Most modern day ones are made from carbon fibre, which is made from lots of little carbon filaments binded and then glued together.This makes it incredibly strong and still light. A material that is similar is chipboard, because it has lots of little bits of wood which are glued together.
For the budget minded home builder / hobbyist I would recommend Marine plywood and fiberglass as the most cost effective.
But whether you go with carbon fiber (which you can get in sheets / rolls too in addition to chopped) or fiberglass (which can also come chopped, matted, or rolls / sheets) both use resins to hold them together, so do some research on epoxy resins and look into vacuum bagging which is a piece of technology the home user can cobble together that can make a huge difference!
Lest you turn your nose up at plywood - it's use for these applications goes back to the 40s and it was not only good enough for PT boats, but also the Spruce Goose.
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"If you want to get somewhere else, you must run at least twice as fast as that!"
thank you.I do not want to build a hydroplane ofcourse but I realy am interested in them and the fascinating speed.would you please shed some light on vacuum bagging