PBO prop test

[Tranona]

Not my problem.
What I am saying is that the test failed in its overall findings because it gave a misleading result concerning performance in actual working conditions.

Bollard pull is the standard test for thrust of propellers and is a valid method of comparison. However it does not cope well with variable pitch propellers such as your Bruntons. That is not the fault of the test, but reflects the way that your propeller works.

Conventional fixed pitch props are sized to give maximum thrust at maximum power and revs of the engine, whereas your prop adjusts pitch to suit the loads placed on the boat so maximum thrust may not need maximum power and may vary according to conditions.

 

[Daydream believer]

Bollard pull is the standard test for thrust of propellers and is a valid method of comparison. However it does not cope well with variable pitch propellers such as your Bruntons. That is not the fault of the test, but reflects the way that your propeller works.
.

Come off it !! The method of testing is flawed. If it does not demonstrate how the prop works in actual conditions then it could be considered a misleading test & certainly not conclusive- is it?
One cannot say " this item will perform in this manner in use" when the method of testing is flawed.
Note I stated "in this manner in use" There would be no point in doing the test if one did not want to know how it performed at sea

If we all sailed our boats tied to the pontoon ( yes I know a lot do most of the time:nonchalance the test would be Ok. However, some of us actually go to sea. So, surely, the test needs to reflect that activity. Should it not?

It is not my problem to say how that should be done. I am not putting forward the tests. I was only pointing out that the test has some flaws in it. It is up to others to decide whether to ignore those flaws or not.

 

[Tranona]

Come off it !! The method of testing is flawed. If it does not demonstrate how the prop works in actual conditions then it could be considered a misleading test & certainly not conclusive- is it?
One cannot say " this item will perform in this manner in use" when the method of testing is flawed.
Note I stated "in this manner in use" There would be no point in doing the test if one did not want to know how it performed at sea

If we all sailed our boats tied to the pontoon ( yes I know a lot do most of the time:nonchalance the test would be Ok. However, some of us actually go to sea. So, surely, the test needs to reflect that activity. Should it not?

It is not my problem to say how that should be done. I am not putting forward the tests. I was only pointing out that the test has some flaws in it. It is up to others to decide whether to ignore those flaws or not.

Bollard pull measures the amount of thrust which is the major determinant of speed achievable. The test used the same boat and engine in exactly the same environmental conditions. So the test is a valid comparison of one prop against another.

There is not a test that can measure how a specific prop with variable pitch behaves in particular conditions simply because there are so many variables.

This does not invalidate the bollard pull test if you understand the principles.

 

[earlybird]

The YM test, in addition to bollard pull both ahead and astern, measured boat speed up to maximum, stopping time from 6kt. and prop walk or side thrust
Fixating on bollard pull seems unfair to an informative set of tests IMO.
IIRC, bollard pull and max. speed weren't consistently related.

 

[Ric]

Bollard pull measures the amount of thrust which is the major determinant of speed achievable. The test used the same boat and engine in exactly the same environmental conditions. So the test is a valid comparison of one prop against another.

There is not a test that can measure how a specific prop with variable pitch behaves in particular conditions simply because there are so many variables.

This does not invalidate the bollard pull test if you understand the principles.

The bollard pull test is not a good test for fixed pitch props because if the prop is correctly pitched to give optimum efficiency at hull speed, it will be over-pitched on the bollard pull. The bollard pull test thus tends to give favourable reviews for props which are actually slightly under-pitched when the boat is cruising at hull speed.

To draw an aeronautical parallel, on aircraft with variable pitch props, the prop pitch is selected to fully fine for take-off and initial climb (equivalent to bollard pull, because airspeed is low). Once the aircraft is established in cruise, the pitch is coarsened to keep the true angle of incidence the same in higher airspeed.

 

[Tranona]

The bollard pull test is not a good test for fixed pitch props because if the prop is correctly pitched to give optimum efficiency at hull speed, it will be over-pitched on the bollard pull. The bollard pull test thus tends to give favourable reviews for props which are actually slightly under-pitched when the boat is cruising at hull speed.

To draw an aeronautical parallel, on aircraft with variable pitch props, the prop pitch is selected to fully fine for take-off and initial climb (equivalent to bollard pull, because airspeed is low). Once the aircraft is established in cruise, the pitch is coarsened to keep the true angle of incidence the same in higher airspeed.

That is not quite right. The prop should be sized so that the engine can reach close to maximum revs. This is not "cruising speed" which is normally at between 60-70% of maximum. If the prop is undersized its performance will be limited by the maximum revs the engine can achieve and may well result in lower thrust. If I remember rightly at least one of the props tested was incorrectly sized which made comparison with others unreliable.

