What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

[GordMay]

Originally Posted by SailingHarmonie

...
In the USA horsepower ratings for small boats are set by rules listed by the Federal government and they are available here: boat hp rating rules

It is true that the manufacturer can put a lower number on the capacity plate. You will also notice that the weight or construction details don’t matter to the regulation.

While the steering performance test quoted above with out any reference information is (sort of) part of the actual rating process, it applies ONLY to boats with remote steering, not the typical tiller steered dinghy.*

Basically, for most dinghies it’s calculated from the dimensions of the boat, and the hull shape—nothing else....

Thanks Harmonie.

* Special Rules: For boats that meet the following qualification there is an optional test course method for determining horsepower:
13 feet or less in length.
Outboard Powered.
2 or less passengers.
Remote steering and 20 inch transom height.

An excellent tutorial, from New Boat Builders:
HORSEPOWER FOR OUTBOARD BOATS WITH ENGINES GREATER THAN 2HP
➥ https://newboatbuilders.com/pages/hp.html

[HeinSdL]

Originally Posted by SailingHarmonie

Wow…. People make up all kinds of stuff when they THINK they know the answer, or present a little piece of it that is only barely true.

In the USA horsepower ratings for small boats are set by rules listed by the Federal government and they are available here: boat hp rating rules

It is true that the manufacturer can put a lower number on the capacity plate. You will also notice that the weight or construction details don’t matter to the regulation.

While the steering performance test quoted above with out any reference information is (sort of) part of the actual rating process, it applies ONLY to boats with remote steering, not the typical tiller steered dinghy.

Basically, for most dinghies it’s calculated from the dimensions of the boat, and the hull shape—nothing else.

What happens if you exceed the number? Probably nothing… if you are a skilled driver and never get in an accident. Of course it depends on the boat… and if you do get in an accident that hurts somebody, you’re going to have a hard time proving you are smarter the the rule writers and your decision to exceed the hp rating had nothing to do with the cause.

We have a 10 foot rib with an engine right at the max rating. With one person aboard, and the right prop, it is scary fast. And not in a good way. I’d not even consider more.

Maybe go easy on assessing the quality of other people's posts, the posters I think are all mainly trying to be helpful. It's only a forum after all and not a court of law or such like. I enjoy reading different opinions and welcome a wide range of participation to discussions in the absence of which this site would become quiet and dull, always waiting for an answer which cannot afford to not be entirely correct. And which therefore never comes...

[SomeGuyInaShirt]

Great answers, everyone. I appreciate the advice. After looking into it more, I'm at an even greater loss for what dinghy to buy next. I suspected Highfield dinghies weren't as good as all the hype and with a little more digging, I found out they really aren't. To complicate matters further I have a tiny boat with an intermediate forestay so upside down on the foredeck isn't even an option. Going to have to do a lot more looking into this I suppose.

[SomeGuyInaShirt]

Originally Posted by Bycrick

I’ve got a 12’ dinghy with an inflatable floor. Rated on the plate at 1600# load and a 25 HP engine. With 2 people and a load of groceries, it will plane easily with an 8HP. With the 25, you can barely open the throttle with one person or it will back-flip. With 3people and scuba gear, it will plane with the 25. I carried both motors.

Which dinghy have you got? How is it in waves? It sounds like the catamaran dinghy I was considering. Great on calm days but a real arse pounding in any kind of waves.

[boatpoker]

Originally Posted by Bycrick

I’ve got a 12’ dinghy with an inflatable floor. Rated on the plate at 1600# load and a 25 HP engine. With 2 people and a load of groceries, it will plane easily with an 8HP. With the 25, you can barely open the throttle with one person or it will back-flip. With 3people and scuba gear, it will plane with the 25. I carried both motors.

Eight, 200lb. guys in a 12' inflatable !!!
What brand is this ?

[Bycrick]

Nahhh. Don’t exaggerate. I then weighed about 150, my wife weighed about 120. The groceries were as much as we could carry. Scuba diving with 3 people and equipment. I doubt if I got above 1000#. The dinghy is a 12’ 3” Achilles that I bought about 2002. With the inflatable floor, it weighs less than 100#. I can lift it on deck solo with the main halyard, but it’s a little clumsy to lift and guide it with one person. With 2, it’s easy.

