Assymetric Cat Hulls

"This is always a question worth asking, but I don't see it as very persuasive."

Exactly, it is in no way definitive, it should just stimulate a line of enquiry.

"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?" "

There is a difference though between a design with a several thousand year history and say cold fusion which needs/needed to establish it's initial viabilty. Endplates are somewhere on that line.

"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."

I think one could argue from side by side paddleboats, to bolger's motor cat, there have been lots of modern examples. Good designs don't stand on novelty alone. It's interesting that your lattest format deals with the difficulties created by a boat large enough to accomodate people in the hulls. As always, more boats!

 

And also, Woody was a glider builder who was able to put together the Polynesian ideas brought back from the war in the Pacific, the advent of plywood, and the ability to build very light but strong structures. It's a weird coincidence, but I was just talking to a plans buyer today who was a Coast Guard inspector in Hawaii back in the days when Woody was sailing off Waikiki Beach in Manu Kai. He told me a sad little story about the old guy. It seems the boat had a delaminating main beam, and Woody couldn't afford to fix it, so the inspector had to pull his certificate, and Woody lost his slot on Waikiki.

History turns on small things.

 

"To me, this is unambiguous evidence that the foils do work."

That's tenuous. Might as well say all racing results come down to differences in boat set-up. Also, I don't think the mono example is convincing at all, because monos heel, so that the end plate will actually behave more like a foil as the heel deepens, which isn't the case with conventional boards. And your example includes poor light air performance where heel would be less pronounced. Of course, cats heel little.

I think you have to refine the concept of "works". Seems reasonable that end plates work, but do they work better than the alternative. Alternatives could include, both, boards with endplates, leeboards, hulls shaped for better leeway prevention etc...

 

Thom, I'm not sure anyone is claiming that they work as well as high aspect daggerboards, just as no one claims that LAR keels work as well as high aspect daggerboards.

All the same, there are LAR keeled boats that are fast and weatherly. They won't win races against identical boats with daggerboards, just as, say Stilettos with central leeboards didn't win races against Stilettos with boards under the hulls., but most boats are not used for racing. LAR keels represent a compromise in the direction of less hassle. So would horizontal foils, and my suspicion is that however the foils work, it's probably on some well-understood principle not yet successfully applied to these particular devices.

I'm curious as to what you would accept as a reasonable demonstration of the utility of these foils, short of tank-testing, which I can't afford. I've considered putting the foils on only one side of the new little boat, but that approach leaves the problem of subjectivity in evaluating the difference, especially if it's a subtle one. It may also be that there would be little difference attributable to placement, if, as I suspect, the foils resist leeway on both sides.

 

Then there's only one answer, Ray.... build two identical versions of the new machine and get someone with nearly the same skills and run them side by side with the different "keel/board/vortex thingy" setups. The recreational nature of the boats will cancel out most of the sailing skills and if you switch boats half way through it will be a virtual wash.

;-)

As if you didn't have enough to do already... just whip that puppy out there and get it on. ;-)

 

"Thom, I'm not sure anyone is claiming that they work as well as high aspect daggerboards, just as no one claims that LAR keels work as well as high aspect daggerboards."

I'm more concerned whether they work better than a foot of DB. A foot isn't too much of a burden in most circumstances, and for some of us there is a lot more water under the keel than that.

"I'm curious as to what you would accept as a reasonable demonstration of the utility of these foils"

Any evidence whatsoever would be worthwhile at this point. A number of these KD designs have been around for a good long while, several hundred plans sold. Not a lot of web presence as far as people building them and reporting back on them, that I have seen. KD has a nifty web sequence on building DBs, so it would be cool if there was some comparison in the fleet somewhere. I'm not calling the lack of info suspicious, more a case of point me in the right direction.

