My son loved the Balloon Boat designed by Keyes, but I wanted to make it faster and more stable. This lead to the idea to modify it into a Catamaran. Two hulls leads to increased stability and two balloons means twice the power!
This can be printed without supports.
1) Narrowed the hull to just under half its original width and made it a Catamaran.
2) Added the bridge to connect the two hulls. The Lego studs on the bridge are spaced so that they can be connected to the studs on each hull (spacing: 9.6 mm higher and 16 mm laterally).
3) I slightly decreased the diameter of the chimney for the balloon mount. Makes it easier to mount while still always being air tight.
4) Decreased the relief for the balloon rim to sit in. This is plenty to keep it on, and makes it easier to remove the balloon.
5) I modified the air channel, so it is more likely to be air tight. I moved the bottom closer to the bottom of the boat (1.6 mm thickness, so print with top/bottom thickness of 0.8 mm or more). The top of the channel is angled like a house roof, so it doesn't' have to be bridged.
Enjoy and if you print one please consider posting a make!