GIS sail tweaking

warm beer · #1 11th Oct 2011, 08:35 AM

I am trying to tweak my sail tuning to improve pointing ability and shape in upper moderate winds (12-15 knots). It almost seems that I give up a bit of pointing as the wind gets over 10 knots, and the luff starts to break earlier on both tacks. The upper third of my leach starts to flap as well. I've got my downhaul tight enough to bend my boom a couple of inches, and my outhaul pretty darn tight (with a 3 to 1 outhaul rig). Has anyone noticed an improvement in these conditions when they switched to a stiffer boom? Any other ideas to try?

I'm beating up on all the day sailers, (it's fun rolling over the stayed gib, and main boats), but I want more.

I've also noticed that my better tack seems to be the port - (with my boom on the port side of the mast). In other words, I point better with the boom on the upwind side of the mast. Other GIS sailers, have you noticed the same?

davlafont · #2 12th Oct 2011, 04:55 AM

73 views and no replies... I guess you're not alone?

BobWes · #3 12th Oct 2011, 09:56 AM

I probably haven't sailed my Goat enough but I notice very little difference in upwind ability between the two tacks. Upwind performance clearly improves with more down-haul. Mine is rigged with a 5:1 down-haul and a reasonably stiff boom. Haven't tried a stiffer boom but the boom doesn't bend when I tighten the down-haul - the yard will visibly bend with full-on down-haul but not the boom.

@ Dave: Silence doesn't necessarily mean agreement.

warm beer · #4 12th Oct 2011, 10:20 AM

crickets chirping

MiddleAgesMan · #5 12th Oct 2011, 12:23 PM

If your sail is loose footed--like several of the recent Goats, the boom stiffness would be very critical. But if you've lashed the sail to the boom per MIK's rigging instructions, not so much.

BobWes · #6 12th Oct 2011, 12:53 PM

Okay, now I am truly confused.

Quote:

If your sail is loose footed--like several of the recent Goats, the boom stiffness would be very critical. But if you've lashed the sail to the boom per MIK's rigging instructions, not so much.

I thought boom stiffness was not an issue for loose-footed sails. MAM, your statement seems to contradict information on the Dabbler Sails site:

1) Support gaffs, and lug, gunter, or lateen yards, at the ends. For masts, support the spar at the tip and where it passes through the partners. (Booms are a special case. If the sail is loose-footed, i.e. attached only at the tack and clew, no bend data is needed. If the foot of the sail is laced, sleeved, or otherwise attached along the boom, bend data may be required, depending on how the main sheet is rigged.)

Am I misreading Dabbler's statement?

davlafont · #7 12th Oct 2011, 01:16 PM

Bob, what Dabbler is saying is that they will shape the foot as they see fit unless it is lashed to the boom. If lashed, they need the bend data to match the foot shape to the boom bend.

MIK has always cautioned that if one wants to deviate from the plans and go loose footed, the boom must be stiffened to suit. It makes sense because a lashed foot distributes its load across the whole boom whereas loose footed stress is at the ends (and the mainsheet block) only.

davlafont · #8 12th Oct 2011, 01:27 PM

Quote:

Originally Posted by MiddleAgesMan

If your sail is loose footed--like several of the recent Goats, the boom stiffness would be very critical. But if you've lashed the sail to the boom per MIK's rigging instructions, not so much.

I just revisited the Warm Beer video: loose footed indeed. WB, there's good chance that the boom was built to plans and should be lashed. That should allow for more downhaul and greater luff tension.

BTW, at 4:10 of the video you state that you're thinking of naming her Feather. Is that official yet?

BobWes · #9 12th Oct 2011, 01:54 PM

David, well, okay... I don't read Dabbler's statement as you do but I don't know enough about it to intelligently explain why that is. I understand (and have re-read Mik's comments re stiff booms with loose-footed sails) we want stiff booms (which lead to some confusion with my reading of Dabbler's statement) with loose footed sails.

What I have seen (sometimes) in the boom when sailing my loose-footed sail is a horizontal bend which suggests it is being bent by the sail and needs to be stiffer in that dimension. I have not seen any hint of vertical bend induced by the downhaul. (I understand that the down-haul, the sail and the boom are all interconnected so my observations are perhaps overly simplistic.) What I saw this past weekend, in the highest winds I've sailed in, was very little bend in the boom (in either direction).

I was planning to build a new boom this winter with the idea of improving the stiffness in the horizontal direction. (As an aside, my boom is a hollow box using 4mm ply sides, 12mm top/bottom, plugs (in the ends) and spacers distributed through the length.) The decision to build a new boom was arrived at this summer after seeing the amount of bend in the boom but this past weekend's observations (and maybe I am misremembering - I'll check the photos and videos) have me wondering why the boom didn't bend so much this weekend.

Any thoughts?

warm beer · #10 12th Oct 2011, 02:51 PM

Yep, her official name is "Feather". We keep a pelican feather lying around in the bottom of the boat as a reminder.

)

After watching my own video, I noticed little deflection from the foot to the boom in around 10 knots max wind. I did not like the look of the loose grommets though, so I lashed them individually loosely. In 15 knots there was a small gap of about .5 inches between the foot and the boom with this arrangement.

I would have filmed all this, but it was nice and wet, and up on the gunwale sailing, so my not waterproof Canon G11 was not up to the task. I plan to pick up a waterproof camera, then it would make this discussion a heck of a lot easier!

