The trampoline has evolved from a coated canvas, to mesh and now a two piece mesh. The initial canvas weighed @ 9 lbs, the mesh @ 4lbs and also allows the water to drain through plus is a little less wind resistant - capsize less easily.
The change to two piece was initiated by Hamish I believe. It allows one to work on or repair one side without taking all of the trampoline off the boat and tends to be a tighter fit when laced.
On the trampolines I have constructed, I have spaced the gromets at 6 inch intervals and find that it takes @ 24 - 25 feet of line for lacing purposes. I also wire the corners into the track as that is usually where they tend to pull out. Rope diameter is critical for the track as too loose and it pulls out and too tight makes for a difficult installation! The advantages of the two piece tramp include easier installation and removal, easier to tighten up, and less prone to tearing.