Oversize outboard on RIB tender

Nick_H

Nick_H

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So having got rid of the Jet RIb (and good riddance), I've now got an Avon 3.1 m RIB, which states a max HP of 15 regardless of whether you use a tiller or have a centre console, which will not be enough to pull a skier, or probably even a ringo.

However the Avon 3.4 m is virtually the same, just 12 kg heavier and the length of a ruler longer, yet that will take a 25 HP with a console, which would be just enough, although still only 15 hp with a tiller.

Now the only things I can think of which dictate max engine size are engine weight, boat weight distribution, and transom strength. So, if I reinforce the transom of the 3.1 which is easily done, stick a 25 kilo bag of ballast in the bow locker, and buy a lightweight 25 hp 2 stroke (which weighs the same as a 15 hp 4 stroke), is that a reckless thing to do?

The RIB was bought used so is out of warranty anyway, and only I would drive it so I'm not too concerned about potential liability issues, so long as I believe the RIB is safe.

What does the panel think?
 
>reinforce the transom of the 3.1 which is easily done
I don't see how. It's a RIB: the transom and floor are an integral construction, and designed and tested to take a 15hp max.

Yes, it might work okay, but you're in uncharted territory on two counts:
1.) Thrust. The thing might just break. Probably not, but why risk it ?
2.) Top speed. It's been tested at max speed with a 15hp, and nobody died. You can be the test pilot that tries it with a 25hp and the sandbag. Rather you than me though.

I spose both could be mitigated by not using max thrust, or not going to top speed, but why bother with an outsize engine, then ? Also, If there were some sort of accident (e.g. hit a swimmer, large 3rd party claim against you), you can bet that the insurance company would try and wriggle out of it if they discovered the engine was oversized, even if it was nothing to do with the accident.

dv.
 
yes tend to agree with dvinell! we did look at overpowering an outboard/rib prior to buying our avon jet rib, so i understand your comments about the jet rib!! but i think, although the design of the rib is flawed in many ways!? it is generally a safe/tested piece of kit so therefore an insurer would pay out.
 
I would ask Avon if they consider it 'reasonble' to install a 25 (be specific re weight) on that boat as is if used with due care..............and if not why not!

Do it by phone and log the basic call details - who/when /what.
 
Thanks for the comments, and I take your point, but the slightly larger model has the same construction and almost the same weight, and can apparently handle the extra power/speed. I'm inclined to think its that most 3.1 RIBS have tiller steering, and 15 hp is the most you would sensibly use that way. There is no comparison with a console/steering wheel set up though.

Reinforcing the transom is easy with some triangular brackets bolted to the deck and transom. You're right about the extra thrust/power, I only actually want it to pull a skier out of the water, the 15 hp would be OK once they're up. Maybe I could use a bit of nitrous to pep up the 15 a bit for that few seconds /forums/images/graemlins/laugh.gif

I've seen a 3.2 m Novamarine fitted with a 40 hp, which is double the reccomended, but of course the fact that I've seen it doesn't make it sensible.

I probably wont do it, but I'm fairly convinced in my own mind that its not such a crazy thing to do, and would be safe.
 
put in a centre console then no need for the cement and will

be safer for your test pilot fun....go for it

cheers Joe
 
I think I've got more chance of them admitting their jet ribs are a pile of [--word removed--], than of them supporting an oversize outboard on one of their RIBS. Some things are just never going to happen.

Ford would not support you fitting a superchip and tubi exhaust to one of their cars either, but it doesn't make it inherently dangerous to do it.
 
having seen a number of letters from Fletcher to individual owners about exceeding the stated limits on craft it's possible but you may well be right. Just thought that with the specific weight of the proposed engine and the console it might have been deemed suitable.
 
[ QUOTE ]
So having got rid of the Jet RIb (and good riddance), I've now got an Avon 3.1 m RIB, which states a max HP of 15 regardless of whether you use a tiller or have a centre console, which will not be enough to pull a skier, or probably even a ringo.

[/ QUOTE ]

I've been in a ringo pulled by a 15hp four stroke, and was pretty okay. I've also seen Mrs. Wiggo skiing behind their old tender with an 8hp on it. Unfortunately Hagrid was at the helm and too busy looking at Mrs. Wiggo to notice the boat anchored directly in front of him. Ouch /forums/images/graemlins/tongue.gif
 
just read my last post and it seemed a bit dismissive, sorry it wasn't meant to be. I'd just convinced myself that in the current legal climate nobody will actually put their name to oversizing an outboard, regardless of whether they think its safe to do so.
 
No i don't think reckless. The "ripping out the transom" level is if you leap over waves etc. I plonked a 15hp on a dinghy raed at 8hp and no probs, and i didn't reinforce it. I also r3eckon that the transom has to be ableto take *any* 15hp and so safty margin etc etc. so I would say max 25.

incidentally, i got a neighbour to kindly loan me his 15hp to try out for an hour tosee that speed and propshft etc all ok and hence avoided being one of the "my dinghy is cr*p" posters ...
 
Take a 25hp and paint over the 2 with 1 on the cowling, that way everybody will be happy and compliant /forums/images/graemlins/smirk.gif
 
Really? The insured loss would surely have to arise directly from the slightly-overpowered dinghy, so unless the transom did indeed get ripped out (very unlikely in most circumstances, and my use of double the rated hp for 3 years with trigger happy self and kids caused not a murmur on a 2.85 zodiac) then it would not affect the claim. The most lilkely dinghy-relatd loss would be theft, and overpowered or otherwise,it would still be covered.

Do u speak from experience in the insurance issue of overpowered dinghy. As far as i can see, boats can be modified, diy-serviced and all sorts of mods added with varying degrees of skill -and yet all fine with the insurance bods...
 
I stuck an 8hp on a 2.7m RIB rated for a 6hp. Engines the same weight as a 6hp so why not? And it goes like stink. But a 15hp on a 2.85? Amazed it doesn't sink. Can I have a go please? /forums/images/graemlins/smile.gif
 
it planed fine with 15hp, not danger of sinking, mercruiser 2stroke almost same weight as yam 8hp.

I later somehow acquired a 3.4 novamarine jetrib which er as standard has 175 hp so that's a bit frightening...
 
Maximum recommended size of an engine on a rib is calculated from this formula, given in the relevant ISO standard. Deviation from this makes RCD compliance difficult:

Max engine size in HP = (Lenght * Width * 10 - 33) * 1.341 (L, W in m at maximum beam, LOA).

I think this is around 22HP for your boat. So then you need to ask why they didn't set it at 20HP. And IMHO this means Avon don't think your boat is safe above 15HP.

Getting insurance for a rib with oversized engine can be a problem.

25HP and even 40 HP engines can be tiller driven (do the RNLI not use tiller driven 40's on their D-classes?)

Finally you should be able to pull a ringo. And with the right prop MIGHT get a skier up. Would have thought you would manage 25mph on a good day.

Finally the engine weight and power are two different issues and will normally be marked with a max wt as well as max hp.
 
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