Surfboard Buoyancy Testing has developed and applied for patents on a standardized buoyancy test for surfboards. Whether a board is made from EPS, epoxy composite, hollow carbon fiber, “S-Core”, or glassed from the dozens of different polyurethane and other blanks available, our “BUOYANCY RATING” will make it possible for you to compare and choose the board most suited to your body weight, surfing ability and wave conditions. SBT has found there is a relationship between a board’s buoyancy and the surfer’s body weight and surfing ability. Selecting a board outside a certain range can hinder performance. It is now possible when purchasing a new board to choose + or - the buoyancy and find out what works for you. There can be a dramatic difference between an identical EPS and standard PU board. With a buoyancy rating you can pick either board based on the buoyancy instead of guessing using the length + width + thickness. Board building has changed significantly in recent years, why shouldn’t there be more specifications when choosing equipment? Retailers as well as the average surfer will find it useful using our “Buoyancy Table” to help choose their next board. Please visit our website for a more in depth look. www.surfboardbuoyancytesting.com . We are currently looking for board builders and selected retailers to add a Buoyancy Rating to their boards. For questions or comments please contact: Curtis Courian at info@surfboardbuoyancytesting.com or phone +64 7 572-1474

I give all my boards a buoyancy test. It’s the belly button test. When I’m sitting on the board it has to float me at the belly button. Give or take an inch or two. Mike

two shortboards with identical buoyancy one made with 21x1 3/4 " tail rocker and wide swallow tail the other with 18 1/8x 2 1/2" pin tail for a 75kg surfer. how much dynamic buoyancy do they create in 1. mundaka 6’ low tide 2. florida 3’ onshore

sorry i call it “milchmaedchenrechnung” without buying the boards

Milk girl calculation is adversely designated the financial planning of a project, with which it is to be foreseen that this will never carry the project and/or with is subordinated that it cannot carry the project, because it is based on fallacies.

The term probably decreases/goes back to the fable “the milk woman” from Johann William Ludwig Gleim. Is told if the history of a farmer’s wife, who already introduces itself on the way to the market, what with proceeds for the milk everything would be feasible, then however the milk buries. Another origin explanation is the following: To the time, when milk was gotten still in cans of farms, said one unite milk girl (milk shop assistants) after to fill up the cans with water if the milk became limited. Since they estimated the full sum naturally nevertheless as money, the term milk girl calculation developed. The first origin explanation aims in particular at the aspect of the self fraud off, while a latter fraud expresses opposite others.

Volume does not equal buoyancy. Two identical boards one made of wood and one of eps would have the same volume but not the same buoyancy

Yes, you are correct: volume factored by average density or weight equals buoyancy. Very complicated. BTW, is that a NZ or international patent you have applied for? Got a patent application number?

When looking at buoyancy we must look at two issues, weight and displacement.

The weight is determined by adding together the weight of the board and the weight of the rider. These two weights determine the amount of “negative buoyancy” created.

Next we look at the volume of the board to determine the displacement created by the board in the water. Archimedes principle states that “a body immersed in a liquid or gas has a buoyant force equal to the weight of the liquid or gas that it displaces”. The volume of the board, not the weight, is the determining factor in understanding the “positive buoyancy” created by the displacement of the board itself. The average weight of seawater is 64.1 pounds per cubic foot so that is to say for each cubic foot of seawater displaced by the surfboard will give you a positive buoyancy of 64.1 pounds. A positive number means you float and the bigger the number the higher you float. A negative number means that the package would sink, at least until the rider and board were separated when the dispalcement of the surfer’s body would create a postive buoyancy to float him to the surface.

To calculate the total buoyancy of the rider/board package one would subtract the total weight of the board and surfer, the negative buoyancy, from the positive buoyancy calculated off the total volume.

That was the simple answer, now for a few complicating factors.

The weight of seawater varies depending on temperature and salinity.

As ones wetsuit changes from winter to summer this will have a huge impact on the total buoyancy of the total package as the weight of the rider may significantly change thus changing the overall calculation of the “boards” buoyancy.

Did you eat a breakfast burrito or just have a cup of coffee before paddling out? This might make more difference on the total buoyancy question then a change from super light to ultra light in the foam.

Keep in mind, how a board floats in the water with a rider and how it glides across the waves while moving are two different things. Once the board is moving bouyancy may be less important then the boards ability to plane well.

Just a few thoughts. Don’t know if any of this helps.

Yeah, water salinity and wetsuit are big factors. My boards fly on the Gold Coast where the water’s warm and salty and I’m 10lb lighter sans wet suit. But they’re still perfect at home in winter with the 5,4,3. I just make sure that they’re around 39 liters volume. I puff the rails and widen the tail for small wave boards and draw the length for bigger waves, but I keep the volume somewhere around 39 liters for my 78kg (170lb?)

Tail kick, width and thickness affect the paddle and ride more than a litre or two of volume or a couple of pounds one way or the other - fitness level is another huge determinant, so some sort of matrix is in order: how fit are you, how heavy are you, how heavy footed are you on the board, what waves do you surf…(hey, that sounds just like a shaper!)

BTW how does a spoon = minimal buoyancy - fit in?

Come up with a standard way of measuring flex (epoxy vs urethane, for instance) and I’d say you have a winner.

The principle of bouyancy explains why a sailboat stays above the surface. The force that water exerts on any immersed object is called a buoyant force. This force counteracts the downward gravitational force that acts on the object. This downward gravitational force can also be called weight and is equal to (“mass” x “acceleration of gravity”)or “mg”. The force diagram for this is shown below. The hand represents the downward push of the “mg” force and the arrows represent the upward force that water exerts on the boat. When these forces are equal, the object will float in static equilibrium.

(Picture from “The Annapolis Book of Seamanship”, Rousmaniere, 1999.)

So when a boat is floating in static equilibrium, the following equation holds true:

B = Fg = Mg

Where B is the buoyant force, Fg is the force of gravity, and Mg is the weight of the object. More importantly, the total weight of the water displaced is equal to the total weight of the object displacing the water (Archimedes’s Principle). Therefore, “displacement” is what we call the volume of water that is moved aside by a floating boat. Displacement is recorded as the weight of the volume of water moved.

(Picture from “The Annapolis Book of Seamanship”, Rousmaniere, 1999.)

We can take this a step further and see that if B = Mg and M =pV, then B = pVg. Where V is volume, p is density, M is mass, and g is the acceleration of gravity. So the buoyant force is a function of density and we can conclude that the density of an object determines whether or not it will float. So if a sailboat had an average density greater than saltwater (64 pounds per cubic foot) it would not remain on the surface. However, if the sailboat has an average density of 32 pounds per cubic foot, it will float on the surface of the water and be halfway submerged. This is why ships that are constructed of materials possessing much greater densities than that of water can still float. The air inside the hull ( air is 1000 times less dense than water) brings the average density of the entire ship below the density of water. Even a cement boat can float and they have![/indent]