QUESTION: With rigid wingsails looking to drive Americas Cup boats to 2.5 times the speed of the wind, do you see any carry-over of this technology to regular sailboats?
ANSWER: This is an interesting question that deserves some space. These Americas Cup boats are the Formula 1 or Space Rocket equivalents for sailing craft and it's true that we do often see technologies from these 'extreme cost-no-object' developments, pass down to the average user, so indeed it's interesting to ask—just what might we see in the coming years that could upgrade the performance of more widespread racing and even cruising sailboats?
For this discussion, I am going to limit myself to the rig, as it's the materials that principally drive hull weight and their performance, and that has already occurred to a considerable extent, since carbon fiber is now used more frequently and in larger volume—so slowly bringing down its cost.
As a starting point, let me put forward a rough comparison to show rig efficiencies for the different mast-sail configurations shown—all based on using the conventional tall mainsail of the Bermudan or Marconi rig.
Here is a table that roughly shows the typical maximum-boat-speed to wind-speed for each rig and mast type so we can better see where we might be heading.
|Ratio of max boat
speed to wind speed**
|** A rough comparative indication only,
as many detail factors are involved.
(One might debate for hours on the accuracy of the above factors, but from my experience and research, they are ball-park enough for the purpose of this discussion.)
Please note that the factors given in the table above, are based on the rigs also being matched to equally advanced boat platforms, that bring out the best of each rig.
Type A covers what most sailboats are still using today. Although going to Type B (by rotating the mast) can show gains in the order of 15%, most cruising boats are designed with fixed masts purely for added safety, as they permit additional standing rigging at different mast heights and therefore offer some redundancy and backup in the event of failure of one stay. One small advantage of the fixed rig is that the mast diameter can be a little smaller as the extra rigging can keep most of the load in a vertical column. Type B is really not more expensive but in order to rotate well, the rig needs to be supported at just one basic point up the foreside of the mast, with typically just a forestay and 2 shrouds being used to keep the mast up. Because the gains are even more significant in faster air speeds, multihulls of all sizes have used the rotating rigs of Type B and even Type C for many years now. But rigging has to be meticulously maintained as one failure will bring down the mast. Going to a wing mast is another step up but with each step there are always pros and cons. Clearly the critical flow over the leeward side of the mast is speeded up—so lowering the pressure there and hence giving the rig more 'lift' towards the wind source. But a wing mast also has a fixed area of its own and although this can be somewhat feathered into the apparent wind, such wind directions may change too rapidly for the mast to follow, therefore providing a form of fixed storm sail, even if the soft mainsail is stowed away. In rough conditions, this might be a source of excess speed, although some proponents of this layout, may argue that this can be used to good effect. But for most conditions, Type C does offer an important efficiency gain over the basic fixed mast. It will ultimately depend on the stability of the boat itself and because of that, is a fairly good option for many stable trimarans. (Chris White's 52' Juniper trimaran did a circumnavigation with twin wing masts without problem.)
Now we come to three configurations that can really step up the ultimate performance but ones that have practical issues to contend with. Type D has been used with a short gaff, in order to be lowered and handled more easily. Long time cat designer James Wharram (UK) has promoted this rig for some time. Even the ubiquitous Laser dinghy uses it as in this case, the sail is small enough to be doused by simply wrapping it around the mast. But for much larger craft, problems of stowage and the need for a mast without stays over its length, limit its practicality for large boats, despite its aerodynamic efficiency.
Before discussing Type E, let me first jump briefly to Type F, the rigs that we are now seeing on the new Cup boats—both for the AC45 World Series cats as well as for the AC72 to be used for the Americas Cup itself in 2013. Being rigid in surface, these skinned wings are far less affected by boat movement, nor can they hang lifeless in low winds as material sails can that are too heavy for the wind conditions at that moment. Also, by articulating the sail through two (or more) panels with a small and sometimes variable gap, these sails can be super-fine-tuned to give a very high lift with low drag and therefore produce the incredibly high performance that we're already starting to see with the AC boats. But there's a huge price to pay for this performance. While these 'sails' can be feathered to some extent in a high wind, the full area is always there, making their use totally limited to boats that are either on a race course or are stored in some protected area. In fact, boats rigged with these wingsails are far safer on the dock when laid over on their side at say 100°—than if left vertical. In practice, these sails have to be lowered and taken under cover as soon as possible should even moderate storm winds approach. Once on the boat in the water, the boat will be like a thoroughbred race horse and it will be hard to contain its desire to just 'take-off'—all of which makes it entirely impractical for any future pleasure boat other than for extremely small ones that can be laid down when at rest, much as one might lay down a bicycle on the grass. So does this mean that the performance limit has been reached with the wingmast or in limited cases, with the luff-pocket rigs?
