What size prop do you have?

Quidi Vidi

Quidi Vidi

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Hi
Sorry to bore you all again with my continuing problem but i could really use some info to verify my own findings. I have come to the conclusion that i am overpropped which is causing black soot and dirt from the exhaust when under load, i have checked all other possible causes. Using the various prop calculators online i reckon my prop should be about 10x6", they all give various answers but this is the average. Using a length of drainpipe with some perspex stuck on the end as a viewer, i measured my prop under water yesterday and it seems to be 14 to 15" diameter. Now i know i can go to various manufacturers to get exact figures but being a cheapskate i want to do as much as i can myself, also i really enjoy this part of owning a boat, ie. solving problems.
Could i ask any of you who have a 22' bilge keeler, powered by 6hp through a 2:1 gear box, displacing about 4000Lbs to let me know what size prop you have. I realise this wont give me the absolute correct answer for my boat but at least i will have a ball park figure and confirm i am going in the right direction. I cant afford to pay for a lift out and then find my prop is the right size after all so i am doing as much research as i can beforhand, also it helps me understand my boat better, not a bad thing in my opinion
 
The important figure is shaft speed, so you need to know your maximum revs. LWL is also more important than LOA as this determines the potential maximum speed. First guess is that a 14" is far too big.

Assuming your max revs are 3600 and LWL at 18ft a 10*6 sounds about right.
 
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Tranona said:
The important figure is shaft speed, so you need to know your maximum revs. LWL is also more important than LOA as this determines the potential maximum speed. First guess is that a 14" is far too big.
Click to expand...

Your absolutely right of course, i used those figures in the calculations but forgot to include them in the OP.
LWL 19'
Max RPM 3600 according to the manual, i dont have a rev counter, engine is at least 20 years old so i doubt if it will still achieve max revs.
The calculators work out the shaft speed but i have forgotten what they came up with, about 1400 RPM IIRC
 
If it is 2:1 then you have a max shaft speed of 1800 rpm - which is why you need a small diameter prop. If it was 3:1 (so shaft speed of 1200) you might have a chance of swinging the 14 " if the pitch was not too steep.

ps am assuming it is a 3 bladed prop.

You could get a recommendation from Sillette-Sonic who are the Radice agents - their props are perhaps the most competitive.
 
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Tranona said:
If it is 2:1 then you have a max shaft speed of 1800 rpm - which is why you need a small diameter prop. If it was 3:1 (so shaft speed of 1200) you might have a chance of swinging the 14 " if the pitch was not too steep.

ps am assuming it is a 3 bladed prop.

You could get a recommendation from Sillette-Sonic who are the Radice agents - their props are perhaps the most competitive.
Click to expand...

Yes it is 3 bladed, shaft speed is reduced by there being i bearing (stern gland) and they calculate to 90% of maximum RPM. I'm almost certain they suggested 1470 max shaft speed

Sillette have been recommended to me by someone else however i was hoping to solve this problem myself and perhaps get a 2nd hand prop from somewhere of about the correct size for now. This would allow me to enjoy a bit of sailing for a change with a view to a permanent solution next winter when i have more time and perhaps more money. I'm kind of hoping to get on a scrubbing grid, do a prop change and then crack on for the season. I realise i have tapers etc to take into consideration but that all adds to the fun

It does make me wonder though, how many people actually know the size of their prop or if it is the correct one. If they had a problem would they know what size to get as a replacement or are they all so well off that they would get someone who can to do it?
 
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One of the main factors is pitch. I googled "prop sizes" and got a lot of info on how to work out your size, but you need lots of info as well.
John
 
Rather than trying to measure it, can you read the dimensions with your inspection tool? They are normally stamped on the flat end of the boss where the nut and washer attach it to the shaft. Usually for example 14 x 10R (Diameter) x (pitch) (R or L)
 
[QUOTE=jmh: "One of the main factors is pitch."

AND diameter.

Only after a damaged prop entailed consulting Clements Engineering did we find that the designer's spec was nonsense - the prop was too big, with insufficient tip clearance under the stern preventing full engine revs.
A new prop made all the difference.
IMHO consulting the experts is the way forward.
 
