Ordering Acrylic in Europe in metric units: which dimensions?


I'm trying to order acrylic in Europe and I'm trying also to understand what is used for what.

From what I understand from the bill of materials I need to order 5 different pieces of acrylic (aka Plexiglass):

  • 3mm blue 24" x 36" - main body
  • 3mm clear 12" x 24" - the caps for the electronics tube
  • 3mm white 12" x 24 "- caps and prop guard
  • clear tube 4" OD/ 3-3/4 ID - electronics box
  • clear tube 3-1/4 OD / 3" ID - props guard?

Unfortunately in Europe we don't sell in inches and feet, but in mm. Furthermore, the provider of plexiglass I found, can cut sheets and tubes to an exact size, so to save money I can order the exact size needed.

Can someone please verify my assumptions and tell me if the sizes are correct?

  • 3mm blue: the PDF says the sheet is 24" x 18" (so half of the sheet required, also build instructions mention this size instead of the other). Can I order 610mm x 460mm?
  • 3mm clear: the drawing contains 10 caps, are they all needed for one ROV? or just 2 x ROV? In that case (2 x ROV) a 240mm x 150mm should be enough, right?
  • 3mm white: looking at the PDF it looks like just a small piece of the sheet is used: 160mm x 240mm should be enough
  • Electronics tube: 4OD - 3-3/4ID doesn't exist in metric unit (101,6mm OD - 95,2mm). The closest I could find is 100 - 94. Also I cannot understand how long it has to be: the bill of material says 3' that would be enough for 5, but not sure about the actual length. Also , having changed the diameter of the tube, I guess I'd have to change the size of the caps and of the electronics structure.
  • Props guard: same as above. 3-1/4 - 3 (82,5 - 76,2) doesn't exist. available are 90-84 or 80-74. But The impeller is 3in exactly, so either I buy the normal impeller and cut to size, or look for a 74mm impeller. Also how long must that tube be cut to?

As you see even if the sizes are metric, many things have been designed to availability in the US. It might be a good idea to make 2 versions of the parts, one for the US makers and one for the makers of the rest of the world? I'm not skilled in cad stuff, so cannot b of any help here



More to come on this, but a few things...

  1. All 10 of the clear circles are needed per ROV.
  2. There's not that much white - just the end of the endcaps and the prop guard. AND it's just the aesthetic we use. You could use clear for all of those cuts, too. It is handy, however, to have white prop guard on the top for visibility.
  3. We had been using cast acrylic tubing but a team in British Columbia is building and found that extruded acrylic works, too (we were worried about visual distortion from the camera but they said there were no issues).

Eric will chime in with measurements.


I made some more calculations using a free CAD tool to measure the actual space needed by the layouts and here is what I found is the minimum sheet size for the 3 colors, including material and price from the provider I found in Germany:

  • Blue, Length: 610 mm Width: 460 mm, 20,86€
  • Clear, Length: 480 mm Width: 200 mm, 11,38€
  • White, Length: 200 mm Width: 240 mm, 9,89€

Total price, including shipping and cement is 65€

To that we might need other 20€ give or take for the tubes (once I find out the length).

As soon as there is a wiki I think we can start creating a BOM for people buying material in Europe.

Can someone verify the minimum size? Thx




I just checked your numbers for the respective sheet sizes and they're all good- that should work!

The 4" diameter of the electronics tube is only because that's what I could get on McMaster-Carr here in the US, but the curved brace that holds it is sized for a 100mm OD. You're right about probably needing to adjust the endcap sizes, but it's possible that we have the same thing just labled with different units. The length of that tube should be 180mm.

There's also enough "wiggle room" on the prop shrouds that anything close to that diameter should work. The circular brace that holds the shrouds is sized to a 41.25mm radius, but perhaps if I can find a way to source truly metric tubes, I can change that to 40mm in the future.

As far as the idea of having two different part sizes for US and (everywhere else), I don't like having to do that. I think (everywhere else) has it right with metric and I'd rather move the US sources to metric as well- we'll just have to work on finding a way to do that at a reasonable price.

Please keep us updated with your European OpenROV BOM, as I hope that will one day be the main one used!



Hi Eric,
I checked the Inventor files and the end caps are 94mm of diameter. They should fit. My only concern is that maybe when we put the O-Ring for sealing (if I understood correctly and the o-ring goes in the small slot between the 94mm caps and over the 90mm cap) the cap might become too difficult to insert and remove.