 

[Daydream believer]

Bollard pull measures the amount of thrust which is the major determinant of speed achievable. The test used the same boat and engine in exactly the same environmental conditions. So the test is a valid comparison of one prop against another.

There is not a test that can measure how a specific prop with variable pitch behaves in particular conditions simply because there are so many variables.

This does not invalidate the bollard pull test if you understand the principles.

So you agree that the test was flawed----- because it cannot replicate the conditions.
One does not sail around in flat water attached to a pontoon.
I also would suggest that when the boat is actually moving the performance or the SAME propellor would change because it is passing THROUGH the water rather than the water passing through it.However, My knowledge of dynamics is insufficient to prove/disprove
However, I have a life to live & do not want to spend it studying the principles of propellor design.
I can only stress that MY propellor does not perform at sea in the way that the test suggests. That is indisputable & I could,- given the will to live,- prove it with a practical demonstration.

 

[NormanB]

So you agree that the test was flawed----- because it cannot replicate the conditions.
One does not sail around in flat water attached to a pontoon.
I also would suggest that when the boat is actually moving the performance or the SAME propellor would change because it is passing THROUGH the water rather than the water passing through it.However, My knowledge of dynamics is insufficient to prove/disprove
However, I have a life to live & do not want to spend it studying the principles of propellor design.
I can only stress that MY propellor does not perform at sea in the way that the test suggests. That is indisputable & I could,- given the will to live,- prove it with a practical demonstration.

Please have the will to do a sea trial, there and back up a measured mile, and record the results.
Then lift the boat swap to a different prop, wait for conditions to be exactly the same as the first test and repeat as many times as required for the number of prop types available for tests.
Publish your results on here for people to pick holes in.

 

[Ric]

That is not quite right. The prop should be sized so that the engine can reach close to maximum revs. This is not "cruising speed" which is normally at between 60-70% of maximum. If the prop is undersized its performance will be limited by the maximum revs the engine can achieve and may well result in lower thrust. If I remember rightly at least one of the props tested was incorrectly sized which made comparison with others unreliable.

No, you are wrong. The mantra that the prop should reach close to maximum revs at cruising speed is a practical simplification of a complicated system, but far from accurate. The reality is that at boat hull-speed (on a displacement hull) the prop should be pitched so that it is operating at maximum efficiency (ie lift/drag of the hydrofoil section is optimal) and also that the driving engine is also operating at its most optimal torque and rpm. The interaction is complex.

I can guarantee to you that the builders of super-tankers do not make their props so that perform optimally on a supertanker equivalent of a bollard-test...

 

[Daydream believer]

Please have the will to do a sea trial, there and back up a measured mile, and record the results.
Then lift the boat swap to a different prop, wait for conditions to be exactly the same as the first test and repeat as many times as required for the number of prop types available for tests.
Publish your results on here for people to pick holes in.

No need to. I am not in the business of doing tests. I trust others to do that.
I would not buy a dog & then do my own barking.
I do know if a product does not match the performance which test result says it should. If i see that, then I do not think that it is unreasonable to point this out others They can then form their own opinion. - Is that not a fair comment???
I do know how my prop compares with the standard fixed blade prop I had before I changed & that was totally different in a short 1 metre chop.

 

[Quandary]

Quandary - in case it’s of use, we have a Flexofold three blade prop. Used it for 3 years so far and been very happy with its performance, in flat water (including Crimean with your help!), punching waves and in reverse Can’t compare directly to any other props, but if the budget allows won’t go far wrong with this.

A respected recommendation, thanks. I am content that the Flexofold 3 blade is the optimum, the YM report is as good a comparison test as could possibly be made, just struggling with putting a £2k prop. on a £35k boat. That sort of money should be spent on sails? Since my issue is mainly the considerable drop off in water speed in even moderately adverse conditions I would expect 3 blades to cope better with that.
I have been here long enough now to exhibit a proper level of canniness but then I had a head start with so many years in Ballymena, where Aberdonians are disparaged for their profligacy.
If my Eber. passes the current examination I might donate the money previously set aside for its replacement.

 

[LONG_KEELER]

To draw an aeronautical parallel, on aircraft with variable pitch props, the prop pitch is selected to fully fine for take-off and initial climb (equivalent to bollard pull, because airspeed is low). Once the aircraft is established in cruise, the pitch is coarsened to keep the true angle of incidence the same in higher airspeed.

