noise from cutlass bearing?

[hans]

I have re-engined my 31`motorsailor this winter. The new engine is a 27hp Yanmar fitted to a 25mm axling with an aquadrive coupling. The stuffing box is one of those newer Volvo Penta ones that last for 500 hours, need 1 cc of grease a year and need to be "birped" on launching. I have also re-newed the cutlass bearing. Miserablejob. The engine runs a 16 x 11 three bladed RH propeller. The problem I am having is a singing, annoying humming at revs 800 - 1100 and 1800 - 2200. Funny thing the humming is not there with the engine doing the same revs but moored to a dock (ie not moving). The noise is there also at certain speeds with the engine off (but in neutral thus propeller rotating) while under sail.

Have I done something fundamentally wrong during installation? Any help very much appreciated. Based in Tromsoe way north of the arctic circle and heading towards Spitsbergen i 3 weeks. Worried about engine trouble on the way...

 

[gunnarsilins]

I have a similar phenomen, if I slow down my engine from cruising speed (1800 RPM/7,5 knots) to tick-over (750 RPM/3 knots) , but still in gear, the transmission begins to "sing" when the boat slowly looses her speed.
This only happens when slowing down, not the other way round, ie going from a standstill to tick-over speed.
The sound can be stopped either by temporary putting the gear in neutral or by giving a short burst of extra throttle.
To my sense this is due to some resonances occuring in the transmission line under certain conditions.
I can hardly see this beeing dangerous and I don´t bother to try out any cure, but I avoid leaving the transmission in this "singing" state, mostly because the sound is quite disturbing.

 

[Guest]

Could be the pitch of the prop? nm

 

[david_bagshaw]

Our previous boat also suffered from a similar problem, which was much cured on getting the shaft alignment to within 2 thou, but she did have 15 foot bronze shafts. Some times a cure can be had by bolting on a flywheel round the shaft to alter its harmonics.

It also got less as the cutlass got older, to the extent when it stopped it was time to renew, usually 1500 hrs.

David

See my web site www.yachtman.co.uk

 

[gunnarsilins]

In my case (see message below) the sound was still there even if the engine was perfectly aligned. The sound is quite high-pitched, a pretty good guess would be about 400 Hz.
My cutless bearing is quite worn, with some play in it, I guess that under certain loads the whole shaft can start to resonate over its whole length because of the play int the cutless. If thats true a misaligned engine would more or less cure this phenomen because then the shaft is forced into the rubber in the bearing and thus dampening out resonances.
In case this is true you definetely got a point about adding a flywheel. On the other side, you say it got less with an ageing cutless.....
So, at least my conclusion is, as long as the engine is aligned and the rest of the transmission is in order and with no unacceptable wear in bearings, glands etc I wouldn´t bother too much.

 

[Guest]

If you hit your propshaft lightly with a hammer, do you get the same pitched sound?

 

[Guest]

It sounds to me like you have the dreaded "singing propeller". It can often be cured by gently filing the trailing edge of the prop.

 

[Guest]

The singing is caused by trailing edge vortices breaking away from alternate edges of the propellor. The cure is as stated earlier in the post but you must make sure that you do the filing at an angle. The angle is determined by the pitch of the singing and the speed at which the singing commences. The object being to have the difference in the chord of the blades set at half a wavelength of the vortices being created, thus calculate the distanc ebetween vortices at singing speed and file the trailing edge so that the difference between the two is the half wavelength. This also increases the efficiency of the propellor as up to 5% of the input energy can be used up in creating the noise.
Fair winds
Ken Gill

Fair winds

 

[hans]

Thank you all for helpful advice. The noise is not the same when I hit the shaft with the hammer. A friend with an engineering background has been out on the boat with his stetoscope (!). He thinks the culprit can be the aquadrive coupling - apparantly these are very sensitive for fore&aft placement, and it seems I have mounted the small bulkhead for the coupling 1 mm too far foreward. So we will try with some thin spacers on the mounting bolts for the aquadrive to move this aft. If that doesn`t help I will start the list of advice from you. Thanks again!

Hans

 

[incognito]

Maybe Prop!!

More probably singing propeller, and yes, it will be at different revs in dock than under way, due to different water flows.