As for the props guards, the other problem is the size of the impeller, not just the brace. And I haven’t found a 74mm impeller yet. Only one supplier, but it comes already with motor and guard.
I’ll send you the link when I find it again :slight_smile:

I agree that having 2 designs, one built to US standard sizes of tubes, and one built to EU standard sizes of tube is a mess and we should try to find suppliers that sell in all continents.



@ #3 is correct, we have done a "dry test" and found the Microsoft Lifecam (even when set to 720p) didnt have any visual distortion , i will post some pictures up in the next day or so. I will note that we actually removed the entire housing from the camera, and are working with just the PCB and a custom USB cable.

My advice on the Cast vs. Extruded tube would be to buy local (if possibe) and just have a look at the extruded. The Extruded tube is MUCH cheaper and if it is good quality it shouldnt be a problem visually. As for strength... well we are going to test it to the limits so we should know soon enough!

Chris @ BC Robotics


This is the only 74mm impeller I found so far:


Has a much higher kv 2570 vs 800, and only 3 blades instead of 7. Might not be ok for running underwater.

So I guess we need to find another solution to use a 74mm ID shroud, or change completely (maybe using directly combos impellers/shrouds/motors) so we don't have to build everything from scratch and will save us from different sizes available


I also started editing the EU suppliers BOM:



Well we have just hat a first wet test with the#153 gaskets with the extruded tube - I don't know if it is our specific tube, but it doesn't hold water out! This is definitely a strike against the extruded tube - I do have some slightly larger gaskets coming in that I ordered just by chance... so we will test that out and keep everyone posted here. Will advise , they should be here Friday!


Do you guy in BC have components in mm or in inches? I was wondering if the original cap (94mm) fits in the mm standard size of 100-94.


everything here in terms of building materials are measurred in imperial just like the states.

I would suggest cutting a set and trying it out - the tollerances have to be so close!

With ours i get the impression that either the tube's tollerances are not good enough or the smaller diameter pieces for the endcaps are between 1/32 and 1/16" too small


Then it would be good for the SI size, which is 94mm vs 95,2mm (3-3/4) :)

Anyway, waiting for my FabLab to open after summer break and then I'll cut stuff


Good morning Simone

Maybe it is easier to fit some parts of the acrylis stuff to the us-system. For the prop guards it would go for widing the pipe a bit. Just heat it and try to make it wider by pulling it over a cone with the right thickness at the end. I did that in a smaler scale for some freediving equipment and i work quite well and easy. I don't think that will work for the main tube scince the optical quality will suffer a bit. but maybe it is possible to sand down the parts that should go into the tube.

It might be even handy to sand the area around the camera and paint it black to avoid reflections and ghost images, maybe that helps also with autofocus.



Hi Tody, thank you.

That was a very old discussion :)

It turned out that the electric tube works quite ok also when a bit smaller...

and now that I have more confidence with making my own parts, I think I'll resize some parts and cut again with the laser cutter




I'm just reading through all this posts and I would define very old a bit different. It was only 4 months. hahah. I made the experience that acrylic, polycarbonate, etc are very good materials to shape with heat. At the moment i'm experimenting om the optimisation of diving goggles.




4 months in a 6 months project is loooong time :)


Thats right. The reason why I'm busy in this forum just now, is that we have a 8 weeks project which is already delayed because of ROV troubles by 5 days.

But it is good to read that the real metric stuff fits more or less, did you solve the propeller issues?



Hey Tobi,

What kind of ROB troubles? Are you using the OpenROV design or another ROV?



no a big one. A ip for all pilots: Never go round a subseastructure 3 times and then through a loop, this creates the most expensive knot you've ever seen. :-)



Hi I ve just discovered this thread as i m having a similar issue - that is, sourcing the acrylic cylinder to house the electronics in the correct dimensions to match the 0-rings so that it is water tight etc. Cutting the other parts in imperial measurements isn’t an issue its more making sure the tolerances and dimensions for the water tight parts are accurate. Has anyone produced a metric design using perspex cut in mm matched to the correct O-ring sizes, so that parts sourced from European sources can be used? If so could you post the link to it, please?
Any help with this would be great.