If you look at the pictures on this page :

http://www.ikarus342000.com/KD860pic.htm

there are a few shots of the plates. They are relatively small, and if they are significant, they would seem to be the single most effective lift device ever conceived. About the size of the board on a sailing canoe, for an ocean going cat. So we have something, that as you say is hard to understand, but that if it works is a miracle of modern engineering. A lot of people have mortgages like that.

By the way, what is the argument for having one of these on the port side of a starboard cat hull?

 

We landlubbers not fortunate enough to have saltwater in our veins must occasionally close with the dreaded dry stuff, so sooner or later, shallow draft becomes a big deal, at least to us unlucky airbreathers.

You could try asking Bernd about this, he's a very accessible guy; in fact, he took the time to sketch up a foil for me when I was considering adding them to Slider.

Enough boats to his design are sailing that it seems likely that similar boats with and without horizontal foils have sailed together. Still, since you rejected as inconclusive an almost perfect and lengthy comparison of two very similar boats designed by Matt Layden, I don't have much hope that direct comparisons between Bernd's boats would satisfy you. Little Cruiser and Paradox have hull forms and rigs that are as close to identical as you're likely to find, and the fact that Paradox was noticeably more weatherly in situations where LC couldn't use her boards is pretty definitive, to my way of thinking. This effect was demonstrated over many weeks and hundreds of sea miles. I don't know how you'd get better data, short of tank testing.

By the way, I don't think the idea that Matt's runners work only because the boats heel holds much water. In the first place, Paradox doesn't heel all that much-- it's a box boat with a lot of initial stability, and a beam of 4 feet on a waterline of around 13 feet. But even if the boat did heel, say, 20 degrees upon encountering a light breeze (which in fact it doesn't) the chine runners are only about two inches wide, so the amount of runner exposed laterally is only a fraction of an inch-- equivalent to a nice grounding strip. I suspect that bilge keels have to be deeper than half an inch to be effective.

You're assuming that these foils work on the same principle as conventional foils. Yesterday that was my position, mainly because Matt Layden's latest chine runner boat, Enigma, used runners of higher aspect than Paradox, and Matt's impression was that they worked a little better. This I took as evidence that the effect does not derive entirely from fencing off crossflow.

However, I think you've talked me into looking more seriously at the idea that these devices serve only to enhance the innate windward capacity of lifting body hulls. This, I think, is Bernd's position-- that his foils serve to reduce drag by preventing a tip vortex on these winglike hulls. He might be right, after all. And Matt's boats may use yet another principle. I don't think I would characterize these foils as "miracles of modern engineering" even if they work well. They represent a technology in its infancy. Even Bernd readily admits that he is unsure of the best planform for his foils. So there's probably a lot of development possible.

I find it strange that with so many Kohler boats sailing, no unsatisfied builders can be located. As a former Wharram owner, I can tell you that most of us, once we got over the whole hippy-dippy shtick that attracted us to the designs in the first place (not that there's anything wrong with that) became quite dissatisfied with being beaten to windward by small monohulls. My Wharram Tane had a big modern fractional rig, and I still got beaten to windward by little chlorox bottles. Of course, it's true that many more Wharram cats are sailing, but the sluggishness of the designs to windward was well-established in early days. For a better comparison, look at the Schionning Radical Bay. This is a boat which has generated some fairly dubious feedback, and there probably aren't as many Radical Bays sailing as Kohler boats with anti-vortex panels.

Maybe all of Bernd's builders lack, for some unknown reason, the gumption to publicly complain about poor windward performance, but Occam's Razor inclines me to believe that there is a simpler explanation.

The usual reason for putting these devices on the inside is to protect them from damage when lying alongside a quay. This bothers me, too, since it seems to me that if these devices function as end plates on lifting body hulls, that they should be on both sides of each hull. But like you, I have no experience with these devices, so I can only speculate. I've thought about putting them on the outside of my little boat, because they'd make a good step to get back aboard and there's enough flare to keep them from hitting a vertical wall, but I hesitate to depart radically from what has apparently worked. Also, beach boats inevitably wind up parallel to the beach on occasion, and that might put undue strain on these structures if exposed on the outside.