In stronger winds with the boom on the leeward side of the mast, I'm getting too much initial curl (body) at the luff. That, I believe, is limiting the pointing ability on that tack. I'm thinking that increased outhaul tension is the only way to flatten this out? That is my main question. If so, would not a stiffer boom allow increased outhaul pressure?

side note: I threw in a 5 inch Viking inspection hatch with fatbag last weekend. It made for a nice dry place to stash stuff on a wet day.

davlafont · #11 13th Oct 2011, 12:41 AM

Ok, so the loose foot theory is shattered. You might be on to something with the outhaul, but consider this too: how is the sail lashed at the throat? It needs to be a fixed connection to the yard such that the downhaul tension pulls on the yard, not on the head of the sail. This could be a hole through the yard or a saddle or an eye bolt, etc. (Note: This is different from the lashing hole at very fore of the yard which is more of an outhaul.) If your throat is only wrapped around the yard like the other grommets are, then your downhaul might be pulling too much cloth from the head instead of tightening the luff.

Quote:

Originally Posted by warm beer

...5 inch Viking inspection hatch with fatbag...

Nice set up. Where did you put it? Horizontal? Vertical? Are you familiar with Bottle Port? It fits with the above and provides a nice way to keep a water bottle handy while still leaving some space in the Fatbag for (dry) goodies. Google it.

paulie · #12 13th Oct 2011, 06:48 AM

I am interested in this topic as well. I haven't sailed my Goat nearly enough to offer advice, but I can offer some observations.

My boom isn't nearly stiff enough to allow a loose-footed sail. At least, not with my 6:1 downhaul configuration and the sheet set up as MIK shows on his web site. The middle of the boom will deflect several inches downward in a moderate wind. That's enough to allow the sail to belly considerably. Great going downwind, but I lose considerable speed upwind. I may try moving both the downhaul and sheet blocks closer to the ends of the boom to see if that helps. But I'll probably have to stick with a lashed foot until I build a stiffer boom.

Upwind performance is fine in moderate winds, reasonable in stronger winds, not so good in very, very light winds. I lose a lot of pointing ability when the wind falls below 5 knots. Not sure what is going on. I'm going to try some experiments with the traveler to see if I can improve on that.

I also find that the rig's balance is very sensitive to the amount of sail before the mast. A couple of times, the downhaul loop has slipped and I've gotten more sail before the mast than the 16" or so I intended. I then find I've got a weather helm as I come into the wind. Fighting this with the rudder slows the boat down. Fix the downhaul and the boat balances out; I can point higher without fighting the helm. I'm thinking that I may fix the downhaul so that it can't slide around. (I have not made any attempt to see if moving the block on the yard has a similar effect.)

Overall, I'm very happy with the performance I'm getting from such a cheap, simple rig. And I'm enjoying the experimentation and the learning curve. But I figure I've got a long way to go before I've got it sailing at peak performance.

warm beer · #13 13th Oct 2011, 08:58 AM

Paulie, I noticed exactly the same things as you did when the boom slipped forward a bit. I saw on one of John's videos that he uses a simple loop of line from the end of the boom around the mast below the cleat to stop this, and did the same myself. It works great.

With regards to the GIS pointing, we really have two different rigs depending on your tack:

(1) boom to windward of the mast: The leading edge of the sail foil truly starts at the mast, and the lifting area of the sail is further aft. The sail in front of the mast acts like an over-trimmed jib? The luff tension is less important I would think. Because the sail bends around the mast, given the same outhaul tension, it is inherently flatter fore to aft.

(2) boom to leeward of the mast: The leading edge of the sail foil is the luff, and the lifting area is further forward. (sixteen inches further forward?) The luff tension is very important. The sail does not bend around the mast, so given the same outhaul tension it has a bit more belly fore to aft.

With my sail, I'm faster, and point higher with the boom to windward. At least, I'm 90% sure I do.

)

warm beer · #14 13th Oct 2011, 09:14 AM

Quote:

Originally Posted by davlafont

Nice set up. Where did you put it? Horizontal? Vertical? Are you familiar with Bottle Port? It fits with the above and provides a nice way to keep a water bottle handy while still leaving some space in the Fatbag for (dry) goodies. Google it.

Hey Dave, I put it in a stern corner about 4 inches off the transom and side to the edge of the port. Hole saw and router. It's not a bad spot, out of the way. I was surprised to see how shallow the depth was in this area to the hull, as the hull curves up here. It's a great spot, but maybe not deep enough for a bottleport? Took me a second to realize the top of the port is the template for the hole. I love the fatbag.

paulie · #15 13th Oct 2011, 10:36 AM

I'm sure the full-length battens were supposed to minimize that difference between tacks, so that sail-to-leeward and sail-to-windward would be more equal. But I found them to rob me of performance, so I ditched them.

Here's a question: In your video, you have your sail set the same way I do, with the yard run almost to the top of the mast and plenty of clearance between the boom and the sheerline. But the pics of Bob and Dave's boats in the Paulina Lake Messabout show their booms so low that they just barely clear the hull -- the forward end is actually below the breasthook. I like the boom high so I don't get hit in the head. But am I doing it wrong?

BTW, thanks for the tip on keeping the boom from sliding forward. I may do the same thing.