Prop size

[Proper Job]

I know it comes up in the forum quite often, but I've looked through many threads and can quite find what I'm looking for.

 

Ailsa Craig specified for their RFR4 engines (40hp @ 1200rpm - 2:1 reduction box) the following prop stern gear requirements:

28 x 21 prop, 3 bladed, 270 sq in blade area.

2" tail shaft with 2. 7/8" stern tube.

 

Allowing 3" top & bottom for clearance, that gives a starting draft of nearly 3 feet.

 

The prop only turns at 600 rpm flat out and will spend most of its time running a lot slower.

 

Is it possible/advisable to reduce the diameter and increase the pitch. Thereby reducing the draft a bit.

 

It will be fitted in a yet to be constructed tug hull (62' - maybe a Jonathan Wilson).

 

I know prop sizing is a mine field, but its expensive to get it wrong!

[-Jack-]

Hi,

 

Crowthers are the guys to contact. They are very good, very helpful and will do all the calcs for you. They are not the cheapest props - just the best. If you have any doubts just do a google search ' crowther props '

 

Click here to link to Crowthers web site.

 

You can always use their calcs etc and buy a cheaper prop. But, having used many props, I know the Crowrthers are excellent.

[Timleech]

Hi,Crowthers are the guys to contact. They are very good, very helpful and will do all the calcs for you. They are not the cheapest props - just the best. If you have any doubts just do a google search ' crowther props 'Click here to link to Crowthers web site.

You can always use their calcs etc and buy a cheaper prop.

 

I think they (understandably) get a bit fed up with that last bit.Wouldn't you??

Tim

 

I know it comes up in the forum quite often, but I've looked through many threads and can quite find what I'm looking for.Ailsa Craig specified for their RFR4 engines (40hp @ 1200rpm - 2:1 reduction box) the following prop stern gear requirements:28 x 21 prop, 3 bladed, 270 sq in blade area.2" tail shaft with 2. 7/8" stern tube.Allowing 3" top & bottom for clearance, that gives a starting draft of nearly 3 feet.The prop only turns at 600 rpm flat out and will spend most of its time running a lot slower.Is it possible/advisable to reduce the diameter and increase the pitch. Thereby reducing the draft a bit.It will be fitted in a yet to be constructed tug hull (62' - maybe a Jonathan Wilson).I know prop sizing is a mine field, but its expensive to get it wrong!

 

You don't really need 3" clearance though it's good to have. 2" is a realistic minimum where draught is limited.You can fiddle diameter & pitch a little bit, but it's not the proper answer.Better is to have the right pitch but larger blades, effectively 'folded' around the boss so they have more or less the same effect at a reduced diameter. Crowthers are the people to go to on that one, it's a problem they've dealt with quite a bit. The results won't be cheap, though, you'll be paying the same price roughly as for one of their full diameter props.

 

Tim

Edited July 29, 2007 by Timleech

[-Jack-]

I think they (understandably) get a bit fed up with that last bit.

Wouldn't you??

 

Tim

 

I think they probably do. But whilst they are known and respected as " the best propeller providers " it is probably accepted by them but not condoned. Of course some of their 'high efficiency' props that they specify as being best for your boat, you cannot buy elsewhere

 

If the OP cannot afford the best prop at least he can get the best info available

 

I hope you never go into a shop, get product info and look at it etc, then buy it cheaper on the net

 

Jack

 

Ps. I don't work for or have any financial connection with Crowthers. I'm just a satisfied customer

[Timleech]

I hope you never go into a shop, get product info and look at it etc, then buy it cheaper on the net

 

Jack

 

I can honestly say that I have never done that

 

Tim

Edited July 29, 2007 by Timleech

[-Jack-]

I can honestly say that I have never done that

 

Tim

 

Would you believe it, I've heard - people will even get useful info from a boatbuilder then not buy a boat from him

[Timleech]

Would you believe it, I've heard - people will even get useful info from a boatbuilder then not buy a boat from him

 

Happens all the time with me, but then I don't sell boats

 

Tim

[denboy]

28 x 21 prop

are your heasure ments for a sea going boat or narrow boat think you may struggle to get 28 " in .

my kelvin according to manufacturers manual says 17 " but i run it with a 22 x 17 after speaking with crowthers who reckomended a 21 x 17 which is doing just fine and that gives me about a 1 1/2" clearance

 

ps i found some info on your engine the other day then couldnt find the thread now i cant find the info typical i was looking for something completely different

[John Orentas]

Hi Steve.