You can see a study plan for Pelican, Bernd's 10.5 meter cruising cat here:

www.duckworksmagazine.com/05/columns/guest/kohler/Pelicstudy.pdf

To boil down my current attitude toward these devices (and it changes daily; I'm no slave to foolish consistency) Bernd Kohler's hulls seem to be idealized lifting bodies, and would almost surely go to windward well enough without any foils at all, vertical or horizontal. However, his experience with these hulls is vastly greater than ours, and he feels that the foils do help windward performance, so who am I to cast stones? It's probably no coincidence that Matt Layden's sharpie-like hulls seem to be monohull versions of Bernd's hulls, though Matt undoubtedly came to these hull forms through a different evolutionary path than Bernd did. (I should add that Bernd's hulls are usually asymmetric-- in other words, half a sharpie hull, stretched out.) It's my feeling that these devices, even coupled with suitable hulls, are significantly less effective than a high aspect ratio daggerboard with a good section... BUT for a day sailing beach boat like the little cat I'm building now, the loss of a few degrees to windward is acceptable in exchange for the greatly diminished hassle. I'm going all out for simplicity, including fixed barn door rudders and a single sprit sail of very modest size.

It may turn out to be a dog. Who knows? But what's the point of designing boats that only differ in miniscule degree from similar boats? No fun, in my opinion.

 

"Still, since you rejected as inconclusive an almost perfect and lengthy comparison of two very similar boats designed by Matt Layden, I don't have much hope that direct comparisons between Bernd's boats would satisfy you."

At least it would be on topic. As you have pointed out yourself, your explanations vary, and some of them are specific to type, so best case the Layden sharpies only offer a partial explanation, which does not seem to be consistant with your attributions to KD.

"Little Cruiser and Paradox have hull forms and rigs that are as close to identical as you're likely to find, and the fact that Paradox was noticeably more weatherly in situations where LC couldn't use her boards is pretty definitive, to my way of thinking."

It perfectly demonstrates they are better than nothing.

"This effect was demonstrated over many weeks and hundreds of sea miles. I don't know how you'd get better data, short of tank testing."

It could be in an asymmetric cat, of which there are apparently many sailing.

"You're assuming that these foils work on the same principle as conventional foils. Yesterday that was my position, mainly because Matt Layden's latest chine runner boat, Enigma, used runners of higher aspect than Paradox, and Matt's impression was that they worked a little better. This I took as evidence that the effect does not derive entirely from fencing off crossflow."

If they are generating lift it would seem to be in the wrong direction, and if they are fencing out foils they would seem to be too short. That's is why this is so interesting. I can live without knowing exactly how it works if the degree to which they help is significant. I've bought lots of plans over the years, and it pays bigtime to question everything. Designers are about as objective as pageant parents.

"As a former Wharram owner, I can tell you that most of us, once we got over the whole hippy-dippy shtick that attracted us to the designs in the first place (not that there's anything wrong with that) became quite dissatisfied with being beaten to windward by small monohulls. My Wharram Tane had a big modern fractional rig, and I still got beaten to windward by little chlorox bottles. Of course, it's true that many more Wharram cats are sailing, but the sluggishness of the designs to windward was well-established in early days."

Testify Brother! And if I may add, the claims and the happy talk of an imense group of people who drank the Kool-aid; are evident all over the net; trade tips on how to get those babies to point up, can seem to some to be open to question. So where we have a sparsely represented technology, at best hard to explain, the same kinds of questions need examination.

"Maybe all of Bernd's builders lack, for some unknown reason, the gumption to publicly complain about poor windward performance, but Occam's Razor inclines me to believe that there is a simpler explanation."

I'm not sure that OR applies, it requires a simple explanation which does not appear to be available for the tech, or the lack of a ripple about their use.

The internet doesn't perfectly replicate the real world, there could be lots of reasons why these guys don't put up sites. Have you spoken to users or found sites we should know of?

