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Old 09 September 2012, 12:19   #1
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Still struggling to set up my Tohatsu 9.8

I recently posted a problem with cavitation on my Tohatsu 9.8hp 2-stroke engine when used with my Zodiac 2.85m SIB.
I had only tried it briefly on a stretch of canal outside a mill where there was corrugated steel banking on both sides of the water, so the wash would not do any damage. In fact I never achieved more than about 6mph as the engine would race and the boat speed would actually reduce.
Now we are on holiday in Cornwall and on the open water there's no cavitation, I have not changed anything on the setup.
If you open the throttle the boat leaps forward and is doing 15mph+ in no time at all, with no pause while it gets out of the hole as with my 6hp engine....very impressive.
So it must have been something to do with where I was on the canal.
However I'm now getting splashback from the engine, water sprays up from the engine on both sides and comes into the back of the boat at higher speeds.

What adjustments can I make to reduce this, would raising the engine and inch or so with a block of wood help? The anti-cavitation plate is a bit lower than the recommended 1 inch below the keel.
It's not the end of the world, but does mean you get a bit wet at higher speeds when sat on the tube.
This never happened with my 6hp Mariner.
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Old 09 September 2012, 15:10   #2
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Here is a picture of the engine set up on the boat. I think it may be the anti-cavitation plate is a bit too low?
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Old 09 September 2012, 15:17   #3
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Yep,raise the engine the inch,once the clamps are still getting a lock on the transom.
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Old 09 September 2012, 15:29   #4
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Surely raising it will make the cavitation worse?? that's what happened when I raised mine on a similar set up. Three up it cav'd in the turns.
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Old 09 September 2012, 16:00   #5
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Yep,raise the engine the inch,once the clamps are still getting a lock on the transom.
Hard to tell but I think youd get away with more like 2 inches ?

It'll go alot better , use less petrol, splash less etc.

If its not cavitating any more then no worries ( you had pulled the wheels up when it did this ? - Been there done that ...)
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Old 09 September 2012, 16:27   #6
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( you had pulled the wheels up when it did this ? - Been there done that ...)
I've done that too - even with them down, the 9.8hp still got her on the plane.

Messy though!
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Old 09 September 2012, 16:31   #7
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I've done that too - even with them down, the 9.8hp still got her on the plane.

Messy though!
I only had 3.5hp when I did it ...no chance.... I do love driving the Bombard up the beach with the wheels down ..impressive rooster tail ( I learnt that from Matt H ...)
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Old 10 September 2012, 04:42   #8
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Yes the wheels were up, but I have also left them down before now with my 6hp, hey why is the boat not getting on the plane....oh....

So do I try raising the engine or not? One reply says the splashback will be worse if I do, another says raise it more than one inch?

I will try it anyway today. I guess that's the only way to really know. Will engine angle affect splashback?
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Old 10 September 2012, 07:09   #9
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Yes the wheels were up, but I have also left them down before now with my 6hp, hey why is the boat not getting on the plane....oh....

So do I try raising the engine or not? One reply says the splashback will be worse if I do, another says raise it more than one inch?

I will try it anyway today. I guess that's the only way to really know. Will engine angle affect splashback?
Try it - it costs nothing ....start with an inch up ..I'd bet you will end up thinking 'hey this is good !' , then try a bit more and see when it starts to feel 'wrong'.

You wont have worse splash .. my Sib had a proper rooster tail ( like a race boat ! ) and a waterfall over the inside of the transom into the boat before I raised the engine.

Just wedge any old bit of wood under there while you try it out and you can make it all neater when you get the height set up Ok .
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Old 10 September 2012, 11:04   #10
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I've got a Zodiac 3.4 with a Tohatsu 9.8. This is a known issue. As others have said you need to raise the engine about 1/2inch or so on the transom. I have stuck some velcro on to the top of the transom and to a strip of wood. This keeps the wood strip in place (I've never taken it off). The engine sits on top of this and clamps to the transom as normal.
This will stop the excess spray etc.

There is also an excellent how to thread on here with pics - do a search.

splash back!
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