 

As you are already in contact with Crowthers there is not a lot more that can be suggested. Crowthers make what they call a Lister pattern prop for just the kind of use that you have in mind, there is of course more to this question than simply pitch and diameter, the effective working area is equally important, it can be expressed as a fraction or a percentage of a full circle when viewed longitudinally, in fact Crowthers sometimes go even greater than the full disk area with the blades overlapping.

 

Another option of course would be to go for a 1:1 gearbox with the prop running at engine speed.

[DHutch]

Another option of course would be to go for a 1:1 gearbox with the prop running at engine speed.

Yeah, i was going to say exactly that.

 

Or engine does about 300rpm, stright though to the prop, and we have a 26*32" prop on her for that. With a 2" propshaft.

- However another boat with very simualay engine (same design, difrent builder) has a 1:2 box to speed it up to about 600, with a 18" or so prop.

 

A large diameter prop is nice though, and if your geting the hull built to the prop/engine its certainly quite possable to have a 26" prop on the uk canal system, we've done it for the last 15 years anyway.

- Going to far 'over square' (pitch greater than diameter) isnt recomended, but you can do it, as well have. (its a ballence thing isnt it, 2'10" is deep enough for now)

- But yeah, talking to crowthers is never abad idea, our prop is a crowthers, and after doin a fair bit of his own calulations/drawings/research my grandad ran his proposal past them, and we've not had a urge to try a diffrent prop yet.

 

I was talking to the owner of Pavo last weekend, and they have a 34*21" prop or something on that, so anythings possable!

 

 

 

 

Because pictures might paint a few words....

 

 

 

 

 

 

Daniel

[Proper Job]

Thanks to all of those who have replied.

 

I will contact Crowthers in due course, I just wanted to get my head around it before I did.

 

A couple more questions around the subject though:

 

Someone (outside the forum) told me that the key thing was to not exceed 70% of the prop blade area v's 'disc' area of the prop diameter. The lower the percentage the more efficient the prop.

Thus Ailsa Craig states 28" with a blade area of 270 sq in.

28" diameter gives disc area of 616 sq in = 44%

Assuming the blade area remains constant at 270 sq in

26" dia = 531 sq in = 51%

24" dia = 452 sq in = 60%

22" dia = 380 sq in = 70%

etc, etc

 

Does that mean that I could run a 22" prop if the blade area is still at 270 sq in. Or is the above total b#####ks?

 

Another question:

2" propshafts and stern gear is relitively quite expensive. 2" for 40hp seems very high.

By reducing the diameter of the prop will the imposed torque on the shaft exerted by the prop reduce (as torque is force x radius) and will that mean that a lesser diameter (cheaper) shaft/gear gear will be required?

 

I'm not sure that removing the 2:1 reduction gear will be possible without chaging the whole gear box. Besides, I thought that the slower a prop rotated, the more efficient it was?

 

Any thought you technical bod's?

[John Orentas]

Hi Steve.

 

Can't see that 2 inch dia stern-gear will be much of an advantage and it certainly will have no effect on the dynamics, probably the only sound reason is 'keeping it traditional'. Yes up to a point for propellers the slower the better is quite valid and no doubt there will be some truth in that 70% rule but I have never heard that one before, but even with a 1:1 gearbox your prop will still be running far slower that that of a modern engine with a 2.5 : 1 box. But you have to start from where you are.

 

There are as many 'Tap room experts' on this subject as any other, I am one myself. At some stage you will probably be putting yourself in the hands of Crowthers, if I was in your position I would do that sooner rather than later. I would also offer a 10% deposit on the prop you intend to order, as someone said he does get plagued by people exploiting his knowledge and spending their money somewhere else.

[Timleech]

Another question:

2" propshafts and stern gear is relitively quite expensive. 2" for 40hp seems very high.

By reducing the diameter of the prop will the imposed torque on the shaft exerted by the prop reduce (as torque is force x radius) and will that mean that a lesser diameter (cheaper) shaft/gear gear will be required?

 

I'm not sure that removing the 2:1 reduction gear will be possible without chaging the whole gear box. Besides, I thought that the slower a prop rotated, the more efficient it was?

 

Any thought you technical bod's?

 

It's not just the hp which determines what shaft size is needed, it's the torque. hp = torque x rpm, so the lower the rpm for a given hp, the higher the torque. Working boats with engines around the 20hp mark with props turning at about 6oo rpm used shafts of 1 3/4" to 2 1/4".

I wouldn't like to go smaller than 2" with your proposed setup. 1 3/4" might be OK, but it's not a common size now.

If your prop is matched to the engine, the torque load will be the same whatever the prop looks like.

 

Yes, in general terms, the slower the prop the more efficient - especially for starting & stopping.

