Board weight, volume and density

General Discussion
1

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.

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Responses to my ramblings are sought.

There’s a hell of a lot more to how well a surfboard paddles and simply generalizing in one single aspect, while interesting, is somewhat impractical.

Let the games begin…

3

hi enuenu, i just wrote something similar a couple of days ago on the firewire thread, resistance to bouyancy is what i imagine it to be ,the volume of the board really does/nt need to be known just a scale of how much weight is needed to sink it a foot ,here is what i wrote,

,your comments on volume i understand,windsurfers used to have their volume expressed in litres to give an idea of flotation but i have always thought that for surfboards a better way and one which i keep intending to try would be to imerse the board under water horizontally to say 1 foot depth and measure the force it takes to hold it there,this would be more like a resisstance to bouyancy test and in this way it takes into account the different materials and cores in use ,two boards the same size one made from lead the other single 4oz have the same volume but wildly different resisstance to bouyancy ,

if it was possible to have a standard ie 20 deg water ,1 ft depth and a scale 1-10 it would take a big part of the guess work out of picking a board with the perfect bouyancy,for instance the surftechs,people say go a few inches shorter and slightly narrower but if in the specs it gave a figure on the 1-10 scale you could match perfectly,pete

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peteuk, you make a very valid point. I tossed my ideas onto a boat building forum and one bloke came up with a similar idea. Apparently there is a standard measurement that naval archtitects use based on force required to lower a hull in the water. Actually measuring it would be a little problematic. Perhaps there is a mathematical formula that would allow you to accurately predict “force required to submerse 1ft” based on board volume and weight.

craftee, I reckon if you had accurate measurements of volume and weight (along with length, width and thickness), you could make a more informed opinion of a board. It would also make comparison between boards easier. For example you could say “OK, this board is 120g heavier than my last board and has 50cc less volume”, it is a small difference that I think you would notice when surfing. However it would be difficult to make such a comparison in the shop based on the board data you are presented with currently. I acknowledge there are other factors, namely shape, at play. Incorporating this variable, while entirely valid, would however increase the complexity of creating a comparison scale by a massive amount. Just knowing weight and volume and perhaps developing some sort of “floatation scale” (based on the physics of buoyancy) would put a surfer in a position where he/she could make much more informed decisions than they can at present.

PS peteuk - could you provide a link to the firewire thread you mentioned please. I am considering buying one when they become available here and would be interested to read it. Thanks.

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http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=58&p=2&f=G&l=50&d=PG01&S1=surfboard.AB.&OS=abst/surfboard&RS=ABST/surfboard

6

So, someone has had the same idea. It just makes so much sense. It was hard to believe that no one has ever had the same thoughts I have. Why manufacturers are not interested in the concept has me stumped. Currently buying a board is like buying a car and not knowing the power and fuel consumption specs. Maybe I should contact this bloke and help get this project off the ground (as I haven’t seen the results in the surfboard market yet)? I wonder if patent laws would stop me developing something similar in Australia as it is quite a generic concept, just involving making measurements. I believe in this concept. Whether surfers in general are interested is another matter.

7

Back in the day (60s? early 70s?), there was a brand whose boards with volume measured. But no longer.

I don’t think most surfers care that much, the typical off-the-rack buyer just looks for the logo, stickers, and paint job if any. And that it be pretty much like his last board.

Most “hotties” buy and ride boards with crappy flotation, and willingly pay the price in poor paddling, less than useful wave catching capabilities, etc. because their tiny little board, once they clamber onto a wave, “rips”.

Myself I like a little more flotation, so my own boards are a good deal larger than most of the guys I surf with. I’m almost always the furthest guy out, and unless someone comes into the lineup from less well-mannered spots, when the set comes, my patience will pay off.

8

Unless I’m still completely asleep this morning, flotation can be directly calculated from volume and weight, so why would an additional measurement be required or useful?

-Samiam

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Knowing the bouyancy of a particular board would tell you little about it’s performance. There are so many factors involved, I could shape you two boards with the same basic measurements and bouyancy but I could make one paddle and surf pretty good and the other a complete dog (probably they’d both be dogs if I was making them, but indulge me for arguments sake) So I can’t think of many situations where this information would be useful, maybe if a shaper had a particular model he wanted to scale up or down for different weight riders?

