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Propellor pitch and dia

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Alan Dixon View Drop Down
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Joined: 07 Sep 2009
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Post Options Post Options   Quote Alan Dixon Quote  Post ReplyReply Direct Link To This Post Topic: Propellor pitch and dia
    Posted: 28 Sep 2016 at 5:00pm
Electrolysis has attacked my prop and I need to replace it
To fix the electrolysis I intend to rewire and replace the anode also for a galvanic isolator. I also suspect the earth braid across the coupling is broken and the shaft has become insulated. thats all fixable.
My question to this wise body concerns the choice of my new prop.
I have eliminated the Darglow from the equation, too expensive and and worried about it becoming entangled in a weed called "dead mens bootlaces"  common NW scotland where I intend to locate.
I currently have a three blade 13 dia x 9" pitch from a 25hp engine max revs 3500 2:1 gearbox
this gives me hull speed 6.6 knots but as the prop is slightly overpitched I can only get 3000 RPM from the engine. normal cruising I have 5.25 knots from around 2000 rpm
In general quite happy with the setup I get good thrust and prop wash, the prop walk is considerable, which can be a good feature some of the time.
I have always had a considerable amount of transmission noise when motoring, some of which I have attributed to the clearance of the 14" propellor within the aperture. the quoted figure is 17% of the prop diameter which I do not quite have.
Would a 13" prop be better? what have other owners fitted?


Alan Dixon "Charlotte"
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Richard Ritchie View Drop Down
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Joined: 13 Jul 2009
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Post Options Post Options   Quote Richard Ritchie Quote  Post ReplyReply Direct Link To This Post Posted: 05 Oct 2016 at 3:26pm
Interesting.
For reference, I have just been told my 2 blade is grossly overpropped: I only get about 2500 revs tops... Then black smoke. But 6 knots in flattest water.   That is a 15 x 12 with 2.2 reduction from 2GM20.  (Note I have and enlagerd prop aperture)  They are recommending 2 blade is 15 x 9 or 14 x 10.  
Clearly 3 blade is different but benchmark may help
Very helpful discussion with Darglow regarding sizes.
Lake Engineering also very helpful.

I will keep an eye on the braid I have just put across the coupling of my new prop shaft...
Richard Ritchie
Badger of Baltimore
IRL 120
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waratah912 View Drop Down
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Post Options Post Options   Quote waratah912 Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2016 at 7:56pm
When we bought her (1989), Waratah had a 15 x 11 three bladed prop, and very good it was too on the delivery trip off Portland against the tide. 
This was replaced with a two bladed 15 x 11 but with higher area per blade from CJR (Europrop?), for some peculiar reason this prop always stopped with the blades horizontal, even if you just ran the engine on the mooring in still water or stopped from astern, precisely what you don't want! 
Last winter, I went for the Featherstream 14 x 12, with two immediate benefits : 
 - the propeller noise is significantly less, though I don't remember the original 3 blader being noisy as noisy as the same diameter 2 blader.
 - she feels more agile under sail, a noticeable difference I wasn't expecting
However each change has resulted in a slight drop in maximum speed and ability to punch into a headwind and while the prop walk with the Featherstream is less than with the others it's still quite pronounced.

Engine is a venerable Volvo 2002 from 1986, so not a quiet beast.

Paul Smith
Waratah
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doug View Drop Down
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Post Options Post Options   Quote doug Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2016 at 9:56pm
I have a two bladed prop but as Richard said it may be of help...

It is 13 * 10 1/2 (standard aperture) with a Beta 20hp engine which should rev to a little over 3400.   I recall that JR told me in an E Mail that the factory normally fit a 14 * 10 with this engine.

I never rev the engine to the max as the whole boat sounds fraught, the steering gets heavy and generally feels as if it is being over pressed. I normally run at c 2200ish, depending, and this works fine.

I do find a fair amount of transmitted noise in the cockpit and, though I have no objective evidence, I suspect that the 14in prop might well be similar and no worse.

Edited by doug - 06 Oct 2016 at 10:01pm
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Brigitte View Drop Down
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Post Options Post Options   Quote Brigitte Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 2016 at 4:25pm
We have a 12" 3 bladed Campbell Sailor prop with a 20hp Beta; they are fixed blade but supposed to create less drag than a standard style 3 blade; we are very pleased with it. It replaced a 14" two blade prop which caused a lot of vibration and ultimately wore an oval hole in our tufnell bearing. The hydrodynamics of a two blade in a cut out are ugly, and we believe 14" is too big.

We now cruise at about 2.2 to 2.4K rpm (beta actually recommend more than that), typically doing about 5.5knots with some power in hand. We think it is about 0.5 knots faster than the old prop at the same revs; fuel consumption has increased a little but there is less vibration (we replaced the bearing as well though). We certainly haven't noticed any reduction in sailing performance by switching to a 3 blade although we never bothered with the faff of aligning the old prop when sailing; we don't race. The prop looks tiny compared to the two blader shovels but was recommended for the Beta: I can remember our tame engineer looking doubtful when he saw it!

They aren't as expensive as a Darglow but they aren't cheap. Given the sudden plummet of the pound, they'll be more expensive still as you have to order them from Canada.


Edited by Brigitte - 10 Oct 2016 at 4:27pm
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