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Engine and prop matching


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Surely by now the prop companies have charts for the different length narrowboats.

 

No, they have charts for different weight boats! The narrowboat market is very small.

 

Actually picking a prop isn't just a case of matching it to the engine (and the rest of the drive chain), but the application. For example my properller is slightly under proped on Victoria to allow the engine to reach full revs on a static pull to allow for pulling loaded butties.

 

The shape, weight, and resitance of a hull can effect maximum speed in the water, and most prop calcs are designed to give maximum speed in open water. On a canal things are slightly different.

 

Cheers,

 

Mike

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Mine is slightly under-propped and I like it that way because it can still turn OK when I have picked up a load of weed or rubbish (especially as it is on a 3:1 gearbox which reduces the speed the rubbish is being spun whilst increasing the torque available to spin it)

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I must be missing something with this prop caper. We cant all be running around with different propellers.

I mean how many do they actually make. Our boats are pretty much exactly the same width exactly the same lengths and made out of exactly the same stuff.

 

I wish someone would do a thread where everyone that knows what prop they have posted dimensions with length of boat and type of engine, with a summary of general characteristics. Of course I accept there are oddities, empty working boats etc, but in general surely there cant be that much difference between what we all have and a trend would I'm sure immerge for the various lengths/type of boat/engine.

Edited by Evo
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Of course I accept there are oddities, empty working boats etc,

 

But occasionally many are filled, and I know no one who changes their prop when loaded.

 

The only thing a propeller needs to do is move water, preferably in an efficient manner - it's the last bit that makes the Voodoo!

 

Mike

 

ps. Here my prop for comparison to Neils above:

 

Victoria_NewProp.jpg

Edited by mykaskin
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Wish I'd found this thread before starting a separate one about prop size.

 

We've got 60ft boat with a Beta 43 and a 20 x 15 right handed prop on a 3:1 gearbox ratio. No idea what the weight of the boat is.

 

Size of the prop was subject of discussion at the time the boat was built between our surveyor, Beta and a prop manufacturer. We were told that the ideal size prop for the boat would be 24" diameter but that meant a draught of around 32" which was too deep.

 

Eventually we ended up with 20" x 15" with a 23" stern post.

 

Move on several years and this year the prop has started singing horribly just at our cruising rev range (1100 - 1400).

 

Tried several things to cure it but to no avail.

 

Neil's suggesting that "dressing up the blade edges with a dolly and club hammer and then bevelling the leading edges approx 45 degrees, with angle grinder" might cure the singing but the engineer who has been trying to cure the problem says changing the prop size is the best solution. Suggestion is that 18 x 14 is the now the ideal size but originally we were told that a smaller prop increase chances of underperformance and noise, plus the gearbox ratio would need to be to 2:1.

 

I'm puzzled as to why the ideal size has now changed so much and the gearbox ratio now doesn't seem to matter. Is it really a black art? Like someone else suggested, it would be really useful to know who's got what size prop fitted to what length of boat with what engine.

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Oliver

 

I have no qualification in prop sizing etc.

 

My thoughts, the prop was the right size then and still is the right size, something has happened to the prop, I know not what, perhaps a blade has been twisted/distorted had a chunk knocked out of it.

 

Changing a prop to a different size will stop the singing because the singing prop is not fitted but the next one could still sing.

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On "Keeping Up" which is 67ft, we have an MC42 engine (ie one hp less than the Beta) with a 3:1 gearbox. The MC42 has a rev limit of 2800, and like most diesel engines the torque is fairly flat over its whole operating range.

 

Originally we had a 19*13 prop, but it sang badly. It was suggested that this was partly because there was too little clearance between the blades and the counter, and we swapped it for a 18*14 which gave very little change in performance but cured the singing. Both props were Crowthers High-Efficiency type. The performance is 1mph per 400 rpm on open waters, or typically 1mph per 450 rpm on most canals except the shallowest.

 

We are definitely under-propped but as I said before, I like it that way because it means we have power to spare for when we pick up rubbish on the prop. Also, if I increased the prop size or pitch we would be travelling too fast at tickover (which is 800 rpm). Our top speed is 7mph on open water, which is good enough for me.

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Lister FR2, 15HP @ 1500rpm, 2:1 reduction gearbox.

60ft NB, 2'10" draft.

20x15 prop.

 

Flat out, the engine does 1450rpm, so we're very slightly over-propped. Lister recommend a 20x13 prop for this engine, which is probably a perfect match.

