Axiom Prop - again

[Tony Brooks]

I do not know if anyone else was disappointed in the recent magazine article on the Axiom prop. After what was said in the last thread I hoped to get something a bit more scientific than GPS speed readings.

 

Experience seems to show that there are loads of narrowboats with props that are not ideal so I just can not see how any comparison can be valid unless steps are taken to ensure the original prop is as near correct as possible.

 

I can not be too critical because unless I saw independent tank test data and was sure about the original prop I would have avoided doing a comparison. Just sorry we seem to be no further forward.

[Timleech]

I do not know if anyone else was disappointed in the recent magazine article on the Axiom prop. After what was said in the last thread I hoped to get something a bit more scientific than GPS speed readings.

 

Experience seems to show that there are loads of narrowboats with props that are not ideal so I just can not see how any comparison can be valid unless steps are taken to ensure the original prop is as near correct as possible.

 

I can not be too critical because unless I saw independent tank test data and was sure about the original prop I would have avoided doing a comparison. Just sorry we seem to be no further forward.

 

I found the photos in the WW piece to be underwhelming, and the speeds of motion at particular engine rpm were meaningless without fuel consumption data. Just fitting a bigger conventional prop would replicate some of the observed effects. The supposed lack of 'prop walk' is intriguing though.

 

Tim

[Troll]

Aixam

 

this is the only Aixam I would fancy, maybe it would pull a narrowboat rather than push

 

http://www.qdoscars.co.uk/megavan.htm

[Postcode]

Aixam

 

this is the only Aixam I would fancy, maybe it would pull a narrowboat rather than push

 

http://www.qdoscars.co.uk/megavan.htm

 

 

Hey thats my boat engine in that! I need to go check its still there! Thanks

[PeterF]

I do not know if anyone else was disappointed in the recent magazine article on the Axiom prop. After what was said in the last thread I hoped to get something a bit more scientific than GPS speed readings.

 

Experience seems to show that there are loads of narrowboats with props that are not ideal so I just can not see how any comparison can be valid unless steps are taken to ensure the original prop is as near correct as possible.

 

I can not be too critical because unless I saw independent tank test data and was sure about the original prop I would have avoided doing a comparison. Just sorry we seem to be no further forward.

Can't agree more, we had a post a few weeks ago discussing and recomending better prop for one member in which someone quoted reducing rpm by 25% to give same boat speed and more relaxed cruising by changing the prop.

 

Although the one thing that I did find interesting is the before photo showing the wake, in this photo the vortex shedding from the prop can be seen in the wake, just beyong the aerated part of the wash you can pick out the surface pattern of the wake going from side to side. I could scan and annotate the picture apart from copyright concern. If this was a truly technical article I am surpirsed that noone picked up on this obvious illustration of one of the inefficiencies of the previous prop. I do not know if this was because the inital prop was a poor design or what but I have not observed it on my boat before, I will look when I am out next. Certainly, the after picture did not show the same effect in the prop so maybe a PSB* reason for it being better.

 

* - PSB = Pseudo Scientific Bulls**t. As I am an engineer, the marketting people used to say, we do not want to get involved in the detail, just give us something impressive sounding and plausible for the customers, i.e. PSB.

 

PeterF

Edited November 19, 2008 by PeterF

[Amicus]

Mmmm, this afternoon I stood in W H Smiths and read this "technical assessment".

Amazing bit of kit, way it alters the hull form and hydro-dynamics 'n stuff, be cheap at twice the price.

Shame the whole thing read like a write up for "Dr Feelgood's All Purpose Embrocation"

Edited November 20, 2008 by Amicus

[Tony Brooks]

Mmmm, this afternoon I stood in W H Smiths and read this "technical assessment".

Amazing bit of kit, way it alters the hull form and hydro-dynamics 'n stuff, be cheap at twice the price.

Shame the whole thing read like a write up for "Dr Feelgood's All Purpose Embrocation"

 

 

Yes - as a mechanic (mustn't upset people ) I am still trying to work out how a propeller can alter the bow wave. I accept that if a prop can not get enough water past the swim then the stern will drop and I suppose form that we can postulate that the bow up attitude might just cause the hull to push more water in front of it so I would love a good explanation.

 

WE are told that wave making of the hull depends upon the water line length, the hull constant (from the designer) and speed, no where does the prop come into this. We are also told that its wave making that tends to limit the speed of displacement boats so with the best will in the world I am still confused about all this. Trouble is I really do not want to knock WW. I find it incredible that the prop vendors have not provided the forum with enough detail so they get some sort of peer review on the product.

[PaulG]

Yes - as a mechanic (mustn't upset people ) I am still trying to work out how a propeller can alter the bow wave. I accept that if a prop can not get enough water past the swim then the stern will drop and I suppose form that we can postulate that the bow up attitude might just cause the hull to push more water in front of it so I would love a good explanation.

