The double hull tender sized of 400cm,420cm,470cm,500cm, we call Grouper boat. Grouper boats are made of 3mm thick powder coated marine grade aluminum for boat hull and 1100 Dtex pvc fabric or 1.0mm hypalon fabric for inflatable tube. Alloy-hull side console ribs and ideal luxury rib tender boat GB420 has double flat boat hull and double front lockers.