Float estimation - PU vs EPS

[Lebowski]

Hi,

My local surf shop is moving premises and selling all it's boards for half price. I'm very tempted by one particular board, an Aloha AF2 turbo shortboard.

My current board is a PU shortboard, 6'6 x 20 x 2 3/4

The new board is EPS, 6'6 x 19 1/2 x 2 1/2

I'm happy with the buoyancy of my current board, but if anything would prefer a slight reduction so duckdives are easier on big days. The new board also has more of a performance (lower) rail, which is what I want.

Both boards are very similar shape. What I want to know is will the new board be floatier than the current one because it's EPS. If so I don't really want it. I would go down a size but this is the only one they have and I don't like to miss a bargain.

[tony g]

Hello Lebowski,
I think this board will give you more performance than your current board. If the rails are more tapered and the new board is a 1/2" narrower, and 1/4" thinner it should be easier to duck dive and offer more performance. I have found EPS boards do float a little higher in the water because the foam lighter. With that said, and if this board is 1/2" narrower, it should give you the performance your looking for. You could also ask them if they have a demo one you could try. That is the only way to know for sure!

[Lebowski]

Thanks for your reply. Unfortunately they don't have a demo, it's a stock clearance because they're moving premises. But I think you're right and it should give me a little more performance. I'll just have to wait till payday and see if I can scrape together enough!

[Rickyroughneck]

Lets work it out.

Using a rough equation for shortboard volume that is actually quite good: http://www.swaylocks.com/forums/calcula ... me-dummies

Volume = 1/2 length x width x height + (10x length above 6')
Then divide the whole lot by 60 (to convert to litres).

The volumes are:

PU = 37 litres
EPS = 33l

A PU weighs roughly 3kg, and an epoxy about 2.5kg (being generous with the difference). This varies with the glassing schedule, blank density, skill of the glasser, what he ate for breakfast etc, but as a ballpark estimate is on the money.

PU = 37 - 3 = 34kg upthrust when submerged.
EPS = 33-2.5 = 30.5kg


Although an epoxy board can be 20% lighter, due to the high density of water it only corresponds to about a 1.5% (for a 35l board using the same equation) increase in buoyancy.

It is the shape, not the materials that confers the buoyancy.

[dtc]

My science was so poor I dropped out at year 10, but bouyancy is basically determined by density and volume.

Epoxy is less dense for a given size than PU (because epoxy has more air in the blank). Thus for the same volume (size of board) it will be more bouyant primarily because it weighs less. However, if two boards are exactly the same shape, volume, and weight, the bouyant force they exert when submerged will be identical regardless of what they are made of. Of course, as Ricky points out, expoxy boards will actually be ligher for the same shape and volume.

However, if Ricky's calculations are correct, the difference in density is fairly minimal. A 2-lb difference in board weights will amount to less than 2 % reduction in total surfer+board mass for a 175-lb surfer.

[Lebowski]

That's an interesting formula and a thorough answer.

It looks like the new board would achieve what I'm looking for.

Thanks very much.

[jaffa1949]

One thing I've found with epoxy boards of similar size to my standard boards is they are more corky and under white sections you have to keep more drive own them otherwise the foam pushes the board out from under your feet really easily!