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

 

[Daydream believer]

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

I had instructions for the Lister LT1 diesel engine that we used to have in our concrete mixers. This included details for air cooling to fit inside a boat. I don't think that It had throttle ( although one could be fitted) It could be increased from its standard 1.5hp in its concrete mixer guise to a 7.5HP by balancing the flywheel & having a different governor etc. Then a simple prop shaft with no gearbox but a watermota propeller with a pitch altered by a lever in the engine bay. This altered the speed & dealt with reverse etc.

 

[Ric]

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

Plenty of the bigger sailing yachts (30m +) and displacement motor yachts have variable pitch props.

 

[Quandary]

I have just completed the purchase arrangements for a 3 blade Flexofold from Darglow, I would like to put on record my appreciation of the service that Nick, their sales engineers provides, three long telephone conversations exploring and carefully explaining the choices available in type of propellor, performance with the boat and engine, sizing, type of anode etc. Nick can fill in the gaps in the YM test and deal effectively with DBs concerns about variable conditions of use.
I do not think that you will get a much keener price for the product elsewhere but the time and care taken to educate me about the choices available makes all the difference, plus the ongoing service available from what is a UK company.
I am well impressed and pleased to recommend them,

 

[awol]

I have just completed the purchase arrangements for a 3 blade Flexofold from Darglow,

I trust your first action after fitting will be to advise the CYCA and send them a cheque. :encouragement:

 

[AntarcticPilot]

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

The Halcyon 27 was equipped with a fully variable pitch propellor coupled to a Sabb engine. The pitch was variable from full astern to full forward. In neutral I THINK the blades were aligned fore and aft, minimizing drag, but it probably didn't matter as the prop was in a small cutout between the rudder and the keel. I don't recall there being any problems with that setup, and no doubt, given the longevity of the Sabb engine, there are many still equipped like that "out there". However, I don't recall us using it to vary the pitch to suit conditions.

A potential drawback it that the prop-shaft is not a simple rod of metal, but has to have moving parts within it to actuate the propellor's change of pitch.

 

[NormanS]

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

The Bruntons Autoprop does that automatically (and brilliantly).

 

[Tranona]

Some years ago now , I remember seeing boats with pitch that could be controlled on passage at sea .

There must be a reason why these became unpopular. Perhaps only being able to control pitch
made insignificant benefits.

I have no experience of them but it seems a good idea

Yes, they were popular for a time because the engines used were low output, low revving and benefitted from the ability to vary the pitch to take full advantage of the limited power available. Avoids the use of a gearbox, particularly because the engines typically revved at under 1000rpm so no reduction was needed to get the appropriate shaft speed. The more sophisticated examples were also able to arrange reverse gear and feathering the blades in neutral. A typical example would be the Sabb 10hp engine often used to drive quite large and heavy boats.

However, they were complicated and needed maintenance to operate reliably - plus the technique to use them effectively was different from a reversing gearbox and fixed prop. Perhaps what really killed them off was the move to higher revving engines which necessitated reduction gears to bring the shaft speed down in order to spin a large enough diameter prop. Typically shaft speeds are in the range of 1000-1600rpm and engine speeds are now mostly greater than 3000, so reduction gears in the range 2-3:1 are common. Although variable pitch stern gear can be used with a reduction box they are bulky, complicated and expensive, although they do find a market in fishing boats where their benefits can be exploited effectively and economically.

As already noted by others, the Brunton design aims to vary pitch according to load and revs and as a result has particular benefits for those who spend a lot of time motorsailng when a fixed prop does not load the engine fully so it runs at higher revs than is necessary. The design also feathers in neutral reducing drag under sail.

As also suggested, variable pitch, and so called "overdrive" feathering propellers are available but usually only on larger boats where the cost and complication might be offset by the efficiency gains. There are also adjustable pitch feathering propellers where the same prop can be assembled with a range of different pitches, and for some with a different pitch in forward and reverse.

 

[Tranona]

I can guarantee to you that the builders of super-tankers do not make their props so that perform optimally on a supertanker equivalent of a bollard-test...

Not sure what props for supertankers have to do with this discussion.

You are making it all too complicated. We are talking about simple propellers with low horsepower engines driving low speed displacement craft and in this type of application the prop is sized to achieve hull speed (assuming there is enough power available) at WOT or close. The sign off for engine installation from all manufacturers of this type of installation requires the boat to achieve this at an engine speed normally within 10% of rated maximum. The calculators used by propeller manufacturers and suppliers to size propellers for specific applications are based on this requirement.

Do not understand why you are referring to this anything to do with cruising speed. The recommended continuous cruising speed in such boats is in the range of 60-70%, which for many engines in this category is in the range of 2100-2500rpm.