Singin' Saildrives.Help please.

[Rohorn]

Singin\' Saildrives.Help please.

Hi....I just launched after fitting two new Yanmar 2Gm20F / SD20 saildrives in my cat, replacing the previous underpowered 1GMSD20s. New motors are super, and drives work well but at about 1900 rpm in gear a high pitched whine sets in. This noise frequency doesn't change with speed, the pitch (musical) stays constant, but gets more insistent as one goes faster.
Out of gear, no noise at any revs, so it's definitely the drives. They both start singing at the same revs. Coming from the gear industry years ago, I suspected a resonance in one of the components in the drive line, but Yanmar must have made many thousands of these things over about 15 years and I don't believe an inherent fault like that.
Dave at Yanmarhelp.com says it's probably coming from the propellers. I assume he means that the props are "ringing". They are bog-standard two blade Radice alluminium. I suppose one could try to measure the frequency while running (using my harmonica), then when stopped, nip over the side and tap the props to see if they ring at the same pitch.
Anyone met this phenomenon?
Cheers....R

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[Aja]

Re: Singin\' Saildrives.Help please.

Rohorn

Dave is probably correct. I have a GM20F with a bog standard Radice prop. I wanted the biggest prop I could fit and the 'flat' part of the blade is only 25mm from the hull - must mention that I have a prop shaft.

Its OK at tick-over revs but anything above that till I get to cruising revs there is a whine. Its cavitation. Dont worry unless its happening at you cruising speed.

I was told that it is due to the shape of the blades - i.e. square shaped rther than round and that one way to get rid of it was to (carefully) file away the corners.

I'm not brave enough to do that.

Discuss the noise with the prop supplier is your best bet.

Regards

Donald

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[boatless]

Re: Singin\' Saildrives.Help please.

Known as prop sing. Theory is that the trailing edges are square, and that alternating vortices shedding from the blade are in synch and moving the thin edge up and down (as it were). Cure is to file the T.E. to 30 degrees from square. Try just 10 degrees to start with, on the outer third of the blade.

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[jleaworthy]

Re: Singin\' Saildrives.Help please.

Agree with boatless. I've found it immediately effective in a couple of cases to gently file the trailing edge of each blade for its whole length. File it to a smooth taper, beginning the taper about 4 or 5 mm from the edge of the blade.

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[snowleopard]

Re: Singin\' Saildrives.Help please.

i think the other replies must be on the right track - i have the same saildrives and don't get any such noise. i use flex-o-fold props.

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[Rohorn]

Re: Singin\' Saildrives.Help please.

Hi people...thanks for your comments. It does sound like prop ringing. I'm just a bit confused about the trailing edges being "square" and need to be sure before starting any drastic mods.
Are we suggesting that 1) the trailing edges are not thin enough, simply cast alu, which causes a vacuum which induces, through cavitation, the ringing resonance, and so they should be filed down to approach a knife edge exit, or 2) that the trailing edge is currently more or less straight from the hub to the tip and should be tapered from about the last third to the tip, like the wing of a Cessna 150?
It's the 10 or 15 degree bit which I don't get. Excuse my few remaining brain cells..they're trying their hardest!....cheers...R

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[boatless]

Re: Singin\' Saildrives.Help please.

Existing trailing edge is the green line, red is the new angle. Just need to do this on the outer third because it's a tip induced effect.

Drawing copyright me. Sorry it's a bit wobbly, but I bashed it out quickly, and it's the first drawing I've ever put on the web. Heartily reccommend Canvas 7, given away on this month's PcPro cover disc.

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

 

[Rohorn]

Re: Singin\' Saildrives.Help please.

Hey, brilliant....I'm most impressed. Not such a radical mod. I can moor in three feet of clear Paros water, unship the props without fear of losing the bits, and sit in the shaded cockpit with a cold beer, carefully filing away to your design, while occasionally glancing at the lovelies on the beach. Excellent and thanks again. I'll post the results in a while.....Cheers....R

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[boatless]

Re: Singin\' Saildrives.Help please.

You're talking to a die hard Greek waters sailor here. Take the beer into the dinghy and leave the props on..

Oh, no wait, park up the beach and let the lovelies admire your skills.

No, no, leave it for a day or two. I'll come and do it for you, where are you at the moment?

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

 

[Rohorn]

Re: Singin\' Saildrives.Help please.

Sorry to be late replying...was distracted.....I moor on Paros, at the head of the bay at Parikia, in front of the Aktia bar. Keep the boat ashore in the winter at Kolymbithres, round in Naoussa bay. Cheers....R

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