RC Submarine 4.0 – propellers (4/10)

I have used different types of Lego propellers in my previous submarines. None of them are very good.

Here are three of of those Lego props and two non-Lego props. I tested thrust for them using two Lego PF L-motors, a Lego battery box, a weight scale and a bucket filled with water.

Here are the results. Each prop was tested multiple times with different gear ratios. I’ll list here the ratio that gives the most thrust. Thrust is in grams.

  1. Orange Lego 3-blade (6041) – 45 grams using 1:25 gear ratio
  2. Double grey Lego 2-blade (2952) – 130 grams using 1:5 gear ratio
  3. Black Lego 3-blade with gear (2740c01) – 200 grams fwd, 50 grams bwd using 1:3 gear ratio
  4. Non-Lego boat prop 2-blade P1.4x40mm 83 serien – 75 grams fwd, 40 grams bwd using 1:15 gear ratio
  5. Non-Lego drone prop Diatone Bull Nose Plastic 4 x 4.5 – 250 grams fwd, 100 grams bwd using 1:5 gear ratio

The big black Lego propeller (2740c01) is quite good. I used it in Submarine 3.0. But you need to wrap an ugly rubber band to rotate it, because it has no axle hole but only a gear with permanent connection to the propeller.

Therefore I decided to go for the drone propeller. This will be the first time I use a non-Lego prop.

Main propeller

For the main propeller I selected Diatone Bull Nose with size 4 x 4.5. It is designed to be used in drones. A bag of 4 pieces cost only 1 euro in HobbyKing, link: https://hobbyking.com/en_us/diatone-bull-nose-plastic-propellers-4-x-4-5-cw-ccw-black-2-pairs.html

I selected this particular drone propeller, because the length is 10.2 cm (4 inches) and therefore will match the 11 cm diameter hull of my submarine. Second criteria was to pick the largest pitch I could find, so that it works with low RPM and I don’t need to add many gears. This has 11.4 cm pitch (4.5 inches). I didn’t test any other drone propellers, so I don’t know if this was the best choice.

The propeller has a 5 mm hole in the center, almost perfect for a 4.8 mm Lego axle. But how to rotate it, since there is nothing to hold on to? Normally they would be screwed tightly to the drone rotors and let friction keep them in place. But I can’t use screws on Lego axles.

I attached a piece of double-sided tape on both sides of the propeller. That should give enough friction.

Then I pushed round Lego plates with axle holes (4032) to both sides.

Lastly I added Lego half-bushes (4265a) to the axle to keep the ends secure.

In hindsight, it worked perfectly. The connection never got loose in 10+ hours of diving under water.

The length of the propeller gave problems sometimes, as the blades would hit pool walls or rocks. I had to add a ramp up to the motor PWM control, to give less power for the first second you press buttons. That helped maneuvering in tight places.

Turn propeller

For the turn propeller, I selected Lego Propeller 3 Blade 3 Diameter with Axle Hole, part id 6041. You can get those for 0.1 euros from BrickLink: https://www.bricklink.com/v2/catalog/catalogitem.page?P=6041

The main criteria was small size. The blades cannot be long as they needs to rotate sideways near the back plate. Little thrust should be enough for turning the submarine.

As this is a Lego part, it was easy to connect it to the Lego axle. Bushes to both sides to secure it.

In hindsight, it was too weak. Or alternatively, the gear ratio I used was too small. The sub would turn too slowly. With the powerful main propeller, the controls felt unbalanced. You were always pressing left or right and sparely clicking forward or backward buttons.

Magnetic couplings

How to transfer rotation through the wall? With the first submarine I used a Lego axle and o-rings. It worked ok but leaked a little. In the next two submarines I used magnetic couplings with better success. So naturally I wanted to use the successful formula again.

Magnetic coupling in Submarine 2.0.

The main benefit of a magnetic coupling is that it is 100% leak free. You don’t need to drill holes to the hull walls.

But it is more complex and requires more space from both sides of the wall. Also, the wall needs to be flat and thin.


I’ll use 24 pieces of K&J Magnetics D38-N52 neodymium magnets. They cost 0.86 USD apiece, link: https://www.kjmagnetics.com/proddetail.asp?prod=D38-N52

The biggest selection criteria was diameter. These magnets are 3/16 inch (4.76 mm) thick, so they will fit almost perfectly in 4.8 mm Lego holes. I put a little piece of packaging tape around to keep them tightly in place.

D38-N52 neodymium magnet. On right side is one with packaging tape wrapped around.

The length is 1/2 inch (12.7 mm), so they will protrude nicely from a Lego gear (8 mm thick).

