tross
Well-Known Member
So, we finally have the answer to why helicopter may or may not fall out of the sky, why maple seeds spin and if spinning or fixed props are best! - look a the the picture on the lower left hand page.
YM prop test
- Thread starter tross
- Start date
johnphilip
Well-Known Member
Just read the article last night
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
Cantata
Well-Known Member
[ QUOTE ]
Just read the article last night
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
[/ QUOTE ]
Dunno about wisdom and folklore. I can however say with absolute certainty that my 3-blade folder performs against head wind and sea a heck of a lot better than the 2-blader that it replaced ever did.
Just read the article last night
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
[/ QUOTE ]
Dunno about wisdom and folklore. I can however say with absolute certainty that my 3-blade folder performs against head wind and sea a heck of a lot better than the 2-blader that it replaced ever did.
rwoofer
Well-Known Member
But your 2 blader could have just been crap full stop. Need to compare similar designs for any 2/3 blade comparison.
RAI
Well-Known Member
Must buy YM...
What did it say about the fixed to foldable performance comparison?
What did it say about the fixed to foldable performance comparison?
Cantata
Well-Known Member
[ QUOTE ]
But your 2 blader could have just been crap full stop. Need to compare similar designs for any 2/3 blade comparison.
[/ QUOTE ]
Quite possibly. But it was the same make as used by the poster to whom I was replying.
Sadly, tests like this can only give indications of what might work best on your own boat.
But your 2 blader could have just been crap full stop. Need to compare similar designs for any 2/3 blade comparison.
[/ QUOTE ]
Quite possibly. But it was the same make as used by the poster to whom I was replying.
Sadly, tests like this can only give indications of what might work best on your own boat.
snooks
Well-Known Member
[ QUOTE ]
Just read the article last night
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
[/ QUOTE ]
What we did find was the 3 bladed props were smoother than the 2 bladed props, but as was mentioned in the text this could have been due to the hull design of the test boat /forums/images/graemlins/smile.gif
Just read the article last night
If it is right then it has saved me a small fortune as I had been considering going from a 2 blade folder (flexofold) to a three blade fold or feather to get more push when struggling into wind and sea. This report shows that the 2 bladed props have a very good "Bollard pull ", ie. slow moving as when into the headwind. I am surprised, as it goes against all conventional wisdom and folklore about 2/3 blade advantages.
I do wonder if the different props give a different thrust at a set revs, say 2000 or 2500 that many people run at. We seldom use the max power that the comparisons are made at.
[/ QUOTE ]
What we did find was the 3 bladed props were smoother than the 2 bladed props, but as was mentioned in the text this could have been due to the hull design of the test boat /forums/images/graemlins/smile.gif
DaveS
Well-Known Member
I have just read the article, which I was greatly looking forward to. I entirely agree that the generic drag curve graph is probably the most useful bit of the article, but rather regret that we are no longer likely to see amusing posts trying to "prove" with increasingly abstruse arguments that a locked prop causes less drag that one free to rotate. /forums/images/graemlins/smile.gif
I'm surprised that no one else has so far reported the howler in the "Prop Facts" box. It states "Diameter is the distance from the centre of the boss to the tip of the blades." and the accompanying diagram diagram agrees with this. Now when I was at school this would have described radius, or half the diameter. The trouble with seeing something set out authoritatively in print like this is that you start to question your own beliefs. Maybe propeller terminology does differ from general usage? A few seconds of practical consideration however, visualising how much of the car boot would be taken up by a 16 X 11 propeller under possible alternative conventions (i.e. a small bit of it or almost all of it) makes it clear - YM's definition is simply wrong.
Another, less obvious. error occurs in the description of the Kiwiprop. It is stated "The blades do not swing completely round in reverse, so the trailing edge becomes the leading edge." This is incorrect. I can see, however, how a casual inspection might lead to some confusion. The blades turn freely from the adjustable ahead pitch position to, and a bit past, the feathering position. If, however, the blades are pushed further round against spring resistance they move into the fixed pitch astern position, with the same leading edge as when going ahead. The need to push against the spring is not immediately obvious, but if the assumption was made that the pre spring compression travel was the limit of blade movement, it should really have prompted a question as to how the propeller could work in astern at all! Given that we are told that a manufacturer's rep for each propeller was present, this could have been easily resolved.
