A double hull is a ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer. Homebuilt pontoon boat / double-hull kayak made from pvc pipe, foam insulation and vinyl siding. design goals (not necessarily in order):. The osprey double and triple kayaks are great light weight tandems for families who wish to kayak with kids or for kayaking with dogs. available as kayak kits or as.