Shaft speed is a direct function of engine revs and reduction ratio. So 3600 revs and 2:1 = 1800. There is very little loss of speed in the bearings - the speed will only be lower if the engine revs are lower.

10" props are rare beasts as they only suit low hp engines such as yours and there are few around. However, there used to be quite a few so you may be lucky and find a second hand one - try the secondhand place in Burseldon.
 
Tranona said:
Shaft speed is a direct function of engine revs and reduction ratio. So 3600 revs and 2:1 = 1800. There is very little loss of speed in the bearings - the speed will only be lower if the engine revs are lower.

10" props are rare beasts as they only suit low hp engines such as yours and there are few around. However, there used to be quite a few so you may be lucky and find a second hand one - try the secondhand place in Burseldon.
Click to expand...

Thanks i will give them a try and we have the Solent boat jumble next weekend to look for one. Another symptom i have is that i can not get anywhere near full revs in gear although in neutral the engine races away, similar to your problem with the oversizes prop. In my case the revs are vastly reduced and lots of soot/unburnt diesel coming from the exhaust if i push the throttle too far.

VyV, although our inspection tool gave a good view of the prop we couldn't get it close enough to read anything, wish i hadn't cut it so short now. I was surprised how well it worked and would recommend it.

John, i googled prop size and used all the calculators available using all the info that they required. 6" pitch was what i came up with but i have no idea what pitch i have on the present prop. However using the recommended 1" altered diameter equals 1.5" -2" altered pitch, if my prop is 14" then it should have no pitch at all.
 
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Bower15 said:
In my case the revs are vastly reduced and lots of soot/unburnt diesel coming from the exhaust if i push the throttle too far.
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That is because the prop is too big. The correct prop should allow the engine to reach near enough its maximum revs and max speed. You won't be able to reach hull speed because you simply don't have enough power, but you should be able to get at least 4.5 knots.
 
Tranona said:
That is because the prop is too big. The correct prop should allow the engine to reach near enough its maximum revs and max speed. You won't be able to reach hull speed because you simply don't have enough power, but you should be able to get at least 4.5 knots.
Click to expand...

Hull speed, according to the calculators should be around 5.7 Knots, at the moment i'm lucky if i get anywhere near 2 Knots although some of this is down to weed growth. All these figures are of course accademic so its good to hear of some real life experiences such as yours to compare with. I would be happy to cruise at 3 Knots with a top speed of 4 Knots although anything extra would be a bonus.
Interesting point you make about 10" props not being very common, looks like i may have trouble finding a cheap one! I'm wondering if i might have better luck finding a 12" one with a smaller pitch, although that would mean a pitch of around 3" which i cant see being a popular size. Its a pity we don't have as many forumites from the prop manufacturers as we do anchor manufacturers.
Interesting too that not many responses from forumites re their prop sizes, perhaps its a black art best left to the gods.
 
My Corribee (21') has a Yanmar 1gm10 with a 2:1 box, and is fitted with an 11" x 9 prop. Even as small as 11" it is still a touch too close to the surroundings, but it works ok at full revs. Any weed growth at all has a marked effect on performance. With everything clean I can easily get 5 knots.

The prop came from Lancing Marine, who did their own calcs to recommend the right size, but I also used the Propcalc spreadsheet, (http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/ look for the link to propcalc.xls half way down the page)
which gave the same result.

A friend of mine has an old classic, again with a 1gm10, and he got a larger diameter prop machined down to a smaller diameter to suit his installation - a simple job provided you have a lathe large enough to swing the prop of course.
 
Bower15 said:
Hi
Sorry to bore you all again with my continuing problem but i could really use some info to verify my own findings. I have come to the conclusion that i am overpropped which is causing black soot and dirt from the exhaust when under load, i have checked all other possible causes. Using the various prop calculators online i reckon my prop should be about 10x6", they all give various answers but this is the average. Using a length of drainpipe with some perspex stuck on the end as a viewer, i measured my prop under water yesterday and it seems to be 14 to 15" diameter. Now i know i can go to various manufacturers to get exact figures but being a cheapskate i want to do as much as i can myself, also i really enjoy this part of owning a boat, ie. solving problems.
Could i ask any of you who have a 22' bilge keeler, powered by 6hp through a 2:1 gear box, displacing about 4000Lbs to let me know what size prop you have. I realise this wont give me the absolute correct answer for my boat but at least i will have a ball park figure and confirm i am going in the right direction. I cant afford to pay for a lift out and then find my prop is the right size after all so i am doing as much research as i can beforhand, also it helps me understand my boat better, not a bad thing in my opinion
Click to expand...