It is inevitable that with tests like these far more data is collected than can be published within the space constraints of a magazine article, and some readers will always want more than others. One vital piece of information that I do feel could have been usefully included without overloading the article is the engine rpm for each prop on each test. This would make comparisons more meaningful. The importance of this is hinted at, e.g. "the Autoprop achieved 6 knots at 2,100 rpm, compared to 2,500 rpm for our standard prop and most of the others on test." and "the engine was over-revving with the smaller prop", but it would be helpful if the full data set was available for inspection. It would be really helpful if this could be made available as a download, so that those who are sufficiently interested to want more detail at the expense of having to wade through more complexity could do so.
Greater detail as above might explain one of the test results which I found rather surprising. "Our bollard pull test shows that the fastest propellers are generally also the most powerful in ahead, and most of the slower ones are among the least powerful." I have always understood that the setting of propeller pitch provides a trade off between speed and thrust - a bit analagous to gear ratio in a vehicle - so would have expected, all other things being equal, the opposite result. An alternative explanation is that each test is simply ranking props by order of closeness of match to the engine, so it is not surprising that in each test a similar order emerges. Comparing the rpm figures would greatly clarify this point.
I'm surprised that no one else has so far reported the howler in the "Prop Facts" box. It states "Diameter is the distance from the centre of the boss to the tip of the blades." and the accompanying diagram diagram agrees with this. Now when I was at school this would have described radius, or half the diameter. The trouble with seeing something set out authoritatively in print like this is that you start to question your own beliefs. Maybe propeller terminology does differ from general usage? A few seconds of practical consideration however, visualising how much of the car boot would be taken up by a 16 X 11 propeller under possible alternative conventions (i.e. a small bit of it or almost all of it) makes it clear - YM's definition is simply wrong.
Another, less obvious. error occurs in the description of the Kiwiprop. It is stated "The blades do not swing completely round in reverse, so the trailing edge becomes the leading edge." This is incorrect. I can see, however, how a casual inspection might lead to some confusion. The blades turn freely from the adjustable ahead pitch position to, and a bit past, the feathering position. If, however, the blades are pushed further round against spring resistance they move into the fixed pitch astern position, with the same leading edge as when going ahead. The need to push against the spring is not immediately obvious, but if the assumption was made that the pre spring compression travel was the limit of blade movement, it should really have prompted a question as to how the propeller could work in astern at all! Given that we are told that a manufacturer's rep for each propeller was present, this could have been easily resolved.
It is inevitable that with tests like these far more data is collected than can be published within the space constraints of a magazine article, and some readers will always want more than others. One vital piece of information that I do feel could have been usefully included without overloading the article is the engine rpm for each prop on each test. This would make comparisons more meaningful. The importance of this is hinted at, e.g. "the Autoprop achieved 6 knots at 2,100 rpm, compared to 2,500 rpm for our standard prop and most of the others on test." and "the engine was over-revving with the smaller prop", but it would be helpful if the full data set was available for inspection. It would be really helpful if this could be made available as a download, so that those who are sufficiently interested to want more detail at the expense of having to wade through more complexity could do so.
Greater detail as above might explain one of the test results which I found rather surprising. "Our bollard pull test shows that the fastest propellers are generally also the most powerful in ahead, and most of the slower ones are among the least powerful." I have always understood that the setting of propeller pitch provides a trade off between speed and thrust - a bit analagous to gear ratio in a vehicle - so would have expected, all other things being equal, the opposite result. An alternative explanation is that each test is simply ranking props by order of closeness of match to the engine, so it is not surprising that in each test a similar order emerges. Comparing the rpm figures would greatly clarify this point.
anniebray
Well-Known Member
I bought a Kiwiprop end of last season & fitted it. Found it was only half a kn faster than the Radice (I think) 2B which it replaced. However it revved up to full engine speed so I'm going to tweek it a touch & aim for 90% overpitch as per Mike Lucas. After all the publicity I'm very surprised the kiwi came out so poorly but I am wondering if Vecta Marine neglected to make the optimum adjustments as perhaps most others did.
snooks
Well-Known Member
[ QUOTE ]
One vital piece of information that I do feel could have been usefully included without overloading the article is the engine rpm for each prop on each test. This would make comparisons more meaningful. The importance of this is hinted at, e.g. "the Autoprop achieved 6 knots at 2,100 rpm, compared to 2,500 rpm for our standard prop and most of the others on test." and "the engine was over-revving with the smaller prop", but it would be helpful if the full data set was available for inspection. It would be really helpful if this could be made available as a download, so that those who are sufficiently interested to want more detail at the expense of having to wade through more complexity could do so.
