1997 Emerald X/p [Archive] - Boat Design Forums

FLUKY

07-17-2008, 06:13 PM

i recently purchased a emerald 18'-6" x/p i have been a boater for
many years, never hearing of this model, if anyone out there has one
or knows about them please respond, i know that seebold had
something to do with this but thats about it. this boat is only
rated for a 150 hp. yet it can carry 7 people with over
1,000 lbs. i really think the horsepower can be upped
they are really built well

marshmat

07-17-2008, 06:48 PM

Hi Fluky,

I'm having a devil of a time trying to find any decent data on this boat. Do you have photos?

Now, about that power thing.

You may think it is built well- heck, maybe it is. But that's not what determines the power rating. The formulae used to derive that rating generally start with hull volume or length, and transom width. Corrections are applied for motor wells, hull bottom form, tiller versus wheel steering, etc. Additional nonlinearities are introduced to allow disproportinately more power for larger craft, and restrictions are applied to certain hull forms and control setups. It is then assumed (and mandated) that the boat will be built well enough to handle the maximum rated power.

The formulae have proven over many years to yield safe ratings that still permit the boat to carry more power than it usually needs. Exceeding the limits is, most of the time, a Very Bad Idea. In the rare cases where it is a possibility, it is up to the person making the change to do the calculations and tests to ensure that it is, in fact, safe.

For most boats, the answer is NO, do not overpower. In a few cases, where the boat has been proven to underperform with its rated power, AND there is good mathematical and empirical evidence that it can safely handle more, AND the failure of the usual formulae to produce an appropriate number can be explained, only then do you have a case for proceeding.

Looking at the Canadian standards, I note that your boat already meets all of the criteria for the most optimistic of the various power rating curves. I also note that 150 hp is quite in line with normal maximums for an 18'6" runabout.

For reference, I routinely carry 4-5 people, or ~600-800 lbs, in my runabout at 20+ mph with only 30 hp. Cut that to 2 crew or 300 lbs and 25+ mph is a breeze. Excess power beyond what a hull is designed for, rarely helps.