# compsand bouyancy--change dimensions?

Hey,

I have a few questions regarding compsands’ bouyancy and the changes one might make in their dimensions.

Obviously, compsands weigh less than PU boards (and possibly less than epoxy). I assume that the lower weight equates to greater bouyacy (unless I severely misunderstand basic physics)… which leads me to believe that people should be surfing smaller, thinner compsands than they would ride if they were on a PU board.

I think I saw a post in which Bert wrote that he rides a 1 7/8 thick board with a concave deck–and he’s about 220lbs, right?

So what dim’s are you compsand shapers using? How small/thin have you gone?

What are the results?

Thanks!

cant see any point reducing the size of your board though trucker

i mean maybe just enjoy the extra float and paddle

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I think I saw a post in which Bert wrote that he rides a 1 7/8 thick board with a concave deck–and he’s about 220lbs, right?

Check the width though, it is like 23" wide.

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Obviously, compsands weigh less than PU boards (and possibly less than epoxy). I assume that the lower weight equates to greater bouyacy (unless I severely misunderstand basic physics)…

Well, yeah, but…

I don’t want to be the guy to rain on this parade, but very simply stated…

Buoyancy is the weight of water displaced minus the weight of the board. Given two boards with the exact same volume, with one of them weighing a pound less, the lighter board is proportionately much more buoyant than the heavier one. But only without any weight added to the system.

When you sit a 150+lb surfer on those two boards and drop the whole system in the water, that 1lb of difference is negligible in a 155 lb+ system. The displacement of the board and the surfer are exactly the same in the two examples, and the total weight is only 1 lb less in the lighter one. The difference is proportionately much less, almost negligible.

Now, when you start talking about swing weight and inertia and momentum and stuff, then you’ll see some major difference with a 1lb lighter board, since it functions as a pendulum at the end of long levers (your legs), and any weight change will be very noticeable. VERY noticeable. Surfers of virtually any skill level can feel a difference between a 6lb board and a 5lb board when riding it, for all the reasons posted above. But a surfboard being ridden is a dynamic planing craft, not a static floating one, so buoyancy is not a factor except for when a surfboard is sitting still.

Again, the differences between boards of different weight are very easily felt… But are NOT due to a difference in buoyancy, and don’t let anyone tell you otherwise.

This really is basic physics, and a pet peeve of mine, since there is no shortage of folks in the surf industry who just don’t get it.

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which leads me to believe that people should be surfing smaller, thinner compsands than they would ride if they were on a PU board.

The major benefit do thinning out a compsand board, and the primary reason to do it, is to free up the flex that makes compsands so very cool! I like my boards pretty stiff, so I make mine to a standard thickness (2 5/8 or so), but the guys who are getting that flex and springback effect are going much thinner than that. If you do that, though, you need to beef up the numbers someplace else, width or length, to get the volume up to where it needs to be to keep the same BUOYANCY.

Don’t believe me? Ask Bert why his “magic carpet” is so wide.

Simple answer: Make your compsand the same dims as your favorite board, get a feel for the difference. Or, alternatively, thin and concave the deck and pull the outline an inch wider so it’ll flex good. Good luck and post photos when you’re done!

what he said!

Interesting Schwuz,

After making a handful of EPS boards and foraying into the balsa comspand land I was coming to the (pre-emptive) conclusion that the only places I will be thinning my boards will be in the nose and tail areas, the middle of the board works fine at “normal” thicknesses and it is in the nose and tail that I think that “corkiness” and flex are to be worked on, just a gremmie though so any wisdom you guys have is readily absorbed…

Hi Trucker,

There’s an EPS design theory thread in the archives where GregL talks about some changes he’s done to EPS compared to PU. HTH.

Cheers,

Rio

here’s Bert’s personal “magic carpet” in all it’s glory.

love that exaggerated “bullnose” tails

I’m 5’8" ~200lbs and over 50

I did a 7’4" x 21" x 1.75" vector and it sucked paddling it out

I did a 6’4" x 23" x 1.75" quad fish and it really sucked paddling it out

riding them both was weird cause you felt every single vibration coming off the bottom and it felt like the board was going to snap at any time.

Funny thing was they rode okay(my brother doesn’t think so though) once you got in the wave

the paddling issue was more of a bad design problem that anything else.

up to you what you want to do

to me

more float = more waves = more fun

just have fun…

Cool, thank you very much!

Although my landlord is now trying to shut down my little garage operation… I’m hoping I can squeeze out a few more before I have to officially say, “okay, yes, I will clean it up!”