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by CapriSun. 11/12/20 09:41 AM
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heavy downhaul in light air #277559
02/12/15 03:32 PM
02/12/15 03:32 PM

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I think someone recently mentioned using heavy down and outhaul in very light air.

I can't find this thread using the search feature

is this correct?
and if so, what is the theory behind it?

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Re: heavy downhaul in light air [Re: ] #277560
02/12/15 03:45 PM
02/12/15 03:45 PM
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SE MI / NE IN
rehmbo Offline
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I've read it in more than a few places. Did a quick google search and found this article - check the bottom two paragraphs of page 1 here.

Key points seem to be not have too much camber/draft allowing premature separation and don't let the leach hook. I saw some of the latter at Charlotte Harbor Sunday when it got really light.

With our square-top mains, I think our boats would need a bit more Cunningham to keep the top of the sail open. Apparent wind up high will be coming from a different direction than what we feel down on the boat.

Not sure that its gospel, but its something I'm going to try a bit more of later this year.

Last edited by rehmbo; 02/12/15 03:49 PM. Reason: Added 3rd paragraph

Jeff R

H18, C2 USA1193
cramsailing.com
crescentsail.com
Re: heavy downhaul in light air [Re: ] #277562
02/12/15 03:59 PM
02/12/15 03:59 PM

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Thanks!

Quote
Apparent wind up high will be coming from a different direction than what we feel down on the boat.

I read that wind on the surface usually contains wind coming from above and bouncing off the water ...

Re: heavy downhaul in light air [Re: ] #277563
02/12/15 04:21 PM
02/12/15 04:21 PM
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rehmbo Offline
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In light air (less than 5kts), the air is usually in a state of laminar flow. The air at water level is essentially at a speed of 0kts and linearly increases to the overall flow speed of the air mass 10-20m up. Maintaining adequate twist is really important in these conditions.

Depending on temperature, pressure, humidity, and a few other variables, the wind trips turbulent around 5-6kts. Then you get a lot more mixing. Then you need more camber and much less twist.

Bethwaite has a big section on this in his 1st book.


Jeff R

H18, C2 USA1193
cramsailing.com
crescentsail.com
Re: heavy downhaul in light air [Re: ] #277565
02/12/15 04:29 PM
02/12/15 04:29 PM

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great info - thanks

Re: heavy downhaul in light air [Re: ] #277566
02/12/15 05:08 PM
02/12/15 05:08 PM
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South Carolina
Jake Offline
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I've heard a lot of theories about that over the years and I'm a believer in it through experience. I think the biggest thing that it does for you is allows you to keep some leach tension on the main, and subsequently the draft of the main in a better position, without hooking the main. Without the downhaul, if you sheeted tightly enough to help the draft form properly in the sail, you would have a hook in the top of the main which points the driving forces aft of the lateral resistance of the hulls through the water...meaning; the top of the sail is trying to sail the boat in reverse while the bottom is trying to sail forward...and that's not fast.

that's my interpretation...so take it with that value.

I once raced a Hobie wave with a badly shaped main and it would hook if I put much sheet tension on it at all (the downhaul on that borrowed boat was not functioning well...it would have helped). If I sheeted one click too far, I could feel the drive in the sail evaporate. The effect of the hook was pretty dramatic.


Jake Kohl
Re: heavy downhaul in light air [Re: Jake] #277568
02/12/15 05:43 PM
02/12/15 05:43 PM

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interesting - thanks

i was reminded about that theory here recently, tried it several times and felt i benefited by doing it

Re: heavy downhaul in light air [Re: ] #277569
02/12/15 06:34 PM
02/12/15 06:34 PM
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I've learned from several H16 national champions that in very light air, flatter is better, so the wind stays attached to the leeward side, aft of the pocket. This also dictates your batten tension (looser is better, which seems counterintuitive to newbies, who usually first learn that bigger pockets equal more power). As mentioned above, tighten the other controls until you get max power without hooking and screwing up the flow in other ways.

Flattest H16 main I ever saw was Enrique at the NAs in RI in 2006, on one of the light days. I never knew you could get one that flat. He had a huge pointing advantage over the rest of the fleet.

Hope this helps.

Mike

Last edited by brucat; 02/12/15 06:56 PM.
Re: heavy downhaul in light air [Re: ] #277570
02/12/15 09:40 PM
02/12/15 09:40 PM
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Tim594 Offline
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Damn did I just learn a big lesson............ I've been doing things 180* opposite. Kudos to you brucat. Outside of fun on the water engineering has gone a long way, love it.


Tim Grover
Memphis TN

Hobie 16
Hobie 18
Hobie 20
Re: heavy downhaul in light air [Re: ] #277572
02/13/15 01:44 AM
02/13/15 01:44 AM
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Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
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Oldish monohulls with stiff masts often go with a deep mainsail in light winds becouse they run out of better options. Much of the sailing litterature is written with these boats in mind.

On a cat my experience is that flat is fast in light winds. Crank downhaul and sheet in. This was confirmed by a former olympic Tornado sailor as well if that counts for anything.


