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Engine and prop matching

Mick and Maggie

By Mick and Maggie
in Boat Building & Maintenance

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Mick and Maggie Rising Star

Mick and Maggie

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I have been mind musing about building a new boat. This could very well happen in two or three year's time. So I am starting to look at the technical engine/electrical specification. Everything else will be down to the Memsahib. Layout and colour co-ordinated toilet paper and holder etc. :rolleyes:

 

A thought that has been niggling me for some time is around the sizing of a boats diesel engine and then matching it to the correct prop. My only experience of this has been gained from our current boat.

 

On our current boat using a Beta 38 we hardly ever, when day-to-day cruising exceed an engine speed of 1200 rpm. Looking at the engine power curve specification, that’s about 10/12 bhp. On a trip along the Trent we never exceeded 1900 rpm even when punching against the tide which is about 23/25 bhp.

 

At about 2200 / 2300rpm (28/30 bhp and the peak of the engine torque) or there about, I note that cavitation starts to take effect.

 

Would this be considered an optimal match for the engine and prop combination?

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WotEver Veteran II

WotEverDonate to Canal World

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Can I ask what size your boat is ? I am wondering what size I would want on mine...

You tell Crowthers your length, draft, engine and prop clearance and they tell you the optimum prop for your boat. Or you practice the dark arts yourself with the aid of various charts that folk will point you to shortly.

 

Personally, Crowthers have a good rep and don't charge for the service, so when I replaced our prop I was happy to let them specify.

 

Tony

 

Actually Midland Chandlers did the specifying, but I believe all they do is ask Crowthers ;)

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mykaskin Experienced

mykaskin

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At about 2200 / 2300rpm (28/30 bhp and the peak of the engine torque) or there about, I note that cavitation starts to take effect.

 

Would this be considered an optimal match for the engine and prop combination?

 

Is this cavitation or ventilation? Two totally different things. Ventilation can be reduced by having a greater depth at the stern.

 

If flatout and the engine is around the max torque then it's well matched, though on a canal boat some prefer to have the boat over prop'ed to allow for slower engine revs. Does it behave as you want it to?

 

Mike

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Proper Job Veteran

Proper Job

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You tell Crowthers your length, draft, engine and prop clearance and they tell you the optimum prop for your boat. Or you practice the dark arts yourself with the aid of various charts that folk will point you to shortly.

 

Personally, Crowthers have a good rep and don't charge for the service, so when I replaced our prop I was happy to let them specify.

 

Tony

 

Actually Midland Chandlers did the specifying, but I believe all they do is ask Crowthers ;)

You'll also need to advise them of gearbox reduction ratio

Edited by Proper Job
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Speedwheel Influencer

Speedwheel

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Actually Midland Chandlers did the specifying, but I believe all they do is ask Crowthers ;)

 

That is certainly not true. MC don't sell Crowther props so Crowthers are not going to spec them.

 

 

Let me have the following info and I will work it out for you.

 

Boat

- Length

- Beam

- Weight (approx)

- Max diameter prop that you can swing.

 

Engine

- HP

- Max RPM

- Gearbox reduction

Edited by Speedwheel
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Neil TNC Rising Star

Neil TNC

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That is certainly not true. MC don't sell Crowther props so Crowthers are not going to spec them.

 

 

Let me have the following info and I will work it out for you.

 

Boat

- Length

- Beam

- Weight (approx)

- Max diameter prop that you can swing.

 

Engine

- HP

- Max RPM

- Gearbox reduction

 

Right ...lets see if you agree with Keith at Crowthers! :rolleyes:

 

15m

3.7m

32ton?

23"

 

75 HP

2600 RPM

1.96:1

 

Shapey double chined barge

Lux%2049%20sketchA.jpg

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WotEver Veteran II

WotEverDonate to Canal World

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People always say you need to know how long, wide deep etc the boat is. I disagree. The engine and the prop do not know what boat they are on and I think it will have no effect on the propeller you need to match correctly with the engine.

Casp'

But surely length, depth, beam etc all contribute to drag? Or are you suggesting that drag is irrelevant in the calculation?

 

Tony

 

That is certainly not true. MC don't sell Crowther props so Crowthers are not going to spec them.

I stand corrected :)

 

I just left the marina's engineer to deal with it and trusted to his judgment. He even ground the prop before fitting it because he 'knew' that if he didn't it would sing like crazy.

 

I guess I should have asked for the old prop and sold it for scrap ;)

 

Tony

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casper ghost Influencer

casper ghost

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But surely length, depth, beam etc all contribute to drag? Or are you suggesting that drag is irrelevant in the calculation?