[/ QUOTE ]
No promises, but I'll see what I can do /forums/images/graemlins/wink.gif
One vital piece of information that I do feel could have been usefully included without overloading the article is the engine rpm for each prop on each test. This would make comparisons more meaningful. The importance of this is hinted at, e.g. "the Autoprop achieved 6 knots at 2,100 rpm, compared to 2,500 rpm for our standard prop and most of the others on test." and "the engine was over-revving with the smaller prop", but it would be helpful if the full data set was available for inspection. It would be really helpful if this could be made available as a download, so that those who are sufficiently interested to want more detail at the expense of having to wade through more complexity could do so.
[/ QUOTE ]
No promises, but I'll see what I can do /forums/images/graemlins/wink.gif
kengill
Well-Known Member
Methinks the graphs were misleading as it seemed to claim that lower propwalk was better than more!. Anyone who's tried to manoever a long keeled boat with little prop walk would I suggest disagree as one can walk the boat around with a little good planning. There again I could be wrong.
Seconly it rated propwalk by percentage of stern thrust and made no mention of what the actual thrust was - clue - do a spreadsheet of their data and regress it to the actual thrust created and you'll seee that the rankings change dramatically. They also failed to give any propwalk figures for the boat in ahead gear.
I think they could have done better, but having said that at least its a first step in the right direction and if one were to draw any general conclusions from their test one could say that Brunton's Varifold 2 blade was the best value for money. Whew! that's what I bought a few years ago. /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif
Seconly it rated propwalk by percentage of stern thrust and made no mention of what the actual thrust was - clue - do a spreadsheet of their data and regress it to the actual thrust created and you'll seee that the rankings change dramatically. They also failed to give any propwalk figures for the boat in ahead gear.
I think they could have done better, but having said that at least its a first step in the right direction and if one were to draw any general conclusions from their test one could say that Brunton's Varifold 2 blade was the best value for money. Whew! that's what I bought a few years ago. /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif
snooks
Well-Known Member
[ QUOTE ]
Methinks the graphs were misleading as it seemed to claim that lower propwalk was better than more!. Anyone who's tried to manoever a long keeled boat with little prop walk would I suggest disagree as one can walk the boat around with a little good planning. There again I could be wrong.
[/ QUOTE ]
The day a bus runs into a shelter killing all the people waiting at it, there will always be one person who suggests that standing in the road is safer.
/forums/images/graemlins/laugh.gif
Prop walk can indeed be used, but only because you have to...if a long keeler wouldn't have her rse kicked round when they go astern, they would be a lot easier to handle in tight spaces. Imagine being able to turn her around any direction you want, without propwalk dictating which way you have to go. Odds are whichever side to you have go against the pontoon, will be the side the prop will kick you away from it.
The Axiom prop was developed to try to eliminate prop walk, if it's such a good thing why would they do that? /forums/images/graemlins/wink.gif
[ QUOTE ]
Seconly it rated propwalk by percentage of stern thrust and made no mention of what the actual thrust was - clue - do a spreadsheet of their data and regress it to the actual thrust created and you'll seee that the rankings change dramatically. They also failed to give any propwalk figures for the boat in ahead gear.
[/ QUOTE ]
Prop walk forwards can be corrected by the rudder. It would hardly be fair if the least powerful prop gave a kick of say 40lbs, while a prop that was giving say twice as much power was only kicking with 50lbs of force...that's why it was done as a percentage, so the reader could see what percentage of reverse power is being used to throw the boat sideways. The same way the crash stop wasn't done from max revs, because the props shifting the boat at 8 knots (for example) would have to do more work stopping the yacht than a prop shifting the boat at 6 knots....So we chose 6 knots as the speed and a level playing field for everyone.
