T-Pain Is On A Boat: Re: [Electric Boats] Re: Trolling motors ==> electric outboards

Friday, May 31, 2013

Re: [Electric Boats] Re: Trolling motors ==> electric outboards

General Thrust Equation: http://www.grc.nasa.gov/WWW/k-12/airplane/thrsteq.html
Since thrust is dependent on the mass, velocity, and density, it would be difficult to put a thrust figure on a motor.

Horsepower: http://en.wikipedia.org/wiki/Horsepower#Nominal_horsepower

When you change the pitch or diameter of a propeller, you change the amount of water pushed to the rear per rotation of the shaft. The pitch and size of the prop are matched to the speed that you want the greatest thrust. IC engines have low torque at low speeds and operate best at high speeds. Electric motors have high torque at low speeds.

IC engines are generally rated at brake horspower (power off the crankshaft). Electric motors are rated off the shaft. Since the Torqeedo mounts the prop right off the motr shaft, there are no losses due to friction, acessories, or drivetrain losses.

1 hp = 746 watts = .746 kw

Joe Murray
Different Drummer #364
Panama City, FL

From: John Francis <surv69@gmail.com>
To: electricboats@yahoogroups.com
Sent: Thursday, May 30, 2013 1:05 PM
Subject: Re: [Electric Boats] Re: Trolling motors ==> electric outboards

I'm going to go out on a limb and pretend I think I know what I'm talking about, simply becuase the thrust and HP is too confusing.

It appears to me that electric motors, while having a HP rating(of the actual motor), in a boat propulsion system ... is rated in thrust.

Evidently THRUST already computes all the prop data and revolutions to achieve a specific THRUST ... like a packaged, computed value.

I've heard that if a person changes over a MinnKota prop to a Kipawa-type prop, the THRUST increases, while the motor in the buld unit remains the same.

So THRUST is the value of pushing power off of the propeller and whatever speed is predetermined as the base-speed(motor).

A 10 HP I.C., is an engine that delivers 10HP of work at some point between the cylinders and the propeller shaft. You can put a double sized prop with a double deep bite, and the motor remains a 10HP motor. If you put a smaller prop on a 10HP motor, it remains a 10HP motor.

So many variations that all come down to pairing up different components.

Personally, I think THUST is the most descriptive value of what the motor can do for us.

This is not that electric mootors need to have available HP ratings, but rather, that IC engines should be computed for THRUST ratings. After all it is THRUST that pushes water astern of you boat.

Probably, none of this is important . . . bigger is better . . . until a group of "green", sailors with displacement hull sailboats, who work in that magic marginal land of decifering THRUST, HP, matching propellers, prop speed and torque.