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Old 21 July 2012, 13:18   #1
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Cavitation problem with new engine

I have a Zodiac 2.85m dinghy, this model has a solid floor with an inflatable keel.

Previously my engine was a Mercury 6hp short stroke and it performed well, took a few seconds to get on the plane but would achieve about 14mph with 2 of us on board.

I replaced the engine this year with a Tohatsu 9.8 2-stroke short shaft, which weighs the same as the Mercury.

The engine initially seems to perform very well, but as soon as you open the throttle to get it on the plane it cavitates badly, the engine races and the boat loses speed.

I did get this very occasionally with the old engine, but it seems much worse now.
The anti-cavitation plate is about 2 inches below the bottom of the transom, the same as the old engine.

How can I stop the boat from doing this? The only adjustment is engine tilt angle which I have not tried yet.

I cannot imagine that I would need a long shaft engine with this small boat.
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Old 21 July 2012, 16:04   #2
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you could check the prop pitch. and diameter. this could have a big effect.
may need to go up a pitch.
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Old 21 July 2012, 16:19   #3
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Try the engine tilt position. I run with the pin in the second hole from the bottom. IIRC the standard prop is a 8"x8", it should be fine.
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Old 21 July 2012, 16:44   #4
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Check to see if you have a spun prop hub. Given that you are using the same boat, but with a different motor, this would be the first thing I would suspect.
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Old 21 July 2012, 17:03   #5
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Quote:
Originally Posted by prairie tuber View Post
Check to see if you have a spun prop hub. Given that you are using the same boat, but with a different motor, this would be the first thing I would suspect.
Yes, except that he bought the engine new, I think?
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Old 21 July 2012, 17:43   #6
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Yes, except that he bought the engine new, I think?

Hmmm... I thought all of Tohatsu's newer 9.8s are 4 stroke.
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Old 21 July 2012, 17:51   #7
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Quote:
Originally Posted by prairie tuber View Post
Hmmm... I thought all of Tohatsu's newer 9.8s are 4 stroke.
They make both. Lightning's is new, it's a long story...
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Old 22 July 2012, 06:31   #8
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What would be the effect of going up a pitch with the prop? We are going to Cornwall in September and l may take a spare prop in case the boat won't plane when we get out on the sea with it. I may only be able to go on the canal between now and then, which obviously means l cannot test it at speed.
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Old 22 July 2012, 06:36   #9
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Check and tell us the pitch of the existing prop (stamped on the hub). Unless they've sold you a Hi-thrust saildrive prop, it won't be making THAT much difference. Where do you have the engine trimmed at? i.e. what hole is the pin in, relative to top/bottom.
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Old 22 July 2012, 06:43   #10
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I will have a look at the prop when l get home. The engine is currently trimmed at third from lowest position, l need to assemble the boat on stands and see what angle the engine is at, as the transom is angled (further back at the top)
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