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Re: Catmaran kayak? I have one --- almost

: Hi Carl,

: Nice exploration in the genre.

: Couple of things.

: 1. There is a small program you can use to determine the amount of wave
: interference between the hulls, contributing drag. It's called Michlet
: (with its companion Godzilla) Michlet is a free download on the Internet
: and was developed, principally, by Leo Lazauskas, with help from many
: experts in the field, notably... famed canoe and kayak design legend, John
: Winters. Michlet and Godzilla will also look at the optimal hull shape for
: efficiency under human power if you configure it to do that.

: 2. As you know, you have given away wetted surface drag for enhanced
: stability by splitting the kayak hull in two. Then, you lost some of the
: gained stability by raising the paddlers COG to a higher point than one
: would see in the typical sea kayak. The raised and out of the boat
: position, though, has given the paddler lots of freedom for wet exit and
: reentry possibilities. In short... you've met, head-on, the business of
: design compromise faced by boat designers the world over.

: 3. About that forward foil... There are several schools of thought on the use
: of an anti-dive plane for a cat-hulled vessel. Sailing craft designers
: face this issue regularly if they work in the field of multihulls.

: One approach is as stated already, where the forward hull connector is shaped
: in a lifting foil configuration and whenever it is immersed, it will tend
: to lift the bows and prevent a pitchpoling exercise.

: The other major approach is to reduce the forward component as much as
: possible to reduce the area of the surfaces that might contribute to the
: same effect of pitchpoling. You see this in cats that have forward
: trampoline, or net surfaces, that can quickly shed water, should the bows
: become immersed, allowing the designed buoyancy to pull the boat back to
: the surface.

: A forward foil is a dandy idea if the following comes into play: The forward
: speed of the boat (and also the speed of the water over the foil) must be
: within the designed lifting mode of the chosen foil profile, or it will
: not create lift. If it is not creating lift, then it is creating drag.
: With an enhanced horizontal surface area beyond what you'd experience with
: a simple round section, you could be increasing the likelihood of a
: pitchpole and not reducing same.

: If the boat is operating, consistently in the envelope of the foils lift and
: it is mounted properly for an optimal angle of attack, then you really
: have something going for you. A reduced aero profile for the frontal area,
: a lifting device to help the buoyancy of the bows and a structural member
: all in one element. That's a big design win.

: Caveat: It's tricky to determine the settings for all those elements.

: The biggest design decision would be to determine at what speeds and in what
: conditions will the foil serve you to its optimal benefit. This will put
: you in the ballpark of foil section to use, angle of attack for the
: mounting and overall chord for the foil for most benefit... at the least
: amount of weight.

: Fun, huh?

: My personal take... you'll almost never see a suitable foiled member benefit
: for this boat unless you regularly surf the boat at speeds at the higher
: end of the design envelope for a paddled craft. Use the smallest tubular
: solution for the bow of the boat. You can revisit this particular issue
: later. Focus on all the basic design issues you will face to arrive at an
: optimal design of hull shape and spacing. You are working on a very
: interesting paddling solution which has all kinds of primary, as well as
: secondary benefits.

: I'm excited to see how this goes for you.

Chris - you make a lot of good points, I've been mulling them since yesterday. Re your major points:

1. I have Michlet but haven't used it yet. I read Lazauskas' papers on hull interaction effects in multi-hulls. I did some envelope calculations covering the hull parameters and flow regime of my design and came out in a good part of his results, i.e. no serious performance sacrifices are expected. I am a fluids experimentalist by training, so I have both respect and skepticism about numerical results, especially when I don't fully comprehend the underlying assumptions and algorithms that were used. But I was initially encouraged and will let a more powerful computer calculate the results (by that I mean the water, of course). If the boat works, I will task some students to get the boat into software and do some modeling.

2. You've got it right about design compromises - I teach intro to engineering design so will be using this project as a concrete example of tradeoffs in the design process. During the build I am going to try to keep everything as adjustable as possible to allow fiddling the design parameters.

3. Re the foil, you bring up several points I hadn't considered, plus there are the comments by Dan and Bill about wave piercing. I agree that a foil doesn't actually lift unless it or the water is moving fast enough - below that speed I expect it to act more like a vane with an upward angle of attack, diverting water downward and so forcing the hull up. Initially, however, I will do what you suggest and just use a minimal round or maybe elliptical section to connect the hulls. The foil is a secondary design component that can be added over the structural connection. I think it would be nice to have a dial or crank by the cockpit to allow the paddler to alter the foil angle on the fly. As far as hull profile, I really conceived of the hulls as piercing the waves most of the time, with some reserve buoyancy high up on the profile at a distance from the bow to pop the bow up only if it is being seriously buried - the foil should be high up for the same reason. I don't really expect to get this right on the first try, and it too will make a good design refinement exercise later on.

Other issues that need thought: as it is a sit-on-top, the cockpit layout is almost completely free. I'm thinking particularly of the relation between foot height and seat height and what relation gives the most comfort and power. For my creaky frame, I think having the footboard a bit lower than in a kayak will help. Also seat angle, rudder control, etc.

BTW, I just got the go-ahead to use the boat shop here at school so I will be starting up next week. I've planned 3 models 14', 16'4" and 18'8" LOA. The long one I see as a racing (non-rescue) version, so will leave that one for later. I would like it to fit within the rules that govern surf skis for competition - racing has a tendency to get students (and other gung-ho types) attention. For ease of construction, I may start with the shortest one - I imagine that the tracking will be good even at 14'. These lengths are pretty random, however - I just pulled them out of the air. The shorter two will fit in my garage and on my car which is a consideration. I wanted them to cover a wide range to reveal differences in performance, too. If anyone has thoughts on what the overall length should be, please let me know. Right now the plan is to use 1/2" marine or birch ply with stringers of cedar and crosspieces of spruce. Again, any opinions on material choices would be welcome, although I don't see the hulls themselves as any different that other SOFs. With no cockpit openings, they should be more structurally sound than usual.

Thanks for all the input so far - Cheers, Carl

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Catmaran kayak? I have one --- almost *LINK*
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