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Old 6th March 2008, 03:35 AM   #1
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Default Why Quote Volume Not Displacement?

Everyone knows that the buoyancy of a board is based on the displacement characteristics and NOT the volume.

I appreciate that quoting the volume for a 'sinker' is relevant because the board is fully underwater then the volume is the displacement, but what about all the other boards? For all the other boards, there is only a minimal link between volume and buoyancy. In many cases it is misleading.

During my earlier naive days, I used the industry's standard buoyancy data (ie width and volume) and bought a new Fanatic Falcon 104 thinking it feel similar to my previous 100L boards. Little did I know the Falcon 89 was much more comparable to my previous boards.

Anyway why does the industry mislead everyone and quote volumes?
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Old 6th March 2008, 04:34 AM   #2
JayBeeJB
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I think it is more accurate to say that the buoyancy characteristics of a board are related to volume distribution.

Any floating mass will displace a volume equal to its mass. Example: An 8kg board will displace 8L of water (1L = 1Kg).
From this you will note that quoting "displacement" is pointless as it is the same as the mass of the board.

The mass of board+rider+rig will tell you how much water you need to displace in order to float - nothing else.

This issue of volume not giving you enough info is the main reason most manufacturers quote "width" as the primary differentiator for boards within a style now. You will also notice that Starboard are also looking at the distribution of thickness as a function of volume this year (Futura etc).

JB
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Old 6th March 2008, 08:37 AM   #3
matt12
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QUOTE "From this you will note that quoting "displacement" is pointless as it is the same as the mass of the board."

I think the original poster means the displacement not with just the board weight and not with the board fully submerged (because at that point the displacement just equals the volume anyway!!!) but I think he means with a standard average rider weight. Say 75kg. Then you have a more accurate assessment of the flotability that takes into account length, width, rail shape, etc.
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Old 7th March 2008, 12:36 AM   #4
Archimedes von Karmann
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Long ago, the case was made, and borne out since then in the science of hydrostatics, that the most significant objective measure is the maximum buoyancy, which would be the volume times the density of displaced fluid minus the weight or the immersed object... And this tells you how much additional weight the hull can support, at rest (i.e. no motion in any direction), before being immersed completely.

Once in motion, it's a different story.... other factors come into play... of a hydrodynamic nature... the basics are simple enough to be within the scope of a modern-day engineering undergraduate course... however a complete and detailed understanding can be... elusive... and as a result practitioners in the field rely on ad hoc generalizations and empiricisms, rather than rigorous proven theory... which is why board shaping and design are said to be more art than science. One hundred years ago, the same was said of aircraft design, which is based on related principles.
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