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Old 01-05-2016, 01:41   #1
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Polars building

I am building polar diagram for my L 400. Boat is heavy by birth, however I keep it loaded in lower range. I use extra ropes to optimise sail shape.

Topic the other day was reaching and broad reaching. I was able to nicely keep above 50% of apparent wind speed for true angles 90 to 140 or so, however, only to 15 kn apparent. After that every increment in wind speed resulted in minimal increase of speed. Hull speed is around 8.4 so that is probably not the reason.

Any idea why this may be ?
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Old 01-05-2016, 01:51   #2
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Re: Polars building

How are you measuring speed?


If your paddle wheel is a little gunked up or not positioned in clean water flow, it could be reading 7.5 when you are actually doing 8.4.
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Old 01-05-2016, 02:30   #3
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Re: Polars building

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Originally Posted by valhalla360 View Post
How are you measuring speed?


If your paddle wheel is a little gunked up or not positioned in clean water flow, it could be reading 7.5 when you are actually doing 8.4.
i have fixed paddle wheel, by entering new values and confirmed with running circles with engines & gps from 4 to 7 kn. And it comes within 0.2 kn discrepancy. So quite confident speed over water has error less than 0.2 kn up to 7 kn.

I have no idea after 7 kn speed over water. Can do 8kn with both engines which i will do next time. Tanks for hint.
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Old 01-05-2016, 05:10   #4
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Re: Polars building

Um, my apologies in advance for the “thesis” on this topic. It’s the kind of thing which gets me excited. Especially when I find people who Want to learn about such obscurities

It would help if you could post the data which you have, especially for the wind ranges & speeds which you're inquiring about. For us to assist you, diagnostically, that is.
And, BTW, BRAVO on doing your own Polars. That’s very uncommon, & also commendable. You’ve restored some of my faith in CF .

Also, how do you know that your boat is one of the heavy ones for her design? As in general, Travel Lifts aren’t a great way to judge the weight of a boat. That, & if you’re curious to experiment, with regards to your Polars, try offloading everything onboard, except for what you need for your sail & speed testing that day. And then compare those numbers to what you have now.



Anyway, some of the "hump" may be due to your headsail being blanketed a lot more by the main, when sailing in that AWA range. Assuming that you're not switching to a larger headsail, a screecher, or a kite. Particularly if they're sprit mounted (especially so with articulating sprits), as they get the headsail sail further out of the shadow of the main, where it can do you a lot more good.

Too, the hump your hitting is definitely affected by weight. That's why all of the downwind machines are so weight conscious. Like the boats designed for the TransPac for example. Or the RTW racers.

But also, you have to remember that wind is a squared function. So that a wind speed increase from 5kts to 10kts, quadruples the force on the sails. Whereas when it goes from 10kts to 15kts, the force only increases by 2.25x.
Thus, above 15kts, each couple of knots in increase nets you a much smaller gain than at lower wind speeds.



Again, were it me, I’d nicely ask the manufacturer (& design office) for Polars on your boat. That way, you can compare the theoretical numbers from them, to the real world numbers which you're getting. Plus, when you’re chatting with them, also bring up the weight issue, & the speed issues as well.
That, & if they wont give them to you when you ask nicely, there are “other” ways of asking. And, too, you can do some inquiring with the owners of other boats of the same type, & year. As perhaps your boat’s not so heavy, or slow, for her model, after all. You’ll never know until…

Then, once you get the factory Polars (or even before), it's worth "bribing" a couple of racers, & or a/your sailmaker to come out with you for a day or three, & work on your sail trim. And see what you can do to improve things. As some of it might be as simple as shifting some of the heavier items on your boat around, or tuning your batten tension.
Plus some guys are wizards with instruments, their calibration, & computers, etc. As your problem could be as simple as the fact that your instruments are getting upwash off of the sails on those wind angles, & it’s affecting the accuracy of their readings.

