Re: [Electric Boats] Re: Power Relationship

 

Roger:

  It is interesting to look at the power losses in an automobile to see just how little energy that is in the raw gasoline is actually transferred to kinetic energy of the car rolling along the ground.

(1) Loss in the engine, from the raw energy available in the gasoline to the mechanical energy available at the flywheel. 63%. This is lost as heat in the exhaust, in cooling water, in heating lubricants, in pumping losses, driving engine accessories, etc. This leaves 37% to power the car.

(2) Idling/coasting losses: 14%. This leaves 23%

(3) Electrical accessories (lights, heat, radio computers, A/C, etc.) 3%. This leaves 20%.

(4) Driveline losses: 6% This leaves 14 % of the energy in a gallon of gasoline put to the tires on the ground.

(5) Air drag of 6% and rolling resistance of 2% leave, finally, only 6% of the energy that you started with remaining as kinetic energy in the moving car.

Yes, the automobile is a pretty wasteful machine. 

Dick

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Roger,

Welcome. As Steve indicated, your 6hp motor is mostly a 6hp motor, not a 6hp propeller. Manufacturers usually measure the motor's horsepower without gears and oil pumps and water pumps and alternators and any other incidental engine load. So electric repowers are usually about 1/2 the watts (horsepower), a little more or less depending on the guessed old efficiency and the guessed or calculated new efficiency. Propeller diameter and RPM make the biggest difference (bigger and slower are most efficient).

The other part of your question: "is electric power more efficient?"
There are several answers:
1. YES, because there is no idling wasted energy.
2. YES, because you use exactly the power you want, when you want (you are not confined to minimum RPMs).
3. YES, because internal combustion engines (ICE) can only harness 10% - 30% (ballpark) of the energy of the liquid fuel, and electric ones get 70% - 90% (ballpark) harness.
4. YES, because the maintenance cost of electric is roughly one tenth the yearly cost of ICE maintenance.
5. YES, because sailors end up sailing much more and motoring much less (starting an ICE is problematic, starting an electric is quick).

Questions are good,
Mark Stafford

--- In electricboats@yahoogroups.com, "roger_russelburg" <rrusselburg@...> wrote:
>
> Hi
>
> I am new to this group.
> My sailboat works fine with 6hp outboard.
> Is there direct relationship (6hp = 4750w) or is eletric power
> more efficient?
>
> Maybe this to complex for a simple answer.
> Point me in right direction.
>
> Thanks
> Roger Russelburg
>

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