Making a Fast Lego Boat 3/3 - underwater propellers

Building a fast Lego motorboat with two submerged Lego propellers. Only Lego parts are used, except for a BuWizz 3.0 battery box. Enjoy!

Propeller attachment: double-sided tape rolled on a Lego axle

More details in my blog, including a part list and 3D model for the motorboat:

[BrickExperimentChannel]

You can read more details and download partlists and 3D models for the three boats here:

[EeekiE]

God tier YouTube channel. Nothing being sold. No begging. Massive effort. Good science and engineering. Additional info provided for anyone else wanting to try out the builds.
Every single view and Like well earned.

[moon_bandage]

Would've been interested to see you try out a rudder instead of using engine power to steer!

Loved the video

[PureRushXevus]

he's preparing for a naval invasion, quickly! build the lego sea mines

[happyhelpfulhoovy]

Similar to RCTestFlight's findings on his fan-made boat propeller challenge: The goal for fastest boat actually ends up finding the correct matching propeller type for your motor and power configuration. Since you have 2A and 12V available for each motor, the propeller must not be more powerful than 24W. Plus the RPM has to match up so the tip speed is not too great. The smaller propeller means a lower tip speed ratio + less energy spent just to rotate the mass vs a larger prop. The greater pitch of C offsets the lower diameter and number of blades, which makes it the best prop for the configuration. Also if you were to use a more powerful motor, there comes a point where a larger diameter at lower RPM is better, but for small scale, what you've found will generally hold true!

[AvatarNeo]

Not gonna lie, that was one fast Lego boat

[Valonia_Nightingale]

Love the 3-Part Fast Lego Boat series 😯

Would love to see an amphibious Lego build that can seamlessly traverse both land and water.

[wobblysauce]

Twin motor, you can get one to run in reverse so you can use the same prop for rotation and normally better control/no need for corrections. Both spin to the middle or outside, the best variance can depend on hull type... but at the small scale, I am unsure.

[TheGahta]

5:14 the two grey sockets look like totems 😂

[Lohandre]

7:25 MAMAMIA. Why didnt you use those?

[marinesciencedude]

So the 'final' comparison is
top speed: airboat 5.0 km/h, paddle wheel 3.9 km/h, underwater propellers 5.3 km/h
approx. journey time (final time visible before overlay disappears): airboat 04:54, paddle wheels 04:53, underwater propellers 04:29
comparing paddle wheel to airboat is a matter of the compromise between speed and steering (we can basically take them to be identical in journey time), but the advantages of underwater propellers do make a proper improvement against both in real-world tests.

[uwainium]

I love these videos, my only critique is that picking the best prop for example might be dependent on the gear ratio and not testing the full matrix (as far as I can tell) irks me haha

[RequiemAeterman]

I suspect we'll get 4/3 next week for a caterpillar drive. :D

[eos_the_goenner]

When you have more than one prop, not even motor, you always want them to spin in opposite directions to cancel out gyroscopic processions... With the smaller props it's not as noticeable but having them spin the same direction will always introduce a little bit of side drift and roll force on the boat

(had to learn that for my sport boating license)

[CrazyChemistPL]

The only issue I see is the elevated position of the engines. Generally the lower the better for stability. I guess keeping them higher up helps with keeping them dry however.

[jackjorgensen]

Never have I ever clicked on a video so fast. Friggen love this channel

[heruhcanedean]

The best prop I've found is using about 15 lego pieces.
You need 2 of the 4 pegs with perpendicular center cross, 4 of the single pin with pin hole (the black ones from your motor mount.) And 8 1x5 technic beams held together with long pins and tied together with string when built. You can build them mirrored of each other super easy. They work great in reverse too.

[micke]

LEGO props was faster than I'd expect. Another great video!

[jussitoivanen]

Extra challenge: could you get any of the used hulls planing? With any prop tech.

[abdulhossain]

Can we give this channel the best lego channel award already.

Also, if anyone asks why we use underwater props instead of air props or paddles, show them these last three videos. Higher speed as well as higher maneuverablility while reducing risk of unnecessary injuries to the users.