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

Monday, August 23, 2010

Re: [Electric Boats] Re: Sailboat Trolling Motor Options

Hi Sir Darnell,

I'm sure that there are people on this forum who can explain this better than me but here goes anyway! The power rating of an engine (or motor) is the amount of energy it disapates over time, this includes work and heat. Engines (or motors) that have a high work to heat ratio are more efficient. A motor dissipating 749 watts is rated as one horsepower no matter what. Your nail in a coil is a one horse power engine, its just very inefficient. This is why electric motors with smaller ratings can replace higher rated ICEs. A 10hp ICE is likely to throw away 6HP as heat leaving 4HP as work. This is the same as a 5HP electric motor that throws away 1HP as heat.

Hope this helps

Perry

--- On Mon, 23/8/10, sirdarnell <sirdarnell@yahoo.com> wrote:

From: sirdarnell <sirdarnell@yahoo.com>
Subject: [Electric Boats] Re: Sailboat Trolling Motor Options
To: electricboats@yahoogroups.com
Date: Monday, 23 August, 2010, 1:49

I didn't mean to start a debate, but was just trying to be helpful.

However, my understanding of the 1 HP = 746 watts is that 1 HP = 746 watts of power output, not input. Otherwise motors of different efficiencies would have the same HP rating even though some might be twice as powerful as others.

Horsepower is power output over time, a measure of work. One definition is 550 foot-pounds per second. I.e. a 1 horsepower gas or diesel engine can lift 550 pounds, 1 foot, in 1 second, in 1 gravity (or the equivalent.)

A motor drawing 746 watts of electric, that is 100% efficient would be delivering 1 HP (I don't believe 100% efficient motor can exist.) However, an electric motor that was 85% efficient and drawing 878 watts would be doing 1 horsepower of work and thus be generating 746 watts of output. This 85% motor if it is designed to draw a max of 878 watts of electricity would be a 1 hp motor (since gas and diesel engines are rated at max hp.) Thus pounds thrust relates to horsepower better than watts of input power.

WARNING sarcasm ahead (I can't seem to resist): Unless a nail and coil of barbed wire drawing power from a 746 watts 60-cycle AC source is just as powerful as a permanent magnet DC motor designed to draw a max of 746 watts using the most powerful rare earth magnets possible, the best possible wire, in the best configuration that science can devise are equally powerful motors? After all they both use 746 watts of electricity.

--- In electricboats@yahoogroups.com, "greenpjs04" <greenpjs@...> wrote:
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> The 4.0 in the Cruise 4.0's name refers to the power input. It is a 4 KW motor. Since by definition, 1 HP = 746 watts, the Cruise 4.0 is a 5.3 HP motor. The preceding is fact. The following is opinion. The Cruise 4.0's efficiency is better than most so it feels stronger.
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> Pat
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> --- In electricboats@yahoogroups.com, "sirdarnell" <sirdarnell@> wrote:
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> > According to Ray Electric (one of the oldest makers of electric outboards) 1 hp = approx 49-54 lbs of thrust.
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> > This would make the Cruise 4.0 a 4hp motor and the Cruise 2.0 a 2hp motor.
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> > Of course these are max HP numbers and an electric motor of the same max HP will out accelerate a gas motor, due to more low end torque. It will feel like you have more power because it has more thrust out of the gate, but an electric motor with a max of 4hp (assuming same gearing and prop) will not ultimately push a boat any faster than a gas 4hp engine.
> >
> >
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