Cavitation noise when starting off

[MoodySabre]

I am currently pootling round Holland in my Moody 31 and had a friend come out for a few days. They have a Westerly Konsort. They were surprised how much noise there is when first starting off and putting the throttle on firmly e.g. when coming out of a lock. As soon as the boat gets up to speed all is quiet. I assume it's the original 14x 17 LH driven by a VP2003.

Is this normal? It's always been like that.

 

[Yellow Ballad]

Is it the wet exhaust making the noise and then clearing as the water flows round it? Or is it actually cavitation?

 

[Spyro]

I noticed a similar thing after changing my cutless bearing, I just ignore it now

 

[jiris]

I am currently pootling round Holland in my Moody 31 and had a friend come out for a few days. They have a Westerly Konsort. They were surprised how much noise there is when first starting off and putting the throttle on firmly e.g. when coming out of a lock. As soon as the boat gets up to speed all is quiet. I assume it's the original 14x 17 LH driven by a VP2003.

Is this normal? It's always been like that.

If the noise is really cavitation, it is perfectly normal. A fixed pitch propeller is normally "tuned" to work properly at certain revs around a certain speed. If you are revving it high at much lower speed, the blades would be cavitating because they operate at too high angle of attack. The blades are actually wings and this behaviour is the equivalent of wing stalling.

 

[sarabande]

A slow speed start to allow water flow velocities over the blades to build up, then increase the revs steadily.

Cavitation noise is usually a whine or high-pitched "zzzing".

If it is a low pitched rumble, then I'd check the cutless bearing for play, or if a moveable blade prop, then the hinges.

 

[scruff]

I had this on my folkboat with a 14hp inboard. The engine was oversized for the boat so if I went straight to max the prop just churned water and accelerated slowly.

I put it down to the nautical version of wheelspin.

 

[MoodySabre]

Thanks all. It is a fixed two blader. Cutless bearing new a few weeks ago.

I shall ignore it.