Folding vs Fixed Prop STW?

[Kings Gambit]

Still muddling over this question from a few weeks back. The question of what a folding prop does to increase STW; by what amount? I refer again to the PHRF time-on-distance handicapping as a starting place. A yacht with a folding prop takes a 3 sec-per-mile penalty to its time-on-distance handicap over one with a two-blade fixed prop. Say I’m at PHRF 105 base rating with the fixed prop. I install a folding prop and take the penalty to PHRF 102 base rating.

From that I’ll refer to the “ideal” race where absolutely everything else is the same between two yachts, including crew, which make no mistakes sailing in perfect trim and steering the shortest course. On a one nautical mile course, the yacht with the folding prop, which functions perfectly as it should, arrives 3 sec ahead of the yacht with the fixed prop. This is a constant (linear) multiplier. So, if the course were 10 n.mi., the folder would arrive 30 sec ahead of the fixed. However, corrected time would be equal. The correction is independent of STW, apparently.

However, 1 knot STW on the short course would be one knot, 1 n.mi./hr= 1 n.mi./3600 sec. But the folder is traveling faster as it arrives 3 sec earlier. If the fixed is at one knot, the folder would be 1 n.mi./3597 sec. Thus, 3600/3597= 1.0008 kt improvement. On a 10 n.mi. course traveling at 1 kt, we have (3600/3570) = 1.008 improvement for the folder. Notice the one decimal place improvement in the correction multiplier. If a100 n.mi. course @ 1 kt we have 3600/3300 = 1.09 kt improvement. I reckon from this that if the fixed-prop yacht is averaging 7 kt, the folder must be averaging 7(1.09)= 7.63 kt for the boats to correct out to equal time-on-distance over a 100 n.mi. course. So I can see where the notion of a half-knot improvement for a folder might come from, especially in our annual N2E setting. But we have only 7(1.008)= 7.05 kt on the 10 n.mi course. Something peculiar here. The whole thing blows up with the next order of magnitude of distance.

 

[kloudie1]

Some hidden issues here.. The biggie is that the handicap is to make things equal and they are using real world race data.. my (barely educated) opinion is that the folks who've fitted folding/feathering props are more likely to be better racers than donkeys like me! so the prop is penalized to a greater extent than would be obvious from actual speed/drag data.. I dunno..

 

[Scott T-Bird]

No handicap will ever be perfect or even fair ... that said, I tend to think that there are way too many variables in wind speed and direction to give you any kind of straight line predictability. For instance, I would assume that if you have a wind that is strong and steady and your sailing direction is constant, the significance of the prop would matter less as the constant speed variability may be insignificant. As the boat sails along basically at hull speed, does the prop matter? It seems to me that the significance of the prop is essentially in acceleration quickness. The more changes in direction, the more variable the boat speed, the more significant the prop. So you can't really compare average boat speed at variable lengths.

 

[jon hansen]

Still muddling over this question from a few weeks back. The question of what a folding prop does to increase STW; by what amount? I refer again to the PHRF time-on-distance handicapping as a starting place. A yacht with a folding prop takes a 3 sec-per-mile penalty to its time-on-distance handicap over one with a two-blade fixed prop. Say I’m at PHRF 105 base rating with the fixed prop. I install a folding prop and take the penalty to PHRF 102 base rating.

From that I’ll refer to the “ideal” race where absolutely everything else is the same between two yachts, including crew, which make no mistakes sailing in perfect trim and steering the shortest course. On a one nautical mile course, the yacht with the folding prop, which functions perfectly as it should, arrives 3 sec ahead of the yacht with the fixed prop. This is a constant (linear) multiplier. So, if the course were 10 n.mi., the folder would arrive 30 sec ahead of the fixed. However, corrected time would be equal. The correction is independent of STW, apparently.

