Exceeding max outboard rating.....

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stevecray

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I am looking at a baby rib for use as a tender (well part tender, part toy if I am honest), the rib I am looking at is 2.9m and has a max engine rating of 8hp. Given most 8hp outboards seem to be identical dimensions / weights to the 10hp versions and very close in price, it seems a real waste to go for the 8hp. Is the hp limit normally driven by weight (in which case I may as well go for the 10hp) or is it driven by the load exerted from the hp? Thanks.
 
stevecray said:
I am looking at a baby rib for use as a tender (well part tender, part toy if I am honest), the rib I am looking at is 2.9m and has a max engine rating of 8hp. Given most 8hp outboards seem to be identical dimensions / weights to the 10hp versions and very close in price, it seems a real waste to go for the 8hp. Is the hp limit normally driven by weight (in which case I may as well go for the 10hp) or is it driven by the load exerted from the hp? Thanks.
Click to expand...

In my experince with larger Ribs the weight on the transom is often a key factor.

FWIW many outboards share bodies, so for example a two stroke 75hp Mariner is the same body (and weight) as the 90hp.

And if you go with a 2 stroke you will get more HP usually because they are lighter. Using the above example a similar weight 4-stroke would have only allowed me to have 60hp.

Of course its hard to get 2 strokes now....:(
 
Weight is going to be the main factor.
2HP is nothing! where does it state the HP is measured, at the prop or up at the block, as if measured at the 'top' the power at the prop can be less.
As I understand it some manufacturers use one and some the other.
In the final analysis it isn't obligatory to jack the throttle wide open ;)
 
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Overpowered dinghies

Here in Oz we have real problems with land lubbers buying dinghies with huge o/bs and driving it like a car because it has a steering wheel. It is fine in flat water at full power. Many come to grief. (even with o/b as specified)
Any o/b powered dinghy is prone to broaching in rough water. Here the bow digs in (possibly into a wave) and the o/b pushing from the back can cause the boat to pivot on the bow turning to right angles to original path very quickly usually resulting in a capsize. This is often exacerbated when running on a breaking wave. Of course the bow can rise so to flip the boat over as well. The lesson is that the throttle is the most critical control on the boat in rough water.
Most Oz surf lifesavers have small inflatable dinghies for rescue. It is amazing to watch them tackle big waves head on with relatively high powered engines. The difference is that they are all prepared for a capsize and dunking and are very experienced at rough water.
Overpowering a dinghy with a large o/b is just going to increase the chances of disaster.
The poster in the link reckoned he tested the rig and found it OK. I will bet he didn't consider the extreme conditions. Sure legislation including HP limitations of dinghies is to cater for the most foolish but can you be sure you as driver or anyone else who drives it will not be occasionally foolish. To exceed the design limitations is to really stick your neck out. (into a dangerous area).
So IMHO it is about thrust and how much can get you into trouble. Weight of course does matter as well.
Be careful olewill sorry do I sound like an "elfinsafety" man?
 
stevecray said:
I am looking at a baby rib for use as a tender (well part tender, part toy if I am honest), the rib I am looking at is 2.9m and has a max engine rating of 8hp. Given most 8hp outboards seem to be identical dimensions / weights to the 10hp versions and very close in price, it seems a real waste to go for the 8hp. Is the hp limit normally driven by weight (in which case I may as well go for the 10hp) or is it driven by the load exerted from the hp? Thanks.
Click to expand...

I like the old 9.8hp 2 stroke tohatsu as that was the lightest most powerful engine.

By the way, the 3.1m Ribeye TL is well worth a look if you have the space for it. Its a bit big for me, so I will be going for the 2.8
Ribeye_Smallboat_4pp3_Layout1.pdf

For a bit extra money, you can get one in Hypalon.
 
Keep in mind than anything over 5hp is a pain to carry. So much so that your new toy might not see the light of day because you can't face carrying the outboard out of the garage!
 
I stuck a 5hp onto an Avon rover. Unfortunately it had no floor stiffening and contorted interestingly before coming up onto the plane!
Added plywood with one of those foam sausage swimming aids jammed underneath like a keel - made all the difference. really good fun but wetter than swimming.
Defo a case of remembering to use the kill-cord though.

The same engine on a 2.7 rib is pedestrian unless one-up and prepared to wait.

My criteria now would be - maximum you/the davits can handle weight-wise, and a reasonable nod to the safety rules. (would you let the grandchildren play with it?)
 
We had a realy short RIB with a 40 Merc, hanging off the back of the company 70ft ketch. Boss liked a bit af skiing and it was just up to that. Never had probs with it. Can't remember the exact size, but around 2.80mt? I used it a few times for commuting but never tried to find it's limits in rough water.
DW
 
I suspect that the official rating will be set on the conservative side so a reasonably lightweight 10hp would be safe. BUT I'm pretty sure that if you had an accident with that set-up, your insurers would refuse to pay out.
 
I had a similar dilemma when I had to get a new outboard for my boat. I found an outboard that was 25hp over the designers specified max HP rating for the boat, but as it was such a good deal I gave my insurance company a call to see if they would insure it. They said they would if they had some kind of proof that the boat was capable of taking the bigger engine. In short I got intouch with the manufacturer and they said that the the boats max HP rating is mainly down to the weight and as the new engine had a simular weight to the original engine then it was ok and they were more than happy to tell the insurance company.
So as long as your Weights are simular then it should be ok but check with your insurer to be on the safe side.
 
I'd imagine insurance validation, rather than manufacturers' warranties, are the critical issue here. I used to wonder at the eleven foot version of the Dell Quay Dory being limited to 25hp. The boat alone, unladen weighed 150kg - not exactly flyaway!

Inflatables' HP ratings always fascinated me, but they invariably highlight one factor - that keel design and degree of hull stiffness, either permit or deny a bigger-than-basic outboard. Hence flat-bottomed roundtail boats are reckoned to cope least well.

Fun to watch though. :)
 
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