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Old 26-05-2021, 07:55   #1
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Sudden thud while motoring

My sailboat with a Yanmar diesel looks and runs fine but recently when motoring I get a sudden hard thump, shaking, then loss of forward motion. There is a movement in rpm but quickly returns to normal. This has happened three times in last two weeks. When it happens I quickly put boat in neutral than re-engage and all seems well. I recently had boat out of water for bottom job and saw nothing odd excepting some minor play in prop strut cutlass bearings. Yard thought I had another season left. I do have a Gori prop but to date no issues. Any suggestions would appreciated.

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Old 26-05-2021, 08:20   #2
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Re: Sudden thud while motoring

Can you give some more information? Nb hours, model, age, gearbox type, screw model, shaft turning or not. I also have an incomplete understanding from the problem based on the information in the above post. But that is maybe me.
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Old 26-05-2021, 10:13   #3
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Re: Sudden thud while motoring

While agreeing with Sailormed that more information is needed, I'm going to bet that the problem lies in the extension/feathering of the prop. It's the only mechanical device you've got down there with the power needed. Could it be hanging up?

Unless, of course, you hit the same log four times in three weeks....

Uh, ouch. Could you be slipping out of gear?

Yeah, need more information to narrow it down. Can you replicate the event?
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Old 26-05-2021, 10:26   #4
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Re: Sudden thud while motoring

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Originally Posted by tkeithlu View Post
Could you be slipping out of gear?

Yeah, need more information to narrow it down.

+1 about possibly slipping out of gear. Check you cable linkage and adjustments to make certain the trans is fully in forward. The clunk could be it coming out of gear while throttled up.

Since it wasn't mentioned, have you checked your trans fluid lately? What does it look and smell like?
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Old 26-05-2021, 14:18   #5
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Re: Sudden thud while motoring

Boat is Tayana 48 with Yanmar 75 horse with 2400 hours. Trans is hydraulic. Prop turns fine. It is a Gori feathering type prop which can change pitch. Engine starts instantly. No smoke. No misses. So not an engine issue. Trans fluid is fine.. no smells and clean. Im beginning to suspect the feathering prop. Although it is perfectly cleans and turns freely. I will check out out transmission linkage as well to make sure it’s not moving out of gear. At first I thought Strut bearing but usually you feel constant vibration with bad cutless. I have none..
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Old 26-05-2021, 14:28   #6
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Re: Sudden thud while motoring

Are you sure that you have a feathering prop? I thought Gori only made folding props. It makes a difference in analyzing your problem.

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Old 26-05-2021, 15:26   #7
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Re: Sudden thud while motoring

I don't own my own boat, altho sailed on many, and I'm not a diesel mechanic. So this is a shot in the dark. My pickup truck was creating a hard thump during acceleration and shifting. Couldn't figure it out for a long time. During an oil change found out one of the motor mounts had deteriorated and needed to be replaced. Solved the problem. Could the same problem occur on your engine?
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Old 26-05-2021, 15:32   #8
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Re: Sudden thud while motoring

Can you get in to watch the shaft while the engine is running? Can you have someone watch in there when you shift from forward to neutral to reverse and vice versa?
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Old 26-05-2021, 18:35   #9
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Re: Sudden thud while motoring

It is a folding prop. It is capable of two different forward pitches - essentially giving you two forward gears.
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Old 26-05-2021, 18:59   #10
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Re: Sudden thud while motoring

Could be a broken motor mount that allows the engine to lift in certain torque conditions, like going thru a swell or making a hard turn. As it passes, the engine sets back down with a thud.
A boat I repaired had a similar sound except it usually happened after a hard turn to port and just before the thud, the air horn would sound briefly. The engine weighed about a ton an couldn't be shifted by hand to check the mounts. So we went for a cruise with hard left turns. Each time the engine lifted the right front corner and then set down with a thud. The horn was caused by some bad wiring where the relay was controlled by the ground not the positive. When the engine lifted, a bracket pinched a wire that was the relay ground, and briefly opened the horn air solenoid.
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Old 27-05-2021, 10:59   #11
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Talking Re: Sudden thud while motoring

Quote:
Originally Posted by Lepke View Post
Could be a broken motor mount that allows the engine to lift in certain torque conditions, like going thru a swell or making a hard turn. As it passes, the engine sets back down with a thud.
A boat I repaired had a similar sound except it usually happened after a hard turn to port and just before the thud, the air horn would sound briefly. The engine weighed about a ton an couldn't be shifted by hand to check the mounts. So we went for a cruise with hard left turns. Each time the engine lifted the right front corner and then set down with a thud. The horn was caused by some bad wiring where the relay was controlled by the ground not the positive. When the engine lifted, a bracket pinched a wire that was the relay ground, and briefly opened the horn air solenoid.
A thud and the horn blowing after a sharp turn is an interesting problem. I bet that was a fun one to find.

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Old 27-05-2021, 11:06   #12
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Re: Sudden thud while motoring

Several years ago I had a similar experience. While cruising on one engine I felt this thump as if I had hit a log or something but didn't see evidence of anything. I carried on to the anchorage and as I shifted to reverse, nothing happened, shifted to forward again and nothing. Luckily I have twin engines and quickly started the other engine to get my boat under control. After being anchored I checked my shaft rotation a saw that it wasn't moving regardless of shifted position. Checked the transmission oil level as my first suspect, all was good. Limped back to the marina on the second engine and proceeded to pull the transmission. My discovery was that the damper plate had all the 16 small screws either broken or fallen out. Looked like the final screw gave up and then caught on the next screw hole. The damper plate was basically falling apart. I ended up replacing both.
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Old 27-05-2021, 12:02   #13
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Re: Sudden thud while motoring

A WAG a key has sheared at the prop. or a coupling.
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Old 27-05-2021, 12:55   #14
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Re: Sudden thud while motoring

Do you have a rope cutter?
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Old 27-05-2021, 12:57   #15
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Re: Sudden thud while motoring

Is your prop keyed onto your shaft with a separate small piece of metal? Is that intact? Or loose?
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