However, 1 knot STW on the short course would be one knot, 1 n.mi./hr= 1 n.mi./3600 sec. But the folder is traveling faster as it arrives 3 sec earlier. If the fixed is at one knot, the folder would be 1 n.mi./3597 sec. Thus, 3600/3597= 1.0008 kt improvement. On a 10 n.mi. course traveling at 1 kt, we have (3600/3570) = 1.008 improvement for the folder. Notice the one decimal place improvement in the multiplier. If a100 n.mi. course @ 1 kt we have 3600/3300 = 1.09 kt improvement. I reckon from this that if the fixed-prop yacht is averaging 7 kt, the folder must be averaging 7(1.09)= 7.63 kt for the boats to correct out to equal time-on-distance over a 100 n.mi. course. So I can see where the notion of a half-knot improvement for a folder might come from, especially in our annual N2E setting. But we have only 7(1.008)= 7.05 kt on the 10 n.mi course. Something peculiar here. The whole thing blows up with the next order of magnitude of distance.

all of that is a whole bunch of nothing. zero logic applied. zero science. zero common sense. you do not understand handy capping. you do not understand sailboat racing.

sorry king, just the facts. i am sure you will beat me 100% of the time playing chess. ................. but dude, you are way out of bounce on all the above misguided conjecture.

 

[Kings Gambit]

No handicap will ever be perfect or even fair ... that said, I tend to think that there are way too many variables in wind speed and direction to give you any kind of straight line predictability. For instance, I would assume that if you have a wind that is strong and steady and your sailing direction is constant, the significance of the prop would matter less as the constant speed variability may be insignificant. As the boat sails along basically at hull speed, does the prop matter? It seems to me that the significance of the prop is essentially in acceleration quickness. The more changes in direction, the more variable the boat speed, the more significant the prop.

Yeah. Thanks. My point really is not about the handicap itself. Only how a folder affects STW using the handicap as a first-cut estimate of the change in STW. It might not be a good choice. Since hull speed is the ultimate limiter, we have to assume that the “improvement” reaches maximum near 7.5 kt for most of us. Once there, is no further improvement with the folder. Maybe PHRF bases the folder penalty in relation to predicted max hull speed.

 

[jon hansen]

sent you a PM with my phone number. i'm not typing all afternoon

 

[jon hansen]

the improvement reaches maximum when the wind goes soft and the boats are going 2knt. your thinking is upside down.

 

[Scott T-Bird]

I don't really know how corrected time is calculated so I am having difficulty in following your mathematics. But, essentially, you are saying that if the boat with fixed prop sails a 100 nm course at an average of 7 knots, than a boat with folder would have to average more than 7.63 knots to win. Intuitively, that does not seem like it would be a good handicap application, because the average speeds over 100 nm should be far closer. More distance = less variability in avg boat speed. Less distance should equal more variability due to the influence of acceleration and deceleration. Intuitively, I would suggest that handicapping favors the more heavily handicapped boat as length of course increases.

 

[Davidasailor26]

PHRF, at least in the time-on-distance mode, can't possibly account for the different advantages that a boat will have in different wind conditions. The ideal PHRF rating provides an estimate of the boat's performance in the expected conditions. That also varies by area, as conditions differ (though the prop adjustment, and other adjustments are usually the same, probably just because they're not scientific enough).

I think your math might be confused in your original example. Also, the fixed prop adjustments I've seen are generally 6 or 9 seconds for a fixed prop (depending on blade count), not 3. So at 1 knot a 2-blade fixed prop boat will cover each mile in 3600 seconds, and the folding prop boat (according to PHRF) would go just faster enough cover that time in 3594 seconds. So PHRF is assuming that the folding prop boat goes just 0.0017 knots faster in those conditions. If the folding prop boat goes 7 knots for an hour it'll cover that mile in 514 seconds, so PHRF assumes the folding prop boat covers the same time in 508 seconds. So the folding prop boat must be going 7.087 knots.

This is where the trouble with PHRF comes in - A constant time/mile offset makes less difference when the times are large (low wind) than when they are small (high wind). That's exactly the opposite of the actual benefit of a folding prop - the benefit is bigger at lower wind. For that reason I don't think using PHRF to predict an actual speed difference is a very good approach. I also don't think their adjustments, even at high wind range, are enough to truly offset the performance increase of the folding prop.

