Rig and Sail Design
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
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
Beach Cat Sails
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.
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.
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).
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".