Gearbox ratio and propeller pitch

[DHutch]

In many waterways a boat with a 24" prop would be called a plough! A modern(!) canal is a shallow waterway with no room for deep draught boats with big props. As the mud comes up something has to go and it's now usually boat draught and prop size.

 

We have no such issues with our 26 prop, have covered almost all of the UK waterways including many of the shallower canals. Ashby, Macc, Llangollen, etc.

[Liam]

What's a modern canal? I thought that they all average around 200 years old, some even more. Not very modern!

[blackrose]

No I disagree. That is how a screw works when screwed into a solid thing like a lump of wood, but a propellor is moving in a fluid, which can result in all sorts of strange effects like cavitation, stalling, recirculation, vortexing etc.

And prop slip.

 

I noticed a slight reduction in stopping power after my prop repitch but I just use a bit more revs now. Starting power isn't really an issue.

[DHutch]

What's a modern canal? I thought that they all average around 200 years old, some even more. Not very modern!

 

Ribble link?

 

- We got up that too....

 

 

 

Daniel

[mick55]

We can theorise about props until the cows come home, best way to size a prop to engine and boat is to ask people what they have got and does it work well?

The other way is to ask Crowther Marine to supply the prop and then you know it is going to perform, had Crowther props on all my boats in the past and they went superbly.

[Dalslandia]

You got that right, experience talks, it is when go outside that knowledge, theory come in, if then theory goes hand in hand with experience it usually get right, but with all theory and experience in the world, sometimes it does not get right the first time. that ad to the experience too.

 

There are so many variables, like real engine power, depending on the setup, exhaust system, alternators ... then boat shape and deplacement, and not the least the owner/user demand, and preference.

 

We can theorise about props until the cows come home, best way to size a prop to engine and boat is to ask people what they have got and does it work well?

The other way is to ask Crowther Marine to supply the prop and then you know it is going to perform, had Crowther props on all my boats in the past and they went superbly.

 

[Jeffers71]

reading previous comments is a 4 blade propeller more efficient than a 3 blade propeller then? also has anyone got one of the axiom propellers are they any better?

[Tony Dunkley]

reading previous comments is a 4 blade propeller more efficient than a 3 blade propeller then? also has anyone got one of the axiom propellers are they any better?

 

No, 4 x bladed propellers are less efficient than 3 x bladers, because each blade is closer to, and working in, the disturbed water left by the preceding blade.

For the same reason, 2 x bladed props are more efficient than 3's, and although the resulting vibration would be self destructing and impossible to live with, the most efficient propeller of all would be single bladed.

I don't buy any of the sales patter, or claimed performance/efficiency improvements for Axiom propellers.

Any perceived improvements are simply down to the huge increase in pitch towards each blade tip, but the overall efficiency must be much less than a properly designed propeller with the same constant pitch from blade root to tip.

[Jeffers71]

 

No, 4 x bladed propellers are less efficient than 3 x bladers, because each blade is closer to, and working in, the disturbed water left by the preceding blade.

For the same reason, 2 x bladed props are more efficient than 3's, and although the resulting vibration would be self destructing and impossible to live with, the most efficient propeller of all would be single bladed.

I don't buy any of the sales patter, or claimed performance/efficiency improvements for Axiom propellers.

Any perceived improvements are simply down to the huge increase in pitch towards each blade tip, but the overall efficiency must be much less than a properly designed propeller with the same constant pitch from blade root to tip.

i didn't see how they could work they didn't look very propeller like if you know what i mean

[Tony Dunkley]

i didn't see how they could work they didn't look very propeller like if you know what i mean

 

I do know just what you mean, . . . . they look to me like an enlarged version of something you'd find in a kitchen food blender.

The apparent improvements that some have reported in boat performance could be better achieved by compensating for the poor, and slower inflow of water close to the propeller boss on most narrowboats, by means of an increase in pitch towards the tips and a decrease towards the roots of the blades of a conventional propeller.

From what I've heard, they're ridiculously expensive too.

Edited December 15, 2015 by Tony Dunkley

[Dalslandia]

 

No, 4 x bladed propellers are less efficient than 3 x bladers, because each blade is closer to, and working in, the disturbed water left by the preceding blade.

For the same reason, 2 x bladed props are more efficient than 3's, and although the resulting vibration would be self destructing and impossible to live with, the most efficient propeller of all would be single bladed.

I don't buy any of the sales patter, or claimed performance/efficiency improvements for Axiom propellers.

