Props & engines

[nb Ditchley]

I am currently mid build with Dave Harris on our 62ft trad boat. We have a JD3 and intend to fit an Axiom prop. I am slightly concerned that the Axion prop efficiency might cause issues with our being able to run dead slow, I do not want to be disturbing snoozing boaters at moorings as I thunder past. Probably worrying about nothing but does anyone have any experience of this combination?

[RLWP]

Wow - what has Dave got to say about that!

 

Richard

[ditchcrawler]

Why not ask the makers what the minimum speed will be with the engine doing 400 rpm times whatever gearbox ratio you are using.

[nicknorman]

Bearing in mind a JD3 at tickover can't produce much power, I very much doubt it will be an issue for causing undue wash or disturbance regardless of the prop. Anyway I think the efficiency of an axiom in fwd may be less than a good conventional prop since the design is shifted slightly more towards stopping power than is normal.

 

The absolute speed of the boat is not relevant, it is how much the water is disturbed and the power of that is linked to the engine power factored for prop efficiency.

 

Anyway, the standard repost is "tie your boat up properly then" or (viz the item in the recent CRT Boaters Update) slip the dog some tranquillisers.

Edited October 10, 2014 by nicknorman

[dmr]

I suspect Nick is right (again) here, props have optimised over many years, Axiom have created nothing new, they have just shifted this optimisation in favour of stopping power for marketing purposes.

We have a JD3 and 21 by 20 prop (not Axiom) and are just a little over-propped.

We go just a tiny bit faster at "tickover" than I might like, but not enough to worry about. We cant really go at tickover but need about 550 rpm, the whole drivetrain goes horribly unstable at tickover (390-450) and I think this is not uncommon, the JD3 is not really designed for very low speeds, it ain't got a huge flywheel like a vintage engine.

What size prop are you fitting????

I don't know if an Axiom would help or hinder this issue (I suspect it will hinder) but do you want to invest your money finding out???

 

..............Dave

[Dalslandia]

By just looking at the Axiom, we can see it will be just as good in reverse as in forward, looks to be perfect for a bow thrust propeller, the blade layout and the twist (difference from tip to hub) isn't optimal for going forward so I suspect it is not as efficient as other normal good matched propeller will be, all that use them say they are good for stopping, and I can see why with the Leading edge (and trailing edge) dropped nose airfoil, the top end efficiency will not shine, But I am not convinced it is bad at dead slow speed, but I think some one said they had to run the engine little faster then tick over to get the same speed as before, (if I remember it correct)

It will be good for acceleration (both ways)

The dropped LE is good for incoming water at big angles, IE acceleration, the other LE that sometimes is a TE will be counter productive, with the end of the S bent the wrong way.

Honest, I have never used one or seen one in the brass, but I designed one similar for a bow thruster

Edited October 11, 2014 by Dalslandia

[Tony Dunkley]

By just looking at the Axiom, we can see it will be just as good in reverse as in forward, looks to be perfect for a bow thrust propeller, the blade layout and the twist (difference from tip to hub) isn't optimal for going forward so I suspect it is not as efficient as other normal good matched propeller will be, all that use them say they are good for stopping, and I can see why with the Leading edge (and trailing edge) dropped nose airfoil, the top end efficiency will not shine, But I am not convinced it is bad at dead slow speed, but I think some one said they had to run the engine little faster then tick over to get the same speed as before, (if I remember it correct)

It will be good for acceleration (both ways)

The dropped LE is good for incoming water at big angles, IE acceleration, the other LE that sometimes is a TE will be counter productive, with the end of the S bent the wrong way.

Honest, I have never used one or seen one in the brass, but I designed one similar for a bow thruster

Looking at one of these on a boat, the blades were flat with no twist from root to tip. Surely this increase in pitch towards the blade tips is not going to be very efficient. The only, and questionable, advantage I could see over a conventional constant pitch design of blade would be the identical performance both ahead and astern.

Can you estimate the efficiency drop for the pitch increase at the blade tips?

Edited October 11, 2014 by tony dunkley

[Dalslandia]

Looking at one of these on a boat, the blades were flat with no twist from root to tip. Surely this increase in pitch towards the blade tips is not going to be very efficient. The only, and questionable, advantage I could see over a conventional constant pitch design of blade would be the identical performance both ahead and astern.

