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Theoretical Considerations
Although the rig allows the boat to move through the water, it unfortunately
also slows the boat down. This is because the spars and rigging create
windage and there will also be an increase in pitching because of the
extra weight high up. So not only has one to balance the driving force
from the rig with the sail carrying power from the hull but one also has
to consider the height and placement of the spars. Furthermore, to be
considered an ideal rig it also has to be easy to handle, simple and have
a long useful life.
The most efficient rig is a semi elliptical shape una rig (ie one sail).
That's what you see on aeroplanes and gliders and it is often referred
to as the "Spitfire Wing". The next best rig is one with a big
mainsail and a small jib that just overlaps the mast. In either case the
aspect ratio of the rig need not exceed 3:1 (which usually implies a mast
height around 1.2 x LOA). But if LAR keels are used instead of daggerboards
then the AR should be less as the rig efficiency must match the foils.
When I studied yacht design during 1975-8 my final year design project
was a 35' cruising catamaran with a short gaff rig as this seemed to be
an easy way of approximating to the semi elliptic shape. But I quickly
realised that there were two problems with this rig. First, one could
only sensibly stay the mast at the top, so the mast was heavier than on
a conventional rig. Second,the gaff was heavy. In fact I decided then
that it was better to have a slightly taller mast that weighed less because
it was stayed better and to use full length battens near the top of the
sail to create the semi ellipse shape. This resulted in a rig like the
Merlin Rocket dinghy and can be seen on the Strider design. This rig also
has the benefit that fewer controls are necessary to control it thus putting
less reliance on crew skill to create the fastest shaped sail and more
on the sailmaker, in other words, it's an easier sail to use.
Back in the 1980's full length battens were still made of wood which
were heavy and easily broken. That's why my early rigs only had long battens
at the top. But as sail hardware and batten design improved I changed
to drawing full length battens over the whole sail. This way the sail
falls neatly and easily into lazy jacks which I consider as essential
as roller reefing headsails on all cruising boats. Having said that, I
still prefer as few battens as possible because they are still heavy and
drag in the sail track making it hard to hoist the sail. In fact for masts
over 13m (45') I recommend ballbearing batten cars on a mainsheet track
up the back of the mast.
Unfortunately, it is not possible to have a mast that is "properly"
stayed with a big roached sail. If you want spreaders and backstays, as
many cruising sailors do, you will have to have a conventional rig with
a small roach mainsail. But these days I'd still have full length battens
because of the easy stowing in lazyjacks and the fact that the sail doesn't
flog when motorsailing.
Twin masted rigs
Traditionally two masts were used on yachts for ease of handling - which
meant some racing rules gave such rigs a rating advantage, eg 1960's ocean
racers and the last IOR maxi Whitbread Race boats. But these days there
is no need to keep sail areas small as lighter and stronger sails coupled
with better sail hardware and self tailing, possibly powered, winches
and ball bearing blocks have all made life easier for crews.
The golden rule when racing is that one should never sail immediately
behind another boat as it is slow, so a 2 mast rig can never be efficient
and is always more expensive than a sloop rig. There is also more weight
and windage and, aggravated by the fact that the spars are nearer the
ends of the boat, pitching will be increased markedly.
Self Tacking Jibs
These sound like a good idea for short handed crews. But they are only
any good when actually tacking or sailing close hauled. They are no good
for example when you want to sail off an anchor and need to back the jib,
while when reaching you will need to barber haul the sail to control twist.
If you have a roller reefing headsail then you can forget it as far as
sail shape is concerned (a rolled sail becomes more baggy the more vertical
the leech as the cloth rolls on top of each preceding turn).
Monohulls like the Star and Etchells have self tacking jibs. But self
tackers work better on these boats because to increase VMG (speed made
good to windward) a monohull has to point higher as it cannot go faster
through the water as it reaches maximum hull speed in only a light wind.
A multihull, on the other hand, can increase VMG by sailing faster even
if it doesn't point as high, so it can usefully use a different style
of rig.
