Axiom - Oh Dear...

[NickF]

less blade area, smaller diameter.

 

will certainly improve the stopping distance - it will probably double it.

I think you are missing the point here, the boat will clearly be much slower with the axiom so will be much easier to stop from such a slow speed.

[MtB]

Much more of this Axiom-bashing (good fun though it is) and Dan will be served with another take-down notice.

 

As happened to the last thread kicking holes in the Axiom hyperbole, IIRC.

[mrsmelly]

Much more of this Axiom-bashing (good fun though it is) and Dan will be served with another take-down notice.

 

As happened to the last thread kicking holes in the Axiom hyperbole, IIRC.

 

Please sir, in fairness I did say in my post that my bro in law liked his axiom The mere fact he now has a Crowther means nowt

 

Tim

[frangar]

Much more of this Axiom-bashing (good fun though it is) and Dan will be served with another take-down notice.

 

As happened to the last thread kicking holes in the Axiom hyperbole, IIRC.

Dan needs to make a stand or this place will become even more bland if we can't have opinions on anything!!

 

They can always put their argument across. Or isn't it a discussion forum??

 

Gareth.

[Neil Smith]

Well i like my Axiom, stops well and seems to have good drive forward so must be the sizing thats important.

 

Neil

[Murflynn]

I think you are missing the point here, the boat will clearly be much slower with the axiom so will be much easier to stop from such a slow speed.

yeah, but ......................... I was trying to avoid criticising the axiom concept because we have no reliable evidence that it is actually slower.

[matty40s]

Much more of this Axiom-bashing (good fun though it is) and Dan will be served with another take-down notice.

 

As happened to the last thread kicking holes in the Axiom hyperbole, IIRC.

 

This isnt Axiom bashing, it is stating facts with pictorial evidence.

The propeller was returned to Axiom who admitted that as the propeller size was not withing their usual sizing requested, and assumed that the customer had got the measurements the wrong way round and made it accordingly.

They are now making another one!!!

[Dalslandia]

Beleiving and assuming have no place in the sience of black magic, but experiance and customer reports is important, to learn a new product and get competence in a wide range of engines and boats.

 

Sizing is the most importance to get a good match of engine/gear/hull, then getting an more efficient propeller out of that is next step.

sometimes it is not easy for the propeller designer/builder/seller, if the engine and gearing isn't suitable for the hull.

Edited November 8, 2016 by Dalslandia

[Geo]

I think you are missing the point here, the boat will clearly be much slower with the axiom so will be much easier to stop from such a slow speed.

 

Well that is not the case with my Axiom props, the speed ahead rose, prop walk decreased, and astern was more controllable.

[matty40s]

 

Well that is not the case with my Axiom props, the speed ahead rose, prop walk decreased, and astern was more controllable.

I think you are missing the point, the supplied Axiom is 4" smaller than the Crowther, so the speed ahead will be less.

[Geo]

I think you are missing the point, the supplied Axiom is 4" smaller than the Crowther, so the speed ahead will be less.

 

It would have been interesting to have tried it surprising things have happened lol

[matty40s]

 

It would have been interesting to have tried it surprising things have happened lol

its a 70 footer with a 2 pot Gardner - I think the surprise would not have been a good one., Axion are now doing the prop with 24 x 28 dimensions.

[alan_fincher]

28" pitch?

 

Surprisingly big difference from 28" mentioned in first post.

 

Or am I misunderstanding?

[Dalslandia]

that company use pitch in degrees, so 28 deg at 70% radii give a nominal pitch in inch. in this case a 24" diameter and 28 deg make, hmmm 28"

 

the first 20x24 if that is degree is 19,6" if that 24 isn't the pitch in inch

Edited November 9, 2016 by Dalslandia

[matty40s]

No, that is what they have specified to match the Crowther. The Crowther was tested in their test pool and showed as more efficient than most normal shape propellor going forwards, but only 15% efficient in reverse.

The owner is hanging on to the old prop for now....in case axiom aren't quite right....

[MtB]

Sounds to me as though they have a 24" x 28" already in stock, and are now trying to palm Matty off with it.

No, that is what they have specified to match the Crowther. The Crowther was tested in their test pool and showed as more efficient than most normal shape propellor going forwards, but only 15% efficient in reverse.

 

How, I wonder, do they come up with an actual figure like that for prop 'efficiency'?

 

15% of what?

 

Thrust in reverse is 15% of thrust in ahead for a given shaft speed perhaps?

[PaulG]

No, that is what they have specified to match the Crowther. The Crowther was tested in their test pool and showed as more efficient than most normal shape propellor going forwards, but only 15% efficient in reverse.

The owner is hanging on to the old prop for now....in case axiom aren't quite right....

Not exactly an independent test then...

[MtB]

Not exactly an independent test then...

 

 

I'm sure it would have been a double blind test...

[Dalslandia]

read that most normal props is 50% efficient in reverse of the forward efficiency, and that most props is 50-60% efficient in forward at design point.

[PaulG]

 

 

I'm sure it would have been a double blind test...

Naturally...

Edited November 9, 2016 by PaulG

[PaulG]

read that most normal props is 50% efficient in reverse of the forward efficiency, and that most props is 50-60% efficient in forward at design point.

If the prop blade is assymetric, like this one:

Then it is optimised for forward efficiency.

When it is in reverse, it is turning in the opposite direction, so the effective shape is "backwards" for best efficiency.

 

Of course, boats spend the majority of their time travelling forwards, so this type of design is popular.

Designs like the "Axiom" look to be symetrical, so should be equally efficient when operating in either direction.

However, for the short periods of reversing, I can't see that reverse efficiency really counts for a lot unless the boat is underpowered. If the prop is inefficient, then just give it more revs.

Extra revs *may* cause more prop walk, however.

But, as in the case of Mr. Brunel, you'd need to test two identical boats side-by-side without knowing which props are fitted to see if there is any difference in reality.

[Dalslandia]

Looks like a medium skewed propeller, but not just the planform will be wrong in reverse, the airfoil will be wrong too.

so an airfoil with two high camber leading edges will make it equal both ways, it's like having 50% water in a single malt

 

It spoil both the water and Single malt

 

https://media.giphy.com/media/mnJRjgXZ2ooh2/giphy.gif​

Edited November 9, 2016 by Dalslandia

[PaulG]

Looks like a medium skewed propeller, but not just the planform will be wrong in reverse, the airfoil will be wrong too.

so an airfoil with two high camber leading edges will make it equal both ways, it's like having 50% water in a single malt

A heinous crime!

[Murflynn]

........................... but only 15% efficient in reverse.

 

hard to believe. have you seen their test results, including the conditions of the test?

[MtB]

hard to believe. have you seen their test results, including the conditions of the test?

 

 

Hence my question about how prop 'efficiency' is calculated or measured. No-one has been able to answer yet.