 

[Davidasailor26]

Here's an actual plot of my boat's speed from a couple seasons of 3 blade fixed prop (orange dots) to 3 blade folding prop -

Notice that at low wind speed the fixed prop slows the boat by about a knot. They converge as the wind increases. At higher wind speed the fixed prop created lots of turbulence in front of the rudder, stalling it, but the folding prop lets it continue to work better.

(Data is from upwind legs of races for a few seasons. To be fair we had more experience with the boat after a few seasons, so some of the speed difference might be improved tactics and sail trim, but probably not all of it.)

 

[jon hansen]

Here's an actual plot of my boat's speed from a couple seasons of 3 blade fixed prop (orange dots) to 3 blade folding prop -
View attachment 192211

Notice that at low wind speed the fixed prop slows the boat by about a knot. They converge as the wind increases. At higher wind speed the fixed prop created lots of turbulence in front of the rudder, stalling it, but the folding prop lets it continue to work better.

(Data is from upwind legs of races for a few seasons. To be fair we had more experience with the boat after a few seasons, so some of the speed difference might be improved tactics and sail trim, but probably not all of it.)

nice info. doubled your speed on the low end. you'll always beat the other boat when your going twice as fast.

 

[jon hansen]

i know these boys think i'm picking on them. really i'm not.

learning about these toys is fun, right?

 

[jon hansen]

most sailors do not sail in 3 knt of wind, or 30 knt of wind. look at david's graph. where most of you sail there is not much difference in speed. so you say, "i don't race. i just cruise" and then you are totally happy dragging around your motorboat prop.


not me 'baby', 'more sail'

 

[dLj]

So my question here lies not in racing but in cruising. An aspect that isn’t being talked about is reliability. The fixed prop is highly reliable. How does that compare to folding props?

the other aspect is maneuverability. It’s my understanding that then really becomes a question of prop design. The folding props that fold flat run quite differently that the feathering props where the blades turn. It’s the feathering props that are said to run better in reverse than the fixed blades and the fixed blades may run better in reverse that the folding props - maybe?

So for me it is the overall advantage - or not - that one prop has over the other and when all those are looked at how does the traditional fixed blade stack up and is really worth the investment in a non-fixed blade prop. Really I only have questions not answers.
dj

 

[Kings Gambit]

I think your math might be confused in your original example.
Also, the fixed prop adjustments I've seen are generally 6 or 9 seconds for a fixed prop (depending on blade count), not 3. So at 1 knot a 2-blade fixed prop boat will cover each mile in 3600 seconds, and the folding prop boat (according to PHRF) would go just faster enough cover that time in 3594 seconds. So PHRF is assuming that the folding prop boat goes just 0.0017 knots faster in those conditions. If the folding prop boat goes 7 knots for an hour it'll cover that mile in 514 seconds, so PHRF assumes the folding prop boat covers the same time in 508 seconds. So the folding prop boat must be going 7.087

Not too confused re: my “point” since we arrive at basically the same outcome by those numbers. But, your empirical(?) data gives a better picture.

 

[Davidasailor26]

So my question here lies not in racing but in cruising. An aspect that isn’t being talked about is reliability. The fixed prop is highly reliable. How does that compare to folding props?

the other aspect is maneuverability. It’s my understanding that then really becomes a question of prop design. The folding props that fold flat run quite differently that the feathering props where the blades turn. It’s the feathering props that are said to run better in reverse than the fixed blades and the fixed blades may run better in reverse that the folding props - maybe?

So for me it is the overall advantage - or not - that one prop has over the other and when all those are looked at how does the traditional fixed blade stack up and is really worth the investment in a non-fixed blade prop. Really I only have questions not answers.
dj

A little bit away from the OP’s question about speed through the water now, but you raise some interesting points. Certainly everything is a compromise, and it seems with props most people are happy with the choices they make, whether traditional fixed, Campbell Sailor style fixed, folding, or feathering.

