RC Submarine 4.0 – hull (3/10)

In previous submarines I used IKEA glass food containers and a plastic lemonade pitcher as hulls. Both worked ok, especially the IKEA snap-on lids were easy to use. But this time I wanted to try something different.


The hull has to be rigid. It cannot bend or compress under pressure, otherwise I’ll lose buoyancy, like with the Submarine 2.0 soft lids.

It needs to be transparent. I couldn’t even imagine building the thing, solving problems and checking everything is ok before dive if I couldn’t see inside.

It is good to be streamlined. As narrow as possible to increase movement speed. But also, I want an on-board camera to the front, so the front has to be flat. The back also needs to be flat for the large magnetic couplings I’m going to use. Not optimal for speed, but fast enough in hindsight.

So, I decided to make a cylinder with flat ends. Plain and simple.

What is the minimum diameter? Magnetic coupling for the main propeller will take 4 cm, as I intend to base it on a 40-tooth Lego gear, like in previous subs. For left/right turn I first envisioned using a linear magnetic coupling, similar to Submarine 3.0, but in the end I changed it to a 24-tooth magnetic coupling. That will take 2.5 cm. Some clearance is needed between the magnets. For the lid sealing I will use a piece of a smaller cylinder that is attached to the flat lid. The lid sealing takes 2 mm and the hull thickness is 3 mm. With this information I draw a sketch with 10 cm outer diameter. In this drawing everything would fit but with little room for error. Therefore I increased the diameter to 11 cm, just to be on the safe side.

Hand-drawn sketch of the cross-section.

What is the minimum length? The syringe takes 23 cm when fully extended. The hose connection to the syringe will take some space also, so a total of 25 cm, a good round number. I was able to find a 25 cm length cylinder. In the end, the hull was a tad too short for the syringe. It extended only 45 ml out of 60 ml, as it would hit the lid supports. Well, good enough.

Acrylic plastic cylinder

I bought two Perspex acrylic plastic cylinders: 11 cm and 10 cm in outer diameter, length 25 cm for both, thickness 3 mm for both. They cost 36 euros + postage. I bought them from an eBay seller simplyplastics, United Kingdom, links:
https://www.ebay.co.uk/itm/254457326025, https://www.ebay.co.uk/itm/254457319827

Left cylinder is the submarine hull. Right cylinder is for the lids. The right one is shorter as I’ve already sawed two pieces from it.

I used a hand saw to cut 2 cm pieces from the smaller cylinder. These were to be used with the lid.

Acrylic plastic is sensitive material. I quickly got some scratches on it and started using soft fabrics underneath the table/stool when working with it.

It also stains easily and is difficult to clean. I first used gloves that left smudge on the cylinders. You couldn’t remove the stains with soap and water. I first though about using a nail polish remover (acetone), but after I used it on the first lid, the plastic lost transparency. So not that. I read some debates online whether you can use alcohol to clean acrylic or not. I decided to try car windshield cleaner (ethanol alcohol, probably 50% concentration), first on scrub pieces. I didn’t see any cracks or clouding, so I used it in a few small places to clean the cylinder. I really dodged a bullet there, because I later found a chart that says more than 30% concentration of ethanol is not recommended for ANY use with acrylic.

SAN plastic sheet

I bought a plastic sheet made of styrene-acrylonitrile, also known as SAN. Thickness 2 mm thick and size 30×30 cm. It cost 5 euros in a finnish hardware store Biltema, link:

The plastic sheet placed at the front of the cylinder.

I wanted the lid thickness to be as small as possible for magnetic couplings. Less gap means more torque. But of course it shouldn’t bend or break. The 2 mm sheet felt rigid enough in my hands when I tried it before buying.

How to cut round pieces from a plastic sheet? I first cut a smaller square piece from the sheet. Then I attached a Lego pin (2460) to the center using double-sided tape.

The pin was then connected to a bunch of Lego bricks. The bricks were clamped tightly on a stool. I clamped a Dremel Lite to the stool leg.

