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Old 04 September 2005, 17:40   #1
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Outboard Choice

Hi all,

I am new to boating and have just bought a rib with a 55hp 2 stroke.(1998 vintage) Weight 103kg.

I have 2 questions.

1) can I exceed the max rated HP for the rib (70hp). If I do, what would the consequences be or does the manufacturer automatically build in some margin for saftey. I am thinking of a 75hp.

2) Am I likely to see marked improvement in performance from 55hp - 75hp bearing in mind the extra weight. current 55hp (103 kg) New 75 HP (145kg)?

Apologies of these questions are simplistic, but I cannot get anyone to give me an opinion.

TIA.
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Old 04 September 2005, 18:29   #2
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Obviously how strong the boat is, is very important.

I am running a 50 HP monster 4 stroke on a 50 HP transom. It seems that weight has a massive impact on the handling characteristics on a boat of this size.

It doesn't seem to be HP that is the limiting factor but weight.

Also you really need to give it a trial run to see how it handles.

A very difficult thing to do.
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Old 04 September 2005, 19:17   #3
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Quote:
Originally Posted by oogopach
can I exceed the max rated HP for the rib (70hp). If I do, what would the consequences
I suppose the worst case scenario would be if you were involved in an accident with a charterboat full of lawyers, and not only were you sued, but your insurance company refused to cover you and the MIAB investigated and prosecuted you so that you ended up penniless and in prison. But look on the bright side

Quote:
Originally Posted by oogopach
Am I likely to see marked improvement in performance from 55hp - 75hp
36% increase in power, I reckon you ought to!

Also consider how much lower in the water your transom will sit. I would suggest adding 42kg in the stern of your RIB and seeing what happens.

What do you want to achieve by replacing the engine, and what length is the RIB?
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Old 04 September 2005, 20:02   #4
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Go for the Yamaha 70hp - 105kgs!!!

Or the Tohatsu - same weight same hp.

I agree weight should be more of a factor than HP. And that is where a 2 stroke wins big style!!! For example the Suzuki 4 stroke 70hp weighs 162kgs!!!
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Old 05 September 2005, 04:47   #5
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Thanx so far for the responses.

The rib is 4.5mtr. it weights 190kgs and with a hp max speed registered on the GPS fitted is 25knts!! The engine has the factory original prop and the leg has a doelfin fitted. I was hoping it would do more than that.

So fitting a bigger engine ..... I should go faster no?

Thanx for the Yam tip.. same weight virtually but 30% power increase.

Does that correlate to a 30% increse in speed (ie 8knts) or is the increase in
HP-vs-Speed relationship non linear?
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Old 05 September 2005, 08:39   #6
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Forget the power-to-weight ratios and linear or non-linear relationships. If your 4.5m RIB only does 25kts with a 55hp engine, there's something wrong with the engine or the way it's rigged.
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Old 05 September 2005, 08:44   #7
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May I suggest you take the doel-fin off and play with the trim?

My little rib is a 4m boat wiv a 40hp 2-stroke on the back and according to my mates speedo (I don't have one) it has reached 36 (I dont know if thats kph or mph). I have clocked it myself on a hand-held gps at 31kph (flat calm, one up, trimmed way out).

I tried a doel-fin, but felt that the top speed was reduced (extra drag). This wee boat comes onto the plane like an express train at all times, so there was no benefit in using the fin - so off it came. Its one of these things you gotta try to see if your outfit gets any benefit.

Try trimming your motor out when you are on the plane - just a little at a time - you should notice the revs pick up each time, with a corresponding slight increase in speed - until you reach the point where your prop starts 'cavitating' ( draws air and loses bite). trim back in a tad and you are then running the best outboard position for your outfit.

At this point note what the revs are and compare to the manual for max revs.
If you are close - leave it alone. If you still have a way to go - you may want to look at a different prop. Have a search on here for 'props' and you will get a lot of good info.

hope this helps....

Bill
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Old 08 September 2005, 11:22   #8
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hp vs speed

Square rule - double the speed quadruple the hp ...
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