 

Tony

I do suggest it's irrelevant. You have a particular engine, it needs a particular prop for peak power. It couldn't turn a bigger prop just because the boat is bigger. The prop goes with the engine, if the engines not right for the boat, that's a different matter.

Casp'

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Speedwheel Influencer

Speedwheel

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I do suggest it's irrelevant. You have a particular engine, it needs a particular prop for peak power. It couldn't turn a bigger prop just because the boat is bigger. The prop goes with the engine, if the engines not right for the boat, that's a different matter.

Casp'

 

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.

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Mick and Maggie Rising Star

Mick and Maggie

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Can I ask what size your boat is ? I am wondering what size I would want on mine and even at only 38hp yours seems more than adaquate.

 

Rosie is a 50' Price Fallows narrowboat.

 

Is this cavitation or ventilation? Two totally different things. Ventilation can be reduced by having a greater depth at the stern.

 

The symptoms are, the wake has a great deal more "white water" in it - the tiller vibrates with a more pronounced side to side movement (much more than normal) and I get a feeling that no extra forward effort is being created, maybe, even the opposite that there is a small reduction. The engine does not hunt at all but maintains the same RPM.

 

If flatout and the engine is around the max torque then it's well matched, though on a canal boat some prefer to have the boat over prop'ed to allow for slower engine revs.

 

Thats an interesting observation as the engine spec sheet on fuel consumption gives 1 Lph @ 1300rpm and 2 Lph at 2300rpm.

 

Engine tick over is about 900rpm - the usual canal cruising speed is around 12/1300 rpm. Peak torque comes at 2300rpm. Engine flat out is 3600 rpm and 7 Lph. But never been that high with the engine rpm.

 

Does it behave as you want it to?

 

That's a good question, to which the answer would be yes. However, I am one of those (blessed or damned) with an enquiring mind.

 

The previous owner, had Rosie re-propped.

 

M&M

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Neil TNC Rising Star

Neil TNC

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Right the size I am getting is 21x13. (Not my personal skill I must add, simply a computer program)

 

Well thats getting nearer! The two programs I used came out at around 20x13

Keith sold us a 22x13, but with blades of a strange??? aspect ratio. I just get the feeling this does not look very efficient and will not pull up terribly well.

 

 

And now for J...the piccy

prop.jpg

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casper ghost Influencer

casper ghost

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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.

The program may change, but if you think about it, it can't make any difference.

Casp'

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Evo Rising Star

Evo

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Posted (edited)

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

Whats the point of asking for weight..??? who the hell has weighed their boat, especially with all the people, water, fuel, food, and beer, solar panels, lumps of tree, push bikes, generators, blah blah.

 

I would expect that it would be done in ranges for the average laden boat. I'm guessing all this of course, but I would hope that if I rang them and said I have a 60ft narrowboat with a BMC1.8 and PRM 2:1 thats all they would need to know really.

 

 

30-40ft @ 30hp = ?

40-50ft @ 35hp = ?

50-60ft @ 45hp = ?

60-70ft @ 48hp = ?

 

oops I forgot the important bit.....I assume I'm wrong..!!!

Edited by Evo
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Neil TNC Rising Star

Neil TNC

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who the hell has weighed their boat, especially with all the people, water, fuel, food, and beer, solar panels, lumps of tree, push bikes, generators, blah blah.

 

 

 

I have, it is useful to know. Most cranes have a pretty accurate weighing device. NB Earnest in full cruising trim (lots of beer, spares and provisions), but with no water or diesel came out at 18.5 tons on P&S crane at Cassio. I thought around 20 tons. This was handy to know, because I saved money when it was dropped back in, in Ireland, as we only had to hire a 20 ton crane.

When it came back, it weighed about the same - full diesel tank, but no beer B)

 

Back on topic, NB Earnest 18.5 Tons, 58ft 6ins x 6ft 10ins x 24" draught, Beta 1903 (43HP) max RPM 2800, 2:1 reduction, with its slipper stern is very slightly over proped with an 18 x 13...just like Crowthers said it would be.

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Proper Job Veteran

Proper Job

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...........

 

 

30-40ft @ 30hp = ?

40-50ft @ 35hp = ?

50-60ft @ 45hp = ?

60-70ft @ 48hp = ?

 

oops I forgot the important bit.....I assume I'm wrong..!!!

It's not that simple though. Power isn't the requirement. It's the torque at the shaft.

 

Mines 56', 27 hp and massively over powered for the boat. 284 lbs ft at the shaft.

 

Compare that with NB Earnest's Beta 1903 (43HP). 162 lbs ft at the shaft.

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