Yes, correct, the lack of twist is not good, the lack of calculated chord lengths is not good, I think the lack of twist make the inner parts non efficient, and can even make negative thrust, the 70-75% radii is average on propellers, so out side that the pitch is (and area) way to big, meaning something ells need to be smaller to absorb the right amount of power from engine. it is a difference in absorbing power and making something useful with it, like thrust

[Tony Dunkley]

Yes, correct, the lack of twist is not good, the lack of calculated chord lengths is not good, I think the lack of twist make the inner parts non efficient, and can even make negative thrust, the 70-75% radii is average on propellers, so out side that the pitch is (and area) way to big, meaning something ells need to be smaller to absorb the right amount of power from engine. it is a difference in absorbing power and making something useful with it, like thrust

 

With a good portion of the blades near to the root doing nothing, the effective blade area will be reduced, and as you say, some of the incoming water stream may even be slowed down, wasting power. It looks to me as though the suppliers are selling bow thruster props for other than the purpose they were designed for. Even the profile of the tips looks to be designed to run with a very small clearance in a tube.

[dmr]

As my previous post was written under the influence of considerable alcohol (Wharf, Fenny Compton) I should perhaps clarify

A JD3 with a conventional prop (21 by 20) will go along very slowly at just above tickover, slower than many other boats.

I would sometimes like to go even slower but thats just me. It will also stop the boat really well with a blast of reverse so I can see no point in risking an Axiom prop.

The stopping power is limited only by the tendency to hoover up the bottom of the canal.

If you want to go really really slow then do some research amongst JD3 owners as to the best driveshaft layout.

 

.........Dave

[Dalslandia]

 

With a good portion of the blades near to the root doing nothing, the effective blade area will be reduced, and as you say, some of the incoming water stream may even be slowed down, wasting power. It looks to me as though the suppliers are selling bow thruster props for other than the purpose they were designed for. Even the profile of the tips looks to be designed to run with a very small clearance in a tube.

 

But we have to fill in, without measuring the inner pitch and knowing the actual average water velocity close to the hub we don't know, normally the water speed here is 50-80% of the boat forward speed, with the propeller induced water velocity.

 

but even if 50% pitch the angle at 25% radii is, on a 20"X20" propeller to pick one.

 

100% radii 20" pitch 17,65 deg

75% radii 20" pitch 23deg

but 25% radii at 50% pitch=10" = 32,5 degree

Edited October 11, 2014 by Dalslandia

[bottle]

nb Ditchly

 

Welcome to the forum.

 

The Axiom has been mentioned many times on this forum and it always get a bad press.

 

I have no experience of the Axiom prop but would not fit one.

 

Have a discussion with your boat builder and Crowther and get a better prop (in my opinion) for less money.

 

 

ps. Crowther's site is not the best but they know props and are very helpful.

 

Had recent communication with them and after all there help, they advised me not to change my prop, so losing them some business, what other company would do that.

[dmr]

I am not any kind of prop expert, just a "jack of all trades" engineer, but my understanding is that the prop needs to be sized to transmit the full power of the engine to the water, that is to just let the engine get to its full power speed, which is usually its top speed. The minimum speed is then dictated by the tickover speed of the engine with this chosen prop. There is a bit of scope for under-propping and over-propping. This is why fitting huge engines is not a good idea?

What I don't know is if prop blade design, plus the balance of diameter and pitch, can vary the relationship between full speed and minimum speed performance......over to you Dalslandia.....

 

.............Dave

[Dalslandia]

I am not any kind of prop expert, just a "jack of all trades" engineer, but my understanding is that the prop needs to be sized to transmit the full power of the engine to the water, that is to just let the engine get to its full power speed, which is usually its top speed. The minimum speed is then dictated by the tickover speed of the engine with this chosen prop. There is a bit of scope for under-propping and over-propping. This is why fitting huge engines is not a good idea?

What I don't know is if prop blade design, plus the balance of diameter and pitch, can vary the relationship between full speed and minimum speed performance......over to you Dalslandia.....

 

.............Dave

 

What to say, you got it right, there is only one optimal propeller for every speed/power, but a change in diameter need a change in pitch to absorb the power, at top speed, the boat and propeller DEMAND is a cube factor, draw that up and it is a concave curve (simply said) but the full throttle power curve is somewhat convex. and if a matched prop is used they cross each other at top speed, well they always do, that propellers top speed.

