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Old 18 August 2014, 16:18   #1
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Yamaha 90 cavitation - any advice on new Prop??

Hi all,
Can anyone suggest best way to figure out the right prop for my needs??

I recently picked up a hard nose 5.6m RIB with a big heavy old engine on the back (200...way bigger/older than I want!), and replaced it with a 2007 2-stroke Yam 90 (very light 3 cyl) with factory prop.
It has plenty of power for what I want, picks up on plane no problem and when trimmed up does 34 knots which is fine, but it cavitates big time when turning and going over waves.

The engine is very light, and with the heavy hard nose I think it is sitting too flat in the water, but I have also heard that the Yamaha 'triangle' K series propellers do not grip well anyway.

I want to keep the same speed, still be able to get a wakeboard up, but want to lose the cavitation... can anyone suggest a better prop to get and ideal pitch/diam?

I may need to put a jacking plate on and push it further back from the transom to get the weight further back.

Existing prop is a 13 1/4 x 17K white alu factory prop. Any help appreciated. Thanks.
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Old 18 August 2014, 17:32   #2
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Just so you know... The ribnet possy doesn't believe this is an osprey...
They've been watching it on fleabay for sometime... So will be interesting to see what they make of it now it's in the hands of one of their own..

I know nowt about props. But pretty sure you need a rev counter to know what's happening. Revs at WOT I believe...

I'd also try dumping some sand bags at the stern to see if that shifts things...
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Old 18 August 2014, 18:11   #3
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Hard to tell from that pic but possibly a little high? I've been told mines too low at about an inch below bottom of transom but have left it alone for fear of this sort of problem (others have complained about props letting go in turns after raising too much in search of extra speed). Can you lower it a hole or is the yolk sat on the top of the transom?
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Old 18 August 2014, 19:23   #4
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Given size of boat, prop pitch, motor and speed achieved, the 17" on there is about the right pitch-it's the same figures as my 5.4 does with a 17" on the Yam. Something else isn't right. It might be worth changing it for a slightly lower (1 to 2") pitch stainless.

Be aware you're probably going to face an awful lot of questions about your boat though-it's been speculated on for a long time.
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Old 18 August 2014, 19:24   #5
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We currently have a similar problem with our boat on hard turns it cavitates quite a bit.

My engine currently has a repaired stock prop and we are looking for a new replacement in the future.

And have been advised to look at 4 bladed props they have better acceleration and supposedly have less cavatation but if your trimmed up. I would look at trimming down as you go into a hard turn and then trimming out again.

But If I were you I would go with the previous answer and put some weight to the stern as this is a easy cheap thing to try and is easy to change if the weight needs adding or reducing. Sand bags work a treat just try to lash them down last thing you want is them shifting as you go hard over.

I have the opposite problem to you as my engine is too heavy having a 135 v6 in the back and mine still cavitates but sorted a lot of the problems by putting 80kg up in the bow in the form of sand bags.
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Old 19 August 2014, 09:35   #6
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As said you can not expect to trim up to the limit B4 she cavitates to then turn hard with it not cavitating! You will need to trim down B4 the turn
how Is your cavitation plate compared to the bottom of the hull?
Higher lower or the same?
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Old 20 August 2014, 16:14   #7
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Hi all, thank you for your thoughts.
I will definitely be trying some weight in the stern to see how that works, but I think height wise it is pretty close with the cavitation plate sitting maybe 1" max from the bottom of the hull. I do also think the boat has a heavy bow, and judging by the way it rides weight in the back of an offset plate to get the engine aft may be the best answer.

I do trim down before turning, as I would expect it to cavitate when fully trimmed, but it does still cavitate when I wouldn't expect it to. It will seem fine then just start slipping, so I will see if I can try a different prop or two.
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Old 20 August 2014, 16:16   #8
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Quote:
Originally Posted by Clamchowder View Post
Hard to tell from that pic but possibly a little high? I've been told mines too low at about an inch below bottom of transom but have left it alone for fear of this sort of problem (others have complained about props letting go in turns after raising too much in search of extra speed). Can you lower it a hole or is the yolk sat on the top of the transom?
The yolk is sat right on the transom, but if I knew it was the problem I would be able to take it down a bit or offset it wit a plate.
Thanks.
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Old 20 August 2014, 16:25   #9
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Quote:
Originally Posted by ShinyShoe View Post
Just so you know... The ribnet possy doesn't believe this is an osprey...
They've been watching it on fleabay for sometime... So will be interesting to see what they make of it now it's in the hands of one of their own..

I know nowt about props. But pretty sure you need a rev counter to know what's happening. Revs at WOT I believe...

I'd also try dumping some sand bags at the stern to see if that shifts things...
I have seen some of the discussions/speculation since buying it.... I am also now not certain what it is now I know more about it. I am hoping to try and find out if anyone has ideas?! I was dubious when it was advertised but I picked it up quite cheap in the end (I think) and planned to change the engine from the start, but hadn't really considered much else about it (spontaneous buy, wife not exactly over the moon!). I'm not too worried about the speed 32 knots+ is fine but just hope to get it set up so it is a better ride and reduce cavitation.

I will def. try some sand bags in the back next time I use it, and notice the WOT revs but I seem to recall they are about 4800-5000 before I start trimming when it gets to about 5500 at full speed before it starts slipping in a straight line. The boat stays pretty flat at this point, I think the heavy bow means that trimming the engine doesn't lift the bow as much as on other boats I have had... it's not a great ride like that to be honest so hopefully some weight in the stern will help!!
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Old 20 August 2014, 16:29   #10
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Does anyone use the horizontal foils/fins that are bolted to the cavitation plate these days?? I haven't really seen them fitted to boats much, is that just because they don't look too good?!
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