prop clearance

[owen]

is there a recognised minimum clearance for a prop from the hull..does cavitation become more of a problem the closer the prop tip is to the hull ? does the prop work more efficiently with a larger clearance? thanks in anticipation

 

[Aja]

I was told that a minimum of 10% of the prop size should be left between tip and hull. Have also been told 15%

On my last boat I fitted one with a 10% clearance. Wanted the largets prop possible.

If its a folding prop, be careful you have clearance all of the arc it opens, if you understand my meaning.

Donald

 

[PCUK]

10% minimum, more for fast spinning props.

 

[Gargleblaster]

When I bought my current boat, the surveyor noted that I only had a quarter inch clearance between the blade and the hull at its nearest point. The propellor is 13inches two bladed. His only solution was to fit a three bladed prop. When I asked him why it was a problem he was concerned that if the drive shaft started to wobble the prop might dig into the fibreglass. Three years later I have not changed the prop and I inspect the fibreglass above it very closely to see if there is any scarring - so far there isn't, despite hitting some very loud clunky thinks and having the odd rope wrapped around the prop and or shaft.
I think the 10% advice is good, but it may not be an absolute criteria. That's my experience anyway.

 

[oldestgit]

I have a 14" x 8 Volvo Penta 2 blade folding prop on a 120s Saildrive and I only have 1" clearance =7%, had it 5 years and never a hint of any problem (Crap in reverse). I have been looking recently to increase to a 15 with more pitch and talked to the Prop specialist at V.P. who suggested a 2" leg extension would do the trick and even then I would still only be left with 13% on a 3 blade folding one. But just look at the price of the 2" extension and the 3 blade folder... wow.

I would guess the under water shape of your hull in the prop vicinity also comes into play here.

Have Fun.

 

[Oldhand]

A small prop-blade tip to hull gap will increase vibration through the hull, the more area in the tip of the blade, the worse the effect will be. The acceptability of such induced vibration will depend if any resonaces are set up at the engine speeds you are likely to use.

 

[damo]

Same here - less than 10mm clearance at the top of 3-blade prop. I don't notice vibration, and there is no damage to the grp.

It's been like that for 25 years so....... /forums/images/graemlins/smile.gif

 

[pampas]

for props up 14" dia a rule of thumg that I use is a minimum of two fingers for the clearance more is a bonus, have passed sales of various vessels because of small clearences,

 

[eebygum]

The three bladed prop on my Contessa 26 also has a quarter inch clearance which was commented on my marine engineer over the winter....he likes to get two fingers between the prop and the hull

But as far as I can tell from previous owners it;s been like that for the last twenty years, no damage to the hull and the three blade is the best prop for the 2GM20 engine and I get a bit more direction in reverse so I'm not planning to change soon.

Cheers