Thread: GIS sail tweaking

Originally Posted by woodeneye

It's not so much that the sail seems to draw cleaner, but more to to with what happens when the airflow reverses, effectively turning the leach into the luff and vice versa. In the conventional situation when the boom is out on the lee side, the forces on the leach are a long way from the mast which is the cause of instability. When by the lee, the mast is the leach, which centres those forces over the middle of the boat which is more stable. .

I was hoping you would drop by Bruce!

Like you say ... for running square the flow around the sail doesn't know where it is going ... there is no gradient. So you end up with "Karmann Vortices" where the flow alternates.

This is water flow around an iceberg .. I was just looking in general for a pic ... but this is great because it shows how generalised the phenomenon is.



This is from the Mawson base website. It pulls the object that is the cause one way and then the other. That's what make rigging hum or the reed in a musical instrument vibrate. Occasionally a bridge.

But if you produce a "gradient" by progressively moving one edge of the sail closer to the source of the wind then the flow will attach and stabilise. What happens in effect is there is a first vortex then the energy of the second one is "trapped" by circulating around the sail itself. The "starting vortex" wanders off to leeward and the trapped "circulation" vortex travels with the sail causing the air to travel faster on the leeward side of the sail and slower on the windward side where it is going in the opposite direction to the wind direction.

So you end up with stable flow .. which is a nice trick.

Best wishes
Michael

But, even headed directly downwind, does anyone ever actually leave the sail perpendicular to it? I don't. I can't tell you when I learned that. Most likely, it was something someone told me when I was so young that I can't remember. It's just "what you do to avoid rolls".

Also, if there is a swell of a length which might cause the bow to plow under, I zigzag a bit to keep both wind and waves more on my quarter, even if that means a longer route. I've seen boats with spinnakers dis-mast themselves when the bow goes under and the foot of the mast gets pulled out of the step.

You can tell that I don't like going dead downwind either.

Originally Posted by paulie

But, even headed directly downwind, does anyone ever actually leave the sail perpendicular to it? I don't. I can't tell you when I learned that. Most likely, it was something someone told me when I was so young that I can't remember. It's just "what you do to avoid rolls".

Dead right Paulie, there needs to be a flow across the sail in one direction or the other that you can control and predict. If the flow is oscillating back and forth across the sail it makes running downwind very precarious indeed. However, a lot of sailors keep doing it and come ashore blaming the designer of the boat and letting everyone know what a pig she is to sail downwind.

I have my stopper knot set so that my boom can go out beyond 90 deg. However I only do this in up to 12 kts. Let's just say that I have my reasons

Mik knows how much I love downwind sailing. As a teenager it was completely different and I hated downwind. Back then downwind was an opportunity to suck in a few breaths and relax a bit and plan tactics for the upwind leg. Nowadays it's the opposite. Downwinds are tougher physically and more adrenaline flows than upwind, especially if you are S-Curving. I'm no Slingsby that's for sure, but it's a real buzz for me and something I look forward to if the wind is up. It's definitely worth learning, because you can really make good gains from it. Occasionally it even earns respect, but not if the gains you make are too large

There are a few things I want to try in the Goat before too long, things that I've learned in the Laser. Am really missing the nice manners of the Goat, but perhaps the things I have in mind will not show that nice side at all

Mik, you are a fountain of information! "Karmann Vortices" hey? I used to like making these patterns in the water with the Dabchick's rudder when drifting around on a no-wind day. When the water was still, they would seemingly go on swirling forever. Karmann Vortices.....must remember that. Anyway, just thinking about those things travelling back and forth across the sail scares me a bit. Visualising them is enough to make you want to avoid having them strut their stuff across your sail, so it's clearly a "don't go there" thing isn't it?

Hi Bruce.
The vortices are a problem running, but when we can get flow over the sail they are responsible for setting up the conditions for lift. If the sail didn't set up shed one vortex and trap the other as circulation then birds wouldn't fly, fish wouldn't swim and we'd only be able to sail downwind (and slowly)

Mik

I get it now. And downwind slowly would indeed be bad.

Mik, I like your page better. It is certainly well explained. Thanks to you too Wood - really great stuff.
I don't have much experience surfing small boats. The funnest time I can remember was surfing our C&C 35 under spinnaker on runs up to Newport. We had an Aussie crewman, (also named Mick BTW), who would hang over the side in the bosun's chair and take pictures while surfing down ten foot waves. Crazy. I can't remember exactly, but I think we would often fly the spinnaker in similar manner to sailing by the lee with unstayed boom. That is wing and wing with the boom on the traditional side and the spinnaker against the grain. (spinnaker by the lee) often in heavy airs.

New subject - I want to move my outhaul adjustment. Right now I have it going through a 3 to 1 with a built in clamcleat at the leach end of my boom. It is a real pain to adjust on the fly. If I run it under my boom all the way to the downhaul, I could combine the downhaul and outhaul with one single adjustment to make, such that I would have half the tension on the outhaul for any downhaul setting. Make sense? It gives me less flexiblity, but has the advantage of simplicity. Any thoughts?

