How much will my tub weigh?

So, based on the sketch I made on the white board (below), I need to calculate just how heavy the tub will be. The main reason I need to calculate this is to be sure the 19kg I’ve taken out by chopping out all that steel and removing the ally panels isn’t then replaced by even more carbon.












I made a trial part, consisting of:

  • 4 * 600gsm carbon (2 above the core, 2 below)
  • 2 * 200gsm e-glass (1 above the core, one below)
  • 1 * 300 gsm aramid (on the bottom, facing the tarmac)
  • 1 * 300 gsm carbon (on the top, to look pretty)
  • 1 * 10mm thick closed cell foam for the core (in the middle)

The layup is symmetrical around the core, apart from the aramid on the bottom and the facing carbon on the top. I have discounted the weight of clear gel-coat applied to the finished part (assume 1kg at the end).

The part measured approximately 103mm * 204mm, and weighed 130g. This meant a unit weight of 0.006 g/mm2.

From this, I fed the dimensions of the panels above into my CAD package. Given a surface extruded to 1mm depth, it will tell me the mass of the panel, to a bazillion decimal places.


Part count Unit Weight Total Weight Running Total
Tunnel Side 2 2.37 4.74 4.74
Tunnel Top 1 1.16 1.16 5.9
Back 2 1.0375 2.075 7.975
Base 2 2.7 5.4 13.375

So, if I go for this, the new tub will weigh 6 kg less than the original steel work.


  • 10mm closed cell foam is used uniformly. This won’t be the case – the sides do not need a 10m core – I will probably go for a 3mm core.
  • the base, back and top of the tunnel need to be strong in bending load, the sides need to be strong in lateral load. As such, I can use a thinner (or even no core) for the sides. I think I will save 1kg there.

5 thoughts on “How much will my tub weigh?

    • I’m fairly sure I weighed the original stack before infusion. That should get me the resin weight. Then add 20% to be safe.

        • Nope. Sorry I didn’t make that very clear. I’m going to wear the cloth are used and then compared the weight of the cloth with the weight of the final part, subtract one from the other and hopefully this will give me the weight of the resin. Once I have the weight of the resin I would then add the 20% buffer for things like infusion mash, pipework, margin for error.

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