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Building a Perfect Kayak: Part Two

In the last installment of Building a Perfect Kayak, I laid out the design criteria for my new kayak and ran into some software problems. Mainly, with the software that I’m using, I couldn’t export rounded stations to build the forms for the kayak. Because Hulls, the boat design program I used, is made to produce plywood panels, it lacked the features needed to accomplish these rounded stations, so I downloaded and tried a program called Sketch-Up.

Kayak Forms

First, for those readers that haven’t built a canoe or kayak yet, I should state exactly what the forms are. Forms, or stations, are simply the cross sectional shape of a kayak or a canoe at a specified location. It’s almost as if you had cut the boat in two from the left side to the right side, and then traced the resulting shape onto paper.

In my case, I specified one-foot intervals for the forms, because the more forms you use the more accurate the final shape of the boat will be. These when drawn together on a sheet of paper allow the boat builder to see the shape of the boat. The next step is to take each station or form and cut that shape out of plywood. These are then mounted according to their intervals to something called a strong back. Simply, a strong back is a solid straight beam that holds the forms in place.

After affixing the forms to the strong back, the general shape of the canoe emerges. And it is this shape that the cedar strips are bent around. It’s actually easier than it sounds to set everything up, but with the boat design program I used, it was hard to output the forms. In this old picture of a Wee Lassie under construction, you can just make out the plywood forms in the last section of the hull that isn’t completed.

The Kayak Design Software

So, because I couldn’t figure out how to output aligned and rounded forms in Hulls, I downloaded Sketch-Up, which is a 3D design program that has an eight-hour trial. After about an hour, I realized that Sketch-Up couldn’t do what I wanted it to do, but it gave me an idea to try in Hulls. By the way, the colorful image above was made in Sketch-Up. If you are in the design business, architectural, landscape, home design, or anything that requires showing 3D designs, this program is easy to learn and use and very flexible. I wish I had a reason to use it.

Using Hulls to Output Aligned Boat Building Forms

The first thing that I did was to add an additional chine to my design. I placed this chine at 20″ height, 0″ width. This extra chine, because of being a straight line, acts as an alignment point in the design.

The second thing I did was to output the forms. In the save dialogue box, there are eight boxes to input the locations of frames, or forms. Note in the figure below, I’ve entered form locations at one-foot intervals. One problem with doing this is that the first form at 12″ actually changes to the location right behind the stem. In my plan, this put my first form at 20.74 inches, so I will probably leave out the form at 24″ and just use the one at 20.74.

Then save your plan, and open the Nesting function of the program. Find the scroll box (Number 1) and scroll down until you find the frames. Then click once on the frame, and right after that, click the flip button (Number 2). Do this with all the frames and you will have perfectly aligned forms. You can then save the file and use the offsets that it produces to loft your boat.

Note, that you will have to go back and save the form locations a second time for the aft section of the boat, and after you loft the forms, you will have to use a flexible batten to round the forms. As you can see below (Don’t laugh about the paper. Believe it or not, you can’t find white paper on a roll in Grand Marais. And no it’s not from a brown paper bag.) I still need to round the forms, but the kayak looks good so far. There is slight distortion in the picture below because of the angle of the camera.

So, What Next

Before going further, I decided to run some tests on the design just to put my mind at ease about stability. This is a rather narrow design, and as you can see from the plan above it has a flat middle section, but is it flat enough and how is secondary stability. Using Hulls, I ran some tests similar to those Sea Kayaker runs. Hulls allows you to test the foot pounds it takes to right the kayak at different degrees of heel. Sea Kayaker tests four different scenarios. All I could test in Hulls was different weights, so I’m not sure they compare perfectly, but, at least, the 200 pound will be comparable to those in Sea Kayaker Magazine. By the looks of it, this kayak should be pretty close to those I cited in the first episode, and similar to the Aquanaut in the August 2005 issue. Also, much to my surprise, I think the Aquanaut’s lines and this ones are pretty close. This should be a good kayak. In the chart below the Foot pounds should actually be 10, 20, 30 pounds, instead of .1, .2, 3 pounds, ext…

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  • Oops! Found the “Hulls” link after all, right where it should have been…my bad! Just didn’t look far enough.

  • No problem. It’s pretty far down the page. :)

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