IOM Luff-line

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Here are some rough schematics showing a currently popular method of attaching the mainsail to the mast. A luff-line passes through the luff of the main, inside the pocket made by the luff tabling. It is fixed at the head of the mast, and at the gooseneck. The fixing at the head may well be made to the "piece of bent wire" that is now legal in the IOM class. The fixing at the gooseneck needs to be adjustable for tension; Keith Skipper tensions his line with a length of thick "roach pole" silicone elastic, for example. Four or so small holes are made in the luff of the sail so that a thin line can be passed around the luff-line to keep it and the sail close up against, but not tightly bound to, the aft face of the mast. The luff-line is a length of wire, not cord, since it needs relative local stiffness. It could be stainless steel stranded cable, or single strand wire. The main is otherwise rigged as normal at the tack, clew, and head.

Overall view Head detail

The idea is that, as the boat bears away to a reach or run, the sail rotates quite freely around the mast while kept in place with the luff-line. The line keeps the luff nicely set, particularly near the foot, without awkward pockets or sags. This allows a wider range of downhaul or Cunningham tensions to be used on the main. The tension on the luff-line itself shouldn't be too high, since it should rotate with the sail, but needs to be firm enough to keep the sail luff close up against the mast face no matter what the wind strength.

Turbulence due to mast for various arrangements

The point of keeping the sail luff as close to the face of the mast as possible is to prevent leakage of high pressure windward air to the low pressure leeward side, which they say tends to reduce the lift developed by the sail.

The point of wanting the sail to rotate around the mast is that the mast is an obstruction which develops turbulence at the luff and helps in the creation of a luff bubble. They say a luff bubble is a "bad thing", and the bigger the bubble the worse the loss of lift. If the sail luff is attached, and remains attached, at the aft face of the mast (eg bolt rope), the luff bubble is larger than would be the case if the sail were allowed to rotate around the mast. This is because "most" of the mast is causing the (leeward) luff bubble. If the sail rotated around the mast, only "half" the mast would be involved, with less turbulence, leaving a somewhat smaller bubble. The ideal in fact is to have the sail luff rotate past the middle to the forward side of the mast, thus having most of the mast interfere with the windward airstream (less of a problem) and leave the leeward airsteam free (much better). In theory, such sail rotation can occur because of the pressure build-up on the windward side of luff and the suction on the leeward side, but it needs exceptionally careful handling of the luff-line tension, the downhaul tension, and the tie tensions.

Peter Spence has posted a comment on the WindPower Internet discussion forum that is worth repeating.

"I see that Lester has given a good description on his website of how the luff wire system is used to attach a mainsail to the mast - I currently use this system on my lightweight GB [Graham Bantock, SailsEtc] A rig on my 1M - but it is worth mentioning that when I was originally chatting to GB about the best way to rig the sail when I picked it up from his workshop, he said that if the wire that is fed up inside the luff goes past the maximum measurements of the sail, then it ‘may’ be out of class if it is classed as part of the sail - therefore both he and I have the eyes at both ends short of the tack at the bottom and short of the head at the top, and the top is then tied to the backstay crane with some twine (I haven’t got round to putting that bent wire back on yet!) and the bottom is attached to another line which is then tied to a rubber loop which acts as a tensioner which itself is attached to another piece of line which has a bowsie so that the tension can be adjusted to suit conditions. Some people I have spoken to have their wire just attached straight up to the backstay crane and seem happy with that because it has passed measurement, but I prefer to play safe rather than turning up at an event only to find that it has been found that it is out of class and requires lakeside surgery before the first race."

My own personal reading of the 1995 IOM class rule is that, because the luff-line is not "attached" to the sail, it is not part of the sail, and so the measurement band restriction does not apply. I think it would be a rather perverse measurer who would not accept the luff-line starting from the backstay crane, but who would accept it if it was tied to the backstay crane with a second piece of line. The 1995 class rule is quite explicit, after all, that any method of attachment of the main to the mast is permitted (double luff excepted).

For the 2002 class rules, the situation has clarified, and the luffline method of attachment is recognised and legal.

There are complementary pages on building a rig, and on building a mast.

2005-12-18


©2011 Lester Gilbert