 

Tim

[John Orentas]

Tim.

 

I have seen many sea boats with a 150 bhp engine with what looks like a 0.75" dia prop shaft. Trawlers have shafts of 3" dia.

Edited July 31, 2007 by John Orentas

[Timleech]

Tim.

 

I have seen many sea boats with a 150 bhp engine with what looks like a 0.75" dia prop shaft.

 

 

 

I'd like to

(i) see evidence for that, and

(ii) know what the actual hp & rpm are

(iii) know how many hours they are expected to last for.

 

The inland waterways environment is known to be much harsher on sterngear than salt water. (abrasives in the water, propellor impacts, ice etc). Ice can be a real killer on inadequate sterngear.

I wouldn't want to put anything much bigger than a daisy on a 3/4" shaft on a canal boat

 

Tim

[DHutch]

A higher blade area helps reduce prop slip, which pitchy small dia propellers are pron to. Hence i beleave the tendancy to increase the dar quite a bit as you approch square/oversquare.

- I dont know the exact figure, and for some reason i seam no to have a photo from directly behind the prop. But we be getting on for 80% area. Certainly you can even see from the side shot that theres some over lap on the blades. Might even be into the 90's.

 

The main reason for our propshaft being 2" is becuase its 10ft long, and the prop is our only 'flywheel', so having it fairly stiff is a good thing.

- Im sure you could get away with 1.5" shaft, for a while, maybe longer. But at the same time, why risk it when 2" shaflts are avalable and nearly as easy as anything else.

- A 26" prop isnt small, and you never know whats going to happen down there. Only last year i was whacking our prop sideways with a lumphammer. Got a 3inch block of wood inbetween the counter and the prop (about 2inchs) running at about 200rpm out of a lock. The prop had stuck deep into the block, traping it pritty much TDC. Only way to get it out was to hammer in further onto the prop, and then rock the shaft back and hammer it off. I was fairly glad the propshaft wasnt 3/4", lol!!

 

 

Daniel

[John Orentas]

I'd like to

(i) see evidence for that, and

(ii) know what the actual hp & rpm are

(iii) know how many hours they are expected to last for.

 

I wouldn't want to put anything much bigger than a daisy on a 3/4" shaft on a canal boat

 

Tim

 

 

OK there was some exageration involved with the 0.75 dia prop shaft. I was simply making the point that a 1.5" prop shaft with a canal boat engine is already grossly over engineered and you do see some frighteningly slim shafts on sea boats. The torque on my gearbox output is in the region of 250 Nm which is very small in the scheme of things.

 

What size shaft passes through the clutch on your average 150 bhp family car no more than an inch or so. The output shaft on a 400 hp truck engine is no more than 1.5 inch dia.

Edited July 31, 2007 by John Orentas

[Timleech]

OK there was some exageration involved with the 0.75 dia prop shaft. I was simply making the point that a 1.5" prop shaft with a canal boat engine is already grossly over engineered

 

I honestly don't agree there, John. It's not over-engineered for the job it has to do, in the environment in which it does it.

 

 

What size shaft passes through the clutch on your average 150 bhp family car no more than an inch or so. The output shaft on a 400 hp truck engine is no more than 1.5 inch dia.

 

That's not really relevant, it's a totally different situation, plus those gearbox shafts will be special steels.

A boat propellor shaft has long unsupported lengths, is often made from the cheapest stainless that money can buy (though it shouldn't be ) and has to be able to cope with all sorts of abuse which the wagon engine shaft would never see or cope with.

 

Tim

[Proper Job]

Dear All

 

Some interesting feedback fro Bruntons propellers. They don't make anything under 30" diameter but still ran the calcs based on all of the info I gave them. ie boat design, displacement etc etc. I did say that I would like to try to restrict the diameter to around 24".

 

There response is below

 

"I have had a quick look at the sizing, and got a similar result to Ailsa Craig at 28", ie 28 x 21, 3 blades. However the optimum diameter is around 30". A 24" propeller will not be particularly efficient becuase of the diameter restriction (a 24" would require around 27-28" of pitch). If you are restricting to 24" I think a four blader would be best, as the boat is so heavy it would reduce cavitation when manoeuvring etc. A typical blade area ratio for a standard four blader is approximately 0.70 (70%)."

 

 

Does anyone run with a 4 blader?

[Liam]

Does anyone run with a 4 blader?

 

The only four bladed prop I've seen is on the Leeds Liverpool Longboat, Ambush, and that seems to go ok...

[John Orentas]

I honestly don't agree there, John. It's not over-engineered for the job it has to do, in the environment in which it does it.