" This bothers me, too, since it seems to me that if these devices function as end plates on lifting body hulls, that they should be on both sides of each hull. But like you, I have no experience with these devices, so I can only speculate."

Good info. Maybe it is like the aircraft foils that are asymmetric and have the tips turned up only on the low pressure side... At least in concept.

"To boil down my current attitude toward these devices (and it changes daily; I'm no slave to foolish consistency) Bernd Kohler's hulls seem to be idealized lifting bodies, and would almost surely go to windward well enough without any foils at all, vertical or horizontal."

That seems to be contradicted by reports re. Choy and some proa guys on lifting hulls.

" However, his experience with these hulls is vastly greater than ours, and he feels that the foils do help windward performance, so who am I to cast stones?"

My default position is to believe in other people's claims and experience. It's true about the greater experience of designers. One experience they may lack is 30 years of buying the plans of others.

"It's probably no coincidence that Matt Layden's sharpie-like hulls seem to be monohull versions of Bernd's hulls, though Matt undoubtedly came to these hull forms through a different evolutionary path than Bernd did."

Could be correct, but it seems like a reach to me. There are so many differences in the way they work. On the non technical front, I have Paradox plans, and it wouldn't be my first choice to cut up with a DB, it's pretty small. I don't see that problem with the KD650 and up hulls.

"It's my feeling that these devices, even coupled with suitable hulls, are significantly less effective than a high aspect ratio daggerboard with a good section... "

That's the key point.

"BUT for a day sailing beach boat like the little cat I'm building now, the loss of a few degrees to windward is acceptable in exchange for the greatly diminished hassle. I'm going all out for simplicity, including fixed barn door rudders and a single sprit sail of very modest size."

Sounds good to me. When making one's own stuff it is time to try some stuff out and see what sticks. Figuring out the minimum that actually works is incredibly worthwhile. It's one of the holly grails. On the other hand, based on your Tane point, prepare yourself for the moment a Bufflehead sailing canoe rips past you upwind.

I would be surprised if the loss is only a "few" degrees, if that means 2ish degrees. I want clarity. What is the claim. Words like "works" "significant" etc... can hide the totality of the performance loss people are concerned about. I could see some cruisers who really don't ever want to point up, take great pains to avoid all that stuff. On the KD 650, for coastal use, I want the maximum ability to claw upwind when required. I wish I lived near some place like Florida Bay where really skinny water was on the agenda.

 

Thom, I was specifically addressing your skepticism regarding the Layden chine runners. You said, in direct response to my retelling of Dave Bolduc's experience with two Layden boats:

Again, I have to ask you what you would regard as definitive proof that Matt's chine runners are an effective anti-leeway device, short of tank testing. I've already acknowledged that these results may not apply to Bernd's designs-- there are in all likelihood completely different explanations. Nonetheless, I think a fair-minded person in reasonable command of the facts would be forced to admit that Layden's chine runners are a proven technology.

You might be interested in Glen Maxwell's account of the sea trials of his beautifully finished Paradox, Zoe. He's a bright guy with a lot of time in multihulls.

I agree, and it's also one reason I chickened out on using horizontal foils on Slider. But I think I was thinking too narrowly, in this instance. It seemed to me impossible to reconcile the two theories of how these foils work, so I rejected both explanations. It may be that they don't need to be reconciled.

Might happen in light winds, but I seriously doubt it would in winds of any weight. For example, my daughter and I went fishing in Slider this morning, in hopes of filling Slider with Spanish mackerel. But alas, the water is still too dirty from the recent flooding rains, and all we caught was a small bluefish and one nice Spanish. The winds were 20 knots gusting to 25, and under full mainsail only we reached across the bay at 6 to 7 knots. It was way too wet and cold for fishing, so after we reached Destin and sailed along the bridge and through the Coast Guard channel, we headed for home, which was a 5 mile beat. Speed rarely fell below 6 knots, and this in a big chop that would have instantly swamped any sailing canoe.

