prop cavitation

[fullandby]

Hello All,

I would be grateful for some advice about my propeller.
I have a MK2 with a Yanmar 2gm 20hp with a 3 bladed propeller mounted midline in front of the rudder.
I am not sure about the pitch or diameter.
My motor should max out at about 3200 rpm, but will only go to about 2800 before belching black smoke. So I think the prop may be overpitched.
But the main problem is over about 1500 rpm there is fine vibration and a sound like stones rattling in a tin can coming from the back end about where the prop is. I think this might be prop cavitation.
The alignment has been checked and is OK and the cutless bearing is new.
I remember a post in the past saying that cavitation can be a problem in Tophats but I cant find it.
Has anyone had any experience with this problem and found a solution?
I am thinking of reducing the pitch of the prop on the next haulout in the hope of reducing the pressures around the prop.
I would appreciate any opinions.

Cheers,

Bart

[8philip8]

hi Bart
I 've my 2 blades prop rebalanced/ repitched 3 weeks ago by DH .Porter & Co
got more knots from the motor at lower rpm.
the motor is running much smoother too.
$ 170 well spend+ the slip cost
worth a look into it.
Sylph Mk III
8Philip8

[storm petrel]

Will your engine reach maximum revs at idle without blowing smoke? If so, your prop is probably over pitched and/or has a diametre which is too large for the boat if it will not reach maximum revs when the clutch is engaged. A corectly matched prop should allow you to reach hull speed (about 6 knots) at max revs. Black smoke is also an indication of your diesel not being able to cope with the load, which can occur when you are over propped. You can try one of the prop calculators on the web, but you really need to know the dia and pitch to do this effectively.

[Dolphin]

G'Day Bart,
I guess you are talking about a clean prop. If it has ANY growth on it it will present a big load to the diesel and of course it wont push the boat through the water. It will also be badly out of balance.
A diesel will blow blacksmoke generally when it's overloaded or if the air cleaner is dirty.
Hope it helps.
Greg.

[lockie]

It's a bit of a mystery - 20HP is a heck of a lot of power for a TH. How many knots is she doing at 2800rpm?

I'd start by making sure the prop is reasonably clean and not a merry-go-round for the oysters. That could explain both the ratttle and the inability to rev out.

Cheers, Graeme

[fullandby]

Thank you for the replies guys,

I think there are 2 problems, the black smoke which I think is too much pitch on the prop. The one that really intrigues me is the rattle.

Graeme, I agree 20 hp is over generous but that was fitted when I bought it. The problem occurs when the prop is clean, she does about 5.8 knots at 2800rpm.
Greg, the filter is clean, I think the motor is getting enough air. I agree the black smoke is probably overload.
Storm Petrel, it willingly revs out to maximum revs in neutral.
Philip, is your prop midline in front of the rudder, or offset to the side of the rudder? I am wondering if my prop has water flow interrupted by the keel. I intend to get the prop checked by a specialist when I slip next.

Cheers,

Bart

[storm petrel]

Bart,

I had a rattling sound from the prop that turned out to be shell grit in the cutless bearing. had to replace the cutless bearing to fix the problem. Interestingly, when I changed from a fixed to a folding two blade prop last year another prop noise disappeared. I think the old prop must have been slightly imbalanced. I have a 2QM15 Yanmar in Storm Petrel and do not have cavitation problems but I drive a 15" x 9 two blade folding prop on an offset shaft. I would imagine having the prop behind the keel could lead to some cavitation at high revs/speed but you have problems at 1500rpm? It sounds like a good plan to have a prop expert take a look. You could also try ringing porters props and having a chat to them about the problem. They were very helpful when I spoke to them about changing props.

[lockie]

Sounds like it's a near certainty to be overpropped. If you know prop spec's (pitch & diam), gearbox ratio and engine rated rpm/hp, you can compare it with what it should be.

Re the cavitation, it is a phenomenon usually associated with higher prop speeds causing pressure drops that lead to bubble formation. I could be wrong, and it could indeed be cavitating, but the fact your engine can't get the rev's up high tends to argue against cavitation.

Graeme

[fullandby]

Thanks for the advice guys, I will get a prop specialist to look at it when I slip next.

Cheers,

Bart