 

WE are told that wave making of the hull depends upon the water line length, the hull constant (from the designer) and speed, no where does the prop come into this. We are also told that its wave making that tends to limit the speed of displacement boats so with the best will in the world I am still confused about all this. Trouble is I really do not want to knock WW. I find it incredible that the prop vendors have not provided the forum with enough detail so they get some sort of peer review on the product.

 

I haven't read the article yet, but their website smells strongly of "snake oil".

 

For example, how can a propeller give "improved steering response" in reverse? In reverse, it's not contributing to water flow over the rudder, so what possible mechanism could provide the "improved response". This claim may be related to their other claim regarding a reduction in prop walk, but this is not the same as steering "response". How exactly did they measure steering "response" in order to justify this claim? Turning circle of the boat at X knots with Y degrees of rudder angle?

 

"Hull lay flatter in the water and through waves (motion reduced)" Puhleezze! How can a fixed propeller stop a boat from pitching in a seaway? Alternatively, of course, they may mean that the forward motion was reduced!

[Liam]

I think they did try Tony, but went about it in the wrong way completely.

[Gibbo]

I will state first that I know Jack sh*t about propellers but............

 

If the prop is more efficient over its whole area so that the entire prop area efficiently pulls water through it at a consistent speed (instead of faster near the outside) then won't that help with all these issues?

 

Just a thought. The reason being that the term "Snake Oil" has, to my knowledge, been applied to the following:-

 

Alternator controllers.

Three stage chargers.

Solid State inverters.

Galvanic Isolators.

SmartGauges

LED lighting.

Lossless split charge diodes.

High frequency inverters.

Microprocessor controlled battery chargers.

 

In all these cases it seems to me that the accusation of "Snake Oil" came from people who seemed to be upset that they didn't understand how they worked, and therefore drew the conclusion that they couldn't.

 

I'm not saying this is the case in this instance but is it a possibility?

 

Gibbo

 

PS. Odd that a Google on "axiom propellor" finds this forum higher up than their own website

Edited November 20, 2008 by Gibbo

[Amicus]

Thinking about this a bit more, what the article says, in essence, is "we changed the prop and the boat performed differently"

Do yous think I could get a start on WW, save them a lot of paper and ink.

 

In all these cases it seems to me that the accusation of "Snake Oil" came from people who seemed to be upset that they didn't understand how they worked, and therefore drew the conclusion that they couldn't.

 

I'm not saying this is the case in this instance but is it a possibility?

 

Gibbo

Definitely, but in this instance we lack the resources/info to form an informed opinion,

we have only snake oil chatter.

 

We need a resource like your Web pages

 

 

 

 

edit; I would dearly like this thing to be kosher, I started work in a boat yard in 1959, but I'm no Luddite, nor am I easily led by the wang. Give us the dope

Edited November 20, 2008 by Amicus

[Tony Brooks]

I will state first that I know Jack sh*t about propellers but............

 

If the prop is more efficient over its whole area so that the entire prop area efficiently pulls water through it at a consistent speed (instead of faster near the outside) then won't that help with all these issues?

 

Just a thought. The reason being that the term "Snake Oil" has, to my knowledge, been applied to the following:-

 

Alternator controllers.

Three stage chargers.

Solid State inverters.

Galvanic Isolators.

SmartGauges

LED lighting.

Lossless split charge diodes.

High frequency inverters.

Microprocessor controlled battery chargers.

 

In all these cases it seems to me that the accusation of "Snake Oil" came from people who seemed to be upset that they didn't understand how they worked, and therefore drew the conclusion that they couldn't.

 

I'm not saying this is the case in this instance but is it a possibility?

 

Gibbo

 

 

Which is exactly why I want to know a lot more about it. I can not just dismiss it out of hand but unless some checkable facts derived from "scientific" tests carried out at an independent & respected establishment any little niggles that do not SEEM to ring true can easily get magnified to a point where they "overwhelm" any other "evidence".

 

I find it particularly concerning that their website only seems to show subjective assessments of performance and few, if any objective ones.

[Gibbo]

I accept that the way they seemed to approach this subject initially on this forum was very bad form. I don't think anyone can disagree with that.

 

But perhaps they have taken an informed decision not to put technical information on their website. I know from my own experience that a huge percentage of people don't understand technical data anyway and all it does is confuse them and stop them buying the product.

 

I'm starting to sound like an Axiom salesman.

 

Gibbo

[Gibbo]

I've gone through the website again. I actually don't see any "Snake Oil".

 

My boat comes out for painting and blacking in April and I have a feeling it will be going back in the water with one of those fitted. Presently it walks like a dog going astern, from around 2mph takes at least 4 boat lengths to stop and cavitates like a son of a bitch at anything approaching 30% power, even in the sea. It can hardly make it any worse!

 

Assuming they do left handed ones.

