Why is it that some boat trailer rails(2x4)are on edge (narrow side up), and some are lying flat? Wouldn't lying them flat provide more support to the boat?
If you take a board and put it on the edge, you have a much stronger beam. A beams strength is in the width of the beam.
If you take a 2X4 and let it lay flat - you can easily deflect the board several inches if you stand in the center with the ends supported. However, if you do the same thing with the 2X4 on edge, you probably can't deflect the board at all.
On a typical boat hull - the key thing is go get a uniform support over a longer length. That is why some of the trailer use a board on edge - to get this support.
You will find, however, that a high quality bunk trailer will use someting like 2X6's lying flat, with suitable supports under the boards to prevent deflection to give a larger area of support on the hull. This is particularly true for the larger trailers; designed to carry heavier boats.
Take care
REW