Massively overpowered dinghies, again

tcm

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i am posting this here with all the fun-loving speed-crazy petrolhead boaties. That's you, that is.

Here in las palmas I bought a 2.1m runabout dink secondhand for 135euros, Suzumar with inflatable keel and single lump of plywood floor and set about looking for a suitable motor. The other dink I have is 3.6metres (12feet) and has modest 25hp on it altho it can take (it says) 40hp - the idea is that the baby dink allows other people to go elsewhere without waiting around cos we only had the one dink, and/or we can take the small dink for easier hoiking it up the beach. Altho it's not too hard dragging the heavy one up the beach - if you sort of drag at the dinghy pathetically whilst beached.....pretty soon some other people always come to help. But anyway, i need a motor for the new baby dink. Hm.

Many hereabout are well-known practitioners of spectacularly overpowering the dinghy. Frinstance i saw that JFM had something like 40hp on the back of a 9-10ft dinghy a while back, very good value, and from a distance his dink was completely lost from view in all the airborne spume and wash. Superb stuff.

Soo, i put the 25hp motor on the 2.1 metre dinghy (max rated 4hp) to see wottle it do. Yeehah! It does 9knots at just over tickover and erm, i saw 24knots on the hh gps at half-throttle but the sea conditions outside las palmas marina here aren't ideal for full-on Coniston-type speed tests. Main thing is the fabulous acceleration, wham, straight up to 20knots in seconds. Also excelent cornering - still bags of power to turn at speed instread of all that slower stuff on corners as the prop sort of loses it if not loads of spare power. I spose it's a bit of a waste cos i can't really see us ever using maximum power, really. But 10hp would be fine, easily.

No sign yet of the transom falling off or any such nonsense. The only thing stopping me from putting gianty motor on it is that the whole idea was a lighter setup. Also, erm, actually carrying out the initial speed tests as above have somewhat reduced crew appetite for a massive motor on that dink, so i think a secondhand 5hp 2-stroke will be just fine. Well, okay, maybe 6. Or 8....
 

Firefly625

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I had a 6hp 4 stroke longstroke (my auxiliary) on a 2.4 dingy with inflatable floor, fine apart from when I stood toward the stern and water was lapping over the transom...:eek: so swapped it for a 2.7m and fine. That engine is around 27kg AFAIK...

.. so a 25hp... around 50kg?? !!
 
D

Deleted User YDKXO

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We all know that safety standards are gold plated in Euroland so IMHO, the factor of safety for the max power rating of the dinghy is about 10. On that basis, if it says max 5hp on the dinghy, I reckon you'll be good for 50hp so I would go for something around that size. Might be a good idea to tie a safety lanyard from the o/b motor to your leg whilst driving though
 

jfm

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Excellent stuff! Someone will be along soon to tell you the warranty on the Suzumar is invalidated and I know you'll be very upset about that.

I agree with Deleted User. The official motor ratings on any dinghy or RIB you buy today are pathetic. You want double the power they say.

IMG_2363.jpg
 

vas

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nice,

so what dingy should I get for my 10hp outboard?
Asking since current dingy is in such a sorry state that needs to be dumped as soon as I can. I'd really like something around 3.2m, going much smaller means I wont be able to get us 4 in (well two are 35kg ea)
 

benjenbav

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On the tcm/JFM index 10 hp would probably be about right for a small surfboard.

Oh, and seriously, I think the OP might have a problem with the warranty on the suzumar.
 

vas

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On the tcm/JFM index 10 hp would probably be about right for a small surfboard.

Oh, and seriously, I think the OP might have a problem with the warranty on the suzumar.

:D

I do have a 3m snark (sailing thingy) so I should add it there for the extra push on 7+force winds... Mind you the snark is less than 40kg
 

Garold

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An excellent thread, and cheered up a bit of a work-heavy day for me too.

But if one buys a new dinghy/ small rib and over-engines it what are the possible downsides?

As a more entertaining tender, I've considered purchasing a new 3.5m small rib that has a max engine rating of 25hp and engine weight of 70kg, but I want to couple it with a 30hp at say 80kg.

The manufacturer strongly advises against it and will not warrant the boat but given the various constraints, the max I can go to is 20hp because none of the quality manufacturers do a 25hp at the right weight with power tilt etc . And I thought that I'd be disappointed with the 20hp performance.

If I am willing to risk my £2-3k on the rib and have no warranty, what are the risks? Do you think it will sink with water pouring in over the back end? Will it self destruct after few weeks? Will I be facing the same problems as Donald Campbell? Anyone had any relevant experience?

Sorry for bit of thread drifitng, but I thought that the audience for this thread may be able to give me some opinions.

