Just bought a custom made parawing/wingfoil board. Advertised (on the board) as an 80 litres. I already have ? 80L board, and it floats when i'm standing on it. This one sinks to about knee-high so I think it might be more like a 60 litres. Is there a way to check ?
Thanks

Not that easy (or water wise) but I did hear of a method that I used for my prone board.
Fill a wheelie bin or something similar to the absolute brim with water.
Measure and mark the centre (lengthwise) of board.
Dunk one end of the board in down to the mark, this will of course displace water.
Then pull board out and refill container, taking note of litres to refill.
Then dunk the other end in down to the mark, remove board and refill again taking note of litres to refill.
Add both amounts together and you will have board volume (and a big water bill).
Tip the wheelie bin of water onto your lawn and you will soon have a nice green patch

I don't subscribe to the idea of buoyancy equaling volume.
I make my own boards & weigh them before glassing to find their volume. I'm 86kg dry so I reckon 90kgs wet & with the added weight of the foil my 100L board sits below the surface when I'm on it. I've done a bit of a investigation into volume & buoyancy but unless someone can assure me that volume=buoyancy as an absolute law of physics then I think there's other factors that come into play.
In saying that I admit I'm bit of a thicko so welcome any arguments to the contrary.

Surface area that contacts the water has a big effect, im sure.

Yes, a larger surface area increases resistance to sinking I would say.
I assume there's a physical explanation behind it

The simplest is length x width x thickness. That should come out as substantially more than 80 litres (coz of narrower nose and tail and rocker etc).



Reduce the calculated volume by 10 to 20% and that should give you a half decent estimate of the actual volume.



If you want to go nuts on it. Run some tape down the middle of the board then divide it into a bunch of smaller boxes. Measure and calculate the volume for them and that should give you a more accurate number.



You could do the rough measurement method on a couple of existing boards with known volumes and come up with some rules of thumb to compare to yours.

Mark's right. Surface area doesn't matter in the steady state, but surface tension does. Surface tension is calculated by knowing the length of the three phase contact line (where the water touches the board at the surface). In this respect, the area doesn't matter, but since the more area you have, the more "perimeter" you have, it matters.

Fun fact: when I was a youngster my father taught me how to gently drop needles and flat razor blades onto clean water so they wouldn't sink. Supported by surface tension.

Edit: Sorry, I meant to add the surface tension force is pretty small and I wouldn't bother calculating it.

Modern phones (e.g. iPhone or Samsung) have LiDAR, a scanning app could be the easiest approach.. I use my phone all the time to measure things.

Very interesting thread!