what governs a boats rated engine size?

[zaroba]

what governs a boats rated horsepower limit?
i'm not planning to go beyond my boats limits or anything, i'm just curious.


is it the weight of the engine so it doesn't sink the boat?
i'm sure its possible to get a higher horsepower engine that weighs less then a lower horsepower engine, it would depend on the manufacturers materials.

is it the transoms ability to handle the horsepower of the engine?
if so, wouldn't replacing the transom with a thicker, stronger one get around this?

or is it the ability of the boats bow to handle the water hitting it at higher speeds?

 

[tashasdaddy]

Re: what governs a boats rated engine size?

it is a combination of all that and more. hull design, type construction, over all weight, length, width, tiller or remote steering. here are the coast guard rules. http://forums.iboats.com/showthread.php?t=191283

 

[zaroba]

Re: what governs a boats rated engine size?

ty for the link...but...lol. the numbers don't match up with the pdf.

according to the doc:
measure the length of your boat and the width of its transom (in feet) and multiply the numbers together to get a factor which decides the max safe outboard horsepower capacity for the boat.

15ft x 3ft = 45
a factor between 43 and 45 shows a max safe horsepower of 10

but the boat's specification plate on it says the max is 50hp

so...doesn't really match up with coast guard rules. unless i'm misreading something (easily possible) or unless they were just drastically different in the 1970s

 

[Willyclay]

Re: what governs a boats rated engine size?

For outboard powered boats only, the USCG has a standard formula.
Try this link:

http://www.uscgboating.org/safety/boatbuilder/downloads/SAFELOADING.pdf

 

[Frank Acampora]

Re: what governs a boats rated engine size?

The Coast Guard also factors in things like steering: an open jon boat of exactly the same size as a runabout with helm steering will take less horsepower safely. And I remember a Go-Fast boat manufacturer adding a thick aluminum angle to their transom in the design stage so the hull would be rated for more horsepower by Coast Guard.

The Coast Guard formula does not compute MAXIMUM horsepower a boat can handle, it computes maximum safe HP under expected conditions of use.

Since "Too much horsepower is never enough" in my opinion it is desirable to exceed the capacity plate specifications. LOL. That should get them going!

 

[JB]

Re: what governs a boats rated engine size?

You don't believe that for a nanosecond, Frank.

I agree that a skilled pilot can usually safely operate a moderately overpowered boat in normal conditions. That does not make it a good idea for even a skilled pilot to deliberately challenge fate. Worst case conditions will make him the late skilled pilot. Skill and wisdom are not the same thing, or even close.

Boat manufacturers have lawyers and statisticians help them rate a given boat for normal (unskilled or semiskilled) pilots in worst case conditions. That covers their butts. If a boat becomes unsafe under adverse conditions with a brain dead knucklehead at the helm the manufacturer becomes vulnerable.

USCG doesn't have liability concerns when setting up recommendations using only technical specs of a boat, yet their formulae almost always rate a boat at a lower HP than the MFG.

I have temporarily overpowered a couple of boats.

A 14' tinny rated for 14HP with an 18hp. It became dangerous because it went fast enough to flex the bottom seriously when hitting chop. Caused my tackle box to become airborne. The boat was too flimsy for that speed and that power.

A 13' Boston Whaler rated for 40HP with a 50HP. It went so fast (over 48mph) that it was chinewalking and not answering the helm.

I repowered both boats immediately, with 9.9 and 25HP respectively and both became safe, controllable boats that would go as fast as I needed them to go.

Yes, I have heard, "You don't have to open the throttle all the way" a thousand times. While true, that is irrelevant to the question, "Is it safe?".

 

[Frank Acampora]

Re: what governs a boats rated engine size?

Actually, JB: What I believe depends upon circumstances!

My every weekend 21 foot cuddy is rated for 170 and I have 140 on it--but only because I can't get a Chrysler 170. However with 140 it still has enough power to (hopefully) outrun adverse conditions on the Chesapeake if they arise.

My 15 footer is rated for 90 and has 90 on it. It is quite adequately powered for skiing.

HOWEVER: My 14 foot 1960-something glass flat bottom has no capacity plate. I suspect it was rated originally for about 35-40. This is my "fun" boat and the minimum it has had on it was 60 hp. It had a 90 and that was not fast enough so now it has a 125. In the future I expect I will test a souped-up 140 on it. It has been rebuilt to take the horsepower and now is nowhere near the original hull. Only the shape is the same.

In all fairness, I don't take it out when there is traffic because while it is very fast on the straight, and acceleration is breathtaking, so little hull is in the water that it will not turn at speed. That's one of my future modifications--Turn Fins. Of course, this is a special use boat and I would never dream of taking any passengers in it. It is a driver only hull and I prefer to use it when there are no other boaters around if possible.

Like I say: It is my "fun" boat and the enjoyment I get is partly the speed but also testing mods to see their results. I take it out for about 1 1/2 to two hours at full throttle, burn somewhere between 9 and 15 gallons then take it home and re-glue it and add another modification for testing the next time.