Propeller - too close to rudder?

[iainmillett]

On a boat I have had surveyed recently, it was noted that the lower blade of the prop. was too close to the leading edge of the long transom hung rudder. There is only a gap of about 1 inch.

Apparently cavitation might occur.

I must say I haven't noticed 'cavitation' whilst under way but would I be losing some power because the design is in error?? If I cut out a piece of the rudder would this weaken the blade? How much gap should there be?

 

[Guest]

Tip Clearance Factor

From what I have read, the clearance between each blade, above and below, should be a minumum of 10% of the prop's length, i.e. 13 inch prop must have a clearance of 1.3 inches at top and bottom.

This is not so much for cavitation as 'bangs' from hull/rudder.

 

[trev]

I would be more worried about things getting stuck- ropes, lumps of wood etc.
Cavitation should'nt be a problem so long as the prop is adequately 'fed' with water, the leading edge won't produce much back pressure.

Trev

 

[Guest]

Re: Tip Clearance Factor

As in most things .... theory is fine, practical often takes a different route.

You say that you haven't noticed any problem before .... if you've lived with it before and had good thrust / speed through water, then why the problem now ?

Chunks out of rudder blades are common ..... look at older barges / long keel boats with prop cut-outs in the leading edge of blades ....... the amount will not probably affect you that much to notice .... but again is it just because some Surveyor has passed comment ? Has he made it a condition of the survey to correct ?

My feeling is that if the boat has been happy enough with it for its life so far, then what's the point ??????

 

[oldharry]

Re: Tip Clearance Factor

I agree with Nigel. The old maxim applies - 'If it aint broke, dont fix it'

If it is really worrying you, then I have always taken the 10% of diamater rule, a 12inch prop requiring 1.2" clearance from the blade tips as the rule of thumb, an increase which can hardly affect the performace of your rudder, provided you are not chopping into anything structural.

But, like Nigel, I suspect if you do that you will find it makes little or no difference - at best a marginal reduction in vibration through the tiller, which you have not mentioned as a problem anyway!

 

[Andy]

well i have got a 32' long keeker and just had a new prop to the same spec but it now seems to cavitate more...the clearance is about 1 " on 17" prop but it is also really bad in reverse.