Cavitation problem with new engine

[lightning]

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.

[jezza2011]

you could check the prop pitch. and diameter. this could have a big effect.
may need to go up a pitch.

[willk]

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.

[prairie tuber]

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.

[willk]

Originally Posted by prairie tuber

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?

[prairie tuber]

Originally Posted by willk

Yes, except that he bought the engine new, I think?

Hmmm... I thought all of Tohatsu's newer 9.8s are 4 stroke.

[willk]

Originally Posted by prairie tuber

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...

[lightning]

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.

[willk]

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.

[lightning]

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)

[willk]

OK. When I got my setup first, I played with the trim for a while. I found that with two on board and the pin in the third hole from the bottom, I got similar problems to yours. I dropped the pin a hole and the problems largely vanished. Yes, the transom angle will enter into that equation too. It's very much "suck it and see" territory.

I also found that it helped a lot to shift weight forward for the hole-shot, especially with 2 OB. A tiller extension was a huge help in this respect. A bit of white sink wastepipe and a bit of tape is good to experiment with to check if one would help you. I wouldn't leave shore without mine fitted. It is the 300/600mm telescopic type, the best of both worlds, IMO.

[lightning]

On the prop it says "F8.5" and according to the instruction book this is the second in a range of five alternatives on this engine F6.5-F9.5 (F9.5 being recommended for the smallest/lightest boat)
F8.5 is the one supplied as standard.

I do have a tiller extension which I use when I am out on my own in the boat. I think I did end up with trim position 2 on my last engine.

[willk]

8.5 is the same as mine (just checked).

[lightning]

Thanks for the advice, I guess it's a case of trial and error next time we are out on the water.

[prairie tuber]

Is there any loose fabric on the underside of the hull - especially near the transom?

[lightning]

No, the boat has an inflatable keel under a solid floor.

Since buying the new engine I've also bought a Bravo air pump, so am now inflating the boat/keel to the correct pressure of 3.4psi. (Well actually I inflate it to 3psi)

Before I found that I was only inflating it to about 1.5psi with my Rule pump, the Bravo has a piston pump which takes over at 1psi

I don't know if this may be affecting the boat and causing it to cavitate more.

[willk]

My advice is pump to the recommended pressures - there will be a safety margin in those anyway. The keel on my QS pumps to 6psi or thereabouts. What are you pumping the deck to? The QS is 11psi...

Correct inflation is essential to get these things on the go properly as the keel and the airdeck form the backbone of the planing structure. Put a bend in that and things are not gonna be right.

[lightning]

The inflatable keel on this boat is a low pressure one, it's just a long thin tube running front to back under the floor (max pressure 3.4psi)
The boat planed fine with my old Mercury 6hp when it was only inflated to 1.5psi. However I agree it should be at the correct pressure, it does say "max 3.4psi" though and not "inflate to 3.4psi" on the boat.
The instructions talk about leaving a margin for temperature if it's going to be a hot day.

[willk]

Originally Posted by lightning

The instructions talk about leaving a margin for temperature if it's going to be a hot day.

Yes. However bear in mind they also sell these boats in countries that have summers, c/w sun. Once you plank it in the water (16C) it's going to be 16C. The keel gets no direct sun (hopefully!!) so, IMO, the section at least risk of overheating. It's probably the most important too.

[lightning]

OK I'll set the pump to 3.4psi.