Beach Cat Sails
Recently its become fashionable for beach cats to have what is called
a "fathead mainsail", often without a boom. Although these rigs
have proved their worth on the race course they are less successful on
larger boats. The boomless sail is obviously attractive on a beach cat,
but larger boats have to be able to be reefed easily. Without a boom the
only safe way to reef is to lower the sail completely - not very seamanlike
if reefing in crowded waters. The reason? Imagine being hit in the face
by a flailing mainsheet block attached to a large boomless sail! It is
just about impossible to control sail twist offwind without a boom. Again
this is not a problem for beach cats as they rarely sail with the apparent
wind aft of the beam, and they need to depower the rig much sooner than
on a larger boat, partly of course because they cannot reef. As I mentioned
earlier, one of the drawbacks to full length battens is that they generate
a forward thrust which makes it hard to hoist the sail as the batten end
tends to jam in the track. Furthermore, it is difficult to fit a sail
cover if the battens are not horizontal, and as battens should bisect
the angle of the roach, fat heads make sail handling much harder.
Wing Masts
No one can dispute the fact that rotating or wing masts increase performance.
However, they do have drawbacks, particularly on larger cruising boats.
For a start, they are not very practical. For example, you can no longer
fit a tricolour masthead navigation light, masthead wind instruments won't
work and it is difficult to lead halyards aft to the cockpit. Furthermore
wing masts tend to be heavier which leads to the pitching problems mentioned
earlier.
Another major factor that needs to be considered is the actual availability
of such masts. Because the multihull market is so small there are few
aluminium mast extrusions available, particularly in boats over 9m (30').
So many people are tempted to make their own. But be warned, they are
a time consuming and difficult thing to make and if, like me, you've just
spent a few thousand hours building your boat what you want to do is go
sailing, not spend a few hundred more hours building a mast. It is not
a cheap option either the basic spar may not cost much, but all the tangs,
halyard exits etc will have to be fabricated as one-offs which will be
expensive. Carbon fibre is now commonly used on wing masts, but the main
problem with this material is that it reacts very badly to aluminium in
salt water. This means the sail track etc should be made in carbon as
well, which is tricky to do. A word of warning, if you do have a carbon
mast paint it white, otherwise the epoxy will be degraded by UV within
two years and your mast will break!
Sail Cloth
A cruising boat wants sails that have a long useful life whereas a racing
boat needs sails that will hold their shape consistently over a wide range
of conditions. So the choice of both cloth and cut will vary depending
on whether you will be mainly racing or mainly cruising.
Mylar mainsails are great for boats up to about 9m (30') while kevlar
is better reserved for larger boats. Mylar in headsails is not so good
as dacron as it mildews quickly which looks unsightly on a roller reefing
sail and the laminates do not like being tightly rolled. In general terms
a mylar or kevlar sail will hold its shape very well for one or two seasons
but will then fail without warning. A dacron sail on the other hand will
distort permanently out of shape within the first season and even when
new will distort under high loads. But it will have a cruising life of
maybe 20 years. New developments in sail cloths are occurring all the
time. Currently the best compromise material is probably Spectra.
Sail Cut
Sail cut is a different matter. In the old days of cotton gaff sails the
cloth was weak and thus seams ran vertically so, if they tore, the sail
would not break in half. With the advent of Dacron/Terylene, cross cut
sails came into vogue partly because these were (and still are) cheap
to produce. More recently computer sail design packages and laser cutters
have made it easy for sail lofts to make sails with more exotic panel
layouts. The most useful of these are the biradial and triradial cuts.
They combine lighter cloth weights near the luff, making the sail easier
to use, coupled with a stronger leech which ensure that the sail holds
its shape in all conditions. If you are on a limited budget then I would
still go for a triradial mainsail even if it was in inferior cloth. The
genoa can be biradial on smaller boats or triradial. The problem with
the latter cut is that there are then a lot of seams at the head and this
makes the sail very stiff so a biradial sail is often better.
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