I started cutting with a 561 MultiPurpose Cutting Bit. I could just rotate the sheet and let the machine do the work. It had to be done slowly because otherwise the plastic would melt and widen the cutting gap. It was noisy and tedious work.

I didn’t need to sand the edge, as the result was fairly smooth.

When I took off the double-sided tape, some glue was left on the center. Also there were smudges from the gloves I had used. I tried to remove them with acetone, but that clouded the sheet. With direct light you can really see the imperfections.

Ok, let’s make another one. This time I’ll use different gloves. Instead of acetone I use ethanol-based windshield cleaner. As I said before, you really shouldn’t use ethanol for acrylic, probably not good for SAN plastic either, but it worked for me.

The second one was a lot better. Here are the round sheet and the cylinder piece on top of each other.

Gluing plastics

I’ll need to glue SAN plastic to acrylic plastic. How do you do that? The bond needs to be watertight and unbreakable.

I decided to perform a little test with four glues:

  1. hot glue
  2. Biltema silicone (a cheap brand)
  3. Bison plastic adhesive
  4. Gorilla super glue

I cut small pieces from the left-over lid sheet. What I am testing here is SAN plastic against SAN plastic.

I let it dry 24 hours in my balcony at 10-20 degrees Celsius.

The results:

  1. hot glue was ok, I needed pliers to remove it
  2. silicone was a joke. It came off clean using my fingers.
  3. plastic adhesive was tough. With pliers and some force I could separate the pieces.
  4. super glue was the best. The glue bond never broke, instead the top piece snapped in half when I twisted it harder with my pliers.

So, super glue it will be. It should work very well with acrylic plastic also, so I’ve read. The ingredient of super glue is called cyanoacrylate.

I poured super glue onto the cylinder edge and pressed the lid on top of it.

This is how it looked 24 hours later.

What is that? It took some investigation online to know it is called frost, fog or clouding, and it is caused by super glue vapor. You need more ventilation.

For the second lid I blew air to it with a big fan. I did this for 24 hours after I glued it. The result was virtually fog free.

I didn’t want to threw away the first lid that had fog on it. The fog is just on the surface, so maybe polishing will remove it? I bought some Dometic Acrylic Glass Polish.

The result was surprisingly good. It required just a few minutes of scrubbing with a cloth and virtually all the fog came off. Although very good, it wasn’t 100% as transparent as the original, therefore I decided to use this lid for the back side.

I tested both the lids for waterproofing. One little spot on the glue seam leaked just a hair. I put more super glue to the spot and there was no leaks thereafter.


The next thing was to seal it watertight. Here is an image of the starting point. Between the lid cylinder and the hull cylinder there is a 2 mm gap.

I first tried window seals. I tested EPDM E-profile, EPDM P-profile and silicone P-profile. I did tests where I filled the 25 cm cylinder full of water and let it stay one hour to see if any water leaked through the seals. Sometimes they didn’t leak at all, but sometimes a few drops went through, especially where the sealing was disconnected. Also, the adhesive would come off after opening and closing the lid multiple times. Not reliable.

Testing an EPDM P-profile window seal between the 2 mm gap.

I experimented with all strange combinations, adding pipe tape, Theraband rubber sheet or Glidex grease on the window seal. Nothing worked well.

Then I bought o-ring cord. Two meters of 2.5 mm diameter, NBR nitrile rubber, 70 shore hardness. It cost 18 euros in a finnish hardware store, link: https://www.ikh.fi/fi/o-rengasnauha-nbr70sh-2-50-orn25

I would have wanted something softer, but couldn’t find any. 70 shore is quite hard and will make installing it more difficult.

I measured the o-ring length. I read it should be 0-5% shorter than needed, to give it a small stretch. But I had trouble installing that, so I cut it 15% shorter. 2.5 mm is a bit too thick for 2 mm gap, so stretching will make it thinner and easier to install.

I made the cut using a thin knife, good for making it straight.