That, I believe, is where the Type E rig might have a place in the relatively near future. So let's look at this rig in some detail.
Variable camber wingsails have actually been around for a long time though very few were sufficiently developed to prove practical installations. Not wishing to make this article excessively long with purely historic happenings, I'll just mention a few of the attempts and give the Patent Nos for those interested to delve deeper into the details.
Way back in 1925, the amazing Lewis Francis Herreshoff filed for a patent (1,613,890) that with some finely detailed engineering, showed sails (both main and jib!) made from two cloths, with their leading edges attached to sides of a fairly large fabricated mast (or spar) that could rotate.
This subsequently led to a number of patents, all somewhat based on the general idea and some 50 years later, (1976), Roberts and Edwards were awarded US Patent 4,064,821 for a rig that in section, looked like this:
Then in 1988. Patrick Johnston of Australia was awarded both US and Australian Patents (US 4,766,831) for his double skinned cambered sail, that changed shape port to starboard by the rotation of a mast that pushed on long sail battens. The mast, that was relatively wide compared to fore and aft depth, fitted snugly behind a streamlined nose piece, not unlike the first Herreshoff design. Over the years that followed, Patrick teamed with this brother Greg and further simplified this rig—see later comments.
In 1990, Nuri Elmali of Florida was awarded Patent 4,895,091 for a double soft skin sail that was ingenious enough to create surfaces of just about any shape one wished but the complexity, associated weight and cost, would surely make this quite impractical.
In 1994, Angel Aquilera of California, was awarded Patent No: 5,279,241 for a double surfaced sail that was to be inflated—and on and on it goes.
But has anything ever come of these ideas in a practical way? Well not much really, until more recently, when the Johnston brothers (noted above) combined their efforts and put some real effort and cash into building several double skinned soft sails in Australia to really work out the details on a variety of craft from windsurfers, Moth-boats, skiffs, Hobie cats, and finally a couple of larger displacement boats of 25' and 28'. They call this a 'VGWS' (Variable Geometry Wing Sail) rig and I personally think this development has a lot of promise for raising performance for sailboats of the future. Unfortunately, many class rules will forbid such sails to be used so this will initially be limited to individual boats or development classes—such as the small International Moth. Sadly, such rules, while keeping costs down, also tend to suffocate new developments and many clever ideas die in the process. The Johnston brothers have a small website at www.advancedwingsystems.com that presently seems to deal mostly with their efforts on Moth sails but there's some interesting VGWS Rigging Details at www.ProBoat.com. But should any reader want to learn more about this, there's a very detailed technical review with photos and charts, recently presented in the Dec/Jan 2013 issue of Professional Boatbuilder under the title of the 'Working Class WING'.
Note: One quick way to check out US Patents is to go to: http://google.com/patents/US### with the Patent No. in place of the ###.
Mike — Nov 2012
ADDED Dec 24th 2012
Since writing the above article, I'd like to take my hat off to Paul Larsen who recently shattered the previous wind-powered record so decisively. Can you imagine 80 mph??? His craft and rig called "Sail Rocket" (designed close to my own birthplace, in Cowes, Isle of Wight, UK) took some 12 years of constant development to reach this amazing pinnacle—way beyond what experts had considered 'the maximum possible'.
I think technical readers will find these two articles of considerable interest.
"HOW SailRocket broke the Record" —by Andrew Redman
And to help further understand the power of wing sails, this interesting treatise on: HOW sails Work" by aeronautical engineer/sailor Paul Bogataj, now with North Sails
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