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Old 25-06-2021, 03:47   #1
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Engine Thumping when Sailing

When under sail one of my engines has been making a very loud thumping noise - Looking at the engine whilst its happening you can see it moving.


The first time it happened I thought I had picked up a cray pot line and promptly stopped and when I set off again it was all good, so I thought I had been lucky and lost the line.


But it happened once again - It occurred when running downwind at around 9+ knots with a short choppy swell coming at 45 or so degrees off the stern (I have a catamaran).


This time I changed direction by 20 degrees and it stopped.


The only other thing is that I have been sailing along with a massive amount of whales - Pretty certain I have not hit one (or 2) but can't be 100% sure.


I have a SD50 Saildrive to a 40HP Yanmar which have no issues under power.
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Old 25-06-2021, 03:58   #2
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Re: Engine Thumping when Sailing

Its probably your prop turning the engine if it doesnt fold or feather. Try putting it in neutral & see what happens or reverse briefly.

Mind you I'm not up with saildrives so you might get a better reply from a cat owner. Not sure if it's recommended to sail with a saildrive in gear or not but someone will chime in
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Old 25-06-2021, 05:48   #3
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Re: Engine Thumping when Sailing

Likewise, do not have any experience w/saildrive motors, but if you can see your motor move, would think that's not good.

Would check the motor mounts before it gets worse.
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Old 25-06-2021, 06:13   #4
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Re: Engine Thumping when Sailing

A prop turning the engine is an impeller, and should slow you down. But, I would not expect it to make a thumping sound unless, as Bill suggests, the engine mounts are completely shot. Is this thumping in turn with the engine turning over (it's a prop doing it) or slower (it's something caught on the saildrive)? Does it still occur if you lock the shaft so the prop doesn't turn? You may be able to do that by putting it in reverse, or you may have to latch onto the shaft (if exposed) or the flywheel, or the front shaft, or whatever turns.

My suspicion is that a quick look underwater will be quite educational.
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Old 25-06-2021, 06:28   #5
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Re: Engine Thumping when Sailing

Motor mount ?
Get a 2X4 and see if you can move that engine by hand.
I've seen mounting studs broken under the rubber ... couldn't be seen until mount removed.
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Old 25-06-2021, 14:33   #6
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Re: Engine Thumping when Sailing

Quote:
Originally Posted by boatpoker View Post
Motor mount ?
Get a 2X4 and see if you can move that engine by hand.
I've seen mounting studs broken under the rubber ... couldn't be seen until mount removed.
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Old 25-06-2021, 14:45   #7
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Re: Engine Thumping when Sailing

A spinning prop, even in neutral, can cause rhythmic thumping. I think it has to do with the prop blade proximity to the hull bottom as it spins past.
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Old 25-06-2021, 17:17   #8
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Re: Engine Thumping when Sailing

The big questions are first, whether or not you have folding/feathering props; and second, do you leave engines in gear.

A fixed prop, with engine in gear, can definitely turn the engine over with a whump! whump! whump! sound with a cat at speed. I've experienced the same sailing in my prior FP 37 cat with Max-Props in rough water, where the prop must have un-feathered.
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Old 25-06-2021, 18:42   #9
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Re: Engine Thumping when Sailing

Put in reverse and see if the crankshaft is turning in the front. I hope it isn't. If the crankshaft is turning, then as others have written, make sure the motor, saildrive housing, are not loose. I think you stated it was just one side. Worse case scenario, leave it running while sailing in forward gear charging batteries.
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Old 25-06-2021, 18:46   #10
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Re: Engine Thumping when Sailing

One blade on a folding prop stuck in open or semi open position will spin the shaft and the other blade/blades will sometimes flop open and closed with the hydraulic pressure against it. Hence a clunking sound. It used to happen with Martecs and others fixed or sail-drives.
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Old 25-06-2021, 19:33   #11
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Re: Engine Thumping when Sailing

Thanks for the answers.


I have a Max Prop 3 Blade that feathers - Always left in Neutral - I do pop quickly in reverse after stopping the engine then back to neutral to feather.


If it happens again I'll try engaging reverse quickly to see if that stops it.


I did dive down to see if I could see any damage from what I thought was a cray pot rope, but could see nothing suspicious.


I'll check the engine mounts today.
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Old 25-06-2021, 22:00   #12
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Re: Engine Thumping when Sailing

FWIW....our monohull sailboat does the same thing in Neutral. As soon as I put our autoprop/transmission in Rev it stops. I have checked, and our propshaft does slowely turn in Neutral...that is the noise we hear.
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Old 25-06-2021, 22:07   #13
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Re: Engine Thumping when Sailing

Also just to clarify, the thumping is pretty violent and very noisy - You can actually see the engine move.
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Old 25-06-2021, 22:24   #14
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Re: Engine Thumping when Sailing

Have you tried shutting down the engine while still in gear then selecting neutral, I have an old PYI booklet and they suggest that as a strategy. Click image for larger version

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Old 26-06-2021, 01:51   #15
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Re: Engine Thumping when Sailing

Ouch,
Bent propshaft and if you're still going, new engine beerers and mounts.


Depends on engine revs when you tangled but may-be a bent crank.
Needs competent, on-the-spot diagnosis, not the musings of several well-meaning amateurs
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