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Old 18-08-2015, 12:36   #31
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Re: what size outboard for dinghy?

15 HP was fine on my 10 ft Caribe double floor heavy rib. 4 people would plane. Might have to shift weight to get it up. A rocket ship with 2 people.
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Old 18-08-2015, 13:03   #32
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Re: what size outboard for dinghy?

Check your capacity plate it gives the recommended HP rating. This is a recommendation not a limit. The limit is the total capacity. Crew, Motor, Fuel and Cargo. If you're carrying 4 200 lb people and a 130 lb motor you may not have capacity for the rest.
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Old 18-08-2015, 13:06   #33
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Re: what size outboard for dinghy?

Quote:
Originally Posted by barnakiel View Post
Think about the forces the engine puts on the transom when trying to lift that 1000+ pounds out of the water.
b.
Didn't the OP mention 400 pounds worth of humans? My 18hp is 100 pounds and the dink is 100 pounds...
Oh, of course, you have included the beer to bring it up to 1,000...
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Old 18-08-2015, 13:08   #34
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Re: what size outboard for dinghy?

I have a Zodiac 3.10 RIB, a Cadet I think.
With the 20 HP Zuk, the stock 10" aluminum prop, it will easily jump on plane with 4 Adults (1,000 lbs with motor fuel etc.), but with one person it jumps on plane and almost immediately hits the rev limiter at about 25 kts.
With a Solas three blade SS 12" prop it crawls onto plane with two adults, but will run an astonishing 35 kts with one on board, never tried, but sure it would never plane with four adults.
Sweet spot is a Solas 11" three blade SS prop, planes two easily, four is a struggle, will still run 32 kts with one on board, but with two people 20 kts is a very comfortable half throttle, can barely hear the engine, low fuel consumption.
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Old 18-08-2015, 14:32   #35
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Re: what size outboard for dinghy?

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Originally Posted by Rick Williams View Post
We just got a used 10' Avon RIB that came with an anemic 6 hp outboard. I need to know what size outboard will make that RIB plane with up to 4 people in it. I expect we'll be doing some longish side trips on our next cruise and would like to be able to cover a lot of ground quickly.
Thanks
Your Avon should have a maximum size outboard to use marked on the transom near where the clamps tighten. Don't use larger.
Contact Avon if in doubt.

My 8'6" RIB is labeled for 6 hp max. With a 5 hp Mercury it will plane at 10.5 knots by GPS with one occupant. With more on board it's down to maybe 4 or 5 knots
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Old 18-08-2015, 14:33   #36
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Re: what size outboard for dinghy?

Along these lines....is it the HP or the weight of the outboard? I have a RIB as well. Solid transom plate says 10hp. A 15 would be great at same weight but is the concern overpowering the transom?

I also keep mine hoisted up under the arch and am adding an extra line to support the motor's mass and save the transom so I feel less guilty about leaving it on. As I am about to buy a new engine I am hoping the group has advise on both.
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Old 18-08-2015, 14:38   #37
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Re: what size outboard for dinghy?

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Originally Posted by MarkJ View Post
You can exceed it by a lot.

And anyway do you mean the US coast guard? Or what country? As the OP hasnt said where he is cruising.
The US Coast Guard only covers ONE country. Guess where that is?? Anyway, if the USCG has that as a rule please screenshot the reference and post it here.

As Mark, ColeMJ, says the parameters are weight, so if the person purchases a 2 stroke OB (country dependant only on NEW OBs) he will find an 15HP 2 stroke is half the wieght of a 15 HP 4 stroke, or thereabouts.

So can so eone in the USa buy a 2 stroke? Yes! If its second hand and they tell me virtually new ones are sold in US from Canada. Also the Bahamas and everywhere south. So it could be well viable for a new cruiser to buy small in the US for his wander down the ICW and repower in the Bahamas....


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I think you nailed it on weight and buying a nasty old two cycle. I love the four stokes but heavier than hell. Quiet but on a tender who cares. I doubt the USCG gives a damn.
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Old 18-08-2015, 14:50   #38
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Re: what size outboard for dinghy?

Quote:
Originally Posted by barnakiel View Post
Think about the forces the engine puts on the transom when trying to lift that 1000+ pounds out of the water. Maybe some users are careful to never open it full up but I have seen a handful that give it whatever there is.

