Kayak and Canoe Design Bulletin Board

Right sized

Posted By: Paul Jacobson
Date: Monday, 17 May 1999, at 12:34 a.m.

In Response To: Kid sized? (Rick C.)

> Hi howdy all,

> A friend and I are building yaks for our 7 years olds. Very simular to
> Glen-L's kid-yak. I have the sides and bottom stitched, and I am wondering
> about the sides. These are flat bottom boats, and the side panels are 9
> inches tall. I don't know if it's an optical illusion or just because it
> is so short,( only 7 feet) but the sides look way too big. I am concerned
> that they will be scraping knuckles as they paddle.

> These boats are supposed to draft about three inches with 100 lbs., and
> both of our kids are less than half of that.

> So, the question is: can we shave 2 inches off the sides without worrying
> about safety and all that? Seems to me, it shouldn't be a problem, but I'm
> not an expert. If anyone can see fault in my thinking, please enlighten
> me.

If the question is ``Will the boat float?'' the answer is ``Yes''. But if the question is ``Will you worry?'' the answer is ``NO!''. If you shave two inches off the sides you WILL worry about safety (and all that, too). Right about now, if the cockpit does not have a coming lip that is at least an inch higher than the sides (preferably 3 inches higher) I would already be worried.

> And don't worry about hurting my feelin's none neither. That's what I'm
> here for. 8^)}

I'm sure if your kid got hurt because you compromised on the design, you would feel a lot worse.

> Input and ideas appreciated. Thanks.

> Rick C.

Here is the reasoning behind these opinions. If you disagree, you can point to a particular number, and we can recalculate things.

First, my assumptions. You did not give the width of this boat, but I am guessing it to be about 22 to 24 inches. I would also guess the weight to be about 40 pounds or less. The paddler, plus their paddle, lifepreserver, and a few drops of water in the bilge is going to be between 45 and 100 pounds. Gross weight would be between 85 and 140 pounds. I also round off the weight of a cubic foot of water to 60 pounds. To make the numbers easier to do in my head, I'll say that it takes 1.5 cubic feet of displacement to float 90 pounds (a bit over the minimum) and 2.5 cubic feet of displacement to float 150 pounds (a bit over our planned maximum).

I do a rough estimate of the displacement of this boat by assuming it is diamond shaped, with a length of 7 feet and a width of 2 feet, with straight sides. I know it isn't really this shape, but I can't do the calculus for curves in my head. Anyhow, if I draw a line through the middle of the cockpit, I can break this diamond into two equal triangles, each with a length of 3.5 feet, and a width of 2 feet. The area of each triangle is 3.5 square feet, (and we have 2 of them) so the area of the top view of this boat is 7 square feet.

Now, If I was to make this boat 12 inches deep (1 foot), and sink it so that all 12 inches were submerged, it would displace 7 cubic feet of water, or about 420 pounds of water. That is a ballpark figure based on simple geometry.

Having this number I can do some more simple math. I can either divide the 420 by 12 to see how many pounds are displaced for each inch the boat sinks in the water (35 pounds per inch of depth) or I can cut it in half to find that at 6 inches It would displace 210 pounds, and then cut it in half again to see that at 3 inches it displaces 105 pounds. Just for the record, when 4 inches of the boat are submerged it would displace 140 pounds.

With canoes, you could calculate the maximum amount a canoe could hold by seeing what the displacement was when the boat had sunk so deep in the water that the gunwale was only 6 inches above the waterline in still water. The amount of boat above the water is called ``freeboard'' the amount of boat below the water is called `draft''. I have paddled in a breeze in a canoe that was so loaded that it had only 6 inches of freeboard. Lots of bailing, and a scary time.

In a kayak, you have the advantage of having a raised deck to deter water entry, so you could calculate `freeboard' in a variety of ways. I like to measure from the sheer chine, or the joining line of deck and hull. If you are trying to picture this in your mind, this is in about the same place as a gunwale on a canoe. While some go down to 3 or 4 inches of freeboard for their kayaks -- and you might see this on white water boats where the paddlers expect the decks to get submerged -- they usually compensate by having a high coaming, or by wearing a sprayskirt. I think you'll want to avoid the cost and hassle of a sprayskirt for a kid, and keep things simpler by allowing for ample freeboard. Lets look at 6 inches of freeboard as a good number to plan around.

Back to the numbers for your boat:

With 9 inch sides you would have 6 inches of freeboard with 3 inches submerged. As we have calculated, that would be 105 pounds of displacement, which is in the middle of the design range of 85 to 140 pounds, but a bit toward the low end of tha range. With 5 inches of freeboard your draft would be 4 inches and the displacement would be 140 pounds. This is is the top edge of your design range of 85 to 140 pounds. It is already one inch less freeboard than my -- admittedly conservative -- desirable size.

I think cutting the freeboard by 2 more inches would be foolish for a kid's boat, unless the kid is an excellent paddler and swimmer. Get out a ruler and measure those 6 inches from the top of your desk. The widest spot on the boat that the kid will have to reach over, in absolutely still water, will be only a bit more than the width of a paddle blade. If the kid has any swivel in their hips, as they shift their weight to one side or the other to paddle, the boat will tilt, submerging another inch or two on that side.

By the way. You will want to put in enough flotation space to ensure the boat is above water when flooded. You could probably get away with 2 cubic feet of space or less, for up to 124 pounds of flotation. You either do this with float bags, foam, or installing bulkheads to create sealed compartments. YOu can use some of this math to help you figure out where to locate those bulkheads, by estimating the volume of a triangular section of the hull at what ever the height is.

If you are still concerned about the kid hitting the boat with the paddle, there are other things you might try. A sheet of 1/4 inch foam (neoprene, minicell or similar) could be glued with contact cement over the area of the hulll most likely to be hit. Or, wrap somthing similar around the shaft fo the paddle. It could double as a handgrip, or work like drip rings, depending on how it is installed.

Hope this helps.

Paul Jacobson

Messages In This Thread

Kid sized?
Rick C. -- Friday, 14 May 1999, at 5:34 p.m.

Right sized
Paul Jacobson -- Monday, 17 May 1999, at 12:34 a.m.

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