor shaft will pay big benefits in efficiency and act like a rudder to enhance steering.

That boat looks like a perfect hull for E power, all you need is a little air compressor to blow a steam whistle! Your motor mount in the rudder tube is clever and should work great.

Don't cut the shaft off until you have used the boat at speed in some waves. The lack of a cavitation plate (like on an outboard engine) makes the trolling motor prop subject to pulling air down and ventilating. Turbulence from the round tube makes this worse but the fairing will help. You may have to get the motor deeper in the water than you first expect.

Denny

----- Original Message -----

From: Kevin Pemberton

To: electricboats@yahoogroups.com

Sent: Monday, February 22, 2010 12:14 PM

Subject: Re: [Electric Boats] Trolling Motor Question

gregory_west2001 wrote:
>
>
>
> After reading your posts on this and other sites, I bought a number of
> the airplane props (9" and 10" with 6", 7" and 8" pitches), but I
> wonder if the blade area will be too small for the boat if we need to
> move through some tide or wind. I am also considering the new Torqeedo
> two-blade kayak prop (8" x 8"). The Torqeedo kayak prop is designed
> for the Torqeedo motor turning at 1200 rpm, which is the rpm that I am
> hoping to get at just under 30 amps. I would appreciate any thoughts
> you or anyone else might have on whether we should worry about not
> having enough blade area with those airplane props for this little
> boat. The airplane props just don't look like boat props.
>
> Greg
>
Hey Greg,

A shrink once told my late wife that worrying was the act of discomfort
over something that hasn't happened.

Largest Diameter that will fit, slow RPM, lowest number of blades is the
formula for best efficiency. Square prop diamensions i.e. 12"x12" have
been cited as best, but that is why you are messing with airplane props.

Surface area of the airfoil creates drag thus the need for more power,
but bite is important as well, so we live with drag.

Fluid dynamics are calculated the same in water or air based on density
of the fluid. The hardest part to deal with is the slower the blade
turns the stronger it must be to handle the torque it is expected to
handle, making a thicker blade thus wider foil, and more drag.

What you will be able to do:

Test different pitches to find optimum pitch.
Find optimum RPM.
Test different Diameters. (/Larger Diameter more bite)

/The cost to you for such great data is very low, so no worries mate.

Kevin Pemberton

P.S. Keep an eye on motor temps, the major reason trolling motor
manufactures don't talk pitch of their blades to the customers.