: That is the question.

Might as well be several questions, as there are going to be many factors to be weighed, and compromised--and each decision in your design process will require some answer from you.

: I'm designing a rough water and day tripping play kayak. I can decide on most
: of the design easily enough, but the one question I keep coming back to is
: how V'd to make the hull - both at the ends and throughout the length. As
: it happens I'm building S&G, but pretty much the same question applies to
: many strip designs - how flat do you make the bottom?

Actually, no, I don't think so. That question is not too common with strip building. With strip building you will rarely build a flat bottom. With plywood you have an essentially flat medium, and if you want it in some other shape, you have to somehow bend it. That's where you start running into terms like "tortured plywood construction", and "developable curves".

A typical flat-bottomed boat which looks like a canoe, or which could be decked so it looks like a kayak, would be a pirogue. There is nothing wrong with a pirogue, but most of them have bracing which supports the sidewalls and keeps the floor from buckling.

A couple of points to start from. Change some of these and you'll have different answers. Let us assume you are thinking of strip construction with strips which are anywhere from 3/16- to 1/4-inch thick. And with S&G construction you'd be looking at using plywood which might be as thin as 3mm or as thick as 5.5mm. Just for reference, let's also throw in a design where you would build with a cotton canvas skin over a wood frame made with ribs 3/8 thick supporting chines which were also 3/8 inch thick. such a design would typically have a thicker keel or keelson strip, which would be perhaps 1 to 1.5 inches thick.

Now lets look at the length and width of a typical boat. Just for the heck of it, I'll start with the numbers of 16 feet long, and 2 feet wide. While we all know that a kayak is made of smooth flowing curves, let me assume for just a moment that this boat is going to be made of two triangles, each 8 high with a base which is 2 feet wide. With those two triangles joined at their bases I get a long diamond-shaped object, 16 feet long and 2 feet wide. The area of this shape is easy to calculate. It is 16 square feet. If this had a flat bottom, and I forced it down 4 inches (1/3rd of a foot) into the water, this shape would displace 16/3 cubic feet of water, or 5-1/3 cubic feet. using a round number of 60 pounds per cubic foot for water, the design displacement with a 4 inch design waterline would be 320 pounds.

That is for a flat bottom, now let's assume the bottom was made from two pieces assembled at an angle. Over the 24 inch width of the boat we'll go down the full draft of 4 inches at the center. Now, instead of a rectangle underwater which is 4 inches high and 24 inches wide (96 square inches), we'll have a triangle with a base of 24 inches and a height of 4 inches (48 square spalces 320 pounds. Using a "V" bottom cuts the displacement in half. We are at 160 pounds.

Because the sides are square above that 4 inch line, regardless of the geometry of the bottom, every additional 80 pounds would cause the boat to sink an additional inch, so a 5 inch draft would displace 400 pounds, or 240 with a V bottom. At 6 inches of draft the displacement would be 480 pounds, or 320 with a V, and probably the extreme would be loading 560 pounds (or 480 with a V bottom) into this to get a draft of 7 inches.

Now let's return to your question. The crude example I've described has a flat bottom and sidewalls which rise from it at 90 degree angles. If this was made from plywood we can calculate the pressure on the wood in a couple of ways. water pressure at 33 feet is about 1 atmosphere, or jsut under 15 psi. Roughly every 2 feet of depth creates a pressure of about 1 psi. 4 inches of depth would be 1/6th that, so the bottom needs to support only 1/6psi, or about 0.16 pounds per square inch. We know from experience that something as flimsy as a layer of paint on a piece of thin dacron can exceed that, and it won't leak. But fabric will flex quite a bit. So will 3mm plywood. Each square foot of the bottom has to support about 20 pounds. How much flexing are you going to tolerate in your supposedly flat bottom?

Try this experiment sometime. Rip an 8-foot long piece of 1/8th inch (3mm) plywood from a standard 4x8 foot sheet. Make this a foot wide. Stand in the middle of it and get two friends to lift the ends as high as they can. I'd guess that they could get each end up at least 2 feet off the ground, without lifting you. It will probably break before it gets you off the ground. If you want, use the 3-foot wide piece instead. It may be a little stiffer to bend, but it should come up almost as much. Imagine how much a 16-foot length would flex if it didn't have some added support!

Consider the same experiment with a piece of canvas. Your friends would lift the ends of the fabric to make a sling, and lift you from the ground. The fabric is limp and flexible, but strong.

Suppose you added a layer or two of glass cloth to this plywood and tried the experiment again. The panel wouldn't fleax as easily, or bend as far--and the glass would probably support your weight, and keep the panel from breaking. If you made a panel from 1/4 inch strips (or even thinner ones) you'd have the same effect. The thinner the strips, the more the weight is supported by the glass fabric, and the more flexible the whole operation is.

