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Old 01-11-2018, 17:47   #1
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Max Power for RIB Dinghy Question

Our Highfield aluminum dinghy is rated to have up to a 25hp short shaft outboard installed.

Question: Why? What happens if the maximum hp is exceeded just so long as the weight of the motor is the same as a 25hp outboard.

For example... what would happen if I installed a 40hp 2 stroke in place of the 15hp motor that I already have? I don’t plan on running the motor flat out, but the weight and size of a 25hp four stroke is about the same as a 40hp two stroke. No need to lecture me on warranty or consumer liability, please save your energy.

What terrible things can happen?

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Old 01-11-2018, 18:56   #2
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Re: Max Power for RIB Dinghy Question

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Originally Posted by Kenomac View Post
Our Highfield aluminum dinghy is rated to have up to a 25hp short shaft outboard installed.

Question: Why? What happens if the maximum hp is exceeded just so long as the weight of the motor is the same as a 25hp outboard.

For example... what would happen if I installed a 40hp 2 stroke in place of the 15hp motor that I already have? I don’t plan on running the motor flat out, but the weight and size of a 25hp four stroke is about the same as a 40hp two stroke. No need to lecture me on warranty or consumer liability, please save your energy.

What terrible things can happen?

Thanks
The USCG specifies that the maximum nameplate horsepower on tiller steered boats of less than 20 feet LOA be determined by a simple formula:
(W x L X 0.8) – 25 = max. horsepower

If the boat has a steering wheel the formula is a bit different:
(stern width in feet x length in feet x 2) – 90 = max. horsepower.

For boats of more than 20 feet LOA, the maximum horse power is up to the discretion of the builder.

The formulas assume "typical" boats handled by "average" drivers. You are a big boy, you can decide for yourself.
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Old 01-11-2018, 19:01   #3
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Re: Max Power for RIB Dinghy Question

So according to your formula, my steering wheel dinghy can handle up to 30hp in North America. Is that different in Europe? The dinghy is in Italy.
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Old 02-11-2018, 10:31   #4
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Re: Max Power for RIB Dinghy Question

Terrible things? Overpowering makes me think of things like trying to leave a dock by steering your motor in reverse and hitting the throttle just a little too much so you slew around (because your boat is lighter than the powerful motor is designed for) and spill your passengers out. Or a friend goes to use it and isn't aware of your "no flat out" idea, and it flips and kills him. Perhaps a 40hp wouldn't be too much, but it might be an expensive way to find out that it is.
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Old 02-11-2018, 10:40   #5
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Re: Max Power for RIB Dinghy Question

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So according to your formula, my steering wheel dinghy can handle up to 30hp in North America. Is that different in Europe? The dinghy is in Italy.
I have no idea how the rules and regs work in Italy or the rest of the EU. I'll bet you can find out with Google.
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Old 02-11-2018, 11:06   #6
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Max Power for RIB Dinghy Question

Assuming you have a brain, and use it, nothing will happen.
If you get real stupid with the motor size, it is possible to simply over-stress the transom and break something. I’ve never seen that, but it’s possible I’m sure.
I don’t understand why people think it’s OK to buy a car that can exceed the speed limit by a wide margin, but are horrified at the thought of a dinghy with a higher than spec engine.

Note the USCG formula has nothing at all to do with the possible safety of that particular boat, it’s simply a formula that guarantees that pretty much no matter how poorly the boat is designed and manufactured it will still be safe. A manufacturer may at their discretion test for higher power, there is a procedure, but if they do so they open themselves up for litigation of course, so why would they?
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Old 02-11-2018, 12:19   #7
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Re: Max Power for RIB Dinghy Question

Just try and collect on any insurance claim.
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Old 02-11-2018, 15:55   #8
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Re: Max Power for RIB Dinghy Question

I have a highfield 310 , max rated HP is 15 , which in suzuki land os the same weight and engine as the 9.9 and 20 HP , guess which one I have ?

I figured that worst that can happen is that the alu transom welds crack ? That should be an easy fix of it happens

Best that can happen is that with 20-25% power extra i get on a plane faster , have an easier ride , get ‘there and back’ faster , can haul more schtuff and have better fuel economy
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Old 02-11-2018, 16:44   #9
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Re: Max Power for RIB Dinghy Question

I have an old Avon 10 RIB powered with an 8HP 2 cycle. One afternoon I was alone in 2’ chop heading into the wind. I had the engine wide open, alone probably close to 20K. A blast of wind hit me while wave hopping and I came close to pitch polling the thing. If I had a larger engine, wide open I know I would have flipped.

