First, a short word on nomenclature. I will be using simple english terms for most parts of the body and the kayak. For Greenlandic terms of these same kayak parts, refer to the most excellent QajaqUSA website.
Objective data begins with a simple height and weight range. By that I mean usual summer and winter weight, to define a high and low. Some folks do not fluctuate, but often northern climed, hot summer, cold winter folks do.
I will need a general but honest description of the health of the individual, including any unusual postural or structural issues that could affect the design.
I also find it helpful, if building for others, to have a full picture set. Frontal and side views, both standing, and sitting with legs fully extended as in a kayak. Be relaxed and natural. I am looking for postural clues here.
Below are pictures that will help to describe relationships that I, usually in person, collect on a story stick, in the case of kayaks, a 1 inch x 1 inch x 8 foot, four-sided stick.
1 The first measurement is simple wing span. A rough rule of thumb for tradtional kayak length is wingspan times three. This is an extremely rudimentary relationship, but as good a place as any to begin. I have quite short arms and legs, proportonally. Other folks have very long arms and legs in relation to their torsos.
2 The second measure is the cubit, from elbow to end of fingers. Adding wingspan to cubit, the measurement from elbow to end of outstretched hand at middle finger, gives a rough rule of thumb for a paddle length, which is often part of a kayak order. A good range check for basic paddle length is, as you are standing, with one arm reaching vertically for the sky, the distance between floor and from end of palm to end of middle finger.
3-4 Next we have dimensions of closed fist and fist with thumb extended. This is the usual range of aft of cockpit depth to shear. Rolling kayaks are more like closed fist deep, ocean boats are more like a fist with thumb deep. No hard and fast here, just real rules of thumb. ;-)
5-6 Next comes width of hips plus two fingers, a simple standard for rolling kayaks. But often, ocean kayaks are very much wider, depending. Function and archetype drive choices here, as the flare of the shear boards in the cockpit provide practical constraints. Peterson offers hips plus four fingers of each hand, as a hunting kayak width, resulting in a wider kayak than I like, all other factors taken into account. I also check this against the same relationship, this time sitting down. This can be significantly different depending on anatomy.
7 Next comes outside point, to outside point, where you thigh bone enters the pelvic socket. I determine this by sitting on the floor ( In the picture, I am standing up for the purpose of showing the bottom view ;-)), and locating these points, with my fingers, on the outside of both butt cheeks. Keeping the fingers in place, stand up and measure this space between the two fingers, This helps identify a range of width where chines and cheeks will be happy in a kayak. No hard an fast here, but an interesting relationship. The point here is that you don't want these tender points grinding against your chines. You want your butt happily inside the chines or happily over them, but not in between. These two different chine locations will affect the handling characteristics of the kayak!
8 Now more in-cockpit relationships. The next picture identifies a ball of foot against footrest to back of sacrum plus a hand depth. I will usually add two fingers to this distance to allow for watersocks, mukluks and/or seasonal clothing. This establishes a rule of thumb for footbeam to back beam, but again only a initial understanding. Depending on other factors this can change.
9 Next, I want to understand the ball of foot to uppermost point on the knee cap, (in relationship to that knee cap high point from the floor) and I also what that floor to kneecap height is.
10 I also want to know the distance from that point on the kneecap to the forward wall of the belly, and...
11 know the distance from floor to top of top of thigh.
These relationships establish rules of thumb for the location of masik and thigh beam, within the kayak, as well as depths required underneath these members for your legs.
12 I want to know what the straight line relationship is from topmost part of knee cap, or patella, to bottom of heel, on the floor. This articulates the clearance necessary to actually get the knees into a kayak, often the fit bugaboo, past all of the structural members of the frame.
13 What sort of mechanical advantage, or margin, is in your knee joint, that extra margin for the reverse fold of the leg as you try to wriggle into an intimate rolling kayak or an ocean kayak with a small hoop. I witness this by holding the thigh and calf to the floor with one arm and bringing my heel off the floor wit hthe other hand. I have almost 3 fingers width under my heel, before my knee REALLY starts to complain. How about you?
14-15 Sitting, legs out, pointing my feet forward, through the toes. I often like to hook the footbeam with my feet, and I need to know the minimum clearance my feet need to clear under. What is the distance from floor of kayak to top most point of foot, and at ankle bone?
16-17 Now come forward folds... In these two shots there is a range of knee to nose postures that define how low volume your foredeck can be. From leg flat, nose to knee, to how far do I have to raise my knee for my nose to touch my knee? I work on my flexibility, (more and more as I age!), and have always had fairly good range, but many folks face real constraints here. If we assume that, for forward finishing, as in aft-finishing rolls, it is ideal to have your upper torso as close to the center of the kayak as possible, knowing this range of flexibility establishes relationships that describe optimal foredeck height for a particular kayaker.
Finally, comes the whole process of putting this all together, weighing all of the above data, factoring in expectations, primary uses, paddler skill level, aesthetic sensibilities, so that a real vision of a particular kayak asserts itself. It is, after all, constraints, that define objects in space. We are, in the modern world, assisted by big systematic mechanisms, by the force of gravity, righting moments, displacement, fluid dynamics, et cetera, yes, of course, but in this tradition, honed by hundreds of years of trial and error, I question the value of spending too much time actually using western equations to calculate too much. The skin on frame archetypes are fairly well-defined and appropriate, or not, to the task of a particular paddler paddling, bracing, or rolling, in their usual environment, alternately fulfilling and challenging the dream world in the eye of the mind. That place in each of us that shouts...YES!...as we engage the waterworld. It is largely melding a body and mind to these archetypical vessels that is most constantly the most interesting puzzle to me.
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