As, for example, in order to avoid this from being as much of a factor in the ’92 America’s Cup, some of the masthead instrument wands were several meters long, & high. So much so that the rule makers had to institute a limit on their size.
That, & Lots of computer time went into modeling sailplans, & wind flow off of them at various wind angles & speeds, so that upwash could be dialed out via tweaks to the instrument software. And this software tuning has only gotten better with time, if you know how to apply it.



Also, there are a LOT of tuning tips on both L-36.com & www.Forums.SailingAnarchy.com Ditto on some of the better sailmaker sites, like North, or Quantum. Albeit, sometimes said info comes & goes, or needs a bit of digging to find it.

And all of that said, usually you can pick up a good quality used; kite, genoa, screecher, or staysail, pretty inexpensively (under $1K, including a snuffer, or accessory hardware). If you really want to add some HP.

PS: Might I also suggest that you get a hand held anemometer. They even make them for Smart Phones. Or you can buy a Kestrel hand held weather station, with loads of atmospheric recording instruments & data built in. Even Blue Tooth setups, so that it can talk to your laptop.
Though if you do snag one, just be aware of the differences in windspeed at 2m vs. 20m, as such is a simple fact of Physics.
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Old 01-05-2016, 05:55   #5
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Re: Polars building

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Originally Posted by UNCIVILIZED View Post

But also, you have to remember that wind is a squared function. So that a wind speed increase from 5kts to 10kts, quadruples the force on the sails. Whereas when it goes from 10kts to 15kts, the force only increases by 2.25x.
Thus, above 15kts, each couple of knots in increase nets you a much smaller gain than at lower wind speeds.


While your math is technically correct, it implies an incorrect assumption. Due to the square effect at high wind speeds a small change in wind speed should result in large change in available power.

To put it into easier to understand terminology, lets say at 5kts wind speed, the sails produce 5hp (exact amount not critical just to demonstrate the principal)
- 5kts wind = 5hp
- 10kts wind = 20hp
- 15kts wind = 45hp
- 17kts wind = 58hp

Even though the last step only increased the wind by 2kts, the additional power was more than 2.5 times as much as the initial 5kts of wind produced.

I do agree though that it is most likely something to do with the measurements. It might be speed thru the water or calibrating the wind instruments to get accurate measurements. Particularly at higher speeds (wind and water), small changes can have big impacts.

I'm still betting on the paddle wheel. If the up current vs down current is 4 vs 7 kts, that's a pretty good current and local eddies or variations in the tide state could be throwing off your results. A 1.5kt difference in the current could easily explain things as you would be running at hull speed and the bow wave is soaking up the extra power that higher winds generate.
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Old 01-05-2016, 06:29   #6
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Re: Polars building

Thanks uncivilised for feedback. I will be mauling thru this so next weekend can try more things. This are polars from older similar boat model Lagoon 41, used for comparision. Polares provided by factory for my model just do not make any sense. French thing i guess.

Circled areas are one that i matched or exceeded using basic sails only.

increase in speed at 110 is due to use of spi, not tested yet, no idea.

Older model does not have some kind of speed barrier at 7.5 kn speed or even 8.4 kn. As new model similar, and I can replicate circled areas, see no physical reason not be able to replicate more, especially speed increase stopping with such a force, like 0.3 kn for extra 2 knots of app wind speed. Up to that point I get 1 kn of speed for 2 kn of app wind speed.

Speedo is sure suspect. However, maybe sail trim need to be changed for higher app wind....
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Old 01-05-2016, 07:46   #7
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Re: Polars building

Quote:
Originally Posted by arsenelupiga View Post
Thanks uncivilised for feedback. I will be mauling thru this so next weekend can try more things. This are polars from older similar boat model Lagoon 41, used for comparision. Polares provided by factory for my model just do not make any sense. French thing i guess.

Circled areas are one that i matched or exceeded using basic sails only.

increase in speed at 110 is due to use of spi, not tested yet, no idea.