That's not really relevant, it's a totally different situation, plus those gearbox shafts will be special steels.

A boat propeller shaft has long unsupported lengths, is often made from the cheapest stainless that money can buy (though it shouldn't be ) and has to be able to cope with all sorts of abuse which the wagon engine shaft would never see or cope with.

 

Tim

 

 

Such question are always impossible to reply to, if someone is saying that a thick one is stronger that a thin one well yes of course it is, however I have yet to see a broken shaft, regardless of the material it was made from, that fact in itself must say something.

 

I sometimes think we have lost our way a little with all this prop-shaft business, there was a time when engines where simply mounted on oak bearers which though not allowing any real movement of the engine did insulate the boats hull from vibration through a wide frequency band, combined with this, a relatively long thin prop-shaft which allowed a bit of flexibility, things were isolated even more.

 

Then came the requirement to mount the engine as far back in the boat as possible requiring ever shorter shafts and the easy to use soft engine mounts began to be used almost universally (and often wrongly). The story was always that more modern higher revving engines needed them. I am not so sure, has anyone seriously tried mounting a modern engine in the old fashioned way, I suspect they probably haven't.

 

Although the manufacturers of these mounts claim that they absorb a lot of energy, I have never been convinced that they do, I have used them in other industries and have always suspected they simply pass energy on and change it's frequency like a spring.

[Proper Job]

Although the manufacturers of these mounts claim that they absorb a lot of energy, I have never been convinced that they do, I have used them in other industries and have always suspected they simply pass energy on and change it's frequency like a spring.

 

A good point. I seem to remember something about 'energy cannot be created of destroyed - its form can only be changed'.

 

A modern engine mount can only change the frequency to that of a less annoying/detrimental one?

 

Before we get to carried away on the prop shaft diameter - I think I'll stick with the 2". I have previously destroyed a prm gearbox by hitting a tree stump (1500 rpm to 0 in 5 nano seconds) and ridden over a submerged car whilst using the prop to open up the roof! The 'ting - ting - ting' sound it made was most disconcerting!.

 

I've not heard of anyone breaking a propshaft, but I have heard of them being bent.

[Rayxt]

"If you are restricting to 24" I think a four blader would be best, as the boat is so heavy it would reduce cavitation when manoeuvring etc. A typical blade area ratio for a standard four blader is approximately 0.70 (70%)."

 

 

With lots more widebeam canal boats being built, weighing 25-30+ tons, and builders only too willing to sell 60-80HP engines (for Continental cruising they say) maybe 4-bladed 20" props (or larger) are the way to go on these boats to handle the power but keep drafts down for UK waterways.

 

Has anyone asked Crowthers about 'small' diameter 4-bladed props for 60HP+ engines?

 

Rayxt

[Theo]

Such question are always impossible to reply to, if someone is saying that a thick one is stronger that a thin one well yes of course it is, however I have yet to see a broken shaft, regardless of the material it was made from, that fact in itself must say something.

 

I sometimes think we have lost our way a little with all this prop-shaft business, there was a time when engines where simply mounted on oak bearers which though not allowing any real movement of the engine did insulate the boats hull from vibration through a wide frequency band, combined with this, a relatively long thin prop-shaft which allowed a bit of flexibility, things were isolated even more.

 

Then came the requirement to mount the engine as far back in the boat as possible requiring ever shorter shafts and the easy to use soft engine mounts began to be used almost universally (and often wrongly). The story was always that more modern higher revving engines needed them. I am not so sure, has anyone seriously tried mounting a modern engine in the old fashioned way, I suspect they probably haven't.

 

Although the manufacturers of these mounts claim that they absorb a lot of energy, I have never been convinced that they do, I have used them in other industries and have always suspected they simply pass energy on and change it's frequency like a spring.

 

 

Theodora's BMC 1.5 is mounted on wooden bearers. I rather think that they are softwood! Is 1983 modern enough for you, John?

 

Nick

[hughc]

Although the manufacturers of these mounts claim that they absorb a lot of energy, I have never been convinced that they do, I have used them in other industries and have always suspected they simply pass energy on and change it's frequency like a spring.

A good point. I seem to remember something about 'energy cannot be created of destroyed - its form can only be changed'.

 

A modern engine mount can only change the frequency to that of a less annoying/detrimental one?

 

We have had some success mounting problem engines on Tico pads. These are sold by James Walker and Co. and completely insulate the engine from the substrate. Tico is compressed cork bonded with some sort of resin and has even solved the mounting problems with a BSD 3 Ford which had eaten a number of high tec flexi mounts from R & D. Regards, H.C.