The 23.5 degree blade angle will result in a 43 degree angle stems. Cut the blank 28 inches long, one per boat. You will get two stems from one 2x3x28.

Set the blade at 23.5 degrees, and the fence at 7/8 inch (x).

Second and third cuts remove the scrap with the same setting. Turn end-for-end.

For a 2x4 use the same setup, but make the first cut twice, removing a parallelogram scrap from the middle.

Angles:
Why 23.5 degrees? Look at the polygon.

1) The inner angles of the polygon add up to 360 degrees.

Rule of geometry

2) Their is one right angle, 90 degrees.

That leaves 270 degrees for the other three.

3) The sharpest angle (pointy end) must be 43 degrees.

Required by the plans, that leaves 227 degrees to share between the two obtuse angles.

4) 227 / 2 = 113.5 degrees

Each obtuse angle.

5) Saw blade angle 113.5 - 90 = 23.5 degrees.

Because the saw blade angle is measured from the vertical.

Fence setting:
Why 7/8 inch?

1) Kerf = k = 1/8 inch

thickness of the blade; the distance between the two parallel lines in the drawing

2) y + k + x = 2.5

from the drawing, adding the three dimensions along the bottom edge

3) y = x + 1.5 * tan 23.5

from the drawing, the top edge of the upper scrap (y) is the sum of x (bottom edge) plus the extra due to the tilt of the first cut. That tilt is 1.5 (the actual thickness of the 2x3) times tangent of 23.5 degrees, (the tilt of the blade). Little bit of trig, here.

4) 2x + k + 1.5 * tan 23.5 = 2.5

plugging 3) into 2), eliminating y

5) x = ( 2.5 - k - 1.5*0.4348 ) / 2 = .8614 = 55/64 ~= 7/8 inch

rearranging 4), remember k is 1/8 (0.125)

6) y = 2.5 - x - k = 1.5 inch

from 2), plugging in known values of x and k

Set blade to 23.5 degrees, and a bit over 1.5 inches high. Using a 2x3 mark its midpoint at the end. Lay that on the saw table, and adjust the fence so the blade goes right through that midpoint. X will be 7/8 inch.

For a 2x4 mark off 1 inch of its breadth, find the midpoint of the remaining 1.5 x 2.5, and proceed as above.