Glued it with super glue. That should be the best glue for bonding nitrile rubber.

I used an angle iron to keep the ends aligned when gluing them together.

Making the alignment correctly is important. At first I used just my hands when gluing the ends together. It resulted in the following image where a water bubble has formed at the slightly misaligned connection point. The alignment error is so small you can’t see it without looking very closely.

The o-ring is installed just by pulling it over the lid cylinder. There is no groove. Nothing keeps it in place, other than friction from the stretch. I though about adding some supports, but couldn’t think of any simple way.

Installing the lid was difficult. You would have to push the o-ring to the gap with your fingertips, otherwise it would slide back. Eventually I became quite good at it. The key was to enter the bottom side of the lid first, and then push the whole lid downwards hard, squeezing the o-ring, while turning the top side in.

To help sliding the o-ring in, I smoothed the cylinder edges with a Dremel 414 Felt Polishing Wheel.

I had some reservations about high pressure. Could the o-ring slide out of the gap when submarine dives deep? Or will it start to leak in high pressure? My previous subs had a better design, as the lid would press the seal tighter when outside pressure increases. Not this time. If there are leaks, they will increase when the sub goes deeper. In the end I tested the submarine only in 1.5 meter depth. In those low pressures it never leaked.

I tested the submarine 10+ hours and the o-ring worked surprisingly well. The interior was always dry after dive. After having so much initial trouble with the window seals, I really thought sealing would be the Achilles’ heel in this submarine. I’m glad I was wrong.


I need a hole in the walls to move water in and out of the syringe. Also, I need access to the outside water for measuring depth. First I thought using the same hole for both purposes, but when I tested the idea, I noticed 30 cm error in depth readings when the syringe was sucking water in. Better make two holes.

I used a 3.5 mm metal drill bit for drilling two holes to the SAN plastic lid. It drilled easily, no cracks formed.

I pushed two silicone tubes through the holes. I used 4mm diameter Lego pneumatic hose, part id 21825. Pushing them through the narrow hole was difficult. I had to carve the end of the hose sharp to help with the task.

Pressure equalization

Later I became aware of a problem with the internal hull pressure. When I installed the lids, the air inside the hull would push back and prevent me from pushing the lids fully to the end.

There is a 3 mm gap between the lid plate and the rim of the hull cylinder. This happens when internal pressure is not equalized.

I could extend the syringe fully before installing the lids. Then when I retracted the syringe, under pressure would suck the lids inside. But there was still a little gap left.

The gaps were problematic, as they would increase hull compression and therefore the submarine would lose buoyancy. Not a good situation when you try to build an accurate depth control.

Eventually I found a solution. An ugly trick but it worked. I would use a thin screwdriver to squeeze the silicone hose. That would form a temporary hole to let air out and equalize the pressure.

Equalizing internal pressure with a screwdriver.

It worked, but I wish there was a better solution. Either there should be a separate hole for equalization that you can conveniently open and close, or the caps should have a locking mechanism that doesn’t care about the internal pressure.


A particular puzzle task is to fit all parts inside the small hull cylinder.

The propeller motors have to be located at the back side to connect with the magnetic couplings. Their location is pretty much set in stone.

The syringe needs space to extend fully, so it has an option to go under or above the motors. Well, the bottom will be filled with extra weight, so let’s put the syringe on top.

The battery is very big. The most available space is near the front lid, so let’s put it there. You can conveniently switch it on/off with you finger when you open the front lid.

Some kind of frame is needed to hold it all together. I first build a long bar placed at the bottom, and everything else was connected to that.

First build of the internal frame.

However, when I got to add extra weights, the frame was in the way. In the second version, I raised the bar higher, and put little legs to both ends. This way there would be room at the bottom for the extra weights.

Tungsten pellets

Okay, how about extra weight. In submarine 1.0 I used steel plates, which need to be correctly sized to fit inside the hull. Not the best option when you are developing and changing things all the time. In submarines 2.0 and 3.0 I used lead pellets. Those are much better, as you can easily change the shape of the bag they are in.

