Prop distance

[Humphrey]

Would appreciate some info without being too technical, what would be the acceptable distance between rear of prop and front of rudder on 57ft with 18inch prop?

[BEngo]

Would appreciate some info without being too technical, what would be the acceptable distance between rear of prop and front of rudder on 57ft with 18inch prop?

 

 

Up to about 1 and a half times the diameter of the prop shaft. Or three inches for a two inch shaft.

 

N

[bizzard]

I'd say roughly 6inches.

[Rebotco]

I'd say roughly 6inches.

 

You can get some blue tablets that might help?

[Keeping Up]

Up to about 1 and a half times the diameter of the prop shaft. Or three inches for a two inch shaft.

 

N

 

Isn't that the guidance for the maximum distance from hull/bearing to prop, rather than from the prop to the rudder which the OP is asking?

[n.b.Goldie]

Up to about 1 and a half times the diameter of the prop shaft. Or three inches for a two inch shaft.

 

N

 

 

Isn't that the guidance for the maximum distance from hull/bearing to prop, rather than from the prop to the rudder which the OP is asking?

 

Not wanting to hijack this thread but I have been considering fitting a larger diameter prop to Goldie. My prop-shaft is 1.5 inches in diameter. Does this mean that the gap between the prop-tip and the counter/skeg should be a minimum of 1.5 x 1.5 or 2.25 inches?

 

Many thanks.

 

Regards

Ditchdabbler

[mrsmelly]

You can get some blue tablets that might help?

 

Luv it Greenie awarded.

 

Tim

[Keeping Up]

Not wanting to hijack this thread but I have been considering fitting a larger diameter prop to Goldie. My prop-shaft is 1.5 inches in diameter. Does this mean that the gap between the prop-tip and the counter/skeg should be a minimum of 1.5 x 1.5 or 2.25 inches?

 

Many thanks.

 

Regards

Ditchdabbler

 

That's something else again.

 

AIUI the distance from stern-tube-bearing to propeller depends on the prop-shaft diameter (and is a maximum distance of 1.5 times diameter), but neither the distance from the prop to the rudder (which the OP was asking about) nor the clearance from prop-tip to counter/skeg, depends on the propshaft diameter.

 

I have always understood that the clearance between blade tip and counter should be at least 10% of the diameter of the prop. So for a 15" prop there should be at least 1.5" clearance, etc

 

I freely admit to be a definite non-expert on all this, and will happily stand corrected by any experts who can confirm the definitive answer

[n.b.Goldie]

That's something else again.

 

AIUI the distance from stern-tube-bearing to propeller depends on the prop-shaft diameter (and is a maximum distance of 1.5 times diameter), but neither the distance from the prop to the rudder (which the OP was asking about) nor the clearance from prop-tip to counter/skeg, depends on the propshaft diameter.

 

I have always understood that the clearance between blade tip and counter should be at least 10% of the diameter of the prop. So for a 15" prop there should be at least 1.5" clearance, etc

 

I freely admit to be a definite non-expert on all this, and will happily stand corrected by any experts who can confirm the definitive answer

 

Many thanks Allan. I had got the 'wrong end of the stick' but I think I understand it now.

 

Regards

Ditchdabbler

[Humphrey]

N - Thanks for the info. Boat has inch and half dia. shaft so distance is three inches.

 

 

Up to about 1 and a half times the diameter of the prop shaft. Or three inches for a two inch shaft.

 

You can get some blue tablets that might help?

 

Brian - Thanks for the info. I have enough trouble without these !! If I took these tablets I would die through lack of blood!!

Edited October 11, 2012 by Humphrey

[Keeping Up]

N - Thanks for the info. Boat has inch and half dia. shaft so distance is three inches.

I may be wrong, but as I said above I do not believe that this is the correct answer to your question.

[bizzard]

I read it as being the gap between prop nut and leading edge of the rudder which would be roughly 6''

[Humphrey]

I read it as being the gap between prop nut and leading edge of the rudder which would be roughly 6''

 

 

Thanks for that info. I will endeavour to measure that distance and see how it fairs

[bizzard]

Thanks for that info. I will endeavour to measure that distance and see how it fairs

This gap can vary hugely boat from boat, but i've found that that gap about right for the more modern style n/boats.

[PaulD]

This gap can vary hugely boat from boat, but i've found that that gap about right for the more modern style n/boats.

 

What happens if it is less, or more? I think mine is more like 3 inches. No comments about blue pills please.

 

Paul

[bizzard]

What happens if it is less, or more? I think mine is more like 3 inches. No comments about blue pills please.

 

Paul

I'd say that's ok, better nearer than too far away, only it might be more difficult to remove the prop in future. Nearer usually makes the rudder more responsive when say reversing and (kicking the stern across)with opposite rudder on as the rudder blade is getting the full thrust stream from the prop. Too far back although of course still in the slip stream but the stream becomes more lumpy and if a very wide gap would be in more turbulent lumpy water cos the clean swirl from the prop will be beginning to break up and can cause excessive tiller shake and vibration. In my experience anyway.

[alan_fincher]

Isn't that the guidance for the maximum distance from hull/bearing to prop, rather than from the prop to the rudder which the OP is asking?

 

 

I may be wrong, but as I said above I do not believe that this is the correct answer to your question.

 

I agree with Allan - I think BEngo has answered a different question from the one being asked, and the answer isn't relevant here.

[nb Innisfree]

Some say thrust stream breaks up further back and some say it becomes less so, don't know but I do know the stream deflects to one side depending on prop handedness, so on that basis is it better if rudder is closer to prop so more of that stream is captured?

Edited October 11, 2012 by nb Innisfree