A Question for the Illustrious Gord

[Chief Engineer]

How does one compare the Horsepower between, say a 9hp four stroke, a 9 hp two stroke, a 9hp diesel and a 9hp electric motor.

I have been pulling out what hair I have left trying to get an answer.

[captain465]

A pound of rocks and a pound of feathers??????????????????????????

[captain58sailin]

I believe it would depend on whether you are measuring, Break hp, shaft hp, or KW.

[senormechanico]

Brake HP is gross, shaft is net, and electric is not directly measurable, although it's roughly equal to 1hp electric+ 746 watts= (mas o menos ) = 3hp of gas or diesel power (shaft) as I remember. Gord will no doubt correct me.

Steve B.

[Boracay]

Surely it would run:-
2 stroke < 4 stroke < diesel < electric

The question should actually ask about torque at cruising rpm. From that derive real hp.

[Chief Engineer]

I hope there is more elucidation down the road.

Originally Posted by senormechanico

Brake HP is gross, shaft is net, and electric is not directly measurable, although it's roughly equal to 1hp electric+ 746 watts= (mas o menos ) = 3hp of gas or diesel power (shaft) as I remember. Gord will no doubt correct me.

Steve B.

[GordMay]

A mechanical horsepower is independant of it's method of generation (elect, gas, diesel).

Power = Work ÷ Time

Units of Power:
1 Horsepower (HP)
= 33,000 Foot-Pounds (Ft-Lb) per Minute*, or 550 Ft-Lb Sec
= 745.7 or 746 Watts (or Joules per second)
= 2545 British Thermal Units per Hour (Btu/hr), or 42.42 Btu/min

* James Watt observed that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. He assumed a horse could do 50 percent more work than a pony, and arbitrarily defined the measurement of horsepower at 33,000 foot-pounds of work in one minute.

Brake Horsepower is measured* at the output shaft of an engine or motor; without the loss in power caused by the gearbox, alternator, water pump, and other auxiliaries. The actual horsepower delivered to the propeller (or driving wheels) is less.

* Measured by a friction brake attached to the drive shaft, and recorded on a dynamometer

Shaft Horsepower is the actual amount of power delivered to the propeller. The latter will be less than the former because of friction losses in the transmission and the stern gland, etc.

As a rule of thumb, shaft horsepower in small craft is usually* between 70 and 90 percent of brake horsepower.
* Nigel Calder quotes “common” shaft horsepowers as about 95-97% of BHP. He probably assumes that BHP includes water pump & alternator losses.

Perhaps, someone would be interested in giving us a basic physics tutorial, describing Force, Work, Acceleration, Power, & Torque, etc.

[LtBrett]

Boracay has it right. HP, wherever it is measured, is a measure of peak engine power. For marine applications, peak hp matters less than hp at cruising rpm. To take it a step further, hp is measured on a curve, ie xx hp at xx rpm. The overall power of the engine is the area under that hp curve. The strongest engine for your application is the one with the most area under the curve within your usable rpm range. I would expect a 9 hp electric to make nearly 9hp from low rpm's, giving it the most area under the curve, making it the strongest motor. As for the other motors, it depends in part on how high they rev. Think about it in terms of how wide is their power band where they make near peak hp. As a broad statement, two strokes have a power stroke every revolution and are generally stronger than four strokes, which have a power stroke every other revolution. Diesels tend to make power earlier than either gas engine, but don't rev as high.

You actually asked a fairly complicated question that lots of knowledgeable people stumble over.

Brett

[Chief Engineer]

Boy oh boy...I knew I could count on you guys/girls.