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It seems I am in the minority of people who think it would be informative to know the hard facts about these aspects of boards. Fair enough. Maybe it would tell you little about performance, but we don’t know because no one has ever analyzed the data. Maybe by measuring many boards over time in this way some insights would be revealed. Perhaps in low volume boards that hotties ride it would be even more beneficial to have accurate data. I heard of a pro going through a selection of new boards “nahh too thick, nahh too thin, nahh too thick, oh this is a good one”. The bystander to this saw no real noticeable difference between the boards. Would have been interesting if each of theses boards had volume and weight dimensions. Knowing these dimensions certainly wouldn’t hurt. I’m not saying such data would provide a silver bullet for choosing a board. As I have noted, shape does play a huge role. However knowing volume and weight would be a useful tool to add to the toolbox when selecting a board. The tow-in brigade seem to be taking a keen interest in board weight lately.

As for volume and weight being a measure of floatation, it seems the “ability to float mass” is indeed based on these parameters. But my investigations have led me to believe the physics is a lot more complex than it seems at first glance. If anyone has a formula for calculating something useful I would be interested in hearing it.

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There’s some good stuff here, I like it. Has anyone contacted a naval architect? They are dealing with this stuff all the time and there are computer programs to do all the work. It is interpreting the results that is so difficult. There are SO many variables…

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The formula you are looking for is (Volume * Specific Weight of Water) - Weight. Note that water is virtually incompressible, so the force required to keep a surfboard submerged is the same no matter what depth.

Example:

A surfboard has a volume of 3.5 cubic feet and weighs 20 pounds. The specific weight of sea water is 64 pounds per cubic foot.

Force to submerge = (3.5 ft^3 * 64 lb/ft^3) - 20 lb = 204 lb.

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A salt water lap pool where a shopper could compare, sit, paddle & duck an otr board before buying? How a shop could offer this without waxing up brand new sticks, might be a problem with my idea. Probably need to have the sides of the pool well padded, also :wink:

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Quote:

The formula you are looking for is (Volume * Specific Weight of Water) - Weight. Note that water is virtually incompressible, so the force required to keep a surfboard submerged is the same no matter what depth……

Approximately true at shallow depths (i.e. typical depths of submersion of surfboards). But to be more accurate, while water is nearly incompressible, a surfboard isn’t. Try weighting down your board to the point where it sinks; lower it to a depth of 200 ft; try to pull it back up (or just try to keep it from sinking deeper); still think the floatation has remained the same? …if you do manage to hoist it back to the surface do you still want to ride on it?

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Yes, though that’d be kinda fun to see. Wasn’t it the Trieste bathyscaphe where they took down some light plastic foam objects of some kind and by the time they’d brought them back up said objects were a lot denser and a lot smaller.

Now, at 200’ or so, that’d have a pressure of something like 6 atmospheres on it. Kinda like the pressure ding from hell. It would actually be kinda interesting to do this, to see where the structure would rupture.

Now, as somebody called for a NA, as a lifelong student of Naval Architecture/Marine Engineering*, will I do? Along with my friend and mentor MTB; who I happen to know is also a lifelong student of that plus physics and fluids and a helluva lot more?

A few things -

Figuring out the weight of a board is easy, as is figuring out its displacement/density. First, you weigh the board on a common spring scale. Then, place it in a vessel of water that’s full right to the top, submerge it completely and measure the volume of what overflows. This, by the way, is Archimedes Principle at work and the origin of the cliche ‘Eureka’. If you’re using fresh and reasonably pure water the volume can be determined by weight: 1 kg of water displaced = 1 liter of volume. Density is simply weight/volume. Like the densities of various foams: X pounds per cubic foot, or grams per liter.

Now, that’s neat, and it can give some useful information. But you want to bear in mind that two boards of identical volume and density can have radically different shapes, say a block of foam roughly glassed and a finished surfboard shape of the same volume and weight. Density might give an overall indication of the ultimate strength of the board…or it might not. The overall floatation potential of a board could be determined by the total displacement it has. But that’s about it.

Awright, ease of paddling - first off, paddling to a plane isn’t gonna happen. Planing speed is defined as ( in feet and miles, I will leave it to someone who has a calculator handy to make the translation to real units ) > 2 x the square root of waterline length ( in feet ) expressed as statute miles per hour. For instance, a 9’ board would have to be going 6 MPH or faster to be planing, a 4’ board at 4 MPH, etc.