[wholybee]

The rated HP stamped on the hull is usually derived by a simple formula.

https://www.law.cornell.edu/cfr/text/33/183.53

I would work the formula and compare it to the number on the boat. If the rated number is much lower than what the formula works to, then there is a reason the manufacture didn't go with the formula (like it failed a test) and I would not exceed it.

If they used the formula, then maybe you can go higher.

Regarding your rollup, however, it most likely has a flat bottom? Is it the one with wooden slats? That dinghy is not supposed to plane, and while it might, you won't have much steering control. I generally think it is a good match for a 3.5hp and stay in displacement and never try to plane.

Money would better be spend on a better dinghy. Something in the 9-10 foot range with a shallow V will plane easily with a 5hp, one person, and gear/groceries. For 2 people + gear you could go up to 8 or 9.9 hp.

[boatpoker]

Originally Posted by Bycrick

Nahhh. Don’t exaggerate. I then weighed about 150, my wife weighed about 120. The groceries were as much as we could carry. Scuba diving with 3 people and equipment. I doubt if I got above 1000#. The dinghy is a 12’ 3” Achilles that I bought about 2002. With the inflatable floor, it weighs less than 100#. I can lift it on deck solo with the main halyard, but it’s a little clumsy to lift and guide it with one person. With 2, it’s easy.

You said it was rated for 1600lbs. not me.

[rslifkin]

Rated for 1600 lbs doesn't mean it's rated for 8 people. It also doesn't mean you could actually fit 1600 lbs of stuff in there without it being mostly lead bricks.

[GordMay]

New Boatbuilders covers everything, we need to know [including formulae] about:
Safety standards for inflatable boats including RHIBs
➥ https://newboatbuilders.com/pages/inflatables.html

”... The U. S. Coast Guard does not directly regulate construction of recreational inflatable boats. They are required to have a
Hull Identification Number (HIN). Other than that there are no US Coast Guard standards for Horsepower, Capacity, or Flotation.
There are standards for all of these published by the American Boat and Yacht Council (ABYC), the International Standards Organization (ISO) and Canada (see below). All inflatable boat manufacturers follow one of these standards and they are all nearly identical. The ABYC Standard is H-28 ...

... Canadian Rules for Inflatables are in TP1332 Section 4.6 ...”

TP1332 ➥ https://tc.canada.ca/en/marine-trans...l-vessels-2010

[GordMay]

CONSTRUCTION STANDARDS FOR SMALL VESSELS
4.6 RECOMMENDED MAXIMUM SAFETY LIMITS FOR INFLATABLE AND RIGID HULL INFLATABLE VESSELS begins on Page 51
TP1332 [Full Document] ➥ https://publications.gc.ca/collectio...7-2010-eng.pdf

[Bowdrie]

Some levity needed.
You've exceeded the rating?
Woe unto you, you miscreant.
You will get hauled into court, and the judge will probably exclaim the same order that was given in a line from a Cheech and Chong album.
"Bailiff, whack his pee-pee".

[roythomas]

Hello, I work at Defender and sell lots and lots of dinghies.



When you overpower your dinghy you negate ANY insurance coverage. Should anything happen even remotely related to the dinghy your insurer will not cover you. They look for ways to not pay and you just gave them an easy one. Liability is your biggest risk. Someone is hurt by your dinghy, you can lose everything. Your dinghy breaks free and people on jet skis hit it cuz its a hazard and someone is hurt you are liable. Intuitively it does not make sense, you weren't operating the dinghy but even one tiny violation of the fine print / clause and they don't have to pay. And they won't.

[David Mathis]

The quality of manufacture is a major factor. Our 10' Hydro-Force inflatable, bought at Costco, was rated at 15p but it was powered by a smaller 9.9 hp two stroke. Within three years, the transom started coming apart which was repaired with screws and 5200. The next year oarlocks came loose and by the fifth year, last year, one of the seams split. It was retired as being unsafe for the kids to use.

[Shrew]

Originally Posted by FlyingScot

You can choose to use less power than available.

Not always. occupants can get jostled. The boat gets knocked around in waves. Many opportunities to crack the throttle unexpectedly. When that occurs. I have watched such situations lead to a occupant getting thrown out.

all you need is a little tiller to port or stb and a unexpected whack of the throttle and a myriad of unexpected and undesirable things occur.