Older model does not have some kind of speed barrier at 7.5 kn speed or even 8.4 kn. As new model similar, and I can replicate circled areas, see no physical reason not be able to replicate more, especially speed increase stopping with such a force, like 0.3 kn for extra 2 knots of app wind speed. Up to that point I get 1 kn of speed for 2 kn of app wind speed.

Speedo is sure suspect. However, maybe sail trim need to be changed for higher app wind....
a. Very likely the factory polar is a prediction, not based on actual sailing.
b. It would be based on a bare boat.
c. Boy, she's slow to windward. I would be scratching my head on keel and sail plan mods. That's where you have potential for big gains. It's probably not so much lack of power as leeway and balance.
d. Make sure you bearings are over ground, checking both tacks. If you can, test at slack tide that is also a big help; correcting is difficult at best.
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Old 01-05-2016, 07:48   #8
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Re: Polars building

Quote:
Originally Posted by arsenelupiga View Post
Thanks uncivilised for feedback. I will be mauling thru this so next weekend can try more things. This are polars from older similar boat model Lagoon 41, used for comparision. Polares provided by factory for my model just do not make any sense. French thing i guess.

Circled areas are one that i matched or exceeded using basic sails only.

increase in speed at 110 is due to use of spi, not tested yet, no idea.

Older model does not have some kind of speed barrier at 7.5 kn speed or even 8.4 kn. As new model similar, and I can replicate circled areas, see no physical reason not be able to replicate more, especially speed increase stopping with such a force, like 0.3 kn for extra 2 knots of app wind speed. Up to that point I get 1 kn of speed for 2 kn of app wind speed.

Speedo is sure suspect. However, maybe sail trim need to be changed for higher app wind....
a. Very likely the factory polar is a prediction, not based on actual sailing.
b. It would be based on a bare boat.
c. Boy, she's slow to windward. I would be scratching my head on keel and sail plan mods. That's where you have potential for big gains. It's probably not so much lack of power as leeway and balance. If I was make 5kn at 15 true, 55 true, I'd be looking to see if I had snagged a crab pot or several. Should be more like 8kn.
d. Make sure you bearings are over ground, checking both tacks. If you can, test at slack tide that is also a big help; correcting is difficult at best.
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Old 01-05-2016, 07:50   #9
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Re: Polars building

To the OP. Sorry for the thread drift here.

Quote:
Originally Posted by valhalla360 View Post
While your math is technically correct, it implies an incorrect assumption. Due to the square effect at high wind speeds a small change in wind speed should result in large change in available power.

To put it into easier to understand terminology, lets say at 5kts wind speed, the sails produce 5hp (exact amount not critical just to demonstrate the principal)
- 5kts wind = 5hp
- 10kts wind = 20hp
- 15kts wind = 45hp
- 17kts wind = 58hp

Even though the last step only increased the wind by 2kts, the additional power was more than 2.5 times as much as the initial 5kts of wind produced.

I do agree though that it is most likely something to do with the measurements. It might be speed thru the water or calibrating the wind instruments to get accurate measurements. Particularly at higher speeds (wind and water), small changes can have big impacts.
Ah, speed changes in the LOWER ranges have bigger impacts on things than they do in the higher ranges. They're squared functions, remember. And this applies to wind instruments as much as anything else.

If his boat speed instruments are within 0.2 kts via GPS comparison, assuming that that's true on all headings. Then the only probable out of cal. instruments are those measuring the wind.

I'm still betting on the paddle wheel. If the up current vs down current is 4 vs 7 kts, that's a pretty good current and local eddies or variations in the tide state could be throwing off your results. A 1.5kt difference in the current could easily explain things as you would be running at hull speed and the bow wave is soaking up the extra power that higher winds generate.

Yes, without question, out of cal. boat speed instruments can have a HUGE impact on everything else.

