Boat Vibrations

[Salty]

As we left the mooring yesterday for sail, I heard a small thump (there was nothing visible in the water) then the boat started to vibrate. It was so bad that I couldn’t go over 1500 rpm. I didn’t realize how bad it was on my way out because I was motor sailing (mostly sailing) on my way out. We decided to enjoy a day of sailing (and what a day it was) instead of going over board to investigate the problem because of poor visibility in the water (overcast day). On our way back we realize how bad the problem was when we took the sails down motored. We can only get 2 knts without excessive vibration, so we limped our way back trying not to break anything. The engine runs fine in neutral, but vibrates in F or R.
My prop is a 14” Martec folding prop (was serviced last year).
My question is what do you think can cause this problem?
My initial thought was the shaft got caught on something, and then I started thinking about cutlass bearing, prop not fully unfolding or defective prop. I am hoping for the former because I don’t want my season to end so early.

Thanks

Salty

 

[Winkfish]

My question is what do you think can cause this problem?
My initial thought was the shaft got caught on something, and then I started thinking about cutlass bearing, prop not fully unfolding or defective prop.

If your prop hit something, it could be the prop itself, the bearings, or the shaft being bent, or any combination. Can you check on the inside where the shaft comes in to see if the shaft is bent? Check for leaks? Hopefully it's as easy as only being a bent prop.

 

[packman]

I Had the same thing happen to me on an inboard motor boat. I was motoring along at cruise on the Sacramento river, thump, then vibration just above idle. I had hit something submerged with the prop and unbalanced it, bent the shaft as well. Just a guess, but I'd bet a folding prop might be more sensitive to such things than a fixed prop.

 

[Salty]

Checked the inside, no leaks or bent shaft.


Thanks

 

[Winkfish]

Checked the inside, no leaks or bent shaft.


Thanks

Have you checked your prop? Any bent blades?

 

[BobM]

It could also be a loose zinc.

 

[Cayennita]

Write again after diving on the prop. I lost a blade in the ICW and had to sail for 3 days before I could get hauled out and change from my folding prop to my fixed prop. Again dive on it, the write. Maybe you did catch a line and one blade does not open fully.

 

[Cayennita]

Also check engine mounts. Dive on the prop and check it out

 

[MJWarner]

Sounds like one of your blades isn't opening. That will cause your boat to feel like you are going to damage everything expensive if it doesn't stop. Since (hope I'm right here) your prop uses centrifugal force to open, when it happens, put the shifter into neutral/reverse/forward. For me, this usually took care of it and the shudder stopped. I found that growth and little barnacles would get into the crevices of the prop, keeping it from opening fully.

 

[Salty]

I finally dived to check the prop and found that the one of the prop broke off. All that’s left is little stub. I didn't realize that the prop could break off like that. Well that's the end of my season but it could of been worst if it was the shaft. I think I'll replace the Martec folding with a fix 2 prop. What do you think? Any suggestions?

Thanks for your suggestions

 

[Stu Jackson]

My thought? Fixed 3 blade, you'll be a happier camper in the long run.

 

[higgs]

A fixed 3 blade will slow you down under sail, but will give you great performance under power.

A Maxi Prop 3 blade is a great way to go, but is very expensive. it is a feathering prop and will give you the best of both worlds.

 

[Liam]

Sounds like kelp.
Every now and then if I hit a big ball of kelp just wrong I hear a thump when the prop chops it off and then the boat will vibrate as the kelp is being dragged by the keel and rudder. If I just stop for a while the kelp will float up the hull and I can pull it free with a boathook. On other occasions I have been successfull going in reverse and backing away. Once it was so bad I had to dive under and pull it off.
Finn keel spade rudder boats are really good at catching kelp. Often you will not even know that you are dragging it along unless the engine is running and you notice the tell-tale vibration.
There is no vibration when sailing but if you were making 6 knots and now you are making 4.5 but the wind hasn't changed, look over the stern and you will most likely see a string of kelp trailing behind.

 

[John Howitt]

The surface area on my three blade, is not that much different that the larger blades that my old two blade has.
If an1/4 of a knot is a problem for you then...
I'm happier with my cambell three blade.

 

[SeaMist II]

Bit Unfortunate

We had a similar occurrence on the River Thames as we were passing Canary Wharf on our way to St. Katherine's Marina by Tower Bridge. Big thud and then lots of shaking and no revs. Sitting in the middle of the river with 2 knots of tide and 30 knot + river trip boats about. Very unnerving experience. A Mayday resulted in the RNLI coming to our aid. By the time they arrived we were making way but it didn't feel right. Managed to get to the marina but the water is to filthy for swimming in so got the boat hook out and managed to pull a kids anoraks off the prop.

 

[Salty]

Liam, its not kelp. One of the blade broke off on the 2 blade Martec folding prop