PS. I'll try to wrap my head around those Karmann Vortices over the next bitter ale. I'm just finishing reading a book on the history of quantum physics, and your description of Karmann Vortices sound a lot like the particle, wave duality of light. I had to read a lot of that book twice too to understand it.

Originally Posted by warm beer

New subject - I want to move my outhaul adjustment. Right now I have it going through a 3 to 1 with a built in clamcleat at the leach end of my boom. It is a real pain to adjust on the fly. If I run it under my boom all the way to the downhaul, I could combine the downhaul and outhaul with one single adjustment to make, such that I would have half the tension on the outhaul for any downhaul setting. Make sense? It gives me less flexiblity, but has the advantage of simplicity. Any thoughts?

Now don't start that again! That's basically the same argument I was making a few pages back with respect to fixing the throat to the yard, only you are talking foot instead of head. If it's wrong up high, it has to be wrong down low. And everyone says it is wrong, so it must be.

To be honest, I still don't understand why. Yes, I understand that separating the actions of outhaul (on either spar) from the downhaul allows you to have different levels of tension along the yard vs down the luff. It allows you to adjust each individually. But when would you actually want to do that? That's the question I never got an answer to above.

(In my business (computer software), we would call that a "use case". What is the "use case"? We programmers are always building in flexibility "just in case". But flexibility always adds complexity. And complexity always adds cost. If there is no "use case" -- if a bit of flexibility will never actually be needed -- you've just spent money for no reason. Then your manager has words with you.)

So, under what set of real-world circumstances would you want to haul away on the downhaul and NOT tighten the outhauls?

Probably driving into sloppy waves with moderate to high wind - where you would want power and lots of downhaul. But, for the sailing that I'm doing, it should work OK ... I think.

I'll take Paulie's invitation to complain. Much like his luff-as-outhaul concept, your use of the downhaul to also tighten the out outhaul compromises your ability to downhaul. Everything I've read about the lug suggests that downhaul tension is critical. What you suggest runs that tension from a block of some sort aft about ten feet (hopefully using--expensive--non-stretch line), around some block/sheave, and to the clew. The "tension" then pulls the whole foot of the sail, acting against the tack. Theoretically, you could pull hard enough to bend your boom rather than tightening your luff. All of this presumes that whenever you want more downhaul, you always want more outhaul AND vice versa. I wish I could answer Paulie's use-case question from experience. But it seems like the downhaul should be adjusted with wind strength to flatten or round out the sail (at both spars). Do you find the boom outhaul needs dynamic tweaking? If so, wouldn't it be better to tweak it on its own without throwing other variables into play such as tension on the downhaul?

Yes, but here's my thought progression.
(1) I want to change the outhaul on the fly
(2) to do this, it needs to be run like a laser, up to a fixed point by the mast
(3) at this point you can have it go through a block/cleat attached to one of three places
(a) the boom
(b) the mast
(c) the deck
(4) (a) might work, but the boom swivels around the mast, changing the block angle, so it's not ideal
(5) (b) could restrict boom movement, and pull the boom forward when tightening, and the tension could change as the boom swivels
(6) (c) is nice, but tightening would act as a second downhaul
(7) if you're going to have something acting as a second downhaul, why not just incorporate it as part of the downhaul?

Originally Posted by warm beer

...
(1) I want to change the outhaul on the fly
(2) to do this, it needs to be run like a laser, up to a fixed point by the mast
...

By on-the-fly do you mean: you're on the rail, mainsheet in one hand, hiking a bit to keep her flat, tiller in the other hand hopefully not scrubbing off too much speed countering any weather helm, and then... tinkering with the outhaul? Sounds a little OCD to me (not that there's anything wrong with that...)

If you're willing to heave to for a moment to make an outhaul correction to match the prevailing conditions, you can bring the outhaul from the clew to about mid boom and cleat it off there (maybe throw a trucker's hitch in between for more purchase?). That's the way it's done with Sunfishes anyway.

Neither the Sunfish's Lateen nor the Laser's Bermuda are good models for how to dial in a Lugs'l (methinks). I would think Keyhavenpotter would have mentioned a technique for dynamic outhaul adjustment if that were the ticket to optimum Lug performance. Instead, he emphasizes kicker/vang as the next most important thing after downhaul. Oh yeah, that and locking in the throat-to-yard connection...

Since I invoked the name of Keyhavenpotter, I thought he might have covered this in his Lug Rig Heaven thread. It's seven pages long, but on page 2, post #21, he mentions this:

Originally Posted by keyhavenpotter

Yes more wind the tighter you want the sail.

ie more wind = less depth!!!!

medium wind = medium depth

light winds = slightly flatter again to keep the air attached to the sail.

