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The alternate model for the 42105 Technic Catamaran set looks a lot like a remote-controlled speedboat.

enter image description here

This set doesn't actually come with any motors, or even propellers, so obviously this model isn't functional. So what would it take to make a motorized RC speedboat from this set?

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With a few Power Functions elements and a pair of propellers, I was able to build a motorized boat similar to the alternate model.

See it in action!


click to watch the video

Pictures


floating, side view


floating, front view

Close-up pictures on how it's built


propeller motor closeup


center structure closeup

Conclusion

Overall it works pretty well, and the hulls are surprisingly resistant to water leaking inside of them. The only major issue is that direct sunlight can interfere with the IR receiver at a distance. As an alternative, the IR receiver and remote could be substituted with something third-party like an S-Brick (uses Bluetooth). I didn't have any of the smaller and lighter LEGO Powered UP controller and motors, but with some redesigning that could also be an option (also Bluetooth).

I also hope that someday LEGO will make some propellers which are a better size/shape for use with water, and maybe this boat could go a little faster!

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    Tons of proper propellers to choose from I’d say: bricklink.com/v2/search.page?q=Propeller. Part 6041 looks like your best match - though the blades are small they are properly tilted and it has an axle-hole. I’d think that running that one on high speed should work. – Phil B. Oct 5 at 10:55
  • Here I am amazed that you happened to build the exact thing the asker was looking for. Then I did a little more research. Awesome build! – JPhi1618 Oct 5 at 14:27
  • Proper propeller does exist - lookup 2740c01 on BL. However it is 30 years old. – Alex Oct 5 at 18:26
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    Great answer, Alexander, though perhaps we shouldn't be surprised it's watertight as LEGO advertises the set as "For the first time ever, this LEGO Technic model toy floats on water!" – Kirk Woll Oct 5 at 19:33
  • I've tried most of the propellers, they've all been either too small or the blades are too long/thin/flat. The really small ones would have to spin really fast losing power to friction, and the really large ones would take much more power than these systems are made for. – Alexander O'Mara Oct 5 at 21:07

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