 

Guys,
I love this forum. Somebody asks a quite legitimate question on design and guess what happens? The discussion degenerates into what appears to a one man promotion of his boat and his prowess as a fisherman. Isn't this just great?

 

I thought I was defending the ideas and boats of Bernd Kohler and Matt Layden, both of whom are much better at designing boats than I am. Sorry to have annoyed you, but if you aren't a better fisherman than I am, you're a pretty incompetent fisherman...

 

How many Kohler designed boats with Anti Vortex panels are out there sailing? I have seen photos of a modified KD860 (Kesmarin KD920) and Bernd's comments on his own Pelican.
If someone knows more references to Kohler boats with AV panels please share.

 

I don't know, but Bernd could probably tell you if you asked him.

Here's a post he made in an older thread:

 

Actually the boat you are referring to is the Kesmarin 910. The modifications to the 910 by the builder, without Bernd Kohler's input, to the stern part of the hulls are so dramatic that the 910 would not be a fair hull evaluation representation as related to this discussion.

I am building the KD 920. A stretched version of the 860, engineered by Derek Kelsall and using the KSS building techniques, with Bernd Kohler's continued involvement and input.

Regards,

Pat Ross

 

"Again, I have to ask you what you would regard as definitive proof that Matt's chine runners are an effective anti-leeway device, short of tank testing. I've already acknowledged that these results may not apply to Bernd's designs-- there are in all likelihood completely different explanations. Nonetheless, I think a fair-minded person in reasonable command of the facts would be forced to admit that Layden's chine runners are a proven technology."

Definitive proof that they work? I wouldn't really require that. I don't see any reason to believe they don't "work". The issue is whether they work well enough compared to the alternatives. Getting overly deep into Paradox seems like a digression, even if we prove that, it won't really help me with KD stuff. This is a quote from the KD650 page:

"Trough the asymmetrical hulls no dagger boards are necessary to go as well to windward as any very good mono hull."

Doesn't really sound like that high a standard. Though at one extreme going to windward as well as a very good mono hull, is like saying going to windward like the best there is, or better than any other multihull known. Are we talking about pointing? Or about getting there due to greater speed over the ground?

All these statements seem to have huge fudge factors built in. Common sense tells us it's a compromise system. That isn't a bad thing for two reasons: Every boat should have a service in mind and be designed to meet that service, that will of necessity mean some things get done better than others; Secondly, an interesting design does things differently than another design would, or why design it? But to then come along and pick out individual components of a design and hold them up to the highest standard can be misleading.

Why should a shoal draft boat point better than any other boat in the fleet. An interesting example is the Gougeon boats. Long history of thrashing boats in their backyard, and occasionally abroad. Favorite leeway prevention device a kick-up centerboard. Not a popular choice on just about any other racing oriented trimaran I can think of, certainly not since the DB gained traction in the post 80s. Doesn't seem to hurt them on the race course, and it has a lot of advantages, but is it the best lift device known to man? Let's be serious. Difference between a good design, and a single component hung out for scrutiny on it's own.

"I agree, and it's also one reason I chickened out on using horizontal foils on Slider. But I think I was thinking too narrowly, in this instance. It seemed to me impossible to reconcile the two theories of how these foils work, so I rejected both explanations. It may be that they don't need to be reconciled."

Since I don't think much about asymmetric hulls, it is easy for me to forget that Hobies have them! Never owned a hobie, and came into multis late 70s as beachcats where yielding to sailboards. So who know maybe the asymmetric hull really is doing a good slug of the work and the doodads are just a small part of the picture.

It's official! You SUCK Ray. Nice kid, nice fish, though she does look like she went undercover to go fishing with her dad! The only ocean fish we get around here are really ugly ballast stowaways. I have three daughters so maybe with a few sliding seats and a pole I won't have to worry about whether the boat goes to windward with the sails.