heavy downhaul in light air

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.

Thanks!

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

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.

great info - thanks

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.

interesting - thanks

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

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

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.

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 <img src="<>/wink.gif" alt="wink" title="wink" height="15" width="15" />

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..

the tighter the outhaul, the more mainsheet tension can be applied before the sail leech hooks to windward

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.

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

It can also be that the effect comes from reducing the damage from a supersize diameter non-rotating mast <img src="<>/wink.gif" alt="wink" title="wink" height="15" width="15" />

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.

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

Light air, big chop: hit the beer truck early...

Mike

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

You guys are killing me. How can I get any work done?

Found some more info on the subject

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.

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

Was it Bethwait who indicated the AWA in laminar flow (light) air is up to 10 - 20 degrees different at the top of the mast than at the water level?

But I see the greatest (personal) need for downhaul to keep the leech from hooking...

That's my reason for pounding on the downhaul in light air, to keep the leech from hooking and keeping a continual shape. Softer battens would also help, but I'm not a huge fan of dropping a main on the water unless it is going to stay down. Singlehanded its a frickin' pain in the butt to pull it up, and keep the boat more or less in the wind.

You should try that with beach wheels still tied under the hulls.

I've heard that works like a champ....

man, that gets funnier every time you say it Jake <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" /> especially since you're a good sailor!

Should have stayed with the Hobie 16, and attended more NAs. In RI in 2006, before the first race (in heavy air and huge seas), one of the Mexican teams intentionally capsized the boat, so they could adjust battens.

Can't make that stuff up...

Mike

when spreader rake was all the rage in the a-cats a few years ago, you would see them capsizing the boat all the time to make an adjustment to the spreaders.

why isn't spreader rake

all the rage

now? Did something change with sail/mast design?

I think they went back to adjusting tension.

I think that whole thing started because Ashby did it once.