[ QUOTE ]
I think they could have done better, but having said that at least its a first step in the right direction and if one were to draw any general conclusions from their test one could say that Brunton's Varifold 2 blade was the best value for money. Whew! that's what I bought a few years ago. /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif
[/ QUOTE ]
We had two days starting at 8am finishing at 6pm with no stops testing 15 props. This meant lifting the boat 16 times, relaunching, tying ropes to do the test in forwards, turning the yacht around doing the pulls in reverse (throughout the full engine revs at 500rpm intervals) then going out, doing speed runs in both directions - to counter any tide, then doing 8 or so crash stops, bring the yacht in lifting her out and fitting another prop. We could have indeed tested lots more, but it would have taken three days, maybe 4, and I doubt Lymington Yacht Haven would have been so generous with their hoist.
Glad you have the right pro for your boat though /forums/images/graemlins/smile.gif
Methinks the graphs were misleading as it seemed to claim that lower propwalk was better than more!. Anyone who's tried to manoever a long keeled boat with little prop walk would I suggest disagree as one can walk the boat around with a little good planning. There again I could be wrong.
[/ QUOTE ]
The day a bus runs into a shelter killing all the people waiting at it, there will always be one person who suggests that standing in the road is safer.
/forums/images/graemlins/laugh.gif
Prop walk can indeed be used, but only because you have to...if a long keeler wouldn't have her rse kicked round when they go astern, they would be a lot easier to handle in tight spaces. Imagine being able to turn her around any direction you want, without propwalk dictating which way you have to go. Odds are whichever side to you have go against the pontoon, will be the side the prop will kick you away from it.
The Axiom prop was developed to try to eliminate prop walk, if it's such a good thing why would they do that? /forums/images/graemlins/wink.gif
[ QUOTE ]
Seconly it rated propwalk by percentage of stern thrust and made no mention of what the actual thrust was - clue - do a spreadsheet of their data and regress it to the actual thrust created and you'll seee that the rankings change dramatically. They also failed to give any propwalk figures for the boat in ahead gear.
[/ QUOTE ]
Prop walk forwards can be corrected by the rudder. It would hardly be fair if the least powerful prop gave a kick of say 40lbs, while a prop that was giving say twice as much power was only kicking with 50lbs of force...that's why it was done as a percentage, so the reader could see what percentage of reverse power is being used to throw the boat sideways. The same way the crash stop wasn't done from max revs, because the props shifting the boat at 8 knots (for example) would have to do more work stopping the yacht than a prop shifting the boat at 6 knots....So we chose 6 knots as the speed and a level playing field for everyone.
[ QUOTE ]
I think they could have done better, but having said that at least its a first step in the right direction and if one were to draw any general conclusions from their test one could say that Brunton's Varifold 2 blade was the best value for money. Whew! that's what I bought a few years ago. /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif
[/ QUOTE ]
We had two days starting at 8am finishing at 6pm with no stops testing 15 props. This meant lifting the boat 16 times, relaunching, tying ropes to do the test in forwards, turning the yacht around doing the pulls in reverse (throughout the full engine revs at 500rpm intervals) then going out, doing speed runs in both directions - to counter any tide, then doing 8 or so crash stops, bring the yacht in lifting her out and fitting another prop. We could have indeed tested lots more, but it would have taken three days, maybe 4, and I doubt Lymington Yacht Haven would have been so generous with their hoist.
Glad you have the right pro for your boat though /forums/images/graemlins/smile.gif
DaveS
Well-Known Member
One of the real advantages of a Kiwiprop is that you can play tunes with it and see what works best. Mine was delivered with 20 degree pitch and worked OK, but I felt the boat speed was a bit fast at minimum engine revs, and at full throttle the engine was well short of its rated speed, i.e. I concluded the pitch was too coarse. Now there's a bit of an "issue" around this, since apparently it's quite a common practice in the UK to deliberately over prop since that means lower engine rpm for a given speed. The downside is, of course, that the engine can never develop its rated HP (so why pay for such a big engine in the first place?)
I have been adjusting my Kiwi over the past 2 / 3 years: I now have it set at 17 degree pitch, and at full throttle I almost reach the engine design speed, i.e. max. power. At tickover, manoevering speed is far lower, which I find useful. The downside is that I have to use higher rpm to attain a particular speed under motor, but at the end of the day I've a sailing boat, and the motor is an auxiliary.
I have been adjusting my Kiwi over the past 2 / 3 years: I now have it set at 17 degree pitch, and at full throttle I almost reach the engine design speed, i.e. max. power. At tickover, manoevering speed is far lower, which I find useful. The downside is that I have to use higher rpm to attain a particular speed under motor, but at the end of the day I've a sailing boat, and the motor is an auxiliary.