That MUST be far too big for a 6 hp engine. My old boat with 7 hp Volvo had about an 11" or 12" three-bladed prop, and my present boat with 27 hp Yanmar has a two-blader at about 15" or 16"approx diameter, though I have not actually measured it. Both reached very close to max revs when in gear with a clean hull and prop, almost 5 knots with the 7 hp on a 27 ft long-keeler (though on a very heavy boat it took time to get there), and over 7 knots with the 27 hp on a modern light 35-footer (not much different in weight to the old long-keeler).

Black smoke and not reaching full revs in gear just confirms your prop is very wrong.
 
srp said:
My Corribee (21') has a Yanmar 1gm10 with a 2:1 box, and is fitted with an 11" x 9 prop. Even as small as 11" it is still a touch too close to the surroundings, but it works ok at full revs. Any weed growth at all has a marked effect on performance. With everything clean I can easily get 5 knots.

The prop came from Lancing Marine, who did their own calcs to recommend the right size, but I also used the Propcalc spreadsheet, (http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/ look for the link to propcalc.xls half way down the page)
which gave the same result.

A friend of mine has an old classic, again with a 1gm10, and he got a larger diameter prop machined down to a smaller diameter to suit his installation - a simple job provided you have a lathe large enough to swing the prop of course.
Click to expand...

I'm surprised your prop is so big, how many HP has your engine? Using the propcalc link in your reply suggests that i need a 10x8" to reach a max speed of 5 Knots and 10x6 to reach 4 Knots which is similar to the figures i had before. I used a prop calculator from a company in Canada which required a lot more info but came up with similar results.
 
It's quite possible to shorten the blades if you find a prop of the right pitch but, say, 11 or 12 inch diameter. A hacksaw and file will do the job, following careful marking out.

Neil
 
pissativlypossed said:
These people http://www.sillette.co.uk/ are very efficient and knowledgeable, ask them to supply a quote for a new prop, you will need to give them full technical details for your vessel, engine, gearbox, etc. Maybe not totally ethical, but then again if you do need a new prop you can ask them first.
Click to expand...

Thanks, I've just completed an online form for a quote and emailed it to them, it will be interesting to see what size they come up with. Looking at their price guide it looks as though 10" and 11" props with 3 blades are £210 which is a lot less than i was expecting but still more than i can afford.
 
Bower15 said:
Hi
Sorry to bore you all again with my continuing problem but i could really use some info to verify my own findings. I have come to the conclusion that i am overpropped which is causing black soot and dirt from the exhaust when under load, i have checked all other possible causes. Using the various prop calculators online i reckon my prop should be about 10x6", they all give various answers but this is the average. Using a length of drainpipe with some perspex stuck on the end as a viewer, i measured my prop under water yesterday and it seems to be 14 to 15" diameter. Now i know i can go to various manufacturers to get exact figures but being a cheapskate i want to do as much as i can myself, also i really enjoy this part of owning a boat, ie. solving problems.
Could i ask any of you who have a 22' bilge keeler, powered by 6hp through a 2:1 gear box, displacing about 4000Lbs to let me know what size prop you have. I realise this wont give me the absolute correct answer for my boat but at least i will have a ball park figure and confirm i am going in the right direction. I cant afford to pay for a lift out and then find my prop is the right size after all so i am doing as much research as i can beforhand, also it helps me understand my boat better, not a bad thing in my opinion
Click to expand...
I had a leisure 23 ,20ft wl with a 6 hp Petter mini 6 , this had a 12x7 two bladed prop and with a clean bottom gave 5.2 knots .This seemed well matched to the engine.
 
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