This will give you a reasonably flat bottom kayak with enough freeboard to handle a fair sized wave, making it a dry boat most of the time.

THE PATTERN

I find this to be the easiest way to explain the rib patterns. If everyone was an experienced boat builder I could simply include the "Halfbreadths" and be done with it. But this is a boat generally built by a person with little experience and simply wants a kayak, not a course in Marine Architecture.
The pattern may be made from most anything, but I prefer a sheet of 1/8" Luann. It does not grow with the moisture as badly as paper and since most time I will be building more then one of a kind, I want the pattern to last a while.
Start by drawing a horizontal baseline across the material you are going to use for the pattern. Then at "exactly 90 degrees", draw a vertical center line, as you see in the drawing below.
The measurements below give an X, Y axis. You will measure to the right from the Center Line and then Up from the Baseline. At this point make a mark with a nail or Pen. ( I prefer a nail as it does not smear.) Do all the positions for one rib and then connect the lines as I have done in the drawing. Be sure to mark the number of the rib plainly in several places along the line. This will make is less confusing a year hence when you decide you simply must have another boat.
I generally will put "all" the plot marks for all the ribs on one piece of luau, however if you find this to be confusing, use a separate piece for each rib.
Once you have completed the patterns go back and drills a very small hole at each position mark. Also drill several holes along the Vertical Center line, making sure one of them is right where the horizontal and vertical line meet. To transfer these line to the plywood you will pin the pattern to the sheet using two small brads through the center line holes to a sheet of plywood. Don't pound the brads all the way in, just be sure they are firmly embedded in the ply. Cut the top off the brads and lift the pattern off the plywood. Using a sharp pencil and laying a straightedge along the pins, draw a line from below the baseline to above where position #5 will be. This will be the center line for the rib.
Now return the pattern to the ply and using a sharp brad, mark each position location. Turn the pattern over and place it on the other side of the center line using the same two pins along the center line. This will ensure that the rib is identical on both sides of the center. Again mark the position Locations.
When finished, remove the pattern and using the straightedge mark the outline from hole to hole. This is the outside outline of your rib.
NOTE: Keep in mind that there is going to be a hole in the top of the boat, so position #5 will not always go all the way back to the centerline as you see it doing in the drawing below.
The thickness of the ribs will essentially be up to you. I will generally make all but numbers 3 and 5 about 1 1/4" thick. It makes a lighter boat and I rarely break one of them. However #3 is where I brace my feet and if it is too light the force applied when I am in rough weather will break it, so I increase the thickness a bit. (see earlier page with picture) And the same applies to #5 as I am lean on it as a backrest. To dimension the thickness, take a straight piece of wood the thickness you wish to use and draw a parellel line along the "inside" of the lines.
Now you are ready to cut out your ribs and begin the building. As you cut, router, sand, and work with the wood, keep in mind that you want to maintain the 90 degrees between the bottom of the piece and the vertical centerline. If you sand or cut incorrectly the ribs will NOT be lined up vertically on the floor board and you will either have a twisted boat or have to redo the rib. It is far, far better to cut a new piece then to try and work with twisted ribs. That can be done, but it is maddening, and not really worth the effort.
Just take you time and be picky at this point, you will save a greater amount of time later in the construction if you do.


These dimensions are from the vertical center line toward the right, and the bottom horizontal line towards the top.

Rib # 1 is OMITTED completely

Rib # 2
                      RIGHT        UP
Position #1.   1"               0"
Position #2.   7 1/4"         1 1/4"
Position #3.   10 1/4"      4 1/8"
Position #4.   12 1/8"      9"
Position #5.   0"              13 5/8"


Rib # 3
                      RIGHT        UP
Position #1,   1 1/2"          0"
Position #2,   8 1/4"          1 1/4"
Position #3,   11 1/4"         3 7/8"
Position #4,   13 3/4"         9"
Position #5,   6 5/8"         12"


Rib # 4
                      RIGHT        UP
Position #1,   2"                0"
Position #2,   10 1/8"           5/8"
Position #3,   13"              3 1/2"
Position #4,   15 3/8"         9"
Position #5,   9 5/8"         10 5/8"


Rib # 5
                      RIGHT        UP
Position #1,   2"               0"
Position #2,   9 1/4"              7/8"
Position #3,   12 1/8"        3 3/4"
Position #4,   14 5/8"        9"
Position #5,   0                 10 3/8"


Rib # 6
                      RIGHT        UP
Position #1,   1"                 0"
Position #2,   4 3/4"          2 1/4"
Position #3,   7 5/8"          4 3/4"
Position #4,   9 1/4            9"
Position #5,   0"                 11"

You will likely have to shim positions 2 and 3, depending on how the wood bends.


If you want a wider boat, here is a quick and dirty way to do it. It is not really the "proper" way, but it works fine on these. Let us say you want a boat that is 4" wider, simply add 2 inches to every measurement in the "to the RIGHT" list. Of course you will need a floor board that corresponds to the new flat spot sizes, but you already know how to figure that out. When you do this, you might want to increase the length of the boat a bit so it does not look stubby and, or paddle like a tub.

Good old rib 6 is a pain in the neck. It will never be exactly right the first time unless you happen to be making two boats from the same plank. Because the kayaks are asymetrical, the bend behind rib 5 will be totally different from the other end.
This causes a bulge that distorts the measurement because no two pieces of wood bend exactly the same. I will generally INCREASE this bulge in order to compensate for the extra weight behind the centerline of the hull. That way even though the boat looks the same from a top view, below the waterline it is wider and will carry my extra weight.
If the boat is for someone light, just let it flow where it wants. If you are heavy as I am, I just sort of push it out a bit at positions 2 and 3. It does not show in the water and it keeps my weight from lifting the bow too much.