Some ribs have very strong transoms, others less so, etc.
No, that is not it. The force required to bring the boat up on plane is the same regardless of the size of the engine, and the engine can only provide a maximum force sufficient to get the boat on plane - after which the force on the transom decreases. Put a larger engine on and the boat simply goes on plane quicker than a smaller engine - with the same peak force applied by both.

You could imagine a very large engine with microsecond acceleration producing some great instantaneous force on the transom for a split second before the boat moves to decrease it - but that is academic only and has no practical basis (we aren't discussing putting a Yamaha 350 on this dink).

One could make the case that smaller engines are worse for transom forces because they keep the force at a higher level for a longer period of time before they get the boat on plane.

The difference in hole-shot forces between a 15hp and a 20hp is pretty much zero - the hp difference comes into play on the top end and in pushing more weight faster when underway.

We just bought a new RIB and the manual mentions several times the maximum weight of outboard allowed. The plate on it says 30hp max, but the manual only makes mention of engine weight - in several places.

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Old 18-08-2015, 18:33   #39
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Re: what size outboard for dinghy?

Quote:
Originally Posted by GordMay View Post
The Avon Rover Compact 310 (16" dia tubes) is only rated to 10 HP; whereas the Rover Rib 315 (17" dia tubes) is rated to 15 HP.
Avon Inflatable Boats and Ribs
Good reminder Gord.

We have a 9.5 Caribe with a merc 15 2-stroke. Its rating is 15 on the transom. With one or two aboard, it scares the heck out of me. I consider it close to dangerous. Too much power is just that. Be careful. Its also easy to put too much weight on the transom and make it very unstable.
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Old 18-08-2015, 18:54   #40
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Re: what size outboard for dinghy?

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Originally Posted by Nicholson58 View Post
Good reminder Gord.

We have a 9.5 Caribe with a merc 15 2-stroke. Its rating is 15 on the transom. With one or two aboard, it scares the heck out of me. I consider it close to dangerous. Too much power is just that. Be careful. Its also easy to put too much weight on the transom and make it very unstable.
Yeah, my previously mentioned Yamaha 15 went from our 10 ft RIB to a 9Ft+ RIB. It was very squirelly. I imagine due to lack of length rather than weight so much.
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Old 18-08-2015, 19:10   #41
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Re: what size outboard for dinghy?

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Originally Posted by Nicholson58 View Post
Good reminder Gord.

We have a 9.5 Caribe with a merc 15 2-stroke. Its rating is 15 on the transom. With one or two aboard, it scares the heck out of me. I consider it close to dangerous. Too much power is just that. Be careful. Its also easy to put too much weight on the transom and make it very unstable.
Quite correct in my opinion. Another thing to remember is that say you needed a 25 hp motor to plane with 4 POB.(just guessing) Then you drop 3 passengers off
and you open the throttle too quickly you are likely to have an Avon sitting upside down on top of you now the "ballast" has been removed.

I can easily imagine the coroner saying This unfortunate death was the result of somebody not taking notice of the manufactures power recommendation for this craft.
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Old 18-08-2015, 20:06   #42
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Re: what size outboard for dinghy?

Quote:
Originally Posted by Rick Williams View Post
We just got a used 10' Avon RIB that came with an anemic 6 hp outboard. I need to know what size outboard will make that RIB plane with up to 4 people in it. I expect we'll be doing some longish side trips on our next cruise and would like to be able to cover a lot of ground quickly.
Thanks
We have a 10.5' AB rib with a 2 stroke Yam 15. Good combo and plenty of poke for my wife and I.

Haven't tried with 4 on board but it would likely still get in the plane.

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Old 18-08-2015, 21:07   #43
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Re: what size outboard for dinghy?

Looks like the ultimate cruisers dinghy 😀


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Old 18-08-2015, 21:09   #44
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Re: what size outboard for dinghy?

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Originally Posted by jwcolby54 View Post

Referring to this in prior post


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Old 19-08-2015, 09:31   #45
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Re: what size outboard for dinghy?

Most 15hp 2 stroke weigh the same as the 10hp. They have this amazing little thing on the handle, it's called a throttle... it doesn't have to be wide open! :>)
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