Fellow Swayloholics - I just received an E-Mail from a friend detailing the specifics of epoxy boards and once again, the buoyancy issue has been raised. This is a subject we have all heard about on this site. So much of what we hear is based on someone’s sales pitch - often from a salesman who is speaking from personal impressions rather than fact. My reply and a brief explanation (directed towards divers) is below… I still contend (and believe that laws of physics support) that objects of equal weight and volume will displace the same amount of water. It doesn’t matter if the interior core of an object (surfboard) is helium, styrene, whatever - if the overall weight is the same, the buoyancy will be equal to another object of the same volume. If someone makes a board lighter, I don’t care what material he or she uses, it’s going to float better. In my opinion, this is not a function of “buoyancy”, it’s a matter of overall weight and flotation - and there is a difference. (see below) -------------------------------------------------------------------------------- Buoyancy Concepts Archimedes Principle Some objects float in water while others sink, and still others neither float nor sink. This is a function of buoyancy. Objects that float are called positively buoyant, those that sink are called negatively buoyant. Neutrally buoyant objects neither sink or float. Archimedes, a Greek mathematician, stated what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. It is from this principle that we can see how a heavy ocean liner can float: it is because of the large amount of water that it displaces. Whether an object floats or sinks, is based on not only its weight, but also the amount of water it displaces. As divers we are mainly concerned with two different liquids: fresh water, and salt water. Because their weights are not the same, they have to be calculated as different fluids when considering buoyancy. A cubic foot of fresh water weighs approximately 62.4 lbs, while a cubic foot of salt water weighs approximately 64 lbs., the difference is due to the dissolved minerals in salt water. Let’s take a moment and look at an object in water and Archimedes Principle. An object weighing 63 pounds will sink when placed in fresh water as it weighs more than the water that it is displacing. It is negatively buoyant - it will sink. It is however being buoyed up with a force of 62.4 lbs, so if we weighed it in the water it would only weigh .6 lbs. The same object in salt water would still weigh 63 lbs, but would be buoyed up by a force of 64 lbs, and it would float. It would be positively buoyant in salt water. By adding 1 pound we could make the object neutrally buoyant if we kept the size the same, (without changing it’s displacement).
Well Put John, In its most simple form, Mass/Volume ratio is what you want to look at when calulating buoyancy. To help solve the debate, we need to make a comparrison of ratios for boards shaped with different blank materials and lams. Does anyone out there have any numbers? When making a board for someone, how do you factor in their weight into this ratio?
But if you break it down. EPS is lighter, floats better, is less toxic, and the finished product is stronger. All those things add up to fun, (at least for me). The down side is that they do flex differently and they are more corky in the water. I don’t mind either. But some will never like them or give them a chance (there are a lot of wierd rumors). There is no use in getting into which is better (and I don’t think that is really the meaning of your post, but I’m afraid people might assume that is what you are getting at). It all comes down to personal preference.
let’s see, salt water weighs 64 lbs. per cubic foot. a clark 6_7R blank has a displacement of 1.525 cubic feet, which if submerged would displace 97.6 lbs worth of salt water. if you glassed it as is, and ended up with a 7.6 lb. surfboard, you’d be about 90 lbs. positively bouyant and it would require a 91 lb. surfer lying on top to submerge it, right?? could you just call the 90 lbs. in this example the “positive bouyancy variable” and simply come up with a percentage range or ‘comfort zone’ to be multiplied by to determine a range of rider weight? like 160% to 180%, so it would be between 90 X 1.6 = 144 pound surfer, and 90 X 1.8 = 162 pound surfer. the positive bouyancy variable is mathematical fact, of course… and the percentage ranges is where the debate would occur, i’m sure.