The best skipper I ever sailed for/with had us calibrate out B&G system weekly, the day before race day, every week. In great depth.
And, BTW, he was the #2 Star boat sailor in the world at the time. Which many serious sailors hold to mean more than having Olympic Medals in most any other class. It is, or was (then) That competitive.
Okay, so despite my insomnia last night, & me as yet being uncaffeinated for the day, I Know that my math's correct. I've been doing this stuff for many, many years. And it ain't complex math.
Re-read your own post, & cross check the text vs. the numbers.

See, what I'm saying is that at lower wind speeds, a small increase (or decrease) in the wind speed has a MUCH bigger affect on how much power is available to drive a boat, than there is with the same level (numerical value) of wind speed change, in the higher wind ranges.

When the wind speed jumps from 5kts to 10kts, you then have 4x the available HP to drive the boat.
To get that same level of increase (in "HP") when the wind speed is at 10kts, the wind speed has to go up to 20kts, so as to get a 4x HP increase again.

And when the wind speed goes from 15kts to 17kts (as per your example), the available energy to drive the boat Only increases by 28%. Period.
But if the wind speed increases from 5kts to 7kts, The SAME 2kt increase, then the energy available (in theory) to drive the boat Doubles!
Make sense?

Force (in lbs) = wind speed in kts (squared) x sail area in square feet x 0.00431
The math don't lie. And you'd have to have Thor hit me with a dozen lightning bolts in order to get that equation out of my head. If then.
http://www.harken.com/content.aspx?id=9096


Now the thing is, that drag. That nasty little thing which a sailboat has to overcome in order to go faster. Also increases as a squared function (pretty much). Especially drag through the water, unless we're talking about planing boats, or skinny hulled, fast multl's. Which Lagoons ain't.

So that up until they break past the hump of their hull speed (a drag limited thing), which takes a certain peak HP to overcome, they're limited in speed, by available HP, as are most other boats.
And at lower wind speeds, as are in question, with the available sail area, the boat wont jump past it's hull speed, & into the double digits on the speedo.
Well, unless asssted by a large wave.
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Old 01-05-2016, 08:19   #10
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Re: Polars building

Quote:
Originally Posted by arsenelupiga View Post

(...)

I was able to nicely keep above 50% of apparent wind speed for true angles 90 to 140 or so, however, only to 15 kn apparent. After that every increment in wind speed resulted in minimal increase of speed. Hull speed is around 8.4 so that is probably not the reason.

Any idea why this may be ?
It is. Or to be more precise, it is on of the factors.

Force required to reach the hull speed are not linear. Compare to hp required to drive a boat at her HS vs. hp required to drive the same boat at 1 x sqrt (lwl).

Chuck in factors related to going from laminar to turbulent, more wave resistance, loses in pitching and poling, etc.

b.
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Old 01-05-2016, 08:53   #11
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Re: Polars building

Quote:
Originally Posted by arsenelupiga View Post
Thanks uncivilised for feedback. I will be mauling thru this so next weekend can try more things. This are polars from older similar boat model Lagoon 41, used for comparision. Polares provided by factory for my model just do not make any sense. French thing i guess.

Circled areas are one that i matched or exceeded using basic sails only.

increase in speed at 110 is due to use of spi, not tested yet, no idea.

Older model does not have some kind of speed barrier at 7.5 kn speed or even 8.4 kn. As new model similar, and I can replicate circled areas, see no physical reason not be able to replicate more, especially speed increase stopping with such a force, like 0.3 kn for extra 2 knots of app wind speed. Up to that point I get 1 kn of speed for 2 kn of app wind speed.
Here, you're likely running into hull form drag issues, & WL length vs. the bigger boat.
Or in english, in order to keep the hulls big enough for the accomodation level desired, they're wide relative to their length. Even as compared to the boat in the Polars provided.
Which I'm guessing is where that "hump" you're running up against is coming from.

