Modifying Outboards

[Boathook]

I want to make our 30hp Honda more suitable for a displacement boat. Has any one tried to replace the gearbox/prop/cavitation plate with one from a 9.9hp saildrive unit ?
Before you ask 9.9hp is not enough, I require around 25hp on a 4 stroke. Also, the expense of buying a Yamaha 25hp 'big foot' is out of the equation at present.

Any comments views ?

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[Joe_Cole]

Maybe I'm being daft but, why not just change the prop? In any event I wouldn't have thought that a 9.9 leg would be man, sorry, person enough to take 30hp (if that's what you are saying)

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[mikewilkes]

We use a 9.9 Yammy 4 stroke on our cat and she moves along at about 5.5 kts. What sort of speed and consumption do you get out of a 30 hp? We were thinking of getting a bigger one with a wee bit more grunt.
Our cat weighs in about 4.5 / 5 tons depending if we have been to restock the booze up!!!

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[Talbot]

My cat weighs in at 4 tons, but the engine I have will push her at abt 4.5 kts against 30 kts of wind! top speed is abt 7kts with a clean bottom The outboard is a 27hp yanmar diesel!

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[Talbot]

Peter, have you thought about adding a silette leg lower end to your outboard. If they can be mated, that would provide the different gearing and larger propellor that you want.

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[mikewilkes]

Looked at a deisel outboard at the boat show but it was Eur 13,000 + 300 for the prop!!
Can you tell me what a silette leg is or where I can find out info on them.

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[Talbot]

Yes they are pricey, but then they are all hand built! The only real drawback in the 11 years I have had mine is the size of the alternator. The option of the <A target="_blank" HREF=http://www.sillette.co.uk/first.htm>Sillette-Sonic leg</A> would not really fit to the design of my boat without some major surgery whereas the outboard needed much less (the boat already had an outboard with too short a leg on it).

The sillette sonic is fitted on a lot of Prouts and would have been my prefered system cause they turn a 15" prop more slowly. My egg whisk is more designed for use with a rib, so I suffer from a fair bit of cavitation, but even so the thrust is remarkable.

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[Boathook]

Re: Modifying Outboards re mikewilkes

Max speed of 6.3 knots, cruise at 4 to 5 knots and about 6 to 7 hours from a 5 gallon tank. Motor sailing can give up to 9 hours per tank. Wind and waves do make a big difference to these figures, normally reducing them !

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[Boathook]

Re: Modifying Outboards re Joe_Cole

Already got a Yamaha dual thrust prop fitted for displacement speeds but it is not the same as turning a bigger prop slower. The dual thrust can still cavitate when going astern so is not perfect but better than the Honda props !

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[Joe_Cole]

Re: Modifying Outboards re Joe_Cole

That's what I thought you were getting at, but I wasn't sure if I had miss-understood your original posting. If it's practical to change the gearing with another leg then I would be interested for my boat. I've always assumed that you had a leg with the outboard and that was that, but if it's not then I feel another project coming on!

Joe

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