 

if the propeller can absorb and make use of the power somewhere between the curves in cruise the engine and the propeller will be more efficient. you get that with over propping, but loosing some of the top speed, and might or might not go to fast on tick over. and a coastal boat might need to be slightly under propped to go well in the waves or when towing.

 

Top speed is not the number one issue on the canals

[nb Innisfree]

As I basically understand it, blade area transmits power and pitch is the 'gearing' i.e. for a more powerful engine the blade area must be increased, preferably by larger dia. if not by wider blades. The pitch dictates the speed and acceleration at tickover. Both interact with each other so it's a bit more complicated than that in practice.

[Dalslandia]

As I basically understand it, blade area transmits power and pitch is the 'gearing' i.e. for a more powerful engine the blade area must be increased, preferably by larger dia. if not by wider blades. The pitch dictates the speed and acceleration at tickover. Both interact with each other so it's a bit more complicated than that in practice.

Simply said the pitch is the speed divided with the propeller rpm, but to make it more complicated boat props have some 30-60% "prop wash"

Half of the prop wash speed increase over forward speed is at the propeller and the other half far behind the propeller.

The incoming air, ah, sorry water, is slowed down by the hull and structure, mostly behind the swim or keel... so the water speed is everything then a steady stream.

to get it all right it takes CFD and tow tank tests, and a very very good propeller software.

 

the cost for this work is soon paid for in lower fuel costs with increase in a optimal propellers efficiency, on large ships, but for more mortal canal boaters, experience from test of series of propellers is good enough, and the experience of people having picked and sold a lot of propellers is gold.

 

A picture say more then 1000 posts, http://web.mit.edu/13.012/www/handouts/PROPELLERSslides.pdf

 

we see on page 2 an old standard layout of propeller with pitch reduced to 80% at the hub, and is then reduced from 70-80% radii to the tip

Edited October 11, 2014 by Dalslandia

[nb Ditchley]

Thank you everyone for your extensive and clearly knowledgable advice, much to ponder on.

[ditchcrawler]

Thank you everyone for your extensive and clearly knowledgable advice, much to ponder on.

Not a cheap prop

[Tonka]

Why don't you contact Wrigglefingers" who is a member of this forum and has a Dave Harris 62ft boat with a JD3 installed. Her boat was moored in Braunston last Wednesday

[RLWP]

Cobbett (wrigglefingers boat) has a conventional prop, and is very easy to handle.

 

Richard

[nicknorman]

On props in general, having gone from a fairly shallow drafted boat to a much deeper one consequentially swinging a well submerged and much bigger diameter prop, the difference was huge especially in terms of stopping power. As far as I know we don't have a Crowther but it seems a good prop none the less.

 

So, depending on what you are used to, you will no doubt find that you new boat with an Axiom is very good. But the unanswered question would be whether it would have been just as good with a much cheaper "normal" propellor, and/or even better with a Crowther.

[ditchcrawler]

I know two people who have tried them and then had them removed, her is one

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

Edit

This is mid way through the story

Edited October 12, 2014 by ditchcrawler

[RLWP]

But the unanswered question would be whether it would have been just as good with a much cheaper "normal" propellor, and/or even better with a Crowther.

 

Not entirely unanswered - a Dave Harris boat with that engine and conventional prop works very well indeed. I'm not sure what the missing thing is that an Axiom prop would supply

 

Richard

[nb Ditchley]

Dear All

 

To the relief of many (most?) of you I am happy to confirm that Dave & I are going with a Crowther prop. Many less than positive stories about Axiom from friends and contacts outside this forum, particularly with low revving engines. Thanks again for all the combined wisdom. Phew, that was close.

 

John

[Proper Job]

On props in general, having gone from a fairly shallow drafted boat to a much deeper one consequentially swinging a well submerged and much bigger diameter prop, the difference was huge especially in terms of stopping power. As far as I know we don't have a Crowther but it seems a good prop none the less.

 

So, depending on what you are used to, you will no doubt find that you new boat with an Axiom is very good. But the unanswered question would be whether it would have been just as good with a much cheaper "normal" propellor, and/or even better with a Crowther.

 

A large diameter prop means a slower running speed which is more efficient for canal operation.

 

Ocelot will stop in its own length, but its throwing a 26 x 21 with a 36" draught (JP3 with 2:1 reduction)

Edited October 13, 2014 by Proper Job