In light winds there is not enough energy in the wind to make it attach to the curve of the sail if we run with the medium wind settings. By flattening the wind dont separate off the sail causing energy loss + drag. Just look at the telltales, over flatten and then release a bit. Separation is pretty easy to spot with telltales at the leech and max draft.

Twist is overrated in cats wink

I believe most sail with not enough downhaul and not enough sheet on in most conditions..

Hooking the leech is a no-no in all conditions. Leech telltales again helps to avoid this but the specific trim settings depends on the mast pre-bend, how much downhaul is used and lastly how hard the mainsheet is set.


Light wind and chop, then it gets interesting.


I miss our Tornado..

Re: heavy downhaul in light air [Re: ] #277573
02/13/15 04:49 AM
02/13/15 04:49 AM
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TEAMVMG Offline
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the tighter the outhaul, the more mainsheet tension can be applied before the sail leech hooks to windward


Paul

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Re: heavy downhaul in light air [Re: Rolf_Nilsen] #277574
02/13/15 06:31 AM
02/13/15 06:31 AM
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Posts: 12,310
South Carolina
Jake Offline
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Originally Posted by Rolf_Nilsen
Oldish monohulls with stiff masts often go with a deep mainsail in light winds becouse they run out of better options. Much of the sailing litterature is written with these boats in mind.

On a cat my experience is that flat is fast in light winds. Crank downhaul and sheet in. This was confirmed by a former olympic Tornado sailor as well if that counts for anything.


In light winds there is not enough energy in the wind to make it attach to the curve of the sail if we run with the medium wind settings. By flattening the wind dont separate off the sail causing energy loss + drag. Just look at the telltales, over flatten and then release a bit. Separation is pretty easy to spot with telltales at the leech and max draft.

Twist is overrated in cats wink

I believe most sail with not enough downhaul and not enough sheet on in most conditions..

Hooking the leech is a no-no in all conditions. Leech telltales again helps to avoid this but the specific trim settings depends on the mast pre-bend, how much downhaul is used and lastly how hard the mainsheet is set.


Light wind and chop, then it gets interesting.


I miss our Tornado..


I've heard that and I think it has more to do with keeping the draft from sagging aft in the sail than it does with flatness...but that's my personal opinion. I don't downhaul to a flat main. I put enough on (about 25%) so that I can keep some leach tension on the sail but it doesn't flatten it all that much.

Air vehicles that are designed to travel slowly do not flatten out their airfoil shapes....they make them as full as they can get it. Granted, the speeds aren't the same but I'm not completely bought into the idea that its the flatness of the sail that keeps air attached in light air. Consider me agnostic on the idea.

Regardless, it works no matter what the correct theory is as to why.


Jake Kohl
Re: heavy downhaul in light air [Re: Tim594] #277575
02/13/15 07:40 AM
02/13/15 07:40 AM
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brucat Offline
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Originally Posted by Tim594
Damn did I just learn a big lesson............ I've been doing things 180* opposite. Kudos to you brucat. Outside of fun on the water engineering has gone a long way, love it.


Can't tell if this is sarcasm? One never knows on this site...

Either way, just passing along what I've learned through the years. When sailing, I actually try not to over think this, especially when racing. Keep the telltales flowing. Never claimed to be a pro, but I've enjoyed moments of brilliance/luck through the years.

As Jake mentions, there's quite a bit of science to this, and pocket placement is one more consideration. There's also no shortage of art, which keeps it interesting and fun.

Edit: There are some great books out there describing sail shape and the effects our controls have on it. As more pro sailors get into cats, and more high-performance monohulls evolve, I'd expect these resources to get even better (for us).

Mike

Last edited by brucat; 02/13/15 09:14 AM.
Re: heavy downhaul in light air [Re: ] #277576
02/13/15 08:10 AM
02/13/15 08:10 AM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
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It can also be that the effect comes from reducing the damage from a supersize diameter non-rotating mast wink

I always find it very difficult to compare settings across boats and classes. We trim to the telltales and percieved boatspeed when flattening and have markings on the mast for the downhaul grommet positions. But what that is in %.. It is a fair bit of downhaul and it also makes it easier to pop the battens over when tacking.

Airfoils moving at cat speeds with our AoA, planform, surface loading, and aspect ratios are out of my experience totally and I dont know if they can be compared at all? Looking at STVOL aircraft they do all kind of funky things like leading slots, high AoA landing/takeoff etc. but still operate in only one medium (air) and can in some instances benefit from drag while a catamaran parks or comes into the risk zone with high drag. At our top speeds an aircraft like the Fi 156 would be at its stalling speed?

Digressions digressions, but oh so interesting!

Speed have its own intrinsic value I would say.

Re: heavy downhaul in light air [Re: TEAMVMG] #277580
02/13/15 08:57 AM
02/13/15 08:57 AM
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Naples, FL
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Originally Posted by TEAMVMG
the tighter the outhaul, the more mainsheet tension can be applied before the sail leech hooks to windward


That's a bit new to me... interesting. Any thought on how that works?