Less than or equal to 1.34 x the square root of the waterline length in feet, expressed as statute miles per hour, that’s displacement hull speeds. For a 9’ board as above, that’s 4 MPH. This, by the way, means that there is no such thing as a displacement hull surfboard, no matter how the term is used. All surfboards in use on a wave are travelling and acting as planing hulls. Calling a surfboard a displacement hull is gonna get any naval architect ticked off and convince him or her that you have no idea at all what you’re talking about. To put it kindly, said NA is gonna think that there really IS a Jeff Spicoli.

Next - the displacement per unit immersion: in boats it’s usually expressed as pounds per inch immersion or kg/cm or similar - that can be calculated by figuring the area of slices of the board that might resemble fillets of a flounder.But the shape of an average surfboard changes so much in that one inch slice that it can’t be calculated but would better be determined by test. And while it might be nice to know how much the deeper the board goes when more weight is put on it, its not gonna help give an indication of performance.

Okay - so what’s a poor boy to do with the few tools available? A modest suggestion:

Determine the displacement of the board in question. Weight it to a standard percentage of that displacement, say 80%. Then, find a river.

You’ll need a bottle or something else that floats, a stopwatch, a long measuring tape and a pal to handle the bottle. Plus a spring scale, the board I mentioned above and a fair length of rope or Real Strong Fishing Line.

You take the bottle, chuck it in the river at one point and time how long it takes to get to a second point - this gives you a speed that the water is moving at: viz-

Okay, having determined the speed the water is moving at, you then take your weighted board, attach it to a line and add the weights. Attach your spring scale and let it pull against the line and take a measurement. Like so;

This gives you a number for drag forces, how much power it’ll take to push the board at the speed the river is going at with that weight on it. Try it at several points in said river where the flow speed varies and you will find that once you get over 1.34 times the waterline length (displacement hull speeds) the power needed goes up a lot until the board starts planing.

And therein lies the fun. See, the thing is that the human body isn’t all that powerful. At most, and we are talking champion cyclists and rowers here, the body develops no more than 1/4 horsepower over any length of time. Note that I said cyclists and rowers…that is using the arms and most especially the legs to generate that power. A kneeboarder with webbed gloves and Great Big UDT fins might be able to put that power to work efficiently, but paddling like a standard surfboard??

With arms alone, paddling, no paddle or webbed gloves, etc, I really doubt that a paddler, even a long distance paddler or Laird Hamilton ( and that sucker is freakin’ built) can generate anything over 1/10 HP for any length of time. The power to drive a planing hull, at planing speed or better, is a quasi-linear function of the weight if I remember right. Double the weight and you double the power requirement.

But that experiment I suggested above would give some interesting results regarding overall drag of a given surfboard hull - it’d be worth it for that alone, much as Naval Architects/Marine Engineers tank test hulls or models of hulls, to find out if their calculations of the power needed to drive said hull at a given speed are correct. It could give some useful measurements that might indicate some aspects of the performance of the board.

hope that’s of use

doc…

  • a Naval Architect is necessarily a Marine Engineer, unlike shore-bound house architects who are often failed engineering students who later studied art history and rely on real engineers to make sure their pretty buildings don’t fall down and have some utility at what they were supposed to do. A NA/ME engineers a vessel from the get-go. Beware of ‘yacht design school’ graduates, as they usually haven’t been trained in the real engineering aspects of it.
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Awesome stuff Swaylockers! This is the sort of stuff that if surfboard manufacturers really start getting stuck into, surfboards will improve in performance for sure I reckon. It would take a while to develop the science, but you have to start somewhere. I notice the people at FCS have started using test flow tanks for hydrodynamic testing of fins with good results they reckon.

Paul_Kotsebue, you said “so the force required to keep a surfboard submerged is the same no matter what depth.” Good point.

MTB, while interesting I don’t think compression of the board will be a factor as it is only submerged slightly under normal usage. What you say is perfectly valid though.

Doc, you are on fire mate! Applying some real science to the “art” of board design is what interests me. I’m not knocking “art”, but combining “art” and “science” creates greatness. You bring up the concept of shape having a big impact on “paddle-ability”, which would correlate to some extent with how it would perform while being surfed. The speed differences between paddling and surfing would destroy the correlation to a certain extent I am guessing. Yes, measuring the resistance to propelling the board forward is a very meaningful measurement. I bet if a design team had access to flow tanks and sensors they could come up with some interesting conclusions. As you mention, a river is the next best thing, it would be interesting to get some results of such an experiment.

