Technical LEGO: Dubble°°Bubble, a twin-screw power catamaran

Posted by Tamas Misik
Dubble°°Bubble is a very seaworthy 0.71 kg, 4.1 W twin-screw catamaran based on a pair of 48x6x5 LU "Speedboat" hulls. She's not the fastest boat I've built, but she's still lot of fun. She's also better-looking than most of my boats. Twin L motors power highly efficient third-party 55 mm counter-rotating props via 1:5 overdrive transmissions mounted on the outdrives. Steering is by differential power to the motors. Maneuverability is quite good considering the strong directional stability of the long, narrow demihulls. Plate and beam construction nicely stiffens the all-important cross-structure holding the demihulls together. Should the cross-structure fail, all of the electricals would surely go under. The battery and receiver had to be cantilevered out to a far forward position to balance out the mass of the motors and outdrives. Since the 7.4V Li polymer battery I would have preferred would have been too light for this purpose, I had to use a 7.2V AAA battery box instead. At the time of this video, Dubble°°Bubble was still using LEGO's exasperating IR-based PF remote control system. Her speed and maneuverability still come through, but the demo is marred by repeated shutdowns due solely to the PF system's dismal range and susceptibity to IR interference from sunlight. When I later switched her to Bluetooth-based remote control via SBrick (, these problems went away, and her already considerable play value went way up. Photos and write-up at
Posted February 1, 2016
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