For me the folding is a good set of compromises. The increased sailing speed is its main pro. It does have its con’s but none of them have ever bitten me - the Flex O Fold is pretty much maintenance free, has never failed to open for me, has the same speed under engine, and still provides plenty of reverse thrust with only half the prop walk of a fixed.

 

[Davidasailor26]

most sailors do not sail in 3 knt of wind, or 30 knt of wind. look at david's graph. where most of you sail there is not much difference in speed. so you say, "i don't race. i just cruise" and then you are totally happy dragging around your motorboat prop.


not me 'baby', 'more sail'

I have the highest density of dots (most often sailing) in 4-8 knots of wind. That’s not the range with the biggest speed difference, but it is still significant, especially in the lower part of that range. Plus I have a boat that was already very competitive in higher wind ranges, so the lower wind was really where I needed the help.

 

[Davidasailor26]

I don't really know how corrected time is calculated so I am having difficulty in following your mathematics. But, essentially, you are saying that if the boat with fixed prop sails a 100 nm course at an average of 7 knots, than a boat with folder would have to average more than 7.63 knots to win. Intuitively, that does not seem like it would be a good handicap application, because the average speeds over 100 nm should be far closer. More distance = less variability in avg boat speed. Less distance should equal more variability due to the influence of acceleration and deceleration. Intuitively, I would suggest that handicapping favors the more heavily handicapped boat as length of course increases.

To calculate corrected time you just take your elapsed time in seconds and subtract (handicap * distance).

I’m not sure I understand or agree with you that wind variability changes the effect of a handicap. A handicap is an estimation of how much faster one boat is than another. It is one number that approximates that speed difference across the full range of racing conditions (at least in PHRF time on distance). What you find when you look at handicap data is that it’s hard to come up with that one right number and have it work in all conditions. A small light boat will be relatively fast in low wind, but fall back to a larger heavier boat once everyone is going hull speed. A fixed prop is a constant drag reducing speed, whereas a heavier keel affects acceleration in a gust more than being a constant drag. All of these types of variables are what PHRF tries to approximate into that single number.

I guess going to your point about longer races, they could help balance out handicap anomalies. If you have a 20 hour race with a wide range of conditions that will tend to balance out the inequities of the PHRF approximations. But if you have, for example, a Beneteau 34 and a 37, largely the same design except for LWL, sail area, and displacement, I don’t think a longer race benefits either from a handicap standpoint.

 

[Scott T-Bird]

I think your graph best represents my theory/opinion. Look on the graph where performance between the 2 prop variables is closest together. It's at the point where you might average over a long distance. Therefore, the advantage would seem to be given to the boat with the larger handicap because the performance is nearly equal to the boat with the lesser handicap. I don't think I agree that a long race helps balance the anomalies, in fact I think it is the opposite. I think that the longer the race, the larger the imperfection of the handicap looms. And I also would suggest that the drag from the prop becomes less of a disadvantage as the boat speed increases, just as you are suggesting that a light boat loses it's advantage as boat speeds increase to hull speed. Intuitively, I think that a folding prop loses it's advantage the longer boat speeds remain relatively constant (also with less maneuvers, typical of a long race) where acceleration is less a factor.

On one thing we agree, that it is hard to come up with fair numbers. That is probably an understatement ... it's impossible to come up with fair numbers. It would seem to be the single largest issue that inflames racers.

 

[dlochner]

Regarding the handicap penalty for a folding or feathering prop, that number 3 sec per mile was pulled from somewhere where the sun don't shine. That's also true for all the other adjustments. There is no empirical data to support 3 seconds per mile vs 6 or 9.

Folding and feathering props cut drag, less drag more speed. I don't know that one brand is significantly better than another. The difference is most noticeable at low speeds.

When it comes to motoring, feathering props beat folding props because the blade shape is better in reverse. I've had a Martec Folding Prop and a Max Prop Feathering prop, both worked well. The Max Prop works exceptionally well in reverse.