Any perceived improvements are simply down to the huge increase in pitch towards each blade tip, but the overall efficiency must be much less than a properly designed propeller with the same constant pitch from blade root to tip.

 

To Tonys comments, I would like to add that Tony is right.

 

the difference going from 3 to 4 or 5 blades is that the vibration mode can change, with a hull that have two dead water areas, above and below shaft, having an even number of blades should make more vibrations, when two blades enter that area at the same time. but even so, more blades even out the vibrations and load per blade.

 

The Axiom works, it push the boat forward, if it does that better then other brand have not been seen in tests. with its non twisted blade I can't see it will be efficient and that is what it is all about to convert engine power to thrust power. The Axiom can be good if that lack of twist fits the hulls slowdown of water profile. but the planform left things to wish for

Edited December 15, 2015 by Dalslandia

[Neil Smith]

I have read from people that have axioms is when they swopped from normal props of various makes that the stopping and reversing wat vastly improved but forward motion was reduced a bit but not much, but there have been posts on this before with for and against arguments but I got one and will report my findings when she is finished next summer.

 

Neil

[Dalslandia]

I have read from people that have axioms is when they swopped from normal props of various makes that the stopping and reversing wat vastly improved but forward motion was reduced a bit but not much, but there have been posts on this before with for and against arguments but I got one and will report my findings when she is finished next summer.

 

Neil

 

That's naturally so, normal? props have a suction and a pressure side, making it more efficient in one direction, the Axiom is equally shaped on both sides/ends.

[Graham.m]

My boat has a Lister HR2M max revs order of 2300rpm and a 2:1 gearbox and the hull swims well.

 

Ever since I have had the boat the engine seemed laboured and under revving. Additionally the prop-walk was worse than on any boat I have ever owned. At cruising revs for example reduce the revs to tick over and the boat dived heavily to port. Astern the prop-walk was enough to walk the stern into a linear mooring. Going astern was a nightmare.

 

The vibration was so bad that I had and checked the prop, there was no damage and on removal the balance was good and no distortion of the blades. The boatyard advice was it was the right prop for the boat and engine. But something was not right. Prop-shaft was free and smooth, none of the bearings catching on rotation, the same applied to the joints.

 

Looking round the prop manufacturers I came across Axiom, went to the Crick show and looked at their prop. As someone who flies for pleasure I have a little knowledge of aero dynamics and could understand the whys of their design and the tests done in the lab at Durham looked convincing, but it was expensive as most tailor made things are.

 

Two years later I ordered a prop. The existing one was a standard 20 inch prop, which rotated with less than ½ inch clearance of the Uxter plate and similar at the strake. I gave Axiom a free hand, just asking for a prop for a 50 foot NB weight 15 tons and max speed target of 6 knots in open water.

 

They asked me to do some test with the existing speed v revs. These were done on the GU between Braunston and Napton

 

Tick over under load. 400 rpm 1 mph

680 rpm 2 mph
1226 rpm 4 mph

 

Maximum revs achieved at full throttle were 1450 rpm with no further increase in speed but a large increase in wash. I suspect that (shall we say) canal effect had a bearing on this.

 

Their recommendation was an 18-inch 22-degree angle prop. This was fitted recently and on the return trip from the boat yard the engine was revving more freely and the speed v throttle setting was the same as during the tests. Unfortunately the boat does not have a rev counter fitted so revs will have to wait until I am not single-handed.

 

Vibration is decrease, speed per throttle setting was similar, engine was not labouring and the wash was down. At full throttle the engine was still running freely and wash was a lot better than it had been. Astern she straight and responding to the helm better.

 

The prop we took off turn out to be a 20 x16.

 

Now to wait for some good weather and get out the handheld rev counter to do some tests with the Axiom. To date it is looking good.

Edited December 16, 2015 by Graham.m

[nicknorman]

My boat has a Lister HR2M max revs order of 2300rpm and a 2:1 gearbox and the hull swims well.

 

Ever since I have had the boat the engine seemed laboured and under revving. Additionally the prop-walk was worse than on any boat I have ever owned. At cruising revs for example reduce the revs to tick over and the boat dived heavily to port. Astern the prop-walk was enough to walk the stern into a linear mooring. Going astern was a nightmare.

 

The vibration was so bad that I had and checked the prop, there was no damage and on removal the balance was good and no distortion of the blades. The boatyard advice was it was the right prop for the boat and engine. But something was not right. Prop-shaft was free and smooth, none of the bearings catching on rotation, the same applied to the joints.

