Outboard H.P.

[jtb]

How is the horsepower rating determined for a boat? I can guess it has to do with beam and length, how about the weight of the motor ? I've noticed a few boats, 16-18', that seem to have low ratings, say 50HP., when I would have thought they coulg take more. Just wondering, thanks.

[Cuyahoga Chuck]

First, it depends on whether you are talking about a dispacement hull or a planing hull. Displacement hulls are very efficient below theoretical hull speed. If you push them up onto their bow waves you can go beyond hull speed but power and fuel use go up geometrically.
With planing hulls the watchword is "liability". For legal reasons I think designers and builders adhere to Coast Guard regulations. There is some kind of CG formula that results in a motor size. That is the HP that appears on the hull tag of a commercially made boat. You are right. It is normally quite modest.
The practicality of it, in part, is the weight factor. Designers can calculate a practical load for the stern of the boat and convert that to a HP using a chart. What they are after has to do with balancing the stern load and the bow load on the center of gravity so that proper trim is maintained at speed and at rest. I saw a rundown of the process by a naval architect and he acknowledged there were a certain amount of intuition and fudge factors included. I am sure there are considerations about wild handling if the hull is pushed to hard but, I haven't got a clue about how they know where the "twilight zone" is.
For insurance reasons it might be unwise to go beyond the motor size specified for a hull until you know that it won't nullify your policy.

Edit:
I just found another discussion on assigning HP.
The designer had assigned a max of 15 HP to one of his boats (a 14 foot vee-bottomed S&G skiff). He said the Coast Guard formula would allow 25 HP with tiller steering but it was contingent upon testing the boat sufficiently to assure that it was safe.
Since the designer was only selling plans he couldn't insure the quality of every boat built nor the common sense of those that piloted them so he fell back on his experience which told him 15 HP would plane this hull with 3 occupants and no-one should ask more of the design than that.

[John Bell]

See the publication Safety Standards for Backyard Boatbuilders (http://www.uscg.mil/d8/mso/louisville/WebStuff/comdtpubp16761_3b.pdf) for information about safe powering. The calculations are pretty simple and easy to follow.

[jtb]

thanks for the info! As usual the knowlege and willingness to respond by those on this forum is awesome!