Is it cavitation?

[Becky]

I bought a nice new inflatable and outboard yesterday. The max limit of power the dinghy will take is 10hp, but that was too heavy for me to lift. But I could just manage the 8hp Mercury. So I went out to play yesterday afternoon. When I accelerated, the boat picked up speed then slowed with the engine running faster. It seemed like the propellor was slipping. So I wondered if it was cavitating? And if so, how can I correct it?

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[muchy_]

I had this happen to me. Just sit further forward. An extension arm on the engine may be required. Once I shift my weight further forward the bloody thing takes off.
Be prepared for the acceleration cos if mines anything to go by it can catch you by surprise.

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[cliff]

Could be. First make sure the transom is the correct height for the engine. I had to "add" a bit on to the transom on the old tender when I changed the engine and what a difference. The "cavitation" plate should be about an inch below the bottom edge of the transom. any less and sever aeration will occur. Fuel tank and some weight forward can also help to get you up on the plane - it is all a matter of trim and the correct engine height - check the manual that came with the outboard for correct setting.

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Stay put when I hit you!

 

[Oldhand]

I trust you are following the running-in instructions fro the Mercury? Doesn't sound like it if you are opening it up straight away.

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[Becky]

'Course I was. I read the instructions first. That is why I am asking the Forum about something that isn't in the book. Believe me, the dinghy weighs 35 kg, so does the engine, though it feels heavier. Bessie (my spaniel) and I don't weigh much more (in my dreams) so we should be able to zoom really well on half throttle, but we didn't. I thought the prop was loose. Worrying! Still, I will try sitting further forward, but I can't reach the steering thingy then. Maybe you can buy a tiller extension? The gears and throttle are all in the handle so I presume that if you can it will be expensive.

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[longjohnsilver]

You also have a pin to alter the angle the engine sits on the transom, experiment with different settings but more likely need to move weight forward, once p[laning then you can shift back again.

Maybe not recommended but you can sit at the bows and steer the boat by shifting your weight from side to side, suggest you have the kill switch attached, but good fun on a calm day with no other boats about (and not going flat out!!)

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[William_H]

For a quick and cheap tiller extension try rubber hose the reinforced type of a size that will fit over the throttle gear select handle and as long as..... clamp it on with a suitable clamp and then get your self and spaniel forward regards will

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[Peppermint]

Re: Sounds like cavitation.

Above the prop is a flat plate which should be just about submerged at cruising speed. As the dinghy gets up on the plane your dragging the prop into the surface water disturbed by the hull. This should be OK with a dinghy but check anyway.

Is the transom vertical? The engine/prop should be pushing in the horizontal. This can vary as trim changes due to load or speed. It's more important to get it right when your at speed and in a usual load state. If the transom is angled adjust the outboard bracket. On the bracket is a quadrant with a spring bar through it, no tools required.

What sort of floor does the dinghy have? If it hasn't a keel the floor can form suction on the water. This results in the engine reving unevenly at fixed rev's in the cruise.

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[Becky]

Re: Sounds like cavitation.

The dinghy is an Avon with an inflatable keel and inflatable floor. The transome is vertical when evrything is stationary, but the bow goes quite high when I start going forward. As a consequence, the engine is tilted backwards. If I move to the seat I can't reach the tiller. I will try it with two people on board. That will make the corect balance easier to achieve. Bit disapppointing at the moment.
Thanks, all of you for such helpful comments.
Regards Becky

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[boatless]

Re: Sounds like cavitation.

As above. They all do that (mine anyway). Either tighten up the friction on the swivel and throttle and enjoy steering by weight, or any old bit of tube that will fit the twist grip to extend it.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[longjohnsilver]

Re: Sounds like cavitation.

Piece of plastic drain pipe, if necessary cut slots in the end and then clamp it to the steering arm, cheap and effective.

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[rich]

Yes, sitting in the bow, flatout, seee the funny looks you get!!!theres nobody STEERING...difrent subjeck .ow do yu git this nuu ieSpell tu werk?

<hr width=100% size=1>http://www.jersey-harbours.com<P ID="edit"><FONT SIZE=-1>Edited by rich on 11/06/2004 23:41 (server time).</FONT></P>

 

[cliff]

Read the instructions on the web site

"Usage instructions:

If you are using Internet Explorer, you can access ieSpell in one of the following three ways:
Under the "Tools" menu, click on "ieSpell" item to start the spelling check or click on the "ieSpell Options" to configure ieSpell.
Click on the ieSpell toolbar button (if enabled) to activate the spelling check.
Right click in the browser and in the menu that appears, click on the "ieSpell" item to start the spelling check or click on the "ieSpell Options" to configure ieSpell.


If you are using a browser based on the IE engine (e.g. AOL, MSN, NetCaptor, NeoPlanet, etc), then only the last option (i.e. using the right-click menu) is available to you. "



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Stay put when I hit you!