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gbclive

Propeller advise for a Kelvin J2

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My first post for some help and probably not the last!

I have purchased a Kelvin J2 powered 64 foot NB that was always going need a lot of work in order to bring it up to a good standard, however it turns out that the amount of work involved is eye watering to say the least!

 

The first item someone may be kind enough to help with is the prop - advise from a respected engineer who is very experienced with vintage engines including Kelvins, is that the boat is significantly "over-propped" and it is therefore planned to return it to the manufacturers which I believe is Crowthers of Bolton, to have its pitch reduced by a nominal 3 inches.

 

So the boat goes into dry dock this week week to have the prop off, so I will hopefully know a bit more about it then from the makers markings etc but in the meantime does anyone have any experience of what would be a reasonable spec for a J2 in a typical 64 foot NB?

 

Very grateful indeed for any advise.

 

Clive.

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Crowthers themselves can probably advise better than most on the best size. You need to know the reduction ratio of the gearbox, give all the info, waterline length, weight etc and trust them.

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Hi

 

We run a J3 in a 60' boat, fairly deep draughted and well ballasted. We fitted a 23 diameter by 19 inch pitch, this worked well but the boat was a bit brisk on tick over. Graham, the builder, took it off and we used a plasma cutter to take about half an inch off each blade, linished it up and job done. Spot on!

 

As has been said, Crowthers are well known for their abilities in this field. You probably know, but the Kelvin box is straight through with no reduction.

 

Good luck with the project, I'm in touch with a Kelvin specialist if you need him. They are lovely engines!

 

Cheers

 

Dave

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Crowthers themselves can probably advise better than most on the best size. You need to know the reduction ratio of the gearbox, give all the info, waterline length, weight etc and trust them.

Thanks Bee. A slight complication is that apparently Crowthers originally supplied the prop for the current boat and engine combination, so any any ball park specification would give a gross check of the design. Something somewhere does not quite add up?

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............I'm in touch with a Kelvin specialist if you need him. They are lovely engines!

Cheers

Dave

Thanks Dave - if you think your specialist might be able to suggest a ballpark spec for a J2 / 64 foot NB, then perhaps you would be kind enough to PM me?

 

Cheers.

Clive.

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Out of interest, what problems are being caused by the boat being significantly over propped?

The primary issue is that the prop is drastically limiting the RPM as opposed to when out of gear. When opening up the throttle in deep and wide water, the engine seems to choke and sounds laboured.

Only subjective so far, but I now have an optical tacho with with reflective tape on the prop shaft, so with a 1:1 gearbox ratio I will try to collect some RPM data. Certainly not getting anywhere near to the rated 1000 RPM though.

 

The engine is also overheating. However this has been diagnosed as due to a 21 year old and poorly designed skin tank design which is on the bottom rather than the side of the hull with no baffles. The pipe work and therefore quite likely the tank its self is quite significantly narrowed with a rock hard deposit. There is only a few degrees of cooling being achieved, so the temperature of the water flowing through the skin tank steadily rises. This issue is being tackled by a completely new thin and baffled skin tank on the outside of the aft swim.

 

There are other problems too, but need to get the basics sorted out first.

 

Thanks for your interest.

Clive.

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My J2 has a 21 in dia 13 in pitch blade in a 60ft boat. This gets to about 650rpm in open water (Thames or Severn), so is a bit too big, but the boat goes well enough and stops beautifully. I would replace it with a 20x13 if I ever needed a new blade.

 

 

The standard Kelvin prop for a J2 was 19 in dia 13 in pitch, though the bats were narrow and almost scimitar like.

 

Are you getting black smoke when you open up? If so that will also contribute to the overheating problem- The cooling depends on the engine speed increasing for the pump to push enough water through the heads.

 

If the pipework is badly scaled the heads and cylinders are also going to be scaled up. Circulate some descaler through them - brick cleaner ( aka brick acid or more accurately dilute hydrochloric acid) is very effective, but if you use that follow it with a strong solution of washing soda.

 

N

 

 

N

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The primary issue is that the prop is drastically limiting the RPM as opposed to when out of gear. When opening up the throttle in deep and wide water, the engine seems to choke and sounds laboured.

Only subjective so far, but I now have an optical tacho with with reflective tape on the prop shaft, so with a 1:1 gearbox ratio I will try to collect some RPM data. Certainly not getting anywhere near to the rated 1000 RPM though.

 

The engine is also overheating. However this has been diagnosed as due to a 21 year old and poorly designed skin tank design which is on the bottom rather than the side of the hull with no baffles. The pipe work and therefore quite likely the tank its self is quite significantly narrowed with a rock hard deposit. There is only a few degrees of cooling being achieved, so the temperature of the water flowing through the skin tank steadily rises. This issue is being tackled by a completely new thin and baffled skin tank on the outside of the aft swim.

 

There are other problems too, but need to get the basics sorted out first.

 

Thanks for your interest.

Clive.

 

Given the other issues I wouldn't be certain that the prop is the reason it won't rev under load.

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I'd be careful fitting one of those Crowther-specced propellers with the thin blades. I took the one off my boat with a K1 as the performance, especially in reverse, was as dire as you'd imagine from looking at the blades.

 

Over-propping is good with a small Kelvin unless you want the engine to be screaming at 750rpm to achieve any sensible sort of cruising speed.

 

MtB

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Over-propping is good with a small Kelvin unless you want the engine to be screaming at 750rpm to achieve any sensible sort of cruising speed.

 

MtB

Screaming??!!

I shared a lock with 'Owl' and was surprised to be told the revs actually were in double figures (well, triple, but only just).

