Cruising Aspect
Back in 1971, I was looking at this question of optium performance to windward by an ocean going CRUISING boats. I was greatly impressed with Lord Riverdale's (Arthur Robb designer) very significant experimentation with twin-keel vessels. I was also suitable impressed with the asymmetrical daggerboards on Bruce King's SORC racing boats.
The combination of the two concepts into asymetrical twin-keels with a preset angle of attack is what I sought in my first monohull drawing:
www.RunningTideYachts.com/monohull/
As I point out on the website, Contrary to the use of symmetrical twin keels as popularized by Westerly, my design utilized asymmetrical foil keels which were placed at a 3 degree angle to the fore/aft centerline of the vessel. The asymmetrical foil shape was more efficient per square foot of keel area at developing the leeway reducing forces, thus the total wetted surface area could be reduced. The 3 degree toe-in was used to avoid the vessel having to crab sideways to its intended direction in order to develop any lift at all (symmetrical foils must be driven at some angle of attack in order to develop lift). The asymmetry and the pre-set angle of attack would act to significantly reduce the 6 to 12 degree leeway experienced by conventional symmetric-keeled vessels.
The combination of the foil shape and the skewed angle of attack was accomplished with very little increase in total frontal area projection over a traditional single fat-foil keel (ed note: remember Charlie Morgan's Out Island 41 was just begining to capture the cruising world's attention at this time). The two keels were attached to the hull such that as one became vertical upon heeling, the other assumed a more horizontal attitude and contributed to the righting stability of the vessel. Thus I had the shallow draft of the Out Ialand vessels with a significant better leeway capability.
And to add icing to the cake I incorporated my then new mast-aft sailing rig. The mast-aft rig's absence of a conventional mainsail imposed less leeway inducing forces so that the keel's surface areas could be reduced (extra wetted surface area was always recognized as a nemesis of twin-keeled vessels). The overall lower center of effort of this sail plan allowed for less voluminous keels (less ballast required).
Should we question the relatively poor contributions of the mainsail? It is most primary in leeway inducing. Per my site, conventional booms excessively flatten the foot of the mainsail, and are often oversheeted, contributing significantly to the leeway forces. I once had a copy of a test on a Morgan 41' Out Island ketch , where upon removing the mainsail, the boat lost only 1/2 knot of speed, but cut its leeway in half (from 11 to 6 degrees). A staysail was then rigged between the masts in place of the mainsail, and the boat regained 1 knot of speed while retaining its decreased leeway.
Or as Tom Speer hints at above , for example, one can reduce the leeway to an arbitrarily small angle just by adding sufficient area to the keel. However, this will not necessarily improve performance because of the excessive wetted area. One can also reduce the leeway angle by sheeting out the sails - but that will not improve performance, either, unless the sails were over-sheeted to begin with. (BE notes: It very often the case that the foot of the mainsail is oversheeted, and is contributing very much to the leeway).
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Or in another posting by Tom,However, I think the important point is the huge vortex coming off the boom...... The vortex is also strongly affecting the region of the mainsail where the chord is the greatest. People usually concentrate on the vortex at the top, but the vortex at the foot may be more significant. It's worth considering how to shape the mainsail so as to reduce the strength of the vortex and to move some of the sail area away from its influence.
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To which I would add Steve Dashew’s observation,
On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.
On our 67' ketch, Sundeer, we were able to pick up FIVE DEGREES in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.
The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats.
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Brian added:
Now if we look at the heights off the deck (and deckhouses) of today’s booms, particularly multihulls, we really have to look at both the inefficiencies and the leeway questions for traditional mainsails, let alone their zone incompatibilities with the headsail.
Finally I might add, that when I drew my monohull concept up, center-cockpits aft-cabin cruisers were all the rage. But I thought it would be much nicer to have a raised center covered deckhouse, albeit a mini one, somewhat akin to the central saloon of the cruising cat, and the same idea that is now so prevalent on today’s monohull cruisers.
I still believe the twin-keeled monohull has a whole lot of potential that’s gone unexplored.
