Assymetric Cat Hulls

Are there any significant gains to be had by having assymetric hulls? Looking at Bernard Kohlers K Designs page;

http://www.ikarus342000.com/CATpage.htm

It appears that here is a man totally dedicated to the hull form. The other interesting thing is his hulls are flat bottomed and very narrow (load carrying?).

I have read elsewhere on this site the benefits/problems with parallel rigs, these seem to be very popular with Mr Kohler. Is there anyone out there in multihull land who has built or knows of anyone who has built and sailed one of these designs and, the biggy are assymetric hulls fast?

 

The difficulty would be in attaining balance and alignment (particularly for the home builder - Many cats are built with a single centreboard as alignment is very difficult and just millimetres of error negates nearly all the effort in having a fair and smooth hull finish - so new home builders are now just putting ONE board.... Asymmetrical hulls offer no significant advantage... If you built an asymmetrical hull cat, you would have great difficulty in selling it when you wanted to take the next step....

Asymmetrical above the waterline with a broad 'chamfer' up to the base of the bridgedeck and on the outer side a bit of extra shoulder/shelving room and can be curved to give the "incandescent light-bulb" shape for a better lateral aerodynamic effect...

This is either a costly process as one must now use strip planking and covered with glass and epoxy or have a female mould made to make the shape and then release it and join longitudinally... A flat bottom is quite popular with several designs where composite panels are used as in the Easy (plywood) or Bob Oram, Schionning and others who produce kits of precut ATL panels to make the build process quicker and easier... - - look at the link at the bottom of my post, and Bob Oram's website also has links to "the Scrumble Project" where you can see the build process in photo reports... also subscribe to "the Coastal Passage" in the Whitsundays - a free pdf download...

May I suggest you take a better look around at local designs and builds, as, for local conditions, local designs are best... Do you need to put it in a marina pen?... where will you sail,... what sort of sailing will you do...

Bay race ? - get a nacra etc and join their club, . . . leisure cruise the bay on weekends ? - go for a used 25 to 32 ft, . . . cruise the Queensland coast? - 30 to 40 ft., . . . go further afield ? - get lots of experience first....

Light and 'skinny' cats are fast until you overload them, so if you have "bower-bird" tendencies, do not think of a cat - get a 100 ft bathtub, as you will fill it up with stuff very quickly.... - New to sailing - get a second hand beach cat like a nacra or whatever you can comfortably tow to the bay for club events as they will help you to learn the ropes....

 

They were good

Assymetric cats were the start of all this cat business. The first proper modern cat was Manu Kai in 1946. Aikane was built in about 1957. She was thin, assymetric, narrow sterned and the first real ocean cat. Beautiful boats artfully design and built. To be truthful I think multis went backwards heaps when Piver then took them down the simple and cheap route - took decades to get back.

BUT

Cats are not usually designed this way today - lack of interior room, hard access into hulls, poor tacking ability, low load carrying ability, lack of transom steps, lack of stern volume for weight to take dinghy, aft deck etc all dictate a different shape. Don't get me wrong - my modern 38 footer could not keep up the the 1957 design Aikane but she is easier to live on.
If you want to do the world a favour build an Aikane replica. She was wrecked and was the grandmother to all of the current cats.

cheers

Phil Thompson

 

BasilD, some of Lock Crowthers cats were set up as asymetric, in that the same hulls where canted at around 8 degrees off the stem/centreplane from vertical, this made for a slightly wider hull centreline to centerline beam, some asymetry in the waterlines & something approaching but not as pronunced as the inboard champher panels we now see on cats . Quite a clever approach & some pretty nice boats too. All the best with it from Jeff.

 

I have not the mathematical skills to demonstrate or prove but do observe and ask questions - thereby learning.... My build will probably have only one centreboard, on advice and experience of other builders and 2 is over-kill so either pull one up for better performance when such should be deployed or only have one and have better space in the other hull... I am sure several simple experiments could be developed to illustrate the differences as well - but not using my boat can't afford that luxury of experimentation....