I've had anoraks taping my engine so they could play it at home - thought they were a bit odd, but I can understand it with a Kelvin.

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Mike

 

Bear in mind that Owl has the K2 Kelvin fitted, not the smaller J series, they run really slowly!

 

Dave

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............ I would replace it with a 20x13 if I ever needed a new blade.

 

Many thanks for this info and advise - I now have ballpark figures for a J2 to compare with mine which I think is 22 inches diameter. I should know the pitch tomorrow once in dry dock.

 

Clive.

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I'd be careful fitting one of those Crowther-specced propellers with the thin blades. I took the one off my boat with a K1 as the performance, especially in reverse, was as dire as you'd imagine from looking at the blades.

MtB

Thanks for that Mike - unfortunately my lack of propeller experience means I have no idea if the prop is thin or otherwise?

Perhaps if I take a photo of it tomorrow you could advise further?

Present advise I have is for Crowthers to reduce the pitch - perhaps by 3 inches or so later this week.

I believe they originaly designed the prop some years ago for the current boat and engine.

Of course modifying the diameter or indeed a new prop are also options depending on what we find out tomorrow.

 

Clive.

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Advice is tricky. A lot of blade choice is just a matter of opinion, in my opinion!

 

I'm just saying that Crowthers are not infallible where kelvins are concerned. They specified, made and supplied the blade I had on mine that was such a disaster. the engine needed to run near it's max speed to get any sort of decent crusing speed, and the brakes were utterly non-existent.

 

so I chose what I thought would work well, and it did. Mainly because I chose to heavily over-prop the engine. This reduced the max engine speed to less than the 750rpm the book says, but also resulted in a comfortable crusing speed at about 450rpm instead. It also massively improved the brakes which was just as important, and often overlooked.

 

But yes by all means post a photo!

 

MtB


P.S. Forgot to make my final point which is that I'm wondering if yours is over-propped for the same reasons as mine curently is. maybe the previous owner has been through the same learning loop as me!

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Screaming??!!

I shared a lock with 'Owl' and was surprised to be told the revs actually were in double figures (well, triple, but only just).

I've had anoraks taping my engine so they could play it at home - thought they were a bit odd, but I can understand it with a Kelvin.

Here's Owl's K2 on tickover. Actually, I don't usually let it run quite so slowly because I want to make sure that the big ends are not starved of oil.

The prop is 28 x 20. Cruising speed on the canals is about 250 and flat out speed - on the Severn - is about 550.

As others have said (pace MtB) , Crowthers will sort you out the correct size prop.

 

http://youtu.be/cj25QFs1V7c

Edited by koukouvagia

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Here's Owl's K2 on tickover. Actually, I don't usually let it run quite so slowly because I want to make sure that the big ends are not starved of oil.

The prop is 28 x 20. Cruising speed on the canals is about 250 and flat out speed - on the Severn - is about 550.

As others have said (pace MtB) , Crowthers will sort you out the correct size prop.

 

http://youtu.be/cj25QFs1V7c

Slightly off Kelvin topic but what size prop did you run when you had the Seffle fitted?

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Slightly off Kelvin topic but what size prop did you run when you had the Seffle fitted?

I'm afraid I've forgotten! However, I've still got it - it's still acting as ballast - so I can measure it when I go up to the boat later this week.

BTW how do you measure the pitch of a prop?

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I'm afraid I've forgotten! However, I've still got it - it's still acting as ballast - so I can measure it when I go up to the boat later this week.

BTW how do you measure the pitch of a prop?

The short answer is " With difficulty".

 

The long answer is :

 

You need to work out the angle of the chord of the blades at a convenient diameter. This is the helix angle, the tangent of which is the pitch divided by the circumference of the circle of your chosen diameter.

 

The angle can be measured by putting the prop on a flat surface, drawing the chosen circle onto each blade and calculating the difference between the height of the leading edge off the surface and the height of the trailing edge off the flat surface. You then measure the straight-line distance between the two points at which your chosen circle crosses the edges of the blades/. The sine of the helix angle is then the height difference divided by the straight line distance.

 

Alternatively, measure the height difference as above and also the angle subtended by the two points at which your chosen circle crosses the edges of the blades, where you measured the height difference. If you work in degrees the pitch is then ( 360/angle of bats) x height difference across the bats.

 

Either way it's difficult to get accurate measurements because of the thickness and curvature of the blades.

 

N

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The short answer is " With difficulty".

 

 

Gordon Bennett, Nigel.

Well I did ask. I thought I'd seen the last of geometry over half a century ago!

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I'm afraid I've forgotten! However, I've still got it - it's still acting as ballast - so I can measure it when I go up to the boat later this week.

BTW how do you measure the pitch of a prop?

Maybe it"s stamped on the boss, the one I had that came from Willow Wren for the Seffle I had was, 28x21 Just curios as there wasn"t many of those about.

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My J2 has a 21 in dia 13 in pitch blade in a 60ft boat. This gets to about 650rpm in open water (Thames or Severn), so is a bit too big, but the boat goes well enough and stops beautifully. I would replace it with a 20 x 13 if I ever needed a new blade.

 

For the first time in quite a while something seems to have fallen into place

Prop off this morning. 20 x 16 stamped on the boss.

Our engineers guestimate was to take 2 or 3 inches off the pitch which ties in nicely with BEngo's advise, so that's what we are going for.

 

Another piece of the jigsaw would be if Crowthers calculations matched, although that's probably too much to ask for. Surgery is scheduled for Thursday. I'll report on the result in a week or so.

 

Thanks everyone for your advise - much appriciated.

 

Now about the waterpump and governor and skin tank and .........................

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