Griffon: noise from prop rotation

[BillGr]

Hi all.

We were out sailing yesterday for a couple of hours, and after about half an hour of being under sail, realised there was a rubbing / scraping / humming noise and a faint vibration in the hull that was neither my tinnitus nor the dredger that was working nearby.

Once Dad and I had realised that we could both hear it, we quickly worked out that it was being caused by the prop turning. When under sail with the engine off, I generally leave the engine transmission in neutral to let the prop turn. When I locked the prop by putting the transmission astern, the noise and vibration stopped. Once I returned to neutral, the noise started again soon after.

Despite taking the cover off the engine and shining a torch into the engine compartment, we couldn't pin down exactly what the source was, so I'd be very grateful for any advice or suggestions?

I left the prop locked in astern for the rest of the sailing time, and the engine started without problem and brought us back through the lock and into the marina without anything giving cause for concern.

I am very concerned about working out what's happening though. Aside from the obvious possible expense of letting a problem get worse before putting it right, around here if our engine fails at a critical moment, our options could be very limited with the tide.

Edit: The engine is a 20hp Beta diesel, installed in 2004.

[Jolly Roger]

Bill

I doubt if there is anything wrong as I expect these normal but due to engine noise you do not hear them. The rush of water over a fixed propeller will cause it to turn, this then rotates the prop shaft and every bearing will cause some noise. The P-bracket, the stern gland, and the gearbox bearing will all generate different sounds.

On my Fulmar I have a folding prop, but if I am travelling too slowly when I switch the engine off the blades will not fold, so I have to put it in reverse and the blades will fold as soon as the speed increases. If I leave it in neutral then the prop will spin and is more difficult to close.

[smudger]

Hi i have the same engine in a centaur and get exactly the same effect ! in forward the prop will spin and rotate the shaft etc causing the rumble ,if you engage reverse it will vanish because the prop wont turn and wear out any bearings unnecessarily.It may slow you down a bit but hey its not a racing boat

[Khiran]

Hi Bill,

nothing new to add than has already been posted but I too had this experience last month & being new to sailing immediately contacted volvo marine engineer who advised me to engage reverse when sailing to avoid prop action & unnecessary bearing wear...no repeat of noise since.

[BillGr]

Thanks guys, the advice and reassurance is much appreciated.

My main concern was that the noise is new: we've sailed around 500 miles in her so far since we took her on at the beginning of the year, and I'm pretty certain I've never heard that noise from the free-wheeling prop before. On the other hand, as long as nothing's seized and nothing's leaking and nothing gets any worse, then I guess the two-bladed prop won't make that much different to our boat speed, even when we are racing her -- so from now on I will lock it astern when we're under sail.

[philipstevens]

If you have a rope-cutter fitted, the noise could have started due to the shim-bearings being worn.

[rhumlady]

If you race then when next dried out set the prop up and down so one of the blades is hidden behind the skeg and mark the shaft so you can set that position when sailing and reduce the drag.

[BillGr]

Thanks Derek, that's a brilliant idea. That's almost worth another trip down back to Porlock Weir or Ilfracombe just to find somewhere to dry out. Not that I really need the excuse

Philip, we do indeed have a rope cutter fitted. Will investigate the shim bearings, thank-you. The thought it might be a possible source hadn't occurred to me.