Luff Pocket experiments

Up until now we have made all of our sails using a luff pocket to fit the sail to the mast.
Upon reflection, when using mylar as the sail material the luff pocket with the mast separating the sides makes a tube that resists torsional forces. We have cut 25 degrees of twist in the sail (we are thinking this is not enough) but feel that the rigid luff pocket resists the sail twist. As many other people use ties to attach the sail to the mast we decided to convert one of our sails to ties and compare the difference. See attached pictures.

Predictably, sailing the two boats with the modified and unmodified sails showed no discernible differences however the thinking still seems correct. We plan some new sails with 40 degrees of twist and no luff pocket. The freedom of the luff to twist may be needed in this case. Is there any definitive amount of twist? In the past Brett has said lots of twist is the way to go. Lots??? There is a picture of Scott’s boat in his hand at Mystic and there looks to be lots of twist in the sail, but how much?

In the past we have reduced the camber in our sails because the camber made the luff pocket difficult to make. This may have been a bad idea. So the new sails will go back to more camber (15% - 18%) as it should be easy to make minus the luff pocket. We’ll see how it goes.

Peter & Clare

Hey P & C, it is great to see you guys posting again, your voices have been missed.

Okay. I have a couple of suggestions for your rigging questions. First, from your photos the luff pocket looks too tight around the spar so I would guess that the sail would bind as the rig reacts to the wind and the mast deflects. On the other rig with the tie-offs, the string loops again look to be too tight around the mast. This will most likely create stress wrinkles when the wind picks up. The string loops should have at least 1.5 mm between the sail luff and the mast when you apply moderate aft tension to the sail, just enough to bring the sides of the loop out of round so the sides look tangental to the mast.

On my rigs twist is controlled with a “leech line” with which I can adjust the tension on the leech thereby the twist in the sail. BTW, lots of twist is used on McRigs because of their flexible aspect. You wouldn’t go with quite so much twist with a more stable rig platform such as a jib/mainsail or swing rig. Now a lot of guys just use a loop of line around the boom to act as a non-adjustable limit for the leech, I guess because its easy to do and they don’t see why accurate control of the leech is important, after all isn’t that the point of the flexible aspect of the McRig, to be self-regulating and slack the leech when the wind picks up? Well yes, to a point, but in light airs adjusting twist when the mast isn’t deflecting makes a big difference in acceleration and boatspeed.

Since I know you two like to experiment (in true Footy spirit) I think you might try a sail luff pocket idea that I believe Ian posted a few years ago (if I have made an error in my credit I apologize to the original designer). I have made a couple of nice sails this way. Basically, the luff pocket is folded over to aline with the greatest depth of camber, usually about 35% to 40% aft of the luff. This large pocket smoothes out the transition from mast to sail creating an airfoil shape without using panels to impart camber, and lets the sail rotate freely around the mast so the sail can adapt to wind variations on either tack. Roger Stollery uses a similar large luff pocket on his swing rig main, but takes a dart in the sail about 1/3 of the way down the sail from the head and back to the edge of the luff pocket to add a bit of extra camber to the sail up high. Sort of a hybrid sail.

Well, this should give you two some ideas to chew on for a while, please post what you come up with.


Hi Niel,

Thanks for the reply.
We have been watching the forum but there hasn’t been much happening and we haven’t been making new stuff just sailing our boats.
The luff pocket is small and could definitely be tight but this was due to the difficulty in making the pocket with a cambered sail with panels.
The sail you describe with the larger luff pocket with a dart in the luff sounds like the sail on the Wee Nip plans. This boat is designed by Graham Bantock another well known English model sailor. Sounds like it is worth a try.
Thanks for the tip regarding the luff ties on the other sail. We had been trying to minimise the gap between the mast and the luff of the sail.

Peter & Clare

Hi Niel,

See attached the first try at a wider luff pocket. The pocket is 40mm wide compared to the previous one of 20mm. The luff is straight ie without a dart. Interestingly when we plotted the panel lines for this sail with 8% camber (low) and 40 degrees twist the panel join lines were virtually straight therefore we just cut out the sail in one piece to give it a try. The ties on the other sail have been loosened to give approx 1.5mm gap but it doesn’t show just sitting there. We’ll try a take some sailing shots when we give the new sail a try.

Peter and Clare

Okay, looking better I think.

Now try a luff pocket with the seam along a line between the 40% location aft of the mast on the foot and the head. The pocket forms the camber of the sail and the sail is free to self regulate it’s shape and relationship to the mast. Trust me, it makes a nice looking sail.

You should also either cut the sails smaller or make a rig with longer spars. Taping the sails in place doesn’t give you any real control over the tensions and therefore control over the sail shape. To be able to adjust the sails for changing conditions is very important, not just for racing purposes but because a good part of learning to get the most out of your boat means that you have to learn to tune the boat. As I see from your photos the head and the outhaul are both taped in place and there is a deflection in the smooth shape of the sail at both those areas.

All of the control points in each corner of the sail should be adjustable and the spars need to be long enough to accommodate these adjustments. In addition lots of guys use a loop of string to act as a control for the leach which is totally unsatisfactory. An adjustable leech line allows the control over the twist in the leech, a loop doesn’t, and an outhaul doesn’t (it is used for controlling the camber of the foot alone).

Yeah, rigging is boring and tedious. But the sails are the engine that powers the boat and needs a lot more attention to detail than most folks give them. In fact, you can spend a fortune going through one new boat after another, but the person that dominates your local sailing will win most of the time because he or she has taken the time to learn how to rig and tune their sails.

A couple of tips; for carbon fiber spars make tie downs by tying a small bowline around a nail, slide it off the nail, and Ca it so it won’t untie. With the extra line on each side of the bowline tie the bowline to the spar with two or three clove hitches with each end at the location that you need the tie down. A touch of Ca will keep it in place. If you don’t like the position slit the clove hitches with an x-acto and pull the tie down off. This way you don’t drill any holes in the spar which will weaken it. These tie downs can work as both anchors for bowsie loops or for fairleads for adjustment sheets.

Tip two; on Roger Stollery’s Bug3 plans there is a design for a simple, lightweight wire bowsie that is a great solution for weight sensitive applications like use on a Footy.

This is not meant as criticism of your efforts so far, your sails themselves look very professionally done. I know that you two are coming to sailing from model aviation so while you all are proficient in airfoils and fixed camber you may not be all that familiar with all the controls needed for a soft sail and sailing in the gradient between air and water. In ways sailing and tuning is less precise but in other ways it requires a good deal of specific eye training and rationalizing to do it well.

Hi Niel,

Thanks once again for your thorough and helpful reply.
There is nothing tedious about foils or learning for us, actually a lot of the fun.
To post the pictures we reduce the resolution so the details may not have been clear.
There is actually adjustment on both the down haul on the tack and the out haul on the clew.
The method is a kevlar line held to the stainless wire at the tack and the carbon boom at the clew by a short piece of small diameter silicon fuel tubing. We weren’t sure if this would have adequate hold on the line but in practice it hasn’t moved whilst sailing but is easily adjustable. Spectra or Dyneema line would definitely slip under the tubing but the kevlar holds.
The idea of the pocket being a constant percentage of the chord instead of a constant size makes sense and can be tried on the next version.
We have sailed the new sail the other day. Pictures attached.
The new sail did appear to tack better than the old one and definitely set easier.
Our next job is a larger rig for very light conditions.

Peter & Clare

Looking good!