Think in terms of the trim tabs used on aircraft to balance their progress by correcting flight surfaces to fly straight when landing in a cross wind etc, not much tab can be seen and felt...

 

Thanks for the input so far guys. But when I'm talking asymetric, I'm talking flat outside hull-forms. Yes Kohlers style of hulls are very narrow with virtually no interior space, but do they work? And as Phil suggests are they hard to tack? Is this due to being asymmetric or something else? I suppose it is due to having to push the flat sided hull across the water. They are definitely way out in left field compared to everything else on the market. Another thing on his web site is the horizontal "vortex boards" he uses. This is yet another subject that is really interesting.

 

The asymmetry in this case is the use of the hull as foil instead of a board, rather than the issue of using one board vs 2. This does work but how well it works on a cat has always been a question to me. On a tri float, or particularly a proa or beach cat flying a hull, the boat is not symmetrical in sailing attitude. With a cat there is the question of the hulls canceling each other out. And the only thing that affects that is the heeling angle. On a cruising boat, particularly at lower speeds, that would seem a pretty marginal effect.

It is true that older boats had this feature, but back then the alternative was rarely a daggerboard, and as a result all multis had relatively compromised performance.

I asked Kohler about the asymmetric hull, and his response was that he was influenced by Choy, who is certainly a major figure. He said that their performance was fine, and their the asymmetry improved the handling in following and diagonal seas.

I checked the Choy thing with one of his co-designers (who can jump in if he wants to). I had conflicting recollections because I thought in a Multihulls article that it had mentioned he used asymmetry on a major modern oceangoing cat. But I also recalled boards. This was Aikane (X-5), a name that has been recycled a lot. The info I got was that they were only slightly asymmetric and relied on big boards. Asymmetry was designed to improve handling in surfing.

I also asked about the flat bottoms because I had designed a boat with flat bottoms that slammed at anchor. Flat bottoms are OK in general and he said his don't slam. I think mine slammed mainly because the boat was small enough and in lake conditions the rollers are not similar in proportion to ocean anchorages.

Load carrying is an issue that has to be carefully addressed in any design. Kohler likes 16-1 hulls which makes sense to me in the smaller designs that shouldn't be overly heavy if they are to be trailered. I think his KD 650 is a very innovative design, though I am skeptical about the sailing aspect. Just can't add anything with my personal experience. I would be tempted to design a multichine, and daggerboard variation if I built one. Once one takes on much higher displacement designs the flat bottom can get too wide. I wouldn't build a 10-1 roomaran boat in that style. But a dory shaped flat bottom hull, not overdone, has been widely used by designers like Marples, Newick, or Woods. I don't consider the boat with a flattish keel section on a multichine, to be all that comparable to what Kohler is doing, except on issues like grounding. Those multichine designs seem closer to round hulls. I think that at least conceptually a boat should also be compared on it's displacement curve (or what one thinks it would be, since they aren't usually published). So a 16-1 square hull with flat bottom is comparable to a round bottomed hull of the same general proportions but 14-1.

Just a note on the Piver thing. He is certainly remembered for the quick build claims and weekly building and sailing of new versions thing. However, he raced a lot of rounded bottomed cats, I think. When he first tried marketing plans he tried to sell rounded bottomed strip planked tri hulls. They didn't sell, but the tris took off in plywood. Strip plank (and shapeliness), it seems to me, really took hold when the combination of stuff like CAD designed stations; CNC cut out station packages; cheaper tools; fingerjointed and coved strips; modern fairing techniques; epoxy; 410; etc... got together and led to a strip building thing where the results and the speed and cost were in line. So I tend to think it was the consumer, and the technology that weren't there with Piver, not that Piver dragged the art down. And for that mater. Choy never really addressed the home build market.

 

The thing that worried me about asymmetric hulls and vortex boards (your new inquiry), is that to anyone who has been around for a while, these are not new ideas. Neither has taken the world by storm in the intervening period. I raised this with Kohler, and his view was just that the work of the likes of Choy is not well understood in Europe. Vortex boards where a very lively Ayers and Wharram community topic for a while, and people did add them to their boats. But virtually nothing has come of that, except in aircraft possibly, where endplates are all the rage on wings. I don't know of any other designer who uses the two together, so we just have Kohler cats to rely on, and despite brisk plans sales, there isn't much in the way of feedback in cyber space.

