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Old 05 September 2004, 06:39   #1
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Getting some air

I think I need a bit of advice on getting airborne, or rather landing.
Normally after a jump the boat just carries on as normal, but 2 or 3 times now the prop has refused to "grip" on landing. I have had to back off to almost tick over before I can get some grip again. This means that the boat has almost stopped and I keep getting looks from the crew.
Each time it has happened the waves have been around 2 - 3 feet high and around 1 foot higher than the general slop. Wavelength has been around 1 boat length and the stall has been on the 2nd of a set of 2 or 3, they were probably a motor boat wake. Speed has been between 15 and 25 kts.
Can I do something to prevent this happening?
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Old 05 September 2004, 12:20   #2
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I have had similar problems usually after a sharp turn. I belive I have had the trim to high and the engine has overeved as the prop came nearer the waters surface, and then the cut out comes in. As you say bring ther revs back to idle usually sorts the problem, so may you could try throttling off at the moment of take off.
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Old 05 September 2004, 12:39   #3
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Aaaah! is that what it is... i was wondering.. i did a 90 degree left and then 90 degree right straight after, boat almost tipped and went on one tube (other tube about at the height of my ear) and then went back flat onto the water, engine went mad, overreved horribly (thought it was going to pop) and then engine switched off... after that it worked fine again... i thought i hit a rock and the rubber slip sleeve thingie had snapped....

Now i know a bit more... Cavitation sucks
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Old 05 September 2004, 12:57   #4
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Crap propeller, not designed for the job. If it used to be fine and nothing has changed then you've got to start looking for faults.
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Old 05 September 2004, 15:55   #5
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Quote:
Originally Posted by jwalker
Crap propeller, not designed for the job..
yo JW whos prop I'm a bit lost now . can't see anyone mentioning their prop

p
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Old 05 September 2004, 15:58   #6
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Landing

How old is your propeller and is it an aluminium one?

I had these symptoms at the end of last year and on looking at my propeller closely could see loads of fine fractures - I reckon that the propeller blades must have been flexing.

Got a s/h Lazer II and the boat has been transformed - not a hint of cavitation / ventilation.

Must be worth a try and anyway SS propellers are shiny!

Duncan
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Old 05 September 2004, 20:11   #7
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Quote:
Originally Posted by jackeen
yo JW whos prop I'm a bit lost now . can't see anyone mentioning their prop

p
Paul, I was suggesting it may simply be a prop that is not good at recovering once it has stalled. Some need the boat to be slowed almost to a stop before they will regain good flow. Backing off the throttle and starting over doesn't do it. However, Mark maybe suggesting that his boat used to be fine and now it isn't. I wasn't sure.
I once had a prop like this which gave a good showing in a straight line but jump it or turn sharply and it was a liability. It had Teddy bear's ears for blades.
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Old 06 September 2004, 03:39   #8
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Thanks all.
It has not got worse, 19 times out of 20 there is no problem. Just the odd occation when it happens.
The prop is ally and a bit rough round the edges, but not chipped or bent.
Good point about engine trim. Thinking about it, with the wavelength around 1 boat length, I may have landed on the down slope of the 2nd wave which would have lifted the stern. I guess, with the engine trimmed out, the prop would have still been in the trough behind and very close to the surface.
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Old 06 September 2004, 05:45   #9
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Quote:
Originally Posted by gingercoastie
I have had similar problems usually after a sharp turn. I belive I have had the trim to high and the engine has overeved as the prop came nearer the waters surface, and then the cut out comes in. As you say bring ther revs back to idle usually sorts the problem, so may you could try throttling off at the moment of take off.

This is very well but the power must be on when hitting the water otherwise the danger is as their is no forward movement the water could be forced up the exhaust into the engine which is not a good thing and definatly kill your engine.
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Old 06 September 2004, 06:15   #10
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Quote:
Originally Posted by Simon Hawkins
This is very well but the power must be on when hitting the water otherwise the danger is as their is no forward movement the water could be forced up the exhaust into the engine which is not a good thing and definatly kill your engine.
Rubbish.

JW hit the nail on the head with the prop and Seariders suggestion definately worth a try. See if Lee Fareweather has a suitable second hand S/S. I have a 19" S/S but is V6 gearbox size so may not fit, check with Lee.

Also worth looking at engine height, high mounted engines do increase speed but when we tried it with a Yam 115 she lost grip to easily, result we put the engine back down a hole. Not a huge change in height but really did make a difference out on the water. Something else we tried was a Suzuki S/S prop with a Merc bush mounted on the 115. Was faster than a Merc prop but cavitated and lost grip in even small waves, so went back to the Merc in the end as the best compromise for cruising with a medium engine height.

Peter
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