Still struggling to set up my Tohatsu 9.8

lightning · 09 September 2012, 17:19

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.

lightning · 09 September 2012, 20:10

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?

Boatnomad · 09 September 2012, 20:17

Yep,raise the engine the inch,once the clamps are still getting a lock on the transom.

Max... · 09 September 2012, 20:29

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.

PeterM · 09 September 2012, 21:00

Quote:

Originally Posted by Boatnomad

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 ...)

willk · 09 September 2012, 21:27

Quote:

Originally Posted by PeterM

( 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!

PeterM · 09 September 2012, 21:31

Quote:

Originally Posted by willk

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 ...)

lightning · 10 September 2012, 09:42

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?

PeterM · 10 September 2012, 12:09

Quote:

Originally Posted by lightning

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 .

Flyguy · 10 September 2012, 16:04

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.

http://www.rib.net/forum/f50/splash-back-49989-2.html

lightning · 10 September 2012, 18:10

I've done exactly as suggested above (raise the engine about 2cm with a piece of wood secured with double sided tape) and there is a definite improvement. It won't go any higher, I tried a thicker piece of wood and the clamps touched the ridge at the top of the mounting bracket, so the engine was not secure.

Adjusting the engine tilt position did not seem to make much difference (second/third/fourth hole from the bottom) the biggest improvement for planing is if I sit on the seat at the front of the boat and use a a tiller extension.

Locozodiac · 10 September 2012, 19:20

With well inflated sib you need minimum 40 cm from transom border to lower keel for that sib to perform as expected, trim engine to look vertical and move your weight forward. Once you have finally matched perfect engine transom height you could re arrange engine plates to match transom border line for engine clamps to rest at middle inner plate.

Happy Sibbing

lightning · 10 September 2012, 21:09

Quote:

Originally Posted by Locozodiac

With well inflated sib you need minimum 40 cm from transom border to lower keel for that sib to perform as expected, trim engine to look vertical and move your weight forward. Once you have finally matched perfect engine transom height you could re arrange engine plates to match transom border line for engine clamps to rest at middle inner plate.

Happy Sibbing

The distance from the top of the transom to the keel is 16 inches. I don't know what you mean by "re arrange engine plates" though. I'm not sure there is any more l can do with this installation.

willk · 10 September 2012, 21:19

He means the engine mounting plate. If you raise it on the transom (if possible) you will have more usable "lift" as the engine screws can be fitted higher on the transom then

So, have you decided yet which one of those props you don't need...

lightning · 10 September 2012, 21:43

I don't think it would be easy to raise the engine mounting plate on this boat, as it is only on the back of the transom.
It seems to be better with the piece of wood to raise the engine, so l will go with that for now. Now l need to try this new propeller.
It seems though, that there is a 6 mph speed limit on the waters around Golant and St Mawes where we are staying.

lightning · 10 September 2012, 23:50

Can somebody tell me how tight the castellated nut should be that holds the prop on? I just removed the prop and the nut was only finger tight, held by the split pin of course so it would never have come off.
As this is a new engine is that correct, just tightened by hand and then put the split pin in?

Poly · 10 September 2012, 23:55

torque will be in the manual - but yes, not particularly high.

Max... · 11 September 2012, 10:04

It's not correct. You need to tighten the prop nut fully and then be able to insert the pin, if it doesn't line up with a hole then add shims until it does.

lightning · 11 September 2012, 15:21

I tightened the nut until the pin could be fitted, but it's only just over hand tight.

This was the same with the original prop.

I'll check the manual for prop nut tightening torque when I get home, but it's not going to come loose with that split pin in place.

Should there be a washer that goes on before the prop is fitted?

Locozodiac · 11 September 2012, 15:26

You should have a inner metal plate for the clamps to be adjusted against, having both matching same height as wooden shim border line would be perfect. By placing a 2 cm transom shim, has spash issues been solved ? All transom heights and sib tubes are not same, so will need to get a perfect marriage between transom height, engine brand and tube diam.

Happy Sibbing