 

Looking round the prop manufacturers I came across Axiom, went to the Crick show and looked at their prop. As someone who flies for pleasure I have a little knowledge of aero dynamics and could understand the whys of their design and the tests done in the lab at Durham looked convincing, but it was expensive as most tailor made things are.

 

Two years later I ordered a prop. The existing one was a standard 20 inch prop, which rotated with less than ½ inch clearance of the Uxter plate and similar at the strake. I gave Axiom a free hand, just asking for a prop for a 50 foot NB weight 15 tons and max speed target of 6 knots in open water.

 

They asked me to do some test with the existing speed v revs. These were done on the GU between Braunston and Napton

 

Tick over under load. 400 rpm 1 mph

680 rpm 2 mph 1226 rpm 4 mph

 

Maximum revs achieved at full throttle were 1450 rpm with no further increase in speed but a large increase in wash. I suspect that (shall we say) canal effect had a bearing on this.

 

Their recommendation was an 18-inch 22-degree angle prop. This was fitted recently and on the return trip from the boat yard the engine was revving more freely and the speed v throttle setting was the same as during the tests. Unfortunately the boat does not have a rev counter fitted so revs will have to wait until I am not single-handed.

 

Vibration is decrease, speed per throttle setting was similar, engine was not labouring and the wash was down. At full throttle the engine was still running freely and wash was a lot better than it had been. Astern she straight and responding to the helm better.

 

The prop we took off turn out to be a 20 x16.

 

Now to wait for some good weather and get out the handheld rev counter to do some tests with the Axiom. To date it is looking good.

I'm glad you are pleased with the Axiom and thanks for the interesting report. However I suspect it may be typical of "positive" reports on Axioms in that you have effectively replaced a wrong sized propellor (diameter too big and too coarse) with one that is correctly sized. I think we can safely put the vibration issue down to insufficient clearance between prop and uxter, and the labouring engine and low revs down to being too coarse. I'm not sure whether or not we can put the excessive prop walk down to insufficient clearance too, but I suspect at least partly.

 

I suppose what I am saying that to be a categoric endorsement of Axioms one would have to replace a correctly sized normal prop with an Axiom and then identify any improvements, but folk rarely seem to do this since folk with correctly sized props don't have any motivation to change.

[Graham.m]

I'm glad you are pleased with the Axiom and thanks for the interesting report. However I suspect it may be typical of "positive" reports on Axioms in that you have effectively replaced a wrong sized propellor (diameter too big and too coarse) with one that is correctly sized. I think we can safely put the vibration issue down to insufficient clearance between prop and uxter, and the labouring engine and low revs down to being too coarse. I'm not sure whether or not we can put the excessive prop walk down to insufficient clearance too, but I suspect at least partly.

 

I suppose what I am saying that to be a categoric endorsement of Axioms one would have to replace a correctly sized normal prop with an Axiom and then identify any improvements, but folk rarely seem to do this since folk with correctly sized props don't have any motivation to change.

 

I think you missed the bit "The boatyard advice was it was the right prop for the boat and engine. " Now as they are one of the best boatyards on the canals with qualified staff, are you suggesting they do not know their job, may I suggest you think carefully as many people know where my boat goes for work. Or could it be that you know everything about everything.

 

Oh I could give you some design reason why the Axiom could be more effective. But as I don't wish to get into a debate with you I shall not.

Edited December 16, 2015 by Graham.m

[nicknorman]

I think you missed the bit "The boatyard advice was it was the right prop for the boat and engine. " Now as they are one of the best boatyards on the canals with qualified staff, are you suggesting they do not know their job, may I suggest you think carefully as many people know where my boat goes for work. Or could it be that you know everything about everything.

 

Oh I could give you some design reason why the Axiom could be more effective. But as I don't wish to get into a debate with you I shall not.

I'm sorry you can't discuss it pleasantly, despite my attempt to be pleasant to you. I know very little about propellers except for what I have gleaned from this site. However I believe the consensus is that 1/2" clearance between prop and uxter is insufficient and likely to cause vibration. 2" is the norm. Also that an engine that fails to get anywhere near max rpm is over-propped. I really don't care which boatyard you are referring to nor see why I should "be careful" since you have already made the case that the previous propellor was too big and too coarse, so it's clear to everyone that the boatyard had got it wrong unless they have a preference for oversized and over propped. Edited December 16, 2015 by nicknorman

[Bee]

Propellers are aerofoils. Modern planes have 'winglets' on the wing tips to stop the low pressure air rolling off the tips, if I had a steel propeller I'd be interested in welding winglets to the tips to do the same. Should I patent this right away or just have another bit of toast and forget about it?

