Choosing the right prop

[Artimis]

Is there a formula for sizing the right prop?

 

What are the factors?

 

Hull size and shape?

Engine power?

Vessel weight?

Torque/power band?

[paulcatchpole]

Is there a formula for sizing the right prop?

 

What are the factors?

 

Hull size and shape?

Engine power?

Vessel weight?

Torque/power band?

 

Yes, and a good dose of black magic.

 

You can download a calculator from here, which gives okayish results when I've tried it.

 

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

 

PC

[GUMPY]

Is there a formula for sizing the right prop?

 

What are the factors?

 

Hull size and shape?

Engine power?

Vessel weight?

Torque/power band?

 

It is part science and part black art and I wouldn't attempt it even though I have spreadsheets for days that claim to predict the correct size prop.

 

I always use these people

http://www.crowthermarine.co.uk/

They will get it right.

 

Julian

[colin stone]

Is there a formula for sizing the right prop?

 

What are the factors?

 

Hull size and shape?

Engine power?

Vessel weight?

Torque/power band?

 

 

Yes.

 

All of them.

 

Get Dave Gerr's book "The Propeller Handbook" for full explanation/calculation. Or get his book "The Nature of Boats" which includes a simple but accurate method of calculating prop size. I used it and was 100% spot on when I went to source my props. TNOB is an excellent general read, so worth having anyway.

[PhilR]

Is there a formula for sizing the right prop?

 

What are the factors?

 

Hull size and shape?

Engine power?

Vessel weight?

Torque/power band?

 

All of the above +

 

BHP and maximum revs

 

Reduction ratio

 

The draft would also be useful and the max diameter prop the boat will swing (with a minimum of 1 inch clearance top and bottom.

 

Also shaft diameter and rotation.

 

Crowthers will give you the definitive answer, but it would be interesting if you quote all of the above and see what answers you get on this board.

[Artimis]

Oh, wonderful answers, thank you all.

 

I see then that I need to gather more info first.

 

We started on the steelwork today, working out the shape of the swim etc, so its going to be a while before I need a prop but wanted to know what size in case I came across a bargain in the meantime.

[nb Innisfree]

The only fixed parameter is prop diameter which is dictated by hull design, that is as large a dia as possible with enough tip clearance. Engine size and power isn't a precise requirement but it's power and gearbox reduction dictates blade area, pitch dictates speed at tickover + prop needs to absorb full engine power which is at full revs but available engine power at the prop depends on other things like alternator(s) size and whether batteries are depleted or not.

 

All in all quite a simple requirement which turns into a complicated business.

[mrsmelly]

It is part science and part black art and I wouldn't attempt it even though I have spreadsheets for days that claim to predict the correct size prop.

 

I always use these people

http://www.crowthermarine.co.uk/

They will get it right.

 

Julian

Agreed, tell crowthers all about boat especialy length of swims etc, engine ,draught,weight they will come up trumps.....

[ymu]

So, our new boat is 65' long and 2' draft, with a Bukh DV36 engine. She has a rudder that looks very much on the small side according to our surveyor but she handles like a dream. However, she is pretty slow (we are easily overtaken by most walkers). Could it be a suboptimal propellor? Or am I just too wimpy with the throttle? I don't run her at more than 1500 revs generally and mostly a tad less. She has a very slow tickover - I usually have to run her slightly faster to get past at a reasonable speed (else fishermen look at me like I'm going to moor in their swim).

 

Any thoughts?

[paulcatchpole]

So, our new boat is 65' long and 2' draft, with a Bukh DV36 engine. She has a rudder that looks very much on the small side according to our surveyor but she handles like a dream. However, she is pretty slow (we are easily overtaken by most walkers). Could it be a suboptimal propellor? Or am I just too wimpy with the throttle? I don't run her at more than 1500 revs generally and mostly a tad less. She has a very slow tickover - I usually have to run her slightly faster to get past at a reasonable speed (else fishermen look at me like I'm going to moor in their swim).

 

Any thoughts?

 

Need at least to know the gearbox reduction (2:1? 3:1?) and engine peak torque RPM...?

 

PC

 

Edited to add 3600rpm for 36hp flat out? Sounds like you're well under 'normal' RPM for that engine...

Edited June 25, 2010 by paulcatchpole

[ymu]

Need at least to know the gearbox reduction (2:1? 3:1?) and engine peak torque RPM...?

 

PC

Not really looking for a specific answer - just whether a suboptimal propeller is a plausible explanation for the slowness.

 

Thanks for the list of info I'll need though. I can ask the other half to get the techie details if anyone really wants to know - but I suspect I'll be asking Crowthers.

[paulcatchpole]

Not really looking for a specific answer - just whether a suboptimal propeller is a plausible explanation for the slowness.

 

Thanks for the list of info I'll need though. I can ask the other half to get the techie details if anyone really wants to know - but I suspect I'll be asking Crowthers.

 

If you're topping out at 1500rpm, that's about 18hp for a DV36SME against its 36hp flat out, so I'd suggest, if you can't exceed that, then you're well overpropped. Is that accompanied by black smoke too?

 

I notice Bukh, rather randomly, suggest 16x11 as an optional prop, but who knows what for?

 

It seems a 3:1 reduction is typical, which sounds about right.

