Power Ratings. What determines this number?

[Unclebob]

As the topic states, what factors determine power ratings on boats?

Wondering why I can't put a 200hp outboard on a boat rated for a 140hp?

[Radioshop]

The USCG runs a formula based on length, beam and, deadrise.   I would really discourage exceeding the manufacturer's reccomended maximum horsepower.  140 is pretty powerful.  Bear in mind most 170's have a 90.

[Jerry-rigged]

Check out -
http://www.uscg.mil/d8/mso/louisville/WebStuff/comdtpubp16761_3b.pdf
and scroll down to about page 5.
You might want to have a pincel and paper handy

This Doc is written for 'backyard builders' but I bet it would give you a good starting point if you don't know the Mfg. rating for your hull.

Jerry

[JimCt]

Excellent reference; that CG doc. should be put posted somewhere permanent here.  It gives a good explanation & formula for guys thinking of souping up their old boats.

[ddd222]

from what i was told, older outboards and newer ones differ from there horsepower ratings as one was rated at the prop, and another was rated at the shaft. I didn't get far enough in the discussion to talk about it intelligently. Just repowered and my boat is rated for 240hp, and i put a 250hp. Been my opinion to always give the max or close to the max hp for resale, and for another reason my mechanic gave me. My 250 strains alot less than a 175 or 200 hp to get me to plane or cruising speed, which may in the long run provide longer engine life. Logical, but not sure its right, just what i've been told