# Kayak design parameters.

Discussion in 'Boat Design' started by LP, Apr 22, 2011.

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### LPFlying Boatman

Leo,

The hull your suggesting then is symetrical about the lateral axis as well as the longitudinal axis. In the equation you gaves for waterline shape, is it the most optimum shape from your research or are there others? If I remember my calculus correctly, the first derivative of a square function is a straight line.(?)

y = 0.5*B*[1 - (2*x/L)^2] = B/2*[1 - (2x/L)^2] = B/2 - B/2(2x/L)^2

y' = 0 - 2B/2(2x/L) = 2x(B/L)

How's my math? I haven't integrated in 2 decades. The point is though that you/we are looking for linear acceleration, or at least a linear delta in the hullform in delta (x). (?)

If a hull needs to be assymetric fore and aft, would you move max beam to a location where it places Center of Bouyancy as needed and replace L with L(forward)*2 for the front and L(aft)*2 for the back?

In your research, do you look at above water form at all? And perhaps there are other that can repond to this also. I've only run across a little of this in literature. How does one want to balance the above water profile with regard to a)CLR and/or b)CG? I've heard that weathervaning in a crosswind can be a real bugger. Maybe, this more of a concern in an open water craft, but there will be moments of exposure in protected waters, too.

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### LPFlying Boatman

Ken,
You're right. I completely neglected the kayak. I think this one would come in at 40 lbs.

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### kengromeSenior Member

Thanks LP. In this case I think I should build exactly to your design without modification. I'm not ready to build yet, but your design looks very appealing, so maybe I should wait until you build yours -- to see what you have to say about its performance ...

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### Leo LazauskasSenior Member

It is an optimum based on my research for hulls in calm water using Michell's integral for the wave resistance.

Remember, in the "design" process you are on the outer arms of the design spiral. Michlet and Godzilla can help to give you a start, but then there are a myriad of other factors to consider that are not as easily quantified.

Fail.

y' = -4Bx/(L^2)

Again, this is a fine design question that depends on many factors apart from hydrodynamic drag.
The choice of maximum beam, and the location of the LCB etc, depends on the paddler's anthropometry (limb lengths, their weights and centres of gravity) and the paddler's style (i.e. how they move their limbs and torso during the stroke) because that will determine how weights shift during the stroke. It's an easier design task than competitive rowing, where the rowers move large distances on their sliding seats, but it can make a difference of a few percent. For recreational paddling, of course, that level of detail is not a priority.

Yes, it is something to consider, but it requires some knowledge of the expected winds and direction and, again, the details of the paddlers height and width and how much their ears stick out.

Sorry I can't be of more help. My work can give you reasonable "optimal" lengths, widths, and shapes for ideal conditions, but then it is up to you to do the fine detailing, i.e. to turn the mathematical shapes into something practical.

Good luck!
Leo.

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### LPFlying Boatman

What is your prefered LOA? I could push the displacement up to 350 lbs. with a different version. We could take it to 15.5'(+/-) with a single scarf.

I've attached a couple of sets of righting curves. One for the original design and the second set where I've filled out the lines a bit to gain some more displacement.

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### cthippoSenior Member

Nice. I like it!

My current boat is an Emotion Edge which is 9' 6" x 28" and I love it because I can get it into little, tiny places. A lot of my paddling is chasing shipwrecks or exploring sloughs where this kind of maneuverability is critical.

The boat I'm building is a 16' SOF 'yak which I envisioned as a long distance cargo hauler for kayak camping. About half way through the build I realized that 1) a 16' boat isn't going to be maneuverable enough for what I really like to do, and 2) I don't like camping. That was about the point where I started work on the kayak tender design to haul me AND my little kayak to the places I wanted to explore, and on which I could sleep in a warm bed with a real head and someplace where my wetsuit has at least a small chance of drying overnight.

So, all that said, 12' (or 11' 12" as your program calls it ) might be a good compromise. The extra L/B would be nice for long transits or when I have to fight my way back upstream against the current, and It would still have room to carry some gear (something my current boat is seriously lacking). From the drawings you posted it looks like she has a beam of 24", correct? Is putting a 20" diameter cockpit ring in going to leave enough beam?