 

Gibbo

[Denis R]

Blimey, it doesn't look like anything I was expecting. I expected it to look more like some intricate multi blade submarine prop than a paddle blade prop off a C130. Intriguing. As per other posters, would love to see some quantified data.

[ditchcrawler]

Aixam

 

this is the only Aixam I would fancy, maybe it would pull a narrowboat rather than push

 

http://www.qdoscars.co.uk/megavan.htm

Saw one of them in Lowestoft this week

[Guest]

Just read the "benefits of Axiom Props." on their website. A magic cure all propellor it is. Never before have I read about a prop that can cure so many problems in one go. Boat sails "flatter," steers better, goes faster but with less wash, uses less fuel, reverses better, stops better, makes less noise, doesn't wear owing to reduced cavitation. A truly amazing device. I'll bet the opposition are shitting themselves.

[Amicus]

I accept that the way they seemed to approach this subject initially on this forum was very bad form. I don't think anyone can disagree with that.

 

But perhaps they have taken an informed decision not to put technical information on their website. I know from my own experience that a huge percentage of people don't understand technical data anyway and all it does is confuse them and stop them buying the product.

 

I'm starting to sound like an Axiom salesman.

 

Gibbo

 

Devils Advocate, first class vocation, where's Uncle Mort.

 

I've been around boats for more than fifty years, been involved in hundreds of hours of sea trials, read every book I could lay my hands on, pestered naval architects and designers to distraction.

Rightly or wrongly I think I know pretty good how a prop works.

Rightly or wrongly I think I have a bit of a handle on how a hull works.

A modern prop didn't just fall out of the casting sand with that shape, a hell of a lot of work has, and does go into it.

I think we all realise a prop has a cross section much like a wing chord and I think we all realise why.

Visualise a wing chord on some graph paper now visualise this new chord alongside, it appears to be a symmetric S shape chord, now visualise air/water flow over those two chords. Thass where I'm at.

Please somebody tell me how this thing works

 

edit PS when thinking about the flow over these chords remember a boat prop has a much higher angle of attack than a wing.

Edited November 20, 2008 by Amicus

[Gibbo]

I think we all realise a prop has a cross section much like a wing chord and I think we all realise why.

 

You mean an aerofoil?

 

I've read several technical papers over the last few years and it appears that, guess what, despite everyone knowing for 75 years exactly why and how a wing works.... low pressure at the top due to longer travel path, higher pressure underneath etc is........... wrong. That's not why they work. So maybe it's time for a rethink on props too?

 

I stress again. I know nowt about them.

 

Gibbo

[carlt]

[wonderdust]

Doesn't everyone a bottle of that in the medicine cupboard!

[Amicus]

You mean an aerofoil?

 

I've read several technical papers over the last few years and it appears that, guess what, despite everyone knowing for 75 years exactly why and how a wing works.... low pressure at the top due to longer travel path, higher pressure underneath etc is........... wrong. That's not why they work. So maybe it's time for a rethink on props too?

 

I stress again. I know nowt about them.

 

Gibbo

I dunno nuffink about aircraft wings. Are you referring to the "equal transit" thing or something more profound. If the latter I would appreciate a pointer? pretty please

[US Marines]

I dunno nuffink about aircraft wings. Are you referring to the "equal transit" thing or something more profound. If the latter I would appreciate a pointer? pretty please

Gibbo might possibly be talking about coanda and 'spinning balls' !

 

......which offers an alternative theory on how wings work, e.g. Clicky!

[Gibbo]

I dunno nuffink about aircraft wings. Are you referring to the "equal transit" thing or something more profound. If the latter I would appreciate a pointer? pretty please

 

For 75 years everyone has known that wings generate lift because the top surface is more curved than the bottom. Therefore the air going over the top has to travel further than the air going underneath. In order to travel further it has to travel faster, and air travelling faster lowers the pressure so the wing ends up with lower pressure on the top than the bottom, ergo: lift.

 

Everyone has known this for years (pilots who flew planes upside down obviously didn't really because it couldn't work).

 

Except it's wrong.

 

So whilst ............. "we all realise a prop has a cross section much like a wing chord" might be true with current props it appears there is no reason whatsoever why it needs to be that shape.

 

So perhaps they have made it symetrical instead of chord shaped and that's why it works better going astern.

 

Remember I know nothing about this.

 

Gibbo

[Speedwheel]

Experience seems to show that there are loads of narrowboats with props that are not ideal so I just can not see how any comparison can be valid unless steps are taken to ensure the original prop is as near correct as possible.

 

I'm so glad someone else has noticed that. So many boats have the wrong prop on. Even those which have the 'correct' prop as defined by the engine manufacturer or calculated by the prop manufacturer can be improved. We have recently changed an (as recommended by a calculation) prop for another slightly larger different pitched one. This has transformed the handling and stopping of the boat and also minimised and slewing in reverse. It is often a case of trying different props and seeing which works best.