Blackmagick Sails
Design and Construction Techniques
As I worked on the sail shapes for these small yachts I noticed the sails that seemed to be
going the best on the water all had a shape or curve that was very even, with the draft a lot
further back than one might first think prudent. Some have suggested in the past that the
maxium draft in a good sail should be approx. 33% of the way back from the luff, but this has
in my mind always been to far forward. I started with draft at 40% aft, but soon found that
even this was to far forward. I got to almost 50% before I knew I had gone to far, so settled
for 45% for an average A rig sail, main and jib. I stipulate A rig here because you can come as
far forward as 40% for sails that are used in heavier airs! It should be noted that sail shape
and draft points do move around in a sail under pressure, and quite often the reverse to what
you may think happens! ie. The sail draft can move forward in a sail due to the suction effect
set up by the air flow over the sail. This is the same force that draws the sail forward through
the air and creates drive. This is also one of the reasons you may have to design a sail with the
draft a lot further aft than looks right to the eye, once on the water and powered up the draft
will move to the correct place in the sail for optimum performance, at least if it's made right it
will. Draft movement is more likely to happen in the lighter weight cloth sails, and one reason
why I feel mainsails need to be made out of mylar, ie. it's much more stable than regular sail
cloth.
The sail example pic I have used below is what I consider a well shaped jib or headsail, with the
draft flowing of the luff in a fair and even curve, which should allow good pointing ability plus
maximum drive. The curve should straighten out a little as it goes past the deepest point in
the draft and end up dead straight at the exit point of the leach. The same can be said for a
mainsail. I don't use battens in my jibs because they should not be needed, in the one metre
class at least headsails have no shape or roach in the leach and should set fine without
battens. Anything added to a sail that is not really needed can distort the way the sail sets,
especially with such small sails and light weight cloths.
Both sails (main & jib) on a 1metre are what we call 'loose footed', this is a help in many
ways because it allows us to alter the sail shape near the foot of each sail to a very great
degree. I have worked very hard over the last few months to build tapers into the sails that
respond most effectively to alterations in the loose foot settings. When you have more than
enough power in your sail, ie. in a fresh breeze, it pays to flatten both sails out on the foot
not only to reduce the power and healing moment that they can generate, but flattening the
lower sections allows you maintain your pointing ablity as well. This is where the placing of
the lower seams in both sails comes into play. I feel if the two lower seams in both main and
jib are placed to low in the sails when you sheet in tight to the centre line for on wind sailing
the tapered shape in each of these lower seams tends to jam the slot between main and jib
slightly. The natural thing for the air flow to do is to follow the tapered shape and this can
tend to backwind the mainsail off the jib if the sail is quite full at these points. Moving the
seams up a little while off setting them as well as putting no more shape or taper than is
necessary makes it easier to flatten out the sail not only in the lower sections, but overall
fullness. I will illustrate this more graphically on the next page.
Shaping the Sails
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