I have a couple book suggestions. The first is Manfried Curry's seminal work on sailing hydrodynamics called "Yacht Racing". This book was written in the 1930's. Curry was able to obtain access to the German Goverment wind tunnels did some amazing research
of sail shape. he tested sails or a wide range of shapes and also tested actual bird wings. He also looked at classes
that used identical hulls and limited sail size but not configuration. This was still a time when gaff rigs were common so that the results of that study included gaff as well as other sailing configuration. (Amazingly enough, the most efficient configution was a rig that looks very much like a modern America's Cup rig or high performance Catamarran rig which is a fractional rig with bendy mast, a large roach mainsail
and a small minimally overlapping jib
In a later book he rigged actual full sized sail boats with a series pressure guages which allowed him to map the positive and negative pressure on a sail while the boat was sailing. His work is well written and easy to understand and clearly anticipates the next 75 years of aerodynamic research
. Unfortunately, both of Curry's books
are out of print but I see them quite frequently at used marine
book stores. I bought my copy for $5.00.
C.A. Marchaj has written two excellent books on the aerodynamics of sails, "Sailing Theory and Practice" and second called "Sailing Hydroynamics and Aerodynamics". He looks at varying sail shapes as well.
Bethwaites' "Performance Sailing" also contains a wealth of information.
You may find Unesco's report on alternative sailing vessels would provide a lot of information on the performance of alternative rigs. This was a 15 year long study of small fishery fishing
vessels all over the world. It looked at the problems associated with third world fisheries switching to petrochemical material constructed and fossel fuel
driven craft. The study was quite comprehensive and included wind tunnel and field tested comparason of a wide range of rigs. Its results were very different than the study shown above.
The article labeled "Fastest Rig" is full of glaring inacuracies. For one, the ideal sail plan form is one that produced an eliptical loading and not an eliptical shaped sail. On slow speed foils this means a plan form similar in shape to a WWII era Spitfire wing of a modern America's cup boat mainsail
Dispite the claims of the article, this was not the first or last time that different rigs were tested on identical boats. As early as the 1920's there were very elaborate testing of varying rigs and plan forms in identical boats with very different results. When you talk about doing testing in a catamarran the windward performance of the hulls and foils will be the limiting factor. Cats have a lot of wetted surface and when you do a study using a heavy cat as the test bed
, the factors that would make a particular rig work well to weather
on a lower drag vessel, do not come into play.
The article ignored the reasons that gaff and lug rigs generally do not do well in most other studies, sag and tip vortex drag. In any boat controling twist is critical to performance on almost any point of sail. Controlling twist is very difficult in a quadralateral sail because the peak wants to sag off to leeward. In the article, the testers rigged spencer vangs to the peak of the sail allowing them to control twist. Spencer vangs act like a barber haul on a jib
sheet and allow the peak to be pulled inboard to control twist. Spencer vangs work poorly on a typically narrow hull form but would improve the windward performance of wide craft like the cat in question and especially over a backstayless masthead bermuda
rig as used in this article. The large forestay sag in the photo's would account for a major performance drop just on its own (especially when the jib is approaching 40-50% of the sail area of the rig).
The other issue with quadalateral sails is tip vortex. When you talk about low speed foils, aspect ratio is a critical factor. Most of the drive of a sail comes from its leading edge. Except downwind, boat are pulled and not pushed by the low pressure that forms on the leading edge of the sail. Close reaching and above, the trailing edge of the sail is only there to prevent this low pressure from being bled off to the higher pressure side of the sail. Because sailboats heel, a lot of the low pressure bleeds off of the top of the sail and mingles with the higher pressure also bleeding off the top of the sail. This creates highly turbulent air, (the tip vortex) and that turbulent air creates a lot of drag. Drag is the enemy of upwind performance. The smaller the tip of the sail, the smaller the tip vortex, and the lower the drag. When you have a typical quadralateral sail, be it sprit, gaff, or lug rig (junks are lug rigs), in order to limit twist the head
of the sail is typically quite horizontal. The head
of the sail becomes the tip over which this vortex forms and its long length means that it forms with a vengence.
The other issue is weight aloft which is higher with these alternative rigs and which is not a problem with a heavy multihull
but of course is a serious problem with monohulls.
The Unesco study was interesting because it looked at the relationship of rig to the hull. Most workboat hulls have comparatively little stability and few need very good windward performance and most need a lot of power reaching. In that application, gaff rigs and sprit rigs do quite well. The real winner was a polynesian rig called a crabclaw rig. This is a horizontal lateen rig with wildly curved yrads and booms. Despite its low aspect ratio, it performed better than all rigs tested(Your bumble bee if you will.) It is not terribly adaptable as a cruising rig because it requires very a specifid shape and proportions which unfortunately precludes being able to reef or even depower.
Enough for now, I don't know about you guys but I want to get cruising.