Rig and Sail Design

 

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 airplanes 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 that used on the Merlin Rocket dinghy since the early 1950's and can be seen on, for example, 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 use 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. Furthermore, 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 often 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! In addition, 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, true fat heads make sail handling much harder.

When I sailed International Moths back in the early 1970's I was one of the first to experiment with "sleeved luff" sails. These are sails where the sail wraps round the mast in a pocket thus fairing the mast into the sail. Much like a windsurfer or Laser dinghy does. There are several drawbacks to doing this on a boat with a halyard (my Moths and the Laser slide the sail on from the top). Rigging can only go to the masthead, so the mast tube has to be bigger and heavier (which of course makes it more important to fair it in!). The extra weight also makes the mast harder to raise on a trailable boat. You cannot easily remove the sail for repair or stowage. True you can use a full length zip, but we all know from salty clothing and oilskins how long zips last at sea.

And finally, and probably the biggest drawback, is the fact that, as everyone who has tried to pull off a wet T shirt knows, the cloth sticks to the mast and won't come down. This is less of a problem with small boats (like Lasers) partly because they don't reef, but also because you are proportionately stronger on a small boat than on a large one.

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! Furthermore it is essential that you fit a good lightning protection system. A metal mast will probably survive a direct lightning hit, a carbon mast probably will not. See my Eclipse Lightning Strike article for more details.

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 spectra 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 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 for a racer/cruiser and Pentex for a cruiser who wants a better setting sail (as we have on Romany). Kevlar isn't used much these days as it has a very short life, instead real racers use a mylar sail reinforced with carbon tapes (as we have on Tucanu).

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. But if on a budget I would go for a crosscut genoa made in a good dacron cloth and spend the extra on a better quality mainsail.

Latest Thoughts

As I wrote above, many of my boats were designed before good quality sail hardware had been developed. So, although I knew all along what sail shapes to use, I knew builders were limited to the materials available. Hence I drew simple rigs with as few full length battens as possible and, on bigger boats, a relatively small roach.

My rigs have been designed for English Channel sailing. One thing that I have learnt from sailing all over the world (I have now sailed in over 40 countries) is that the average wind speed in other countries tends to be lower than in the UK. Not only that but the seas tend to be smoother so boats pitch less. Thus my boats tend to be underrigged compared to others. So if you think your sailing conditions warrant a bigger rig then please let me know and I can draw a new sailplan for you.

These days sail cloth and batten hardware have improved significantly and it is now possible for anyone to have an efficiently shaped sail. However such a sail is only feasible if you are prepared to spend the money on good quality cloth and good solid rod or foam battens. Except in the very smallest boats do not have a big roach sail with a cross cut dacron sail and definitely do not use the cheap flat grp battens.

Having said that, for those who want better performance I, or most good sailmakers, will be happy to draw you a big roached mainsail.

The video below taken on board the Transit 38 catamaran shows just how easily the mainsail drops, even when sailing downwind, when using modern rig hardware. OK, we were sailing in only about 15 knots of true wind, but we have lowered sail just as easily in more. You cannot drop a sleeved sail as fast as this!!

My favorite sailmakers, who are experienced multihull sailmakers and whose products and service I have been happy with over the years, are Dolphin Sails in the UK and Dave Calvert in the USA.

These two photos show what I mean. Both are Romany mainsails. I think you will all agree that the Calvert one, on the right, is the better, faster sail. After spending all that time, effort and money building a boat surely your boat deserves the best sails you can get, after all it is the boat's "engine".