But could I improve on the size? There is never too much free space inside the hull.

Tungsten is much heavier than lead. The tungsten alloy I’m using weighs 18 g/cm3, whereas lead weighs 11.3 g/cm3. So I’ll save 40% in volume.

I bought 2 kg of tungsten pellets, 2.5 mm diameter. It cost 236 euros in a Finnish gun store, link: https://www.aawee.fi/ammunta-ja-aseet/tungsten-18-0-5kg-2-50mm-hauli-tss-1-lk/p/TSS250/

That was very expensive. Tungsten pellets cost 10 times more than lead pellets. Not a good decision in hindsight. I could have used lead pellets without big problems.


Later in the process, when all other parts had been installed, it was time to weigh and add the extra weight.

I calculated the hull volume. The hull cylinder diameter is 11 cm and length is 25 cm. Add 4 mm from the lids and some extra from the propellers to get a total length of 26 cm.

  V = π * h * r2
    = 3.14 * 26 * 5.52
    = 2470 cm3

Displacement is the volume of water displaced by the submarine.

  m = V * ρ
    = 2470 cm3 * 1 g/cm3
    = 2470 grams

I weighed the submarine using a kitchen scale.

Submarine weight without extra weight.

The base weight is 826 grams. Subtract it from the 2470 grams of displacement, and you get 1644 grams for the extra weight.

  extra weight = displacement - base weight
               = 2470 - 826
               = 1644 grams

But that would lead to neutral buoyancy. We want the submarine to be slightly positively buoyant, so that it floats when put into water, and the syringe ballast makes it dive.

The syringe ballast range is 42 ml. I think a good neutral buoyancy position is 30 ml, so that it has more range for surfacing, just to be safe. So take out 30 grams. The final number for extra weight is 1614 grams.

Put tungsten pellets in a plastic bag while measuring the weight.

Place the bag on the bottom of the hull.

The internal frame slides on top of the bag.

Now the weight is in the right ballpark. When you do the first dive in water, you need to tune it. I had to take out some 30 grams. Therefore the final extra weight was 1580 grams.

Bottom pads

I installed furniture pads underneath the hull to protect it from rocks. The lower front is probably going to have the most damage, so I put two pads there.

Safety line

In earlier submarines I attached a fishing line to the sub when doing dangerous dives. The fear is, the submarine could run out of battery, you could lose radio connection, the sub could tangle up with vegetation at the bottom, and surely a dozen other possibilities. You need a way to lift it from the bottom.

The problem with a permanent safety line is, it will get tangled in the propellers. You would be limited to just dive and surface.

So I invented a more convenient way. I’ll tape a magnet inside the top of the hull and another magnet to the end of the safety line. This way, I can use the submarine without the line attached. If the submarine is stuck at the bottom, I just lower the line, the magnet will snap onto the hull, and then I’ll pull it up. Of course, with this rescue plan, you need to always see the submarine position.

The magnets I used are neodymium ring magnets, K&J Magnetics R844-N52, link: https://www.kjmagnetics.com/proddetail.asp?prod=R844-N52

The line is Westline Dyneema 50, rated for 65 kg. It is the thickest fishing line I could find in a local hardware store.

I measured the pull force to be about 200 grams when there is hull wall and some felt pads in between. That wouldn’t be enough to lift the submarine if it is filled with water. So for the most dangerous dives I would still need a permanent safety line.


Oh, I almost forgot the most important part. The hero, the maniac, the brave soul to test this engineering deathtrap. Lego part id col154.

2 thoughts on “RC Submarine 4.0 – hull (3/10)

  1. Todd

    This was way more fascinating than it had any right to be. Turns out I had a ton of questions in my head about this details here and you answers them all. Very cool

    Liked by 1 person

  2. MH

    The propellers etc outside the sub also take up volume, so total volume is slightly more than the 2470 cm3 of the cylindrical hull.



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