Authors: Flyboy Wakesurf
That picture of the 180 pounds of elevator counter weights was pretty dramatic wasn’t it? Here it is again, as a reference.
The strength to weight ratio, when the force is applied directly against the honeycomb column is amazing. Now that doesn’t have any external or internal lamination, so it would be equivalent to do the same thing to a piece of EPS foam, in terms of similar construction medium. We’ll do that comparison for another post, but here is EPS doing the same thing.
The honeycomb is a bit stronger and stiffer, the point being that we can “engineer” a similar weight, strength and stiffness with a different construction. Love your EPS board, this method can probably duplicate that feel.
So that wasn’t what we wanted to talk about, instead it’s the concept of buoyancy. It’s a ridiculous complicated theory and concept, but we all know how it works. If the density of an object is lighter than the density of the water that object tries to displace, the object will float. So take something like a fishing bobber, it’s lightweight and most are of a generous size and shape so they weight less than the same amount of water and so float on the surface, partially submerged, but mostly out of the water.
So there are a few concepts in there. One is that the water is a fluid, so is gasoline but gasoline or ethylene glycol don’t interest us. Realize that the concept applies to all fluids, not just water. Also, it involves gravity. Gravity is always pulling us DOWN and that includes when we are over water. You could force that fishing bobber under water with your hand, or if you tied it to a string and swung it crazy fast, the velocity would most likely take it under water. Those forces are not part of buoyancy, they would in fact be overcoming buoyancy.
Let’s revisit the floating structure.
The skins that we used were kinda over-sized, so that would be an aid in creating MORE buoyancy. You’ve no doubt felt it on a larger board. If you’ve been on an ocen long board, you’ve felt how buoyant they are behind the boat. More area, more volume equates to more buoyancy.
Let’s visit density now. Density, in an over-simplification, is some measure of weight per volume. It’s usually expressed in terms like 1 pound density, which means that it weighs 1 pound per cubic foot of the material. Foam materials are almost always graded in terms of density. So the EPS foam that our wakesurf boards will be made from 1 pound density foam, or 2 pound density foam. Water has a density of slightly more than 62 pounds per cubic foot. Holy crap, right?! Anyway, you get the idea, anything that has a density of less than that 62 pounds per cubic foot with float in water, possibly submerged somewhat, but it will still have a portion above the water.
The perfect example is a bottle. If it’s empty, it will fall over and float on it’s side. Fill it partially with say rocks and partially with air and it will float, but some of it (where the rocks settle) will be under water. Now, fill the whole bottle up with rocks, so that it exceeds that 62 pounds per cubic foot density and it will sink to the bottom of the lake.
When we are wakesurfing, we fall in that partially air, partially rocks situation, don’t we? If you stand on your board without the boat moving, you’ll sink the board, but YOU will be floating typically above the water, probably from mid-chest up.
So what you ask? We want to draw a comparison to this honeycomb structure and EPS foam. EPS foam is very much like a bunch of random honeycomb cells in a structure.
The way that EPS foam is made, is that small beads of plastic are filled with what is termed a blowing agent. It’s a gas that will expand when heated. The beads are pre-processed to expanded somewhat. But there is a point there, they all start with the same basic amount of the gas inside the plastic bead. There isn’t MORE of the gas added to make a lighter weight foam or plastic bead. Now lighter density EPS foam typically has larger beads of that inflated plastic. Remember our concept of density? Some weight per some specific volume? So if the bead was LARGER it would have a lower density than a smaller bead.
That’s how EPS densities are managed. Lower density EPS foam has a smaller number of LARGER beads and higher density EPS foam has a LARGER number of smaller beads. Over a specific volume measurement, the higher density EPS will have more plastic because there will be more beads jammed in that same space.
What’s interesting and makes this sort of difficult to process for most of us is that the lower density foam beads don’t have more internal gas than the higher density beads. That gas is LARGER but it’s the same amount. Whattttt?????!!!! It gets expanded when it heats up, but that cell is sealed and so the gas doesn’t escape or get replaced, it just gets BIGGER.
So larger EPS beads become more buoyant only due to their change in shape. The larger beads increase in volume and hence change density, but weight is the same as the smaller beads and the amount of plastic and of the internal gas remain the same between both the larger and small beads.
Now is that true with a larger honeycomb cell? It isn’t is it? If we say a 1 cubic inch honeycomb cell is too small and we increase the cell size to 2 cubic inches, what happens to the amount of air or internal gas? It’s doubled hasn’t it? So while we have doubled the volume, we’ve done that not by expanding the air molecules, but by adding MORE and our honeycomb cell isn’t sealed upon construction, only later after it’s built.
SO, what happens is in comparison to EPS foam, and it’s associated plastic beads, is we actually add MORE of the lighter material (internal gas in this case air) into the larger cavity of the increased honeycomb cell. In effect, we wind up with the honeycomb structure being lighter and stronger than EPS foam, at least in resisting forces in certain specific directions. The EPS beads, being complete spheres will have some structural resistance in all directions. In fact pentane gas, the blowing agent in EPS foam is in a liquid state at room temperature, so is obviously a higher density than air, by itself.
Fun with physics! Ok, enough of that for now, the point we are trying to make is that varying the size of the honeycomb structure can dramatically improve buoyancy in our test panel.
Thanks so much for following along, we REALLY appreciate it!