serene – 3, part 5

he progress is really slow lately, several weeks passed, but little get done 😢! Things started moving anyhow, I “quickly” transfer the “offset tables” onto the plywood boards, draw all the bilges, bulkheads, and other parts. The greatest thing of all is that now I’ve purchased very good sheets of plywood, not truly marine – grade (there’s no such in Vietnam), but high – grade water – resistance ones. I could feel it when do the sawing, the boards are quit tough, not fragile as with my previous ply!

There would be an immediate consequence with the new plywood, I would just use less epoxy to pre – fill the boards, and since the boards is stiffer, the glassing would be done on the outside only, that would significantly save the boat weight, I hope. All drawing is completed quickly, I finished in just less than one day, next come the steps of cutting and jointing the bilges. Everything has been done many times before already, so I didn’t have to think or consider things much, just repeat it!

4th image: the cockpit coaming frame cut from a piece of 18 mm MDF. This cockpit is drawn using the mathematic formula mentioned in my last post. It came very closed to the shape of Serene – 2‘s cockpit, but slightly smaller on each sides by about 5 mm, so my existing spray skirt should fit tightly with this new coaming (on Serene – 2, it’s too tight). The coaming would be constructed from 2 layers of 4 mm plywood, and I would later omit out the (a bit tricky) glass reinforcement on this part!

how to draw a kayak cockpit mathematically?

ver the years of designing and building wooden kayak, often a recurring question comes to my mind: how can we precisely construct a kayak cockpit shape in a mathematical way. Today, I spent 30 minutes to figure out the problem, it turned out to be quite easy indeed. But first of all, there are so many different shapes for cockpits, and everyone may have his / her own preference on how it should look like. Here I try to plot a shape that is most suited to my eyes, and closed to what’s usually called “an ocean cockpit” found on Greenland kayaks.

I went to the Wolfram Alpha website (wolframalpha.com) and entered an ellipse function. Apparently a cockpit shape is not elliptic, but rather an asymmetric “egg shape”. I tweaked around the equation for a while, changing its components, and the shape came out, closely resembles that of my Serene – 2. I’d also tried to construct other types of “egg shapes”, for example, an 3 – ellipse that has 3 foci, something that could be drawn with closed thread of rope running around 3 nails pinned on a wooden board (similar to drawing an 2 – ellipse with 2 foci).

But it’s hard to determine the dimensions of that 3 – ellipse, and the positions of its 3 foci. Finally, I found out the formula that is best suited for me, something that could be determined numerically to ease out the actual drawing. The formula finally: x2 + 3.5y2 + 1.5xy2 = 1. Upon close inspection, the “egg shape” turned out to be very satisfactory. One might try changing just the “weight” parameters (e.g: 3.5, 1.5…) and retaining degrees of the polynomial’s terms, to experiment with different other shapes, to figure out what is best according to one’s need.

Next come the question of how to draw this shape onto the wooden board that would actually construct the cockpit coaming? A bit of thinking has the problem solved too. The Wolfram Alpha website provides us with numerical formulas. The general steps are like this:

1. Draw a bounding box in the dimensions you want, mine is: 39×72 (cm), the dimension that your spray skirt would fit into (and the spray skirt vendor usually supply you with this information before – hand). The x value would have the value range of [-1, 1].

2. Draw a dense grid of x & y inside that bounding box, for each value of x from -1 to 1, calculate the y value with the formulas given above. Giving very fine incremental values of x, say every 1 or 2 cm, it would be possible to construct a good looking “egg shape” then.

Of course, I want to find out a way to draw the “egg shape” more precisely and more conveniently, e.g: print the plotting actual size on paper, but found it a bit cumbersome as I have no printer of that size! Anyone finding out a good way, please kindly let me know! 😀