noise from propeller area

[Jerzys]

I have a problem with a noise coming from propeller area. My sailing boat is 37´´ long with long kill (similar to Vanderer). For 3 years ago I changed motor (35hp) to lager one (50 hp) and propeller (three blades) as well. I’ve installed aquadrive too. Up to 1800 revolutions per hour all is OK. Above it starts unpleasant noise coming from propeller area. It does not changing, it is increasing on higher speed . I do not feel any signs of imbalansed propeller. I controlled the sound at the different number of revolutions of the motor alone, separately motor + reverse gear (with separated aquadrive), after that motor + aquadrive (with separated propeller shaft) and at last altogether: motor, reverse gear, aquadrive and propeller at different speed then come the noise.
Motor installation, aquadrive and propeller installation were checked by local boatyard and I was told that installation is OK. Recently under discussions with different people has come a suggestion that noise is caused by too little hole for propeller between keel and rudder and even more from edges of propeller hole which are round. They should be sharpen to about 30 degree especially aft of propeller blades but even forward. I was told that this is most important.
The distance from propeller blades to upper and lower part of hole is 1´´ , forward to keel is 4´´ and aft part of hole ( in rudder) - 2´´. The thickness of keel and rudder in this area is 2 1/2 inch. Distance to bottom of hull from upper part of hole is 6 –7´´. I can not make the propeller hole larger (attachment of rudder at upper and lower part of hole) but I can change the profile of edges of the hole to 30 degree.
Has anyone experience about this problem? Does this suggestion make a sense? Will it diminish the noise?

 

[longjohnsilver]

Usual theory is to have 15% of prop diameter as clearance between top of blade and the hull. What you are experiencing could be just cavitation, or not enough clearance between prop and hull.

 

[Guest]

It sounds like you have a "singing" prop. Its when the prop resinates at certain revs. It can nearly always be stopped by filing the edge of the prop I have done this and it worked, but please seek proffessional advice before you attack the prop.

 

[johnt]

AHEM........"Just Cavition" eats the prop!

you didnt give us the prop size Jerzy, if you have ..say ..a 16inch (40cm) prop, which possible from the hp, you should have a clearance of at least 2.5 inches (c 6.5cm)

This would give the reccommended 15% clearance, I'd work on an absolute minimum clearance of 5cm.

So it seems the answer might be a smaller propellor

As for "singing" props , that sounds like harmonics to me and would point to a relatively thin blade! ..any comment on this VYV?

 

[Jerzys]

Size of prop is (Mikado) 17” x 14”, engine Yanmar 4 JH2 50 hp . Distance to hull (V-form in this area) is 25 cm (10”) but distance to upper edge of the prop hole, as well to lower edge of the prop whole in keel and rudder is only 2,5 cm (1”). Distance to front edge – 9 cm and back is 7 cm. I do not see any sign of cavitation on prop blades, any erosion on them. Do you think, that these small distances to the upper and lower edge of prop hole can be reason of the noise? So the most simple solution is to change the prop to smaller one (16” x 15”-16” ?). Thank you for your suggestion. I think I’am going to try.

 

[Jerzys]

I have never heard about this phenomenon. Which edge of the prop should be filed? May be it can contribute to the noise. Should it be some “tone” in this phenomenon? I try to talk with producer of the prop (Mikado 17” x 14”).

 

[Jerzys]

Distance to hull (V-form in this area) is 25 cm (10”) but distance to upper edge of the prop hole, as well to lower edge of the prop whole in keel and rudder is only 2,5 cm (1”). Distance to front edge – 9 cm and back is 7 cm. I do not see any sign of cavitation on prop blades, any erosion on them. Do you think, that these small distances to the upper and lower edge of prop hole can be reason of the noise? So the most simple solution is to change the prop to smaller one (16” x 15”-16” ?). What do you think?

 

[johnt]

the cavitation damage doesnt happen overnight ..it just shortens the life of the of the prop.

and from what you say you need to go down to a 15 inch prop ..but you should be able to go up in pitch to compensate, at least to 15 inch pitch, probably 16 inch

 

[vyv_cox]

Reasons for noise from propeller area

Changing the prop is a very drastic step, unless you have a cheap supply of a range of sizes available. Two possible reasons for the noise:

1. The relatively small clearance between the prop and hull can lead to blade passing frequency noise. With a three-bladed prop there will be three pulses per rev but the noise may not become apparent until revs reach a threshold level, perhaps 200 rpm at the shaft. Then I would expect the noise to be initially fairly low frequency but for frequency and perhaps volume to increase with revs.

2. Prop singing is a high frequency noise that normally occurs within a fairly narrow rev range. It is due to vibrations at the natural frequency of each blade, energised by water flow. The good thing about this one is that it is easily solved - file a flat about 2 mm wide on the edges of each blade, removing the sharp point. No deterioration in performance will be noted, in fact in theory the opposite will happen as there is less drag on the departing flow. I doubt if you will notice it, though!

If the problem is the first one, there is not a lot that can be done except to improve the size and shape of the hole in the deadwood. Enlarge its clearance from the end of the prop as much as possible. Fairing the edges of the hole may be helpful. One possibility is to add a shroud, an open ended cylinder about 25 cm long that fits inside the hole. This would stop the noise but unfortunately would almost certainly reduce power as well.