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
if you have a pool, 60kg of dive weights, a few milk crates, a couple of people to help and a sense of adventure, then yes, its fairly easy.
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 
get the free version of Shape3D and re-create your board in it. A rough approximation based on length/width/thickness will get you close on volume even if you don't make the exact shape. There are lots of foilboards you can download as a starting point. The difference in dimensions between a 60L and 80L board is not subtle
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.
The physics is that if something weighs less than the weight of the water it displaces it will float, if it weighs more than the weight of water it displaces it will sink.
1 liter of fresh water water weights about 1kg so that's where the rule comes from.
The volume of the board and foils needs to be higher than the weight of the board and foils and rider in gear in kg or it will sink in fresh water when not moving until the additional volume of your submerged body makes up the difference.
To add a little more to my post, the law of physics we are talking about is Archimedes' principle:
en.wikipedia.org/wiki/Archimedes%27_principle
This is a great interactive blog post that takes you through it starting from the basics:
ciechanow.ski/naval-architecture/
I actually found this blog from his post on airfoils which is directly applicable to how our foils work as well:
ciechanow.ski/airfoil/
They are long reads but I find them really informative.
Yes, a larger surface area increases resistance to sinking I would say.
I assume there's a physical explanation behind it
Thanks for the answers. I'll try the water thing, and maybe if i can find a lazy way to do it, the computer thing
Thanks
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.
Select to expand quoteFrankieboy said..
Yes, a larger surface area increases resistance to sinking I would say.
I assume there's a physical explanation behind it
Only in the sense that it will sink slower due to water resistance. but it will still sink - you can't change archimedes law with surface area.
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.
There's only the Archimedes principle, the rest is 'sensation'.
So if you want to test the volume the easy way, go to a body of fresh water standing still. Bring weights and keep adding weight to the board upto the point where the last piece of 'skin' is still just above the surface. The weights you've added + the weight of the board itself equals the volume.
A more techical solution would require a 3d scan of the board or pretty accurate measurements in autocad.
The easiest? Just stand on it in that body of fresh water. If it sinks, the volume is less than your weight. You could do the same in salt water, the difference is marginal.
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!