My feeling is that there isn't a lot to loose from proceeding down the Kohler path. I don't think the asymmetric hulls and VBs will be bad, it is more a question of whether they are enough. It would be pretty easy to lay the foundation for one of the three major board types, and then just add them if they seem required. Kohler does use boards on some designs, which is interesting in itself. He has a pretty simple construction process. So he might be able to help a builder down this path if the builder insisted.

 

Bernd Kohler used horizontal foils on his personal boat for years and reports that the boat was close-winded and fast. It seems unlikely to me that he would continue to use these foils if they were not as effective as he says, since with the number of plans that he sells, it would soon become obvious if his claims were overstated.

There is a yahoo discussion group for Bernd's designs:

http://groups.yahoo.com/group/k-designs/?yguid=215961385

The American designer of small cruising boats, Matt Layden, has used horizontal foils to excellent effect. He won the Everglades Challenge one year in Paradox, a heavy cruising sharpie of less than 14 feet. Of course, these boats are not multihulls, but I know an experienced multihull guy who built a Paradox, and from his reports, the boat is quite weatherly.

I'm building a small simple cartop cat design right now that will use horizontal foils. We'll see how they work, but a larger boat of similar hull form does pretty well with her daggerboard up, so the foils won't have to do very much to yield adequate windward performance.



While it is true that Wharram builders were for a time enamored of these foils, it may be that Wharram's hull forms were not really suitable for this approach.

 

Those are all interesting data points. The flip side of it is if they work all that well and they aren't a big secret, why don't we see more of them? John Marples once told me of his CC 26 and a day of sailing it he had in the keys in skinny water, how impressed he was with how well she sailed, and he only noticed towards the end of the day that he had never dropped the centerboard. Boats don't require a board but most as improved by them.

People are easily pleased with their own creations which is at the one time a good argument for designing more of one's own boats, and looking for more critical sources of info.

I'm a member of the KD Yahoo group, but it doesn't seem too tough a crowd like most single purpose groups.

I think a more likely explanation for the "vortex generators" falling on tough times among the Wharramites is that they didn't produce a noticeable change. These were mostly retrofits so they would have seen before and after results. May have even impressed their owners, don't remember, but not much traction over time. Wharrams would seem good candidates since the hull is deep, otherwise there were no other boards at the time, and the angle of the hull would seem fairly conducive to passing water from side to side.

As I understand it the point to these endplates is mostly that they hold whatever water pressure is in position, in place. That presumes there is some pressure difference that it is causing sluicing from one side to the other. There will also be a small outright board effect the greater the angle of heel. One option that might be interesting, would be the full length external chine log. Bolger puts these on rowboats. and claims they do not increase drag appreciably. (I always delete them when I build his designs so I don't know one way or the other). But as full length endplates, they might stabilize pressure the length of the hull. They would presumably only need to stick out on the outside of each hull, which would reduce drag.

I forgot to mention another designer who at least once used asymmetric hulls on a cat - Chris White. No idea how well it worked. He said that the Choy experience indicated that a board was also necessary, but that there were two other reasons. One is handling, and the other is the offsetting of the per hull CB outward for a stiffer lateral stability at no greater beam. So far it all sounds good, except I might still want that board.

I look forward to hearing how they work on you new boat.

 

the point of asymmetric hulls is to generate lift to windward by flying the weather hull. if both hulls are in the water, little or no lift. with the lift comes increased drag. even if you fly the hull, a dagger board is vastly more effective hydrofoil.

the advantage of asymmetric hulls is in their ability to sail well near a beach. much easier to get through a beach break if you can point immediately.