 

Flat out we do 5mph, at 1200rpm which is the fastest I'd run all day we do 4.5mph. Our normal cruising rev range at 800-100rpm which is 3-4mph.

 

All speeds in deep water, a shallow canal slows us down a lot!

 

MP.

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Bottle

 

So we could put another 20 x 15 on and the singing might stop or we could put, say, a 19 x 16 on and it would also sing?

 

What actually is causing the singing?

 

Oliver

When Wot Ever was re-propped last year I left the engineer (Kevin) at Springwood Haven to deal with it. When he received the prop I said "Oh goody, so it'll be back in the water in an hour then?". He replied nope, not until he'd angle-ground the prop first to stop it singing. Not a lot of help, I know - but maybe you could talk to him about what he knows?

 

Tony

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When Wot Ever was re-propped last year I left the engineer (Kevin) at Springwood Haven to deal with it. When he received the prop I said "Oh goody, so it'll be back in the water in an hour then?". He replied nope, not until he'd angle-ground the prop first to stop it singing. Not a lot of help, I know - but maybe you could talk to him about what he knows?

 

Tony

 

It's a black art. ;)

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When Wot Ever was re-propped last year I left the engineer (Kevin) at Springwood Haven to deal with it. When he received the prop I said "Oh goody, so it'll be back in the water in an hour then?". He replied nope, not until he'd angle-ground the prop first to stop it singing. Not a lot of help, I know - but maybe you could talk to him about what he knows?

 

Tony

Why angle grind a new prop to stop it 'singing', when it's never been used and you wouldn't know if it would 'sing'?

 

Obviously re profiling the leading edge of the blades would be a bit of belt and braces 'just in case'.

 

If its such a big problem, why don't the manufacturers profile the edge properly (and I use that term a bit tongue in cheek) as part of the casting/finishing process?

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If its such a big problem, why don't the manufacturers profile the edge properly (and I use that term a bit tongue in cheek) as part of the casting/finishing process?

Exactly the question I asked him - he didn't know the answer. As for "why grind before you know it'll sing", I asked him that too. He answered "They always do".

 

Tony

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But occasionally many are filled, and I know no one who changes their prop when loaded.

 

The only thing a propeller needs to do is move water, preferably in an efficient manner - it's the last bit that makes the Voodoo!

 

Mike

 

ps. Here my prop for comparison to Neils above:

 

Victoria_NewProp.jpg

 

That prop is a work of art, may I ask what the dimensions are - I am guessing somewhere near 27 x 20?

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Reading the posts and other stuff on the net - it's not clear just what data do we need to decide on the size and pitch of the prop. How to use the info to make the calculation.

 

1. Boat Weight.

2. Boat Length.

3. Boat Beam.

4. Boat Draft.

5. Engine BHP.

6. Engine RPM Range.

7. Engine maximum torque RPM.

8. Gearbox Ratio.

9. Prop Diameter.

10. Prop Blades.

11. Prop Pitch.

12. Prop Slip.

 

Alternatively there is the Castle Marine prop pitch calculator.

 

http://www.castlemarine.co.uk/pitch.htm

 

M&M

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When you use a prop calculation program it requires you to put in all this info.

 

If you change the length, width, weight etc it can change the prop size calculation quite significantly.

 

I cannot give you an explaination why but it must make a difference.

 

I would suggest that drag and displacement are vital bits of the calculation as they determine the maximum hull speed for a given engine power.

 

This speed (through the water) can then be used as the basis for calculations on prop slip relative to the actual prop pitch?

Edited by OptedOut
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That prop is a work of art, may I ask what the dimensions are - I am guessing somewhere near 27 x 20?

 

Hi, you're close - 28 x 16, matches the JP2 quite nicely.

 

Judging by comments on here nobody's an expert (me included), but I understand that bigger diameter props are more effecient (losses around the blade edge as the water slips around the edge), but does this start to cause other problems with regards to getting the most out of the engine power - for example weight, drag of blades, structural issues?

 

As to singing, I guess it all depends on what you call singing? It isn't the swirly swishing noise you hear many props make which are often down to dented edges on the blade, or rubbish, but a vibrational noise:

 

http://www.hydrocompinc.com/knowledge/whitepapers/HC138-SingingPropellers.pdf

 

Interestingly caused from the trailing edge, not the leading edge. I'm not sure if propeller manufacturers normally provide an anti-singing edge on their props.

 

Mike

 

Do you find that the Vauxhall creates much wash?

 

MP.

 

Actually it's a Peugeot! and it depends on the depth of the puddle! :-)

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