On a kayak, where the paddler's weight is roughly in the center of the craft, a flexible boat will sag a bit in the center when the paddler is aboard. If the boat is built to be laser stright along the keel line, when it is put into the water and loaded up, the flexibility of the boat will create rocker. A highly flexible boat will curl up like a banana--or like that plywood panel you were standing on. In a canoe, where you have two paddlers, the weight is located closer to the ends, but balanced around the center. When aluminum canoes were more common, you could go to any summer camp and see abused boats which were "oil canned" or "hogged" when the flat bottom caved in, and the center collapsed.

Part of the purpose of the design of your boat is to counteract that flexibility. A flat bottom doesn't help you here. A slight "v" bottom does. So does a gentle curve.

People building with strips want to use a medium which lets them create round, oval, or egg shaped bottoms to their hulls. And, people who want a hull with such a shape know that the easiest way to build it is to use thin strips of wood, and sand away the rough edges. Whichever side of this want to start from, you end up arriving at the most common shape of a strip-built kayak hull: A mostly rounded bottom. Structurally this is much stronger than a flat bottom. Turn it over and a cross section resembles an arch, such as the Romans used for centuries for their aquaducts. It is a very strong shape. Water pressure on the hull tries to compress the arched shape, and within the limits of our designing, usually it can't do it. Make the arch deeper and it gets stronger. Make the arch shallow and the bottom of it approaches a flat shape--and the design gets weaker.

Scroll down and take a look at the illustration of an oval hull cross section. It is scaled to be about 24 inches wide and 12 inches high. The height is fairly typical for a recreational kayak, and the width is too. Also, I picked that width so I could match the math in my previous example.

What we want to concern ourselves with is the "magic" 4 inches at the very bottom of the boat. Most designers seem to want a design waterline right around there, so, most sea kayaks are designed to have a draft of about 4 inches. Whitewater kayaks, and low displacement kayaks will sink deeper. Some playboats are designed so that almost all the boat is submerged. If you figure human feet are 10 to 12 inches long, those boats would draw over 9 inches, and of course that means they would be narrower and shorter. But except for these, let's stick to working around the 4 inch mark.

In the illustration I have 3 tint boxes which are scaled to 4 inches. The oval shape itself effectively makes the bottom into an arch which is about 4 inches high. At the waterline the width of the boat will be about 20 inches. This is going to be much stiffer than a flat bottom.

Now take a look at the shallow "v" shape created by the red lines (B). These lines atart tangent to the curve, and come together about 1 1/4 inches below the lines of the oval. The amount of added volume that the "V" shape adds is not great. However, since it now lowers the center of the boat, if we stay with that 4 inch draft, you'll see the amount of the boat which is now displacing water. Overall, it is considerably less.

If we stay with using 4 inches as our "magic" design waterline when we are designing the boat, then to achieve the desired displacement with a shallow V, we need to increase the width of the boat, or the length. A longer, narrower boat will give you better tracking and higher speed. But it is a totally different design than you started with. Narrower boats are also less stable. All of these traits have been mentioned in other postings about your concern with making a V shaped hull.

As an extreme example, look at the deep "V" created by the blue lines (A). With an angle like that, at a 4 inch waterline (look at the tint box) you are looking at small displacement, and a very top-heavy boat. Tracking resembles a ride on rails, but stability is bad, and so is the ability to turn.

As has been mentioned elsewhere, when you load up these boats they sink lower and stability improves. Again, this seems to be a common trait with "v" bottom boats.

As I see it, the problem is working with that "magic" 4 inch design draft. Get rid of that concept and I think you'll have a nice boat. If you look at the illustration, adding that 1 1/4 inch "v" to the bottom of the oval makes the bottom stiffer. But if you stay with the 4 inch waterline on the oval shape you could match that by starting to design this boat with a 5 1/4 design depth. The added displacemet added by the V would probably balance out at obout a 5 inch draft. You would have a bit more wetted surface area than the oval hull, but you would be able to sit a bit lower in the boat, and your center of gravity would be lower in the water as well.

Let's go back to flat bottom boats. Klepper's SOF has a flat bottom, but it is supported on two ample-sized rails. The fabric curves in a bit between them, acting like an inverted V. As a waterski has a fin, or skeg, a snow ski has a groove down the center. Both of these shapes help to keep the things going straight. On boats, v and inverted v shapes act the same. On boats built by aboriginal peoples there is generally a single keel or keelson over which the hides are stretched. The keel is atached by ribs (or frames) to the gunwales, which are sturdy parts. They carry most of the weight on the boat. To keep the
hides (or canvas) from dragging on the ribs, longitudinal chines or stringers serve to keep the covering away from them. The thick, strong keel, however, gives most of these boats a hull profile resembling a shallow "V". They have some flexibility, but it is planned for. They don't "oil can". These hull profiles have worked well. I don't see any reason to ignore the historical precedents.

Just some (rather longwinded, sorry) thoughts on this matter. Hope it helps.

PGJ