If you had an adjustable "outhaul", the clew line going through a block to a cleat forward on the boom, which meant you could adjust the sail when changing course out on the water, you would flatten the sail for upwind and then release some line to make the sail fuller when sailing on a reach and a run.

Brian

So here's an instance where he's advocating adjusting the outhaul on the fly. What I notice is that he mentions doing this during changes of course and that could be done with a mid-boom cleat. It probably couldn't be done while rounding a mark though...

I'll keep sifting through that thread and add anything else that may shed light on this discussion.

On page 4, post #46, Keyhavenpotter describes setting up for the start of a new season:

Originally Posted by keyhavenpotter

Just a small update on setting the sail on the yard and boom at the start of the season. Having tried to set them in the same place as last season, neither were quite correct. Changed them for the weekend sail and boom was too tight and yard too slack. So adjusted them again - we are talking about 1/2" adjustment.

He goes on to describe the thrill of competitive sailing; he was pleased with this set-up and doesn't mention adjusting the outhaul enroute.

In response to a question, he writes (in red):

Originally Posted by keyhavenpotter

Are the adjustments you're talking about tension between the peak and throat or the head lashed to the yard?
Talking about tension when rigged along the yard and along the boom .

I'm assuming if the peak to throat tension is too high there are wrinkles parallel to the yard and if too loose wrinkles perpendicular to the yard when close hauled.
Yes that's correct.

Can you judge the correct tension by looking for wrinkles or do you use sail shape as your guide?

Both. Wrinkles will clearly show too little or too much tension. So you can adjust to that indication. However when it looks ok it could still be improved by the following check. Rig the sail on dry land with downhaul set, and set the boat as if beating to windward, mainsheet tension applied as you would sailing to windward. Now, holding the mainsheet steady, and standing at the back of the boat, luff the boat into the wind just enough to luff the sail. If the sail luff first at the middle of the yard, the peak needs releasing slightly. keep releasing and re-trying the luff until the sail luffs at the throat, the intersection of yard and luff of the sail.

When lashing the head (between the peak and throat) do you get it as tight as possible or leave some slack?

You must leave some slack. This is the key setting on the whole of the lug rig. Here's why. The throat of the sail is tacked to the yard in some way. Either by lashing or shackle the throat clew will move forward as the downhaul is applied. It will pull down some and then stop. At this position the yard tension has to be the tension you want it to be. This is why it is trial and error to achieve a good setting.

I guess it has to be the case that this setting is the setting for one wind strength. Hopefully with good spar bend to sail round the setting will do well enough for a wide range of winds. I tend to have mine set for a nice breeze, which leaves it slightly tight for very light breezes and still fine for strong winds.
Also I find using two bits of string to tie off the clews to the ends of the yards helps. One goes round the sail and yard to take the strain of the wind and the other is used to adjust the clew to spar end, ie the tension we have been talking about. This separates the two functions.

Brian

So Brian tends to set the outhaul tension of his yard to accommodate what he anticipates will be the prevailing conditions and expects the yard bend/downhaul tension to handle changes in wind strength. He doesn't explicitly say so, but I infer that he treats the boom the same way.

Warmbeer, basically you can rig it the way you suggest and then report back here to tell us if it works or not! Do I think it will work? Yes, but only up to a point. That point will be when your downhaul tension is so much that it tears your sail in half! I'm being a bit dramatic, because in theory you could put a stop knot somewhere to limit the absolute tension that is applied to the foot. Therefore, beyond that certain tension, the foot isn't affected. However, your boom could bend. The truth is, I can't think of when you'd have so little tension in the downhaul that the foot would be loose enough? It may be possible to achieve somehow. However, there will always be a time when you'll want plenty of downhaul and a loose foot, such as when reaching and you want the sail powered up with a tightish leach. This will be when you have a crew in 25 kts, you are both hiking, well in control and you want to scream past that Laser in front of you and watch the Laser skipper's face Hehe.

You mentioned the laser setup, but what you described was the NEW Laser setup. I think what is needed is the OLD laser setup, where there was a loop handle hanging down off the boom and you pulled that to release from the cleat. Instead of the loop around the mast, you could run the outhaul around a block screwed to the boom. I've never tried this, but this system was employed on around 100,000 Lasers before the new turbo rigging came in, so I guess it must have worked. This system gives a 6:1 or 8:1 purchase depending on how you attach the clew, although there is some friction involved because of the loops instead of blocks. I guess you could always employ some cheapie blocks instead of having those loops. See here:
Attachment 199190

EDIT: Just thinking some more about the OLD laser outhaul. The laser rules said the outhaul had to run through the cleat on the boom. However, you really would not need that cleat if you just tied a series stopper knots in the outhaul line near the loop handle. The knot could simply "catch" on the last loop and be held there by tension. Pulling the loop handle in a forward motion would release the knot and allow adjustment that way. No cleat required. Just a thought...

PS. Make the loop handle smaller than the circumference of your crew's head