 

Based on that, and your boat specs, 18x15 is what I'd say... Decent max speed of 7.8knots.

 

PC

[Big COL]

Why is it that only the first two figures are ever quoted when talking about prop sizes.

There are three figures used in prop sizing dia x pitch x % of cover, the latter seldom quoted but can and is equally important, I had handling problems with a 70' boat that was propped correctly on pitch and diameter but according to Crowthers it required more cover.

Their recomendation cured the problem and the boat was a dream to handle after the change, so be aware that pitch and size are not the only thing that matters.

[nb Innisfree]

Why is it that only the first two figures are ever quoted when talking about prop sizes.

There are three figures used in prop sizing dia x pitch x % of cover, the latter seldom quoted but can and is equally important, I had handling problems with a 70' boat that was propped correctly on pitch and diameter but according to Crowthers it required more cover.

Their recomendation cured the problem and the boat was a dream to handle after the change, so be aware that pitch and size are not the only thing that matters.

 

Of a few 'experts' I consulted only Crowthers considered blade area, once max. diameter had been decided (not a job for them really as it is the hull that dictates dia) then blade area was calculated according to max power and shaft speed. Ours was 33 bhp @2000rpm with 2:1 reduction which resulted in 17 x 14 x 75% BAR, they weren't really interested in draught and length of swim etc. They were spot on, engine pulling just under max revs with no smoking. Some of the others said blade area was irrelevant, if a slower engine is fitted then only pitch is altered, which is nonsense.

 

Edited to change optimum dia to max dia.

Edited June 26, 2010 by nb Innisfree

[colin stone]

Blade diameter is determined by power and reduction ratio, and there are nomographs around which will easily determine diameter. If you are designing/building from scratch, then you have some options on where to place the propshaft. Non optimal diameter can be compensated for by pitch and blade skew.

 

Faster shaft = smaller prop diameter = less prop walk = less draft.

 

Larger prop diameter = slower shaft = greater efficiency = greater draft = more prop walk

 

Prop tip speed should also be less than 30 metres/sec to avoid cavitation.

Edited June 26, 2010 by colin stone

[Josher]

Got them to replace the perfectly good looking prop on my boat before I paid for it, because of this:

 

 

High Frequency Vibration Present (250 to 300 Hz) – Noise from Propeller Singing.

 

Replacement prop fine.

 

ps this post much too sensible from me ... so far ..

 

[nb Innisfree]

Blade diameter is determined by power and reduction ratio, and there are nomographs around which will easily determine diameter. If you are designing/building from scratch, then you have some options on where to place the propshaft. Non optimal diameter can be compensated for by pitch and blade skew.

 

Faster shaft = smaller prop diameter = less prop walk = less draft.

 

Larger prop diameter = slower shaft = greater efficiency = greater draft = more prop walk

 

Prop tip speed should also be less than 30 metres/sec to avoid cavitation.

 

Crowthers will tell you what diameter, pitch and blade area you need but it's up to you to tell them of any possible dia. restrictions with your hull, they will then amend spec with larger blade area.

[Trev]

I was talking to an ex merchant seaman yesterday. He has just had a new bespoke boat built and specifed a four bladed prop. His arguement being that with three blades in the traditional 'y' shape that one (at the bottom of the rev) is actually doing the work while the others( at the top) are 'coasting' as they are too close to the surface to be effective. So a four bladed prop gives better drive through the water? Anyone got one?

[PhilR]

I was talking to an ex merchant seaman yesterday. He has just had a new bespoke boat built and specifed a four bladed prop. His arguement being that with three blades in the traditional 'y' shape that one (at the bottom of the rev) is actually doing the work while the others( at the top) are 'coasting' as they are too close to the surface to be effective. So a four bladed prop gives better drive through the water? Anyone got one?

 

In my experience most ex merchant seamen talk a load of b*llsh*t when they boat on the inland waterways.

[soldthehouse]

Someone mentioned clearance is there a golden rule???

[nb Innisfree]

Someone mentioned clearance is there a golden rule???

 

A rule of thumb is a minimum of 10% of diameter, top and bottom.

Edited June 26, 2010 by nb Innisfree

[soldthehouse]

A rule of thumb is a minimum of 10% of diameter, top and bottom.

Thanks, It may be touch and go!!!!!!

[nb Innisfree]

Thanks, It may be touch and go!!!!!!

 

I think it's mainly to stop drumming on the counter plate + minimising things getting jammed.

[soldthehouse]

I think it's mainly to stop drumming on the counter plate + minimising things getting jammed.

Suppose a slow revving prop will help a little. I will be running a 2:1 reduction on a slow revving old engine so hopefully vibes will be at a minimum. The whine off the mechanical gearbox and some soot in my mush should take my mind off any drumming from the prop

[smileypete]

Why is it that only the first two figures are ever quoted when talking about prop sizes.

There are three figures used in prop sizing dia x pitch x % of cover, the latter seldom quoted but can and is equally important, I had handling problems with a 70' boat that was propped correctly on pitch and diameter but according to Crowthers it required more cover.

Their recomendation cured the problem and the boat was a dream to handle after the change, so be aware that pitch and size are not the only thing that matters.

Would have thought more cover makes the flow from the prop more directional, making the rudder more effective.

 

There must be a down side to it, maybe efficiency at higher engine speeds?

 

cheers,

Pete.