[Graham.m]

Propellers are aerofoils. Modern planes have 'winglets' on the wing tips to stop the low pressure air rolling off the tips, if I had a steel propeller I'd be interested in welding winglets to the tips to do the same. Should I patent this right away or just have another bit of toast and forget about it?

 

nar have another piece of toast The cost of proving and development would be an arm and a leg. I suspect the winglets was patented years ago anyway. lol Thanks for the smile on my face

[Graham.m]

I'm sorry you can't discuss it pleasantly, despite my attempt to be pleasant to you. I know very little about propellers except for what I have gleaned from this site. However I believe the consensus is that 1/2" clearance between prop and uxter is insufficient and likely to cause vibration. 2" is the norm. Also that an engine that fails to get anywhere near max rpm is over-propped. I really don't care which boatyard you are referring to nor see why I should "be careful" since you have already made the case that the previous propellor was too big and too coarse, so it's clear to everyone that the boatyard had got it wrong unless they have a preference for oversized and over propped.

 

Actually I have not made the case as you write. I have totally change to a prop of a totally different design and concept from the standard prop so simple comparison is difficult or impossible. Another manufacturer came up with a standard design prop of 20/16 so unlikely two experts are wrong. But hey what do I know only been boating for over 60 years and fitted umpteen props on boats. I have my own opinion on the why, but will hold that until I have done a lot of testing.

As for you attempting to be pleasant leopards don't change their spots; I tried to open communications with you by PM with no positive result.

[nicknorman]

Actually I have not made the case as you write. I have totally change to a prop of a totally different design and concept from the standard prop so simple comparison is difficult or impossible. Another manufacturer came up with a standard design prop of 20/16 so unlikely two experts are wrong. But hey what do I know only been boating for over 60 years and fitted umpteen props on boats. I have my own opinion on the why, but will hold that until I have done a lot of testing.

 

As for you attempting to be pleasant leopards don't change their spots; I tried to open communications with you by PM with no positive result.

So you would maintain that a prop with 1/2" clearance and that limits a 2200 rpm engine to 1400 rpm is correctly sized? Interesting! All I can suggest is that you chose the wrong "experts".

[Graham.m]

So you would maintain that a prop with 1/2" clearance and that limits a 2200 rpm engine to 1400 rpm is correctly sized? Interesting! All I can suggest is that you chose the wrong "experts".

I think you see threats in even the simplest comment about something and now you can't even keep a PM private.

 

For anyone who wonders the remarks in the PM were about a different world, different age, and country and no threat implied or actual it was a description of life then at that time of the beauty and the ugly.

 

I apologies to the OP for this deviation

[mrsmelly]

Prop sizes mmmmmmmmmm the 18 x 12 with a modern 40 ish engine is fitted with the usual 150 gearbox etc etc quite simply because it works well with the average steel bath tub that most forum members drive. I have had this configuration on several boats which includes my present 68 foot Colecraft, a previous 70 foot Hudson and a57 foot Pinder to mention just three. So I see it as a case of it it aint broke it dont need fixing. As for axiom props my bruvver in law had one fitted at silly money he says it is certainly is better than his previous standard prop but that was a crap one and he believes it is probably no better than a Crowther which would have been cheaper......just his opinion, we all have them.

 

Tim

[ditchcrawler]

There are some reports of one owners experience with the Axiom prop

 

http://noproblem.org.uk/blog/axiom-propeller/

http://noproblem.org.uk/blog/axiom-propeller-mk2-at-streethay/

http://noproblem.org.uk/blog/the-spring-cruise-continuous-at-last/

[DHutch]

Axiom appear good at specing a suitable size and pitch, and the blades appear to work well in reverse and fine in forwards, and I expect for most boats out and out efficiency is fairly none critical as well has hard to get a good measure.

 

Another big name in the industry is obviously Crowther, who use more conventional designs, are very good at specing suitable pitch and size, and produce a very solid prop which suits canal applications, etc.

 

There are lots of other prop suppliers and manufactures, some of which also give advice on what to fit.

 

 

The point raised about narrowboats having poor flow to the centre of the prop is an interesting one but while the distance to the swim is often compromised this is not unique to our boats, nor confined to the boss area.

 

 

Graham is obviously happy with his new prop, but like Nick I would expect he would be equally happy if he had gone through the same process with Crowther.

If money where no object it would be very interesting do the test, and similarly to fit an Axiom to EmilyAnne, but as money is an object and bith now have satisfactory props I expect we will never know!

 

Daniel