inalized my new kayak design, a considerable amount of work on optimizing the hull shape, adjusting it back and forth to find out the minimum drag numbers (based on the Kaper algorithm) while keeping stabilities and other parameters under control. Some numbers: LOA / LWL (length overall / waterline): 5.500 / 4.945 m, BOA / BWL (beam overall / waterline): 0.452 / 0.420 m. I reduced the boat width to exactly my hip plus 4 fingers (that is: 36 + 9 cm, my fingers are quite big, 9 cm for 4 fingers).

Designed draft: 0.11 m, Designed displacement: 95 kg, Cp (prismatic coefficient): 0.5475, Cb (block coefficient): 0.4064, LCB (longitudinal center of buoyancy): 0.5225, VCB (vertical center of buoyancy): 0.0690 m, LCF (longitudinal center of floatation): 0.5136, Cw (waterplane coefficient): 0.6445, S (wetted surface area): 1.699 m², Aw (waterplane area): 1.329 m², Am (midship section area): 0.034 m², Cm (midship coefficient): 0.7466, KMt (vertical transverse metacenter): 0.196 m.

Predicted drags at 0.11 m draft (95 kg of displacement): 3 knot ~ 7.122 N, 4 knot ~ 12.622 N, 5 knot ~ 23.763 N. Predicted drags at 0.121 m draft (110 kg of displacement): 3 knot ~ 7.679 N, 4 knot ~ 13.611 N, 5 knot ~ 25.506 N. The hull is heavily optimized for speed in the [4 ~ 5] knot range with some sacrifices in stability and load capacity. Now that all “theoretical calculations” has completed, it would take a few months to build the physical boat to tell the real truths about the design! 😀

ade some good progress in the process of modeling my new kayak… For a human – powered watercraft, water doesn’t like complex curves and shapes I think, so they need to be as simple as possible (it is not too simple to come to that simplicity though). The model is then decomposed by Free!Ship into several “developable” plates, at this point, professional builders could just output the plates to a large CNC machine, which would precisely cut the plywood accordingly.

I print those plates to paper to make a 1:15 paper model. Cut the plates with a pair of scissors, then stitch them together using transparent duct tape. Making a paper scaled model would help verifying about the “develop – ability” of the product, double check if there is any design mistake, and give a clear view on how we should compose the pieces into the final boat. For simplicity, I didn’t draw details such as the cockpit, skeg, lines or hatches, just the basic shape of the kayak.

The paper model is a bit ugly (my hands are not too skillful though), but the pieces come together perfectly, the shape just looks like a bamboo leaf! This gonna be a long, thin kayak with very little freeboard! Next would be finding some free time to materialize all these drawing onto the plywood sheets. I don’t have a printer that’s large enough to print the plates in their real sizes, so I would just redraw them using offset table exported from the CAD software, actually, I prefer this manual method!

t tooks just a few hours to learn the new software and construct the basic 3D objects: hull & deck. The time – consuming tasks are adjusting the shapes and playing around with hydrostatics. Some basic measures: LOA / LWL (length overall / waterline): 5.50 / 4.44 m, BOA / BWL (beam overall / waterline): 0.483 / 0.451 m, Draft: 0.1 m, S (wetted surface area): 1.66 m², Cp (prismatic coefficient): 0.5619, LCB (longitudinal center of buoyancy): 0.5150, LCF (longitudinal center of floatation): 0.5192.

Additional hydrostatics parameters, VCB (vertical center of buoyancy): 0.0590 m, Cb (block coefficient): 0.4684, Cm (midship coefficient): 0.8336, Cw (water plane coefficient): 0.6385, Displacement: 0.096 tonne… Well, I wouldn’t pretend that I fully understand those parameters above, cause they contain insights into a boat that can only be correctly interpreted by an experienced designer. However, I’m tweaking around to optimize the parameters toward a higher Cp, higher LWL, and lower S.

Drag (resistance) predicted by the KAPER algorithm looks fine till now, however, the displacement is sacrificed already, 96 kg, barely enough for me (65 kg) plus 30 kg of gears. This gonna be a demanding boat with low primary stability, not recommended for beginner. It takes some real world experiences to understand why low initial stability is indeed a good thing, and why a kayak which appears to be very stable on flat water could probably throw you up side down in bumpy conditions.

n the progress of learning to design my new kayak… you know its name already. I’m using Free!Ship, a CAD software running on Win XP virtual machine (with VirtualBox) on my Macbook. I haven’t used any CAD software before, haven’t designed anything 3D, not to say about a watercraft. So why designing a kayak!? Well, first, just for the fun of doing something yourself from A to Z. Second, though I’m no naval designer in any sense, I believe I have some guts on how a good kayak should be!

Many kayaks are designed for 70 ~ 90 kg paddlers on average, I’m not that bold, and I need something slimmer, lighter, with the drawback of sacrificing some load capacity of course. I’m re – modeling my kayak after Björn Thomasson’s Black Pearl, using just some publicly – available pictures of the boat. And it’s not a copy, there’re some modifications: slightly narrower beam, slightly less rocker, and slightly deeper V – bottom. And I would stick still to my familiar stitch & glue construction method.

Why stitch & glue!? Strip build generally offers best boat shapes, but look at the Inuit people’s SOF (skin on frame) kayaks, those “hard chines” suit naturally to S&G, the method is simpler and takes less time (which I don’t really have much for now). It gonna be not an easy process: just for the hull, adjust the 60 control points back and forth, recalculate the stability and performance parameters, repeat again and again until you’re satisfied with the results. I hope I can finish the design in about a month or so.