Has anyone done accurate weight measurement comparisons of boards constructed in various ways with various materials? How would the weights between a traditional polyester 6’4” and a compsand of equal length and volume compare? I reckon board weight is an important dimension that is not generally known accurately. The same goes for volume. Ideally every board you look at buying would display accurate weight and volume dimensions. It would be interesting to then calculate the overall board density and compare this dimension as well.

I dream of developing a scale that can be used to compare the ability of various surfboards to support various weights of user. The scale would measure the comparative ability of various surfboards to support the weight of various sized people. I believe the theory behind my question is well trodden in the field of naval architecture.

Such a scale would allow surfers to determine precisely the extent to which a particular surfboard will support their weight in water when compared to another surfboard. The ultimate goal is to give the surfer a comparative estimate of how much energy they will have to expend to paddle the board at the speed needed to use it effectively. At the moment when a person is comparing different surfboards, it is only possible to estimate how well one surfboard will “float” them in comparison to another. It would be good if you knew a precise answer to this question. Surfboards do not reveal their weight or volume. They should, or give an indication of ability to float mass. Measuring volume accurately is problematic, but not impossible.

I foresee a scale working something like the following;

Surfboard X has a “FloatFactor” of 100 and surfboard Y has a “FloatFactor” of 110. Thus the prospective purchaser could quickly conclude that surfboard Y will float their body weight 10% better (and therefore be easier to paddle). I admit I am not sure how to quantify “better”. By 10% better, perhaps I mean it would sink 10% less distance when the body mass is applied to it?

At first guess I would say that a scale graduated in kg would be appropriate. The kg rating being the mass that when added to a surfboard would make the “surfboard + added mass” unit neutrally buoyant. Alternatively, maybe displacement is a simple measurement that would convey the characteristic I am grasping at? I have seen formulas used by ship designers that use a ratio of displacement to hull length to estimate resistance to moving forward.

However I am not sure how linear a scale based on “mass to make neutral” would be. I am thinking there may some exponential or other non linear aspect to a “Float Factor” curve based on this measurement as body weights, board volumes and board weights vary and interplay with each other.

Can a meaningful, linear scale for floatation comparison purposes be established if the only three known variables are surfboard weight, surfboard volume and water density? Does the weight of the surfer have to be known?

For example (I theorize), as surfboard volume decreases perhaps the effect of body weight becomes more significant to the amount it sinks (or energy required to paddle it at a planing speed). For example, variation in a passenger’s body weight has an insignificant effect on the sinking and thus hydrodynamic hull drag of the Queen Mary II when they step aboard. However on a 5 foot long surfboard, small variations in body weight will make a huge difference to how much energy will be required to make the board plane when paddling it.

Inversely as body weight decreases, perhaps the effect of board volume becomes less significant. Perhaps a 40kg surfer would have to transfer 10% more energy to the water to paddle a 5 foot long surfboard at 8 knots than to paddle a 12 foot board at the same speed (shape of course will matter, but I will have to ignore this effect). Whereas an 80kg man may have to transfer 70% more energy to equally paddle the 5 foot board over the 12 foot board, and a 120kg man 150% more. Then again the comparative weights of the 5 foot surfboard and the 12 foot surfboard will also effect the equation. This is getting complicated!

As I write this, the more I am starting to think that "displacement” is the term that is critical to the concept.

I don’t have the naval architecture mathematics knowledge to be certain how the variables work together to affect sinking/paddling energy required. It may well be as simple as using a measure of “mass required to achieve neutral buoyancy”.

The variable of volume distribution may also be a factor. For example if a larger % of the surfboard’s volume is located in the areas that support the body weight (i.e. under the chest when lying on it), perhaps the surfboard will “float” them better? Even if this is the case I will have to ignore this aspect as it would overcomplicate the process. I know shape also plays a role in ease of planing, but I will also have to ignore this effect. All things being equal, a flatter board will require less energy to paddle at a planing speed than a board with more rocker.

Responses to my ramblings are sought.

Thanks.