EDIT: Never mind on that last question, I was reading the diagram wrong. What is the design beam?

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### LPFlying Boatman

CT, The waterline beam is about 2'2" and max beam is 2'4"ish. It's a little wide, but I actually wanted a little extra capacity and stability because I want to take my young daughter out with me. I believe i will continue to work this latest version. I believe I will trim down the beam on the earlier model to mKe it a little more efficient. I really like the stability numbers on the CT version because of it's intended mission. I believe the increased capacity makes it more versatile.

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### kengromeSenior Member

LP, will you be posting the Freeship file for the version shown in posts #12 and #14 in this thread? Just curious ...

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### cthippoSenior Member

Having the slope away from the cockpit is nice. I occasionally manage to bash my hand on the side of my current boat because of it's beam and that doesn't feel too good.

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### LPFlying Boatman

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### Easy RiderSenior Member

LP,
The commercial kayak I thought was much like yours is the Eddyline Skyhawk. http://www.eddyline.com/eddyline-kayakshttp://www.eddyline.com/eddyline-kayaks
12' x 26" hard chine and a bit full in the ends. Like cthippo I like a very maneuverable kayak like the Sandpiper. That little boat is a blast to paddle. Most serious kayakers think any yak over 22" wide is for sissy's. We're living in an age of extremes. Can't even pedal a bicycle without spandex.

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### LPFlying Boatman

Agreed. I'm get very self conscience every time I ride my 30 year old 12 speed in my bluejeans and t-shirt.

The Eddylines look like a nice product. It's good to have some design correlation with an off the self item. I imagine they are going to be conservative in their design spec unless they are offering a specifically all out performance model. I'm aiming on the conservative side as well.

Thanks for your interest ER.

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### cthippoSenior Member

Most "serious kayakers" don't have a pelvis that's 19" wide. That's bone, it doesn't squish very well. I'd be hard pressed to physically fit into anything less than 24" and I prefer 25-28".

I love my Emotion Edge and that's part of why I'm excited about LPs design is that it's the only other boat I've seen that shares the design philosophy.

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### LPFlying Boatman

Leo, sorry to come back to this. I'm looking for a purely intuitive and/or theoretical answer based on your experience. I guess, more specifically, when you move away from a symmetrical hull (fore and aft), how does that effect your mathematical hull form equation?

Also, if you are willing, this was stated in the http://www.boatdesign.net/forums/boat-design/thoughts-differntial-cp-foreward-aft-37842.html thread.

"... I guess I ought to at least try to answer the original question regarding the merits of a different fore and aft Cp. If you envision two hulls, one consisting of the fore section mirrored front to back and the other consisting of the aft section mirrored front to back, you can look up the ideal Cp of each based on length for each and the design speed. ..."

Intuitively, it makes sense and falls right in with my questioning about the assymetry of most hulls. I'd like to put it in my tool box for designing the "ideal" hull form. For lack of a tactful way to ask, I'll just ask. In your opinion, does this statement merit consideration?

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### LPFlying Boatman

This isn't really relevent to your comments. It's a little more teaser for you though. I've been doing some fine tuning. Shaping the panels to reduce edge and area errors and other tidbits to get them to nest better on the plywood. I'm really squeezing to try to get them into two 4' x 8' panels with a single scarf. I'm on the verge of upping the BOM to a third panel. That would free up design latitude, but then there would be a lot of excess material. What are your thoughts on two vs. three sheets of ply? It's not a huge expense in the overall picture.

Right now, I've got everthing where it would fit into the two sheets, but I've got two deck joints on either side deck and the canted frame that goes behind the seat would have to be pieced together. Also, I have never tried to form 4mm ply in the arc of the forward deck. I believe that I can, but it might be best to go with a 3mm deck material. Lastly, if a person wanted to laminate the cockpit ring from excess ply, there would not be enough in the two sheet BOM. I think I've just convinced myself to go with three.:idea:

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