Aeolus_IV
Well-Known Member
[ QUOTE ]
Prop walk forwards can be corrected by the rudder.
[/ QUOTE ]
Well, actually, no, not everyone can correct prop walk with the rudder. A traditional long keel yacht with the prop in a "hole" in the rudder moves the rudder out of the thrust as it swings to port or starboard. Many older yachts have their props offset to port or starboard. A rare few bizarrely have the prop behind the rudder.
Mind you, I do have to agree that less intrinsic prop walk does make life easier.
Now I must get a copy of YM and actually read the reviews.
Regards,
Jeff.
Prop walk forwards can be corrected by the rudder.
[/ QUOTE ]
Well, actually, no, not everyone can correct prop walk with the rudder. A traditional long keel yacht with the prop in a "hole" in the rudder moves the rudder out of the thrust as it swings to port or starboard. Many older yachts have their props offset to port or starboard. A rare few bizarrely have the prop behind the rudder.
Mind you, I do have to agree that less intrinsic prop walk does make life easier.
Now I must get a copy of YM and actually read the reviews.
Regards,
Jeff.
anniebray
Well-Known Member
I have a Yanmar 30 hp saildrive with Kiwiprop & Spurs ropecutter. The propeller HP curve intersects with the engine HP curve at (or maybe beyond) 3600. I plan to increase the pitch so that max engine speed is about 3300. I use my engine at 2500-2800 and I want to get better speed at that RPM . I will still have a margin of power for coping with headwinds etc. I sail rather than motor but when there's no wind & I'm crossing the Channel I want good speed & economy.
damo
Well-Known Member
Maybe I couldn't see it for looking, but where are the drag results for the Axiom? This is a prop I have been curious about for a while, and may be one of the few alternatives I have to a fixed 3B (prop in aperture, so little room for a folder) - but ONLY if it has significantly less drag.
snooks
Well-Known Member
[ QUOTE ]
Maybe I couldn't see it for looking, but where are the drag results for the Axiom? This is a prop I have been curious about for a while, and may be one of the few alternatives I have to a fixed 3B (prop in aperture, so little room for a folder) - but ONLY if it has significantly less drag.
[/ QUOTE ]
The drag (at a guess) would be similar to a fixed 3 bladed, if not slightly more, because by looking at it it seems to have a larger surface area with its square blades rather than the rounded blades of normal props. But I'm no expert I just take photos /forums/images/graemlins/wink.gif
If you can't fit a folding prop in your aperture, worry not, your other alternative is a feathering prop, and that have a lot less drag than a standard 3B fixed prop.
I'm going to put a gallery of extra photos from the prop test up soon so watch this space /forums/images/graemlins/smile.gif
Maybe I couldn't see it for looking, but where are the drag results for the Axiom? This is a prop I have been curious about for a while, and may be one of the few alternatives I have to a fixed 3B (prop in aperture, so little room for a folder) - but ONLY if it has significantly less drag.
[/ QUOTE ]
The drag (at a guess) would be similar to a fixed 3 bladed, if not slightly more, because by looking at it it seems to have a larger surface area with its square blades rather than the rounded blades of normal props. But I'm no expert I just take photos /forums/images/graemlins/wink.gif
If you can't fit a folding prop in your aperture, worry not, your other alternative is a feathering prop, and that have a lot less drag than a standard 3B fixed prop.
I'm going to put a gallery of extra photos from the prop test up soon so watch this space /forums/images/graemlins/smile.gif
snooks
Well-Known Member
Gallery of extra images now available /forums/images/graemlins/smile.gif
Folding, Feathering and Fixed propeller gallery
Folding, Feathering and Fixed propeller gallery
damo
Well-Known Member
Thats great, thanks. Shame about the probable drag on the Axiom - the best feathering props would cost me twice as much /forums/images/graemlins/frown.gif
Maybe I'll stick with the old one for a few years, although the thought of the extra days added to long passages is a bit disheartening.
Maybe I'll stick with the old one for a few years, although the thought of the extra days added to long passages is a bit disheartening.
DaveS
Well-Known Member
Very interesting. What's shown in picture 9766?
Looking again, is it an underwater camera?
Looking again, is it an underwater camera?