Cheers

Garold
 

jfm

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An excellent thread, and cheered up a bit of a work-heavy day for me too.

But if one buys a new dinghy/ small rib and over-engines it what are the possible downsides?

As a more entertaining tender, I've considered purchasing a new 3.5m small rib that has a max engine rating of 25hp and engine weight of 70kg, but I want to couple it with a 30hp at say 80kg.

The manufacturer strongly advises against it and will not warrant the boat but given the various constraints, the max I can go to is 20hp because none of the quality manufacturers do a 25hp at the right weight with power tilt etc . And I thought that I'd be disappointed with the 20hp performance.

If I am willing to risk my £2-3k on the rib and have no warranty, what are the risks? Do you think it will sink with water pouring in over the back end? Will it self destruct after few weeks? Will I be facing the same problems as Donald Campbell? Anyone had any relevant experience?

Sorry for bit of thread drifitng, but I thought that the audience for this thread may be able to give me some opinions.

Cheers

Garold

Good stuff Garold. Fact is, nothing bad will happen. 3.5m can easily take 40hp let alone 30. My pic above is 3.3/3.4m walker bay weighing 110kg and rated for 30, but that's a new yam 40 efi on it. It went light stink and was quite flippy/flighty with just one passenger at 90% + of WOT, so you had to drive a bit carefully, but it was good fun and nothing went wrong with it. To hell with the manufacturer's warranty - who ever made a warranty claim on a blimmin tender dinghy anyway? They're disposable. As Deleted User said above, these days there is a big "safety factor" in products sold on the cnsumer market

Only downside I can see is if there is an accident insurers might get difficult. Insurers always investigate big claims very hard and try to escape liability
 

Wiggo

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An excellent thread, and cheered up a bit of a work-heavy day for me too.

But if one buys a new dinghy/ small rib and over-engines it what are the possible downsides?

As a more entertaining tender, I've considered purchasing a new 3.5m small rib that has a max engine rating of 25hp and engine weight of 70kg, but I want to couple it with a 30hp at say 80kg.

The manufacturer strongly advises against it and will not warrant the boat but given the various constraints, the max I can go to is 20hp because none of the quality manufacturers do a 25hp at the right weight with power tilt etc . And I thought that I'd be disappointed with the 20hp performance.

If I am willing to risk my £2-3k on the rib and have no warranty, what are the risks? Do you think it will sink with water pouring in over the back end? Will it self destruct after few weeks? Will I be facing the same problems as Donald Campbell? Anyone had any relevant experience?

Sorry for bit of thread drifitng, but I thought that the audience for this thread may be able to give me some opinions.

Cheers

Garold
I have had one bad experience doing this. A while back, we wanted a light but fast RIB tender, and on the dealer's recommendation bought a feather weight Bombard RIB with a Johnson 15hp 2 stroke. The RIB was then fitted with a lightweight console by the same dealer.

The rig went like stink. Unfortunately, the speed it was capable of meant that a very lightweight hull took rather more of a pounding than it was designed for. The transom was fine (and the boat sat well in the water, so the weight of the motor was OK), but the hull started to stress crack. This was exacerbated by the console, which caused stress concentrations round its forward edges, and the hull came very close to breaking in half and started to let in water.

Great fun for a few months, but the hull was ultimately trashed.
 

Nick_H

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Only downside I can see is if there is an accident insurers might get difficult. Insurers always investigate big claims very hard and try to escape liability

The answer to that is to buy the outboard second hand, preferably from a private seller, then buy a lower hp sticker from a model that outwardly looks the same. For belt and braces buy the sticker cash, so there's no traceability back to you.

That way if the worst happens, and the insurers aren't fooled by the sticker alone, you can always claim that you didn't realise the motor was overpowered as the previous owner must have changed the sticker.

Having said that, not many 25 hp outboards look outwardly the same as their 4 hp siblings!
 

Nick_H

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The rig went like stink. Unfortunately, the speed it was capable of meant that a very lightweight hull took rather more of a pounding than it was designed for. The transom was fine (and the boat sat well in the water, so the weight of the motor was OK), but the hull started to stress crack. This was exacerbated by the console, which caused stress concentrations round its forward edges, and the hull came very close to breaking in half and started to let in water..

But IIRC the rib was single skin and wasn't supposed to have a console at all, so the problem was not so much the overpowering as fitting a console on a rib that wasn't designed for one??
 

Wiggo

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But IIRC the rib was single skin and wasn't supposed to have a console at all, so the problem was not so much the overpowering as fitting a console on a rib that wasn't designed for one??

Good memory. Yes, it was single skinned hull, but ultimately it broke up because it got too much of a pounding from going too fast. The console concentrated the damage into two small areas which meant it failed very quickly, but even without the console I think it would have broken up in time.
 
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