HP rating on outboards

[Tnstratofam]

I have a question about which number means what. My 1982 Chrysler outboard is a 75hp. Now I think that rating for my engine is at the prop. I believe older outboards are rated at the crankshaft. So here is my question when looking at an older outboard how do you determine the hp rating at the prop, and on a newer outboard how do you determine the rating at the crankshaft. Which should the average boater be considering when looking for a repower. I know weight of the engine plays into max hp for a boats rating. I'm just curious about the hp ratings, and how to determine what you've really got.

 

[metriccrescentwrench]

Don't overthink things. True, outboard manufacturers went from rating at the crank to the prop somewhere in the early to mid '80s (depending on the manufacturer). That has zero bearing on what you should repower your boat with. It's not a large difference, I've read that the actual numbers are within about 5 to 7 percent. If you're racing that much difference matters, for the rest of us we wouldn't be able to tell.

 

[Tnstratofam]

Thanks for the reply. I was reading another thread earlier today where the poster was wondering if their motor was powerful enough for their boat, and it got the wheels turning in my head. I am very guilty of over thinking stuff like this. No more than 5 or 6% isn't even worth worrying about. I was more curious than anything.

 

[roscoe]

Power loss is typically 3-6%, depending on the number and type of gears and bearings. So not really worth worrying about.

Additionally, manufacturers are allowed 10% +/- when they make a motor, to allow for manufacturing variances.
So your 50 hp motor, would need to fall between 45 and 55 hp to wear a 50 hp badge.

 

[steelespike]

The Chrysler is rated at the crank.Chrysler and Force were all rated at the crank.

 

[64osby]

The law doesn't consider crank or prop either. If the boat states X than X it is for max.

 

[Frank Acampora]

I forget the exact year, but after either 1982 or 1984 the NMMA decreed that ALL outboards would be rated at the prop. NOW for actual numbers. The Chrysler 105 was downrated to 90 to comply with the new regulations Thus, the change in rating from crankshaft to propeller resulted in a 15 horse loss in rating--that is, the water pump, seals, and lower unit gears absorb 15 horsepower in this case.

ALL Force engines were rated at the prop.

 

[boobie]

OMC started doing it in '85-'86.