No prop clearance

[bhallworth]

I am about to remove my 3 bladed propeller 13x8 (estimated 8) after discovering it had zero clearance between the hull (you couldn’t get a piece of paper in there!)

I experience no vibrating or other obvious effects from this arrangement, except fairly poor performance (engine produces full power & reaches full revs). The correct size should be a 11x8, is it possible that this down size will actually improve performance by creating the 10% clearance???


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[Rick]

I'm not sure about how the clearance impacts performance, however if current prop is 13x8 and you are getting full revs, then an 11x8 would seem too short on the pith by several inches - what type of hull / length / and power?

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[supermalc]

From my experience this year, changing from a 13 to 11 of the same pitch will make the boat very underpropped. I have had the same problem of too little clearance. I was told the danger was of wear to the boat, from the water against the hull, and damage from debris being trapped.

I have been told, although I have no mean, and little experience to judge, that there should be 2 to 3 inches of clearance.

My 15 x 14 was massively too large, and the engine would not rev much above tickover. Fitting a 15 x 10 is usable, but I think a 14 x 9 would be the correct size.

Have a look at my site for pictures of the boat etc.

<hr width=100% size=1>Malcolm.
<A target="_blank" HREF=http://geocities.com/malcsworld/>http://geocities.com/malcsworld/</A>
Take a look at my boating pages.

 

[bhallworth]

She's fin keeled, 22' LWL, 5000lb, producing 7.5hp @ 3600 with a 2.21 ratio. Hull speed should be 6.5 kts but I'm lucky to get 5kts at full rpm (cruise around 3.5kts).

I have compared my existing 13" prop against various others and estimated at 8" pitch, but I know of no accurate method of doing this?

Even going down to 11" I would just have 1" clearance, although I reckon I can gain a bit more by moving it down the propshaft slightly!





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[oldharry]

You can retain the correct measurements for your prop/engine installation by simply adding to the pitch what you take off the diameter (within limits!). For example a 12x9 will perform almost identically to a 13 x 8. An 11 x 10 will start being too steep a pitch for the speed, and would only work efficiently at higher hull speeds.

Clearance as fine as yours is definitely not a good thing, even without vibrations, as the impact of the fast moving water on the hull where the blades pass it will cause erosion and damage long term.

For a definitive answer on the correct size of prop for your engine/boat combination, contact a specialist firm like Lancing Marine, who will either have it listed, or be able to do the calculations for you. I am tiold there are diownloadable prop sizing programmes available on the Internet too, but never tried them.



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[Juggler7823]

I find it very surprising to have no prop clearance at all. The clearance should be about 10% of prop diam. Is it a new installation? Has any thing changed? Has the engine dropped on its mountings?

If you are convinced that everything is OK, PM me and I will send you a file of prop calculator.

<hr width=100% size=1>LOM

 

[snowleopard]

apart from changing to a smaller coarser prop, can you alter the foer & aft postion of the prop to get some clearance?

i'm sure the designer would never have drawn a setup like that- has the engine size been increased?

if the engine gives max revs at full throttle the prop is the correct size. if you reduce diameter without increasing pitch you'll lose a lot of performance.

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[capt_courageous]

There used to be a good prop matching computer program on the Westerly user group site. It certainly worked well for our 26 footer. I put your boat values in and it gave the answers of 11 by 7 three blade or 12 by 7 two blade. It also recommends 3 blades where possible unless you are really worried about possible prop drag Good luck

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[l'escargot]

You can download "Propcalc" <A target="_blank" HREF=http://www.castlemarine.co.uk/pitch.htm>here</A>. It seems to give reliable results when I have checked existing setups.

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[bhallworth]

Thank you all

Using 3 different calculators and having spoken to a few propeller shops I have answers ranging from 11x7 to 11x9 and a 12x8.

I estimated the current pitch by lining it up against an 11x8 and the curve looked like a good match but I don’t know how subtle the difference is? I had hoped it would be dramatically less and would explain the lack of power.

I think I will have to remove the old prop again and take it along to someone who can tell me the exact pitch.


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[sailorman]

for every inch removed from dia add an inch to the pitch


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[sailorman]

prop size normally stamped on prop boss either the side or aft face

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[Rick]

The two calculators I have gave me results of 10.8 x 8.8 and 11 x 9 .... but both agreed that max speed will be 5.2 to 5.3 knots with your available horsepower - and that you'd need 12 to get to 6.3 knots (SQRT22*1.34)

I guess it's likely that 2.21 and 7.5 means you've got a Volvo? Also what is the shaft angle like - ie how far from horizontal would the shaft be?

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[bhallworth]

Its a Yanmar 1GM, in good running order.

Unfortunity the size isn't stamp on (unless its hidden under a layer of corrosion) however 11x9 seems to be coming out as the average?

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