The art of the sailmaker

tim_sailing_sDon’t get me wrong – the junk sail is a great sail for what it does.  It’s for people with not a lot of money, it reduces in storms beautifully and is so easy to use.

It’s problem though is its flatness.  Flat sails have no power, especially to windward because what a sail needs to go, just like an aircraft wing or a woman, is curves.

This is the sailmaker’s art.

If you look at the picture below right, what you see is a curve.  Not only that but the top is twisted away from the wind more than the foot and that is not a quirk – it has to be for efficiency, for flow over the whole sail.

In the Little America’s cup, they had great wingsails but what was lacking in a rigid wing was twist.  An aeroplane has it designed into the tips but sails are more dynamic.

sail-craig-merkSo the LAC boys spent thousands of dollars getting twist into the rig, something the soft sail does as a matter of course.

Camber

All sails hoping to go to windward need camber of a certain amount.  This is the curvature of the sail in its base position.

If you lay a sail on the floor, it should be a bit baggy, not lying flat.  This is because of the clever way the panels have been cut and “bunched up” a little in the sewing, a principle not unknown to dressmakers too.

It’s very precise, as you’ll see in this clip from the series: How to Draw the Camber of a Sail — powered by eHow.com.

Basically, the NACA tabulated precisely what sailmakers had known for centuries and came out with a seried of curves which are the most efficient in certain conditions and for certain foils. Here is an interesting article on foils, specifically those for keels and rudders but the principle is the same.

They discovered certain things about the position of maximum camber.

camber

Mast and sail combined

If you have a flat[tish] wing mast of 20% of the length of sail and mast, it is less efficient than a 10% wing mast [and 90% sail].  It only gets more efficient at 35-40%.  Therefore, one either goes the whole hog with a wing or else sticks to the traditional mast sail.  That’s why the old sails will never die because, within a small angle of attack [the angle at which the wind makes the sail fill and operate], they are almost as efficient as a solid wing at its best.

wingsail.zefyr

The advantage of the solid wing is a wider angle of attack, meaning it will be efficient either let out or pulled in and is more forgiving that way.  On the other hand, it won’t de-power and every sailor knows there are moments every few seconds when one wishes to de-power to keep the boat upright.

So, it’s all a compromise.

20050906_025_Johanna_1024_dots

Vertical is not horizontal

The greater the camber, the more the power and here another factor comes in.  A sailboat is always in the wing-effect zone close to the water whereas a horizontal aircraft wing is there for a second or two.  The top of the sail is meeting different wind patterns to the foot but that is not so with an aircraft.

Therefore, in the sail, the top must be cut flatter for less power and it must allow twist.

There have been attempts with the junk sail to design camber into it and it works well although adding to the cost and defeating much of the appeal of the traditional junk.

Again, it’s all a compromise.

8 comments for “The art of the sailmaker

  1. October 11, 2009 at 17:10

    A work colleague spent a week in the Galapagos Islands on the Alta… lucky b******

    http://www.surtrek.com/alta_galapagos.html

    If they invented the steam train today… they would manage to make it look ugly in the name of efficiency.

  2. October 11, 2009 at 18:33

    Oh yes … lovely machine.

  3. October 11, 2009 at 21:07

    Good piece. The scene: a very posh Swedish boatyard. I am collecting a posh Swedish boat for delivery to new owner, putting the sails on PSB with help of handover agent.

    He says, in a rather droll Swedish way, that X company ‘should not be allowed to make XXXXing hankerchiefs’. The in mast reefing main was nasty by the time we made Southampton.

    • October 11, 2009 at 22:17

      I really dislike in mast reefing and even in boom reefing. For a start, it can’t be good for the fabric, as you mentioned. This is why I’ve turned against the plastic fantastic and think the modern junk sail is the way – less to go wrong and good for cruising – so easily reefed – untie, drop. The only value of a Bermudan, in my eyes, is for course racing. I’m currently looking at the Viking longboat with a modern junk – could be the way to go.

  4. Andrew Duffin
    October 12, 2009 at 10:57

    In-mast reefing terrifies me.

    What happens when it jams? Big sail, too much wind, can’t stop – omg.

    Don’t say it wont’t jam – if you say that, you are not a real sailing man, and Sod will get you with his law.

    • October 12, 2009 at 11:14

      Couldn’t agree more. The KISS principle is everything. Remember Scandia and its twin keel problems in the Sydney to Hobart? We’re not talking here about racing, with rescue boats about – we’re talking, aren’t we, Andrew, about blue water, no one about and a gale upon us. I know I go on about the junk sail and of course, all sails can jam as long as there is running rigging but that sail reefs in seconds and the sheets are reduced to two instantly. Heart attack? Let the halyard go and the sail drops into the topping lift cradle.

      Whatever, I think we’re agreed – when the chips are down out there, we want to know our simple running rigging is going to work smoothly every time.

  5. Peter Mc
    October 13, 2009 at 09:58

    “I’m currently looking at the Viking longboat”

    You’ll cause everyone heart attacks if you come Whitby way with one of those.

    • October 13, 2009 at 10:35

      Precisely what I plan to do, Peter. I can tell you now – it’s 53′ long, 8’6″ wide, has one outrigger on the starboard size with the stores in there for ballast and uses two junk sails [given camber between the bamboo battens]. There are four compartments – sleeping, living, my sleeping and the cockpit. Very simple design, using clinker/lapstrake. I’ll post on this today.

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