By the way, I purchased my Avon (not made any more) way back in 1986! Yes, I keep it covered on my front deck. Seams still tight, no noticeable leaks, fiberglass floor with a small bilge to keep my feet dry. Many of my friends are into their 3rd dingy over these same years. Too bad the original Avon is gone.
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Old 02-11-2018, 16:48   #10
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Re: Max Power for RIB Dinghy Question

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Just try and collect on any insurance claim.
I’m guessing you failed to fully understand post #1. In fact, the dinghy with a larger motor would be able to properly tow the mothership, which might actually prevent a serious insurance payout.

I’ve been in several situations where more power is beneficial in emergency situations.
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Old 02-11-2018, 17:07   #11
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Re: Max Power for RIB Dinghy Question

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I have an old Avon 10 RIB powered with an 8HP 2 cycle. One afternoon I was alone in 2’ chop heading into the wind. I had the engine wide open, alone probably close to 20K. A blast of wind hit me while wave hopping and I came close to pitch polling the thing. If I had a larger engine, wide open I know I would have flipped.

By the way, I purchased my Avon (not made any more) way back in 1986! Yes, I keep it covered on my front deck. Seams still tight, no noticeable leaks, fiberglass floor with a small bilge to keep my feet dry. Many of my friends are into their 3rd dingy over these same years. Too bad the original Avon is gone.
I highly doubt you would have flipped , why ?

Because flat out with a larger engine you would have been going 30-35kts , which is not something you (I !!) do in high winds / chops , so you ( I !!) would have throttled back to the same speed as with the smaller engine , result would have been the same ( = slightly elevated heartbeat , and away you go again)
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Old 02-11-2018, 17:13   #12
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Re: Max Power for RIB Dinghy Question

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Originally Posted by ReneJK View Post
I have a highfield 310 , max rated HP is 15 , which in suzuki land os the same weight and engine as the 9.9 and 20 HP , guess which one I have ?

I figured that worst that can happen is that the alu transom welds crack ? That should be an easy fix of it happens

Best that can happen is that with 20-25% power extra i get on a plane faster , have an easier ride , get ‘there and back’ faster , can haul more schtuff and have better fuel economy
The worst that could happen is you have an accident and damage a person or property (damages possibly in $millions); try to claim insurance; only to be rightfully denied any insurance compensation.
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Old 02-11-2018, 17:19   #13
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Re: Max Power for RIB Dinghy Question

Well , keeping with ‘no need to lecture me on liability’

I am not sailing in US waters , so claims and insurance payments / claims are not quite the same

But how about ‘don’t be stupid and dont cause millions of $$’s worth of damage’ ?

Its not what this thread is about now , is it ?
Or am i feeding trolls now I wonder
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Old 02-11-2018, 17:24   #14
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Max Power for RIB Dinghy Question

I’ve had two RIB’s with this same 20 HP Suzuki with a SS 11” pitch Solas prop. Both rated for 20 HP.
The Zodiac 3.10 would run a little over 30 kts. GPS speed off of my SH HX-870 handheld.
My new 10’6” AB will run 20 kts, 1/3 slower, same engine, same prop.

Zodiac was a much flatter hull, and I assume it ventilated the tubes, that is air was forced under the boat and provided lift, and reduced drag.
The AB is a much deeper V and much more drag.
Point I’m trying to make is that the Zodiac was likely about as fast as it is safe for the average person to operate, whereas the AB could use likely 10 more HP, but according to the formula, there is no difference.

Latest version of my Wife’s car will crack 200 MPH, yet its sold to the public, with no problem. Why? Because 99% of the people have enough sense to drive like an adult.
Why should a dinghy be any different? Are cruisers stupid? Incapable of not running wide open regardless of conditions?

If you want the bigger motor, get it. If your getting it to tow with, get a big diameter four blade low pitch prop, that will both seriously reduce top speed and increase max low speed thrust.
The low speed thrust difference will be tremendous
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Old 02-11-2018, 17:32   #15
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Re: Max Power for RIB Dinghy Question

@a64pilot

Few more thing to check to get max speed
1) height of the engine
2) trim of the engine

Maybe the suzuki was perfectly matched to the zodiac but transom height and angle are slightly different with the AB ? If you adjust for that the deeper V should (should !!) be faster that the flat bottom with the same engine

Ps i am doing 30+ with our highfield 310 and 20hp w/ non optimal prop (goes in high rev limiter when full open) so i see no reason why your AB shouldn’t achieve the same results ?
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