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Old 07-01-2024, 19:19   #1
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What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

So I've got a cheapie West Marine roll up dinghy and a 5hp Honda 4 stroke. I top out at 14 knots empty and a slug's pace when loaded with fuel and groceries. I'm thinking of a proper RHIB but something small still and a 15hp 2 stroke engine. But I'm wondering, what happens when you exceed the maximum horsepower rating of a dinghy? Presumably the forces on the dinghy itself are minimal once on a plane and a 15 hp will get on plane easier with groceries etc. The 2 stroke motors are only 20lbs heavier than the much weaker 4 strokes so that seems like a minor concern but I am just surmising. Anyone tried this? I was considering a 3 meter Truekit catamaran dinghy though I heard they aren't good in waves or maybe a proper v-hulled Highfield ultralight 3 meter. I want to go fast and preferably not get beaten to death every time I run over a ripple like I do in my flat-bottomed dinghy. Advice?
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Old 07-01-2024, 19:26   #2
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

It depends on the dinghy, although depending on where in the world you are, there may be legal implications to exceeding the rating on any dinghy (especially if you get into an accident). Sometimes the limit is just the regulatory limit based on the dinghy's dimensions, not a structural limit. Other times the builder is worried about weight on the transom, or they're concerned about structural strength of the transom with too much power. Or they may have handling related concerns at high speeds and limit the power rating accordingly.

The speed you'll achieve with a given amount of power and weight will also vary depending on the dinghy. Some are easier to push than others.
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Old 07-01-2024, 19:27   #3
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

When you exceed the HP rating of the dinghy you are putting more force on the transom then designed for. In the extreme you could possibly fold the transom dramatically, but more likely it will take repeated use to break it.
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Old 07-01-2024, 21:36   #4
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

Yes, it makes the glue bonds at the transom weaken, then sometimes, if you weren't paying attention, the transom falls off. Depends on who's around under the water how serious an event this is.

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Old 07-01-2024, 21:42   #5
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

Quote:
Originally Posted by SomeGuyInaShirt View Post
The 2 stroke motors are only 20lbs heavier than the much weaker 4 strokes.
I've had quite a few outboards. All my 2 strokes have been about 20lbs. lighter than comparable 4 strokes.
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Old 07-01-2024, 22:05   #6
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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I've had quite a few outboards. All my 2 strokes have been about 20lbs. lighter than comparable 4 strokes.
Sorry, I meant to say the 15hp 2 stroke is 20lbs heavier than my 5hp 4 stroke
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Old 08-01-2024, 02:29   #7
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

The Quick-Turn [performance] Test:
Setting the throttle at a low maneuvering speed, and facing straight ahead, you then turn the wheel 180 degrees, in half a second [or less], and hold it there.
If you can complete the 90-degree turn, without losing control of the boat, or reducing the throttle, your boat & motor has passed the test.
Repeat, increasing the turn in speed, until you can no longer pass the test, or you reach the maximum throttle.
The maximum horsepower the boat can use, while still completing this test, is defined as the maximum horsepower capacity
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Old 08-01-2024, 05:20   #8
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

This thread brings to mind Boeing door plugs.
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Old 08-01-2024, 05:48   #9
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

You gave a Highfield as an example. I took a quick look at the specs for the Highfield Ultralite UL-310FD Aluminum RIB and it's rated for max 15hp so you would not be exceeding the spec.
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Old 08-01-2024, 06:10   #10
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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.
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Old 08-01-2024, 06:14   #11
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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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.

That's exactly what I was getting at with my post, but you explained it better. There's the regulatory limit for HP, then manufacturers have the option to further de-rate it for structural reasons.



Our dinghy (an aluminum skiff) is rated for 15hp, but we only have a 6hp mounted on it. Something around 10hp would be better, but the extra engine weight would eat into payload capacity a bit and I didn't want to lift the extra weight in the davits (our setup lifts the dinghy with the engine mounted). It's an old dinghy, so they were expecting the weight of a 2 stroke. Based on that, I think a 15hp 4 stroke may be a bit heavy for it, and honestly, with a single person it would probably be bordering on scary fast. The 6hp will do 13 kts with 1 person, by my best estimate a 10hp would do somewhere in the 16 - 18 range, a 15hp would likely do 20.
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Old 08-01-2024, 07:50   #12
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?
The dinghy becomes very squirrelly. I had a friend who overpowered a dinghy. You had to be very careful with the throttle. I went at no wake speed and tried to jump on plane and came very, very close to flipping the dinghy (Think back-flip), despite there being two people i it.

It also becomes very twitchy on plane at high speeds and also very jerky around the docks. Close quarters you were limited to Fwd, Neutral, Rev. Any additional throttle had too much response and made you look like a greenhorn with a touch of whisky throttle.
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Old 08-01-2024, 12:41   #13
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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I've had quite a few outboards. All my 2 strokes have been about 20lbs. lighter than comparable 4 strokes.
Yes I agree wth that answer
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Old 08-01-2024, 13:17   #14
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

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.
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Old 08-01-2024, 15:04   #15
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Re: What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

What happens? Excitement and a little danger.

As discussed above it's mostly about dimensions and hull shape (flat or V).

As with most things I would be concerned about too much weight as you can't change that. You can choose to use less power than available.

Modern 4 strokes keep getting closer in weight to the their two stroke counterparts.

Comparing short shaft no frills.
20 HP Suzuki 4 stroke with Fuel Injection is 97 pounds.
18 HP Tohatsu 2 stroke is 90 pounds.

The gap narrows. I bet the Suzuki uses half the fuel at mid throttle cruising.

Of course there are other concerns.
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