To hold the magnets, I’ll use a 40-tooth Lego gear for the main propeller and a 24-tooth Lego gear for the turn propeller. 8 magnets for the big gear and 4 for the small one. I set the polarity to change after each magnet. I have also tested “+ + – -” combination for the 40-toother, but it transfers less torque.

Note that the pull force between two magnets drops very rapidly with distance. The distance in my case is about 2.5 mm (2 mm plastic sheet + two tapes). Two D38-N52 magnets at 2.5 mm distance have 249 grams of pull force, but only 95 grams when you double the distance to 5 mm. That is why I used a thin plastic sheet for the lid. Magnet calculator: https://www.kjmagnetics.com/calculator.asp

Friction reduction

To reduce friction, I’ll use TapeCase 423-5 UHMW Tape. A roll 2 inches wide, 5 yards long, cost 17 USD, link: https://www.amazon.com/TapeCase-423-5-UHMW-Tape-Roll/dp/B00823JDXM

The friction coefficient should be 0.1-0.2 based on this source. I have also tried PTFE (Teflon) tape, which should have 0.1 friction coefficient, but it didn’t feel durable enough.

The tape is attached to both sides of the wall. The magnets will slide against the tape.


I did proper tests before going forward because I’ve had bad experiences. In Submarine 3.0 the magnets stopped rotating, as the tape would lose some of the properties in use. The problem was fixed when I removed two magnets to reduce the pull force.

It is a sensitive balancing act. With more pull force the friction becomes too high and the coupling stops rotating. With less pull force the torque transfer is too low, and you’ll see the secondary coupling stay in place while the primary coupling keeps rotating, which will cause bouncing and rattle as the magnets attract and repel.

So, I tested the setup properly in water and measured RPM as well. At first the main propeller rotated at a low 200 RPM rate, the motor being almost stalled. The turn propeller rotated at 500 RPM which was ok.

Then I added silicone spray on the magnets. The one I used is this: https://www.motonet.fi/fi/tuote/6003442/AT-HD-Silikonispray-400-ml

That increased the main propeller RPM from 200 to 300. I was quite happy with the result. But does it degrade under water? I tested it multiple times and let the surfaces dry and tried again the next day. No degradation. Why on earth didn’t I try lubricants before with my earlier submarines? And why is the low-friction tape performance so bad when not lubricated?

Lastly I also replaced a liftarm with a Lego Technic brick to hold axles and gears. That increased the rpm up to 375. I guess those tiny uneven surfaces on the liftarm introduce extra friction.

Just for comparison, the propeller rotates 450 RPM without magnetic coupling, and 375 RPM with magnetic coupling. So not many RPM’s are lost to friction. Good efficiency.

If I stop the propellers with my hand, the motor will keep rotating while the magnets bounce. This happens both with the main and turn propeller. So the couplings are too weak for the motor max torque. Not the way I’d have wanted it, but I didn’t have any more magnets slots available. In the end, is worked well in actual usage.


After gaining confidence from the tests, I continued building the lid. I first attached a low-friction tape on both sides of the lid.

Then added silicone lubricant on the magnets. I used a Q-tip to avoid messy stains.

Added magnetic couplings.

Inserted a Lego Technic brick. Added a 8-tooth gear (10928) and an axle for the turn propeller. Then a connector piece (48496) to make an L-turn with two bevel gears (6589).

After inserting the bevel gears, I got a 3:1 gear ratio for the turn propeller. Added a 8-tooth gear, a long axle and the prop, and got a 5:1 gear ratio for the main propeller.

I used toothed half-bushes (4265a), that have in my experience the highest friction with Lego axles. Also, I hand-picked axles that seemed to stick tightly with the bushes. This was because I’ve had problems with axles falling off in Submarine 3.0.

Nothing holds the construct in place other than magnets. In my previous subs I had additional supports, but this time I went with this simpler construct. In the 10+ hours of testing it under water, it worked well. One time the big magnetic coupling bounced into a misaligned position, and I had to lift the submarine out of water to fix it. But even in that case, the construct didn’t fall off, which had been my biggest fear when diving close to branches and rocks.

One little problem I found in swimming pool testing, was that the submarine veered left. After investigation I found that the main propeller axle tilts when torque is applied.

Main propeller tilting.

I added two Lego liftarms on the shaft to support it better. Thereafter the submarine moved straighter.

Main propeller tilt fixed.

5 thoughts on “RC Submarine 4.0 – propellers (4/10)

      1. Burak

        Can you create a basket with only the lego pieces needed for this project and share the link?



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