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Old 05 September 2004, 05:39   #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, 11:20   #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, 11:39   #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, 11:57   #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, 14:55   #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, 14:58   #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, 19:11   #7
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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, 02:39   #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, 04:45   #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, 05:15   #10
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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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Old 06 September 2004, 05:17   #11
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I was soooo hoping you were gonna comment on getting old spice aired out
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Old 06 September 2004, 05:17   #12
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Originally Posted by Pete7
Rubbish.

JW hit the nail on the head, but 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.

Peter
Which is why I am about to order a transom jack!!! More I think about it the better the idea seems!
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Old 06 September 2004, 05:18   #13
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Hydraulic or manual?
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Old 06 September 2004, 05:38   #14
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I was soooo hoping you were gonna comment on getting old spice aired out
oh she can go airbourne , but she is an old girl so tend to drive her with a little TLC however doesn't suffer from ventilation problems like an outboard, but then it is a 16.75" dia prop turning quite slowly.

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Old 06 September 2004, 06:05   #15
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A good prop should be able to leave the water and then take a grip as it comes back in. In most cases, you should be applying power as the boat re-enters.
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Old 06 September 2004, 06:47   #16
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Stainless would be nice. Current prop is a genuine Tohatsu A18 which I think translates to 13.5 x 18. It needs to go up a pitch or 2 because I can max out on revs before WOT. The prop was not changed when the extre 20hp were added. If I go s/s I must be looking around 13-14 x 20-21.

Anyone know what else fits a Tohatsu spline?
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Old 06 September 2004, 07:01   #17
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Unless Porter boats are very light, it's not likely you'll want a 21" pitch on a 140hp engine on a 6.5mtr. 19ish max I would have thought.
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Old 06 September 2004, 07:52   #18
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19" would seem nearer the mark. I was working on 18" +1" to bring revs down +1 to 2" for going stainless. Depends on what diameter I can swing though.
The dry weight is around 1000kg.
Time to re read all the prop threads 'cos this has been done to death before, and then phone round the suppliers.

I am now happy that my original question came down to trimmed out too much for the wave in question, fluffy prop and iffy driving.

Mr 7. I shall give Lee a ring. Are you going to Church' next Monday nite?
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Old 06 September 2004, 08:24   #19
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Hydraulic or manual?
Hyrdraulic of course!

http://www.cook-mfg.com

The PL65 - would 10" setback be too much with a 225hp on an 8.5m rib?
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Old 06 September 2004, 08:38   #20
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Difficult to say. Could be great for max top speed (with the right prop too), but worse in the sloppy stuff unless you start adding a bow tank, or other shiftable ballast. Do you have to trim up way above neutral at the moment to get max top speed? As always it's a compromise, but IMHO a good one if you are prepared to spend time setting the boat up around it. Just bolting it on you might well be disappointed.
I would also consider either a low water pickup on the gearcase or transom mounted water pickups, plus also a water pressure gauge on the dash. It'll be very easy to run the stock water pickups above the water when you are going for max top speed, assuming you have a prop that can handle running near the surface. (I'm assuming you are running a merc with stock gearcase). I guess you could just about get away with putting those flaps on the stock pickups which force feed the water, or I've read about some people blocking off the top 2 holes on the pickup too, but they always seem a bit marginal.
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