 

This is always a question worth asking, but I don't see it as very persuasive. When I was building Slider, I can't tell you how many times I heard some variation of this: "Ray, if a little trailerable open cat with seating inside the hulls is such a great idea, how come no one else has done it?" As I recall, in another thread you pointed out that the idea was not actually new, that double canoes had been sailing for thousands of years in Oceania. And yet, it hadn't occurred to other modern designers to take that particular approach.

Multihulls themselves have suffered from a similar phenomenon. Lately I've been reading about some of the early attempts to develop cats in the Western world. Herreshoff built Tarantelle well over a hundred thirty years ago and the boat was clocked at 17 knots. This with plank on frame hulls, gaff rig, and cotton sails. Despite this amazing performance, many decades would pass before the modern multihull movement got off the ground. This was a case in which one of the greatest designers who ever put pencil to paper had drawn and built boats that were, by the standards of the day, capable of other-worldly performance... and the boats were still ignored completely by the yachting mainstream.

By the way, I came across this Outings magazine compilation the other day:

http://books.google.com/books?id=4h...X&oi=book_result&ct=result&resnum=1#PPA483,M1

It's a piece by A.J. Kenealy on the evolution of the "double-hulled" craft, published in 1898. He ends up describing a sort of tunnel-hulled scow, which he is at great pains to distinguish from the failures represented by Herreshoff's cats of 20 years before. Interesting stuff.

 

I should also say that when I was drawing Slider, I originally intended to experiment with horizontal foils. I eventually chickened out, for several reasons. One is purely strategic: if you build a new design that has too many new features, and the design is a failure, it's very difficult to tell which of the new features pushed it over the edge. Except for the sprit rig, Slider is a completely conventional design, and even the sprit rig was successful on Wharram's Hinemoa design.

Another reason was experiential. I'd spent the year before Slider was finished sailing around the bay on a little dinghy that did not go well to windward. The nearest pass to the Gulf is many miles dead to windward on most summer days. The confluence of these two factors nudged me toward a big deep daggerboard, which I knew would work well, instead of experimenting with horizontal foils.

Finally, I don't think that a completely plausible explanation for why these foils work really exists. Some view this as a fence effect. Bernd has his explanation. I don't find either explanation convincing, so I found it difficult to design for a feature when I had no clear idea how it worked. But I think it has to be acknowledged that the foils do work.

Matt Layden's boats leave little doubt as to the efficacy of his "chine runners." This is due to certain circumstances in the evolution and ownership of his boats. His first such design was a narrow, light, 13 foot boat called Swamp Thing, with what amounted to external chine logs. In it he cruised the Bahamas extensively. His next design, Little Cruiser, had dual boards instead, one forward. Then he built Paradox, with more foil-like chine runners. LC passed into the hands of Dave and Mindy Bolduc. They later cruised the Bahamas in company with Matt in Paradox for weeks. It was Dave's observation that there was no difference in the weatherliness of the two boats, except under two circumstances: in very light air, LC with her boards was more weatherly, but in water too shallow for LC to use her boards, Paradox was superior. Both hulls are almost identical in form, though LC is slightly larger than Paradox. To me, this is unambiguous evidence that the foils do work.

 

The issue still stands in the monohull community with regards to multihulls. If it weren't for the fact that Ernesto and Larry have actually toyed with the idea of settling their haggle with, "Oh, My God!", giant multihulls, the issue would remain buried for some time to come.

I'd be willing to bet that there have been many discreet forays into the multihull potential since Herreshoff. They did not get any attention in the press and the designer/builders did not have the clout to wangle any lasting evidence of their efforts. The monohull establishment is pretty potent when it comes to pushing down that which causes them possible grief.

It took a different economic time in history to dislodge the conventional wisdom enough to get things rolling. When coupled with an exploding boating population of post WWII folks and their attitudes about the self-sufficient concept of "hey, I can do that", there would be fresh new ideas along with fresh new building methods and the boating consciousness was opened like a floodgate.

Woody Brown stands as a guy with the right kind of creative personal drive, the right collection of experiences and knowledge and the pure status as a Waterman who simply loved the sea and the wind and what he might be able to do with them together.


Out came the previously closeted wacky ideas for cats and tris and the rest is history.