I was having a bit of difficulty last weekend in the light stuff. Seemed I couldn't get all the telltales to read the way I wanted.... especially difficult was getting the jib to match the top windward telltales.

Perhaps I was running too much draft at top and should have downhaul and cunningham tighter?

I almost wondered if I had the jib sheeted too tight since i couldn't get the main to match... except that in addition to the mis-match, I couldn't point quite as high as I probably could have (without pinching of course)

but this discussion is starting to make a bit more sense and I must try at the next light air outing.


So, the consensus for lighter displacement, rotating mast multis would be:

Light Air, clam water
- flatter is better

Light air, chop
- adjust for more draft

Medium air
- moderate draft depending on water

Heavy air
- flatter is better



Jay

Re: heavy downhaul in light air [Re: ] #277582
02/13/15 09:16 AM
02/13/15 09:16 AM
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brucat Offline
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Light air, big chop: hit the beer truck early...

Mike

Re: heavy downhaul in light air [Re: brucat] #277583
02/13/15 09:25 AM
02/13/15 09:25 AM
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Posts: 381
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rehmbo Offline
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Originally Posted by brucat
Light air, big chop: hit the beer truck early...


+1 - there is a magic ratio that when crossed is like jumping off a cliff.


Jeff R

H18, C2 USA1193
cramsailing.com
crescentsail.com
Re: heavy downhaul in light air [Re: ] #277584
02/13/15 09:35 AM
02/13/15 09:35 AM
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rehmbo Offline
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You guys are killing me. How can I get any work done?

Found some more info on the subject

[Linked Image]
This image illustrates the change in wind speed as a function of height in different conditions. The 1st, nearly linear line is for wind speeds less than about 5kts.

Point is that optimal trim angle at the bottom of the sail would be different than the top - thus the need to allow twist and open the top portion of the leach.

Once the breeze goes turbulent, then you get the curves to the right in the above image, and no need for twist until you're overpowered.

Some good articles here:
http://www.ockam.com/2013/06/02/wind-shear/
http://www.skysailtraining.co.uk/sail_trim.htm (the bottom section)


Jeff R

H18, C2 USA1193
cramsailing.com
crescentsail.com
Re: heavy downhaul in light air [Re: rehmbo] #277585
02/13/15 09:54 AM
02/13/15 09:54 AM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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Originally Posted by rehmbo
You guys are killing me. How can I get any work done?

Found some more info on the subject

[Linked Image]
This image illustrates the change in wind speed as a function of height in different conditions. The 1st, nearly linear line is for wind speeds less than about 5kts.

Point is that optimal trim angle at the bottom of the sail would be different than the top - thus the need to allow twist and open the top portion of the leach.

Once the breeze goes turbulent, then you get the curves to the right in the above image, and no need for twist until you're overpowered.

Some good articles here:
http://www.ockam.com/2013/06/02/wind-shear/
http://www.skysailtraining.co.uk/sail_trim.htm (the bottom section)


Again, these are my theories that I've assembled based on tidbits of information from various sources - including Bethwaite's book, coaches, and "common sailor knowledge":

I don't think twist and light air are as directly related as many believe. In smooth water and light air, I don't see any reason to put a significant amount of twist in the main. There are some apparent wind effects due to the difference in velocity that will want to see a little twist but it's not much. You might also want a little twist on a jib/main boat because the jib will change the angle of the breeze that flows around the bottom of the main while the top needs to realize a different, unaffected, breeze angle. However, there are differences in the shape of the mainsail cut shape from top to bottom that help accommodate the affect of the jib - so that difference needed in trim angle from top to bottom due to the jib effect is pretty small. While the velocity may be different from top to bottom, the angle of the wind isn't different by much - so you are not necessarily looking for a lot of twist in the main just because the wind is light.

However, when you introduce wave chop onto the water surface, the boat starts to swing back and forth like a pendulum (aka on my boat as "tomahawking"), THIS is where twist really starts to become important. As the rig swings back and forth, it's seeing large changes in velocity and apparent wind angle. You couldn't possibly trim fast or accurately enough to account for this so introducing a little more twist than usual will at least ensure that some portion of the sail is properly trimmed at any point in that swinging motion.

The tradeoff to more twist, however, is that the fuller sail creates more drag and reduces pointing angle. As the chop increases at higher wind speeds, a fuller sail, over normal trim shape, is beneficial to help provide a little more driving force to keep speed up through the waves and you can make up for the loss in efficiency.


Jake Kohl
Re: heavy downhaul in light air [Re: Jake] #277588
02/13/15 11:08 AM
02/13/15 11:08 AM
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rehmbo Offline
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Originally Posted by Jake
In smooth water and light air, I don't see any reason to put a significant amount of twist in the main. There are some apparent wind effects due to the difference in velocity that will want to see a little twist but it's not much. You might also want a little twist on a jib/main boat because the jib will change the angle of the breeze that flows around the bottom of the main while the top needs to realize a different, unaffected, breeze angle.


Agreed on both accounts. I think the leach telltales are the feedback for when enough is enough.


Jeff R

H18, C2 USA1193
cramsailing.com
crescentsail.com
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