Speedo is sure suspect. However, maybe sail trim need to be changed for higher app wind....
There's not an easy way to say this, but... After peeking at the Polars. The bottom line is that you're suffering from; a heavy boat (or rather, design), with keels & rudders which produce low lift & a lot of drag, & also, a lack of enough sail area.
The latter of the 3 is fixable, somewhat. The foils too, but for much more $.

If you compare your Polars to something reasonably fast, you'll see the difference. Especially upwind. Albeit, they're lead mines.
And too, while it's not the clearest, you can compare their theoretical Polars, to their real world speed data via the Ockam #s (in light ship form).

Also, what the Polars for these boats don't show, is that from 070 AWA & further aft, if there are any sort of waves & 2-digit winds, they start to surf, & or plane, like dinghies.
Aerodyne 38 | Rodger Martin Design
http://www.rodgermartindesign.com/wp...A38-Polars.pdf
http://www.rodgermartindesign.com/wp...Ockam-Data.pdf

Aerodyne 43 | Rodger Martin Design
http://www.rodgermartindesign.com/wp...ne4polars-.pdf
http://www.rodgermartindesign.com/wp...ne4polars-.pdf
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Old 01-05-2016, 12:36   #12
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Re: Polars building

arsenelupiga,
My apologies if I was short earlier, & ditto on my style of delivery of the information which I posted.
I don't know that it'll help with how you may or may not be feeling, after my previous post, & the news that it contains. For I know that I surely wouldn't be happy to hear such.
But below is a link to an article which explains why some cat's, those designed akin to yours, sail the way that they do.

And, if you ever desire to, I'm happy to offer up tips & ideas on how to make her sail better than she does now... Without making too big a dent in your wallet... well, maybe a middle sized one (within reason).
And there Are some possibilities, especially if you're handy with tools.
So, basically, I'm saying that I'm sorry for having to be the bearer of bad news, earlier. And or, if I was a horse's backside, via my delivery style in my previous post.

Anyway, John Shuttleworth is a noted, talented, multihull designer. With many types of boats out there, a good number of them being high performance ones. And when paying his dues, in establishing himself as a designer/NA (Naval Architect), he penned many an article, which explained, what made a boat fast or slow, whether or not it pointed well, & why or why not.
Plus listing off other performance defining, design characteristics too; in both layman's & engineering terms. Which is what this one covers. Considerations for Seaworthiness And perhaps some of the sailing & design wisdoms in it may prove to be helpful.
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Old 01-05-2016, 13:44   #13
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Re: Polars building

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a. Very likely the factory polar is a prediction, not based on actual sailing.
b. It would be based on a bare boat.
c. Boy, she's slow to windward. I would be scratching my head on keel and sail plan mods. That's where you have potential for big gains. It's probably not so much lack of power as leeway and balance. If I was make 5kn at 15 true, 55 true, I'd be looking to see if I had snagged a crab pot or several. Should be more like 8kn.
d. Make sure you bearings are over ground, checking both tacks. If you can, test at slack tide that is also a big help; correcting is difficult at best.
for 50 true, it does 6kn in 15 kn wind - that is how much i manage in reasonable conditions.

It does 5 in 15 kn true at 43 true angle.

i only use AP.
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Old 01-05-2016, 16:13   #14
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Re: Polars building

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for 50 true, it does 6kn in 15 kn wind - that is how much i manage in reasonable conditions.

It does 5 in 15 kn true at 43 true angle.

i only use AP.
I understand you are driving in fully built up seas?

Does not sound too slow to me. I found it difficult to sail fast upwind when the seas are proportional to 15 -20 knots of wind: the wave shape is so that it makes for either very wet and bouncy or else SLOWER ride.

I know only one boat that sailed truly well upwind in 15 to 20 knots. Not a cat either.

b.
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Old 01-05-2016, 16:48   #15
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Re: Polars building

Nope, that is slow. Quite Slow. Really Slow.
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