While I would love to see some in depth flow tank experiments, realistically this probably won’t happen unless Quicksilver or Billabong start investing in such research and development. So I will go back to my humble “floatation/buoyancy/volume/displacement/weight” measurements. Doc, I believe that displacement and volume are different (unless the board is totally submerged). For example a wafer board will be totally submerged when sitting on it, thus the displacement and volume would be equal. However on a big mal that does not fully submerge when sat upon, the displacement is a measure of the portion of the board that is submerged. Do I have this correct? I guess planing displacement is important in surfboards. The point about displacement hulls versus planing hulls was enlightening. I guess we know this stuff on some subliminal level, but need an expert like you to point it out.

Doc, you said “The overall floatation potential of a board could be determined by the total displacement it has. But that’s about it.” This is getting close to the essence of what I am driving at. Let’s look at some examples. Again let’s ignore shape, I know it is vitally important and a huge factor but it adds a huge amount of complexity. You would have to use your eyes to allow for shape. Maybe one day you could get a computer program to analyse the shape data (like the data a CNC shaping machine uses) that would spit out a drag factor for that shape? Until flow tank testing results are collected and interpreted that remains a fairy tale, but it is do-able. OK here are the examples (don’t forget I am ignoring shape, except in the last example);

1) Two boards of equal volume, one is slightly heavier. It won’t float as well and therefore not plane as readily or paddle as easily, but it will have more momentum once moving.

2) Two boards of equal density. One is a 6 foot thruster and the other a 6 foot 10 inch thruster. The 6 foot 10 inch will paddle and plane easier.

3) Two boards of equal weight. One has more volume. It will float, paddle and plane easier.

4) Two boards with the same bottom shape and outline, one thicker than the other. The thicker one has 200cc more volume and weighs 150g more. A 60kg bloke rides the thinner one and a 70kg bloke the thicker one. Assuming the same paddling power, which one will paddle faster? Which one will go faster across the water at surfing speeds? Being able to reach an accurate answer to this question quickly and easily would be groovy baby.

I originally thought that “force to submerge” would be the best all round measure of buoyancy. I am still leaning towards that measurement for its simplicity. But maybe the “displacement per unit immersion” mentioned by Doc is more meaningful? Again the surfers’ weight throws the spanner in the works. Like I mentioned in my opening post, a 40kg kid won’t notice as much difference in paddle-ability or planing speed between a 5 foot 10 inch and a 6 foot 4 inch board. But an 80kg man would notice a huge difference between these boards. This is why I wondered whether the curve of “force to immerse” versus “board volume” would be linear. Buggered if I know.

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I just knew a naval architect would get this going, great post DOC. As a dinghy sailor one of the big things we watch is TRIM. Keeping a planing boat flat both laterally and fore/aft is vital to top speed. The wonderful tow / river experiments mentioned above would need to take into consideration the longitudinal trim of the board as the drag factor increases greatly with improper trim…am I right or just burbling for the hell of it?

18

Awright, let me tackle these in reverse order;

Rikds, you’re absolutely right in both your use of the terminology and in your thoughts on trim. A poorly trimmed boat needs a helluva lot more power to plane, if it ever does plane, than one that’s trimmed right. Same deal with a surfboard, somebody going slow and standing right on the tail is gonna stall and stall badly in most cases.

Now, enuenu, there’s a lot of stuff. Let me do these one at a time.

Quote:

You bring up the concept of shape having a big impact on “paddle-ability”, which would correlate to some extent with how it would perform while being surfed. The speed differences between paddling and surfing would destroy the correlation to a certain extent I am guessing. Yes, measuring the resistance to propelling the board forward is a very meaningful measurement. I bet if a design team had access to flow tanks and sensors they could come up with some interesting conclusions. As you mention, a river is the next best thing, it would be interesting to get some results of such an experiment.

Okay- the differences between paddling as a displacement hull and surfing a planing hull are important. See, a flex spoon kneeboard, with barely enough float to float itself, will surf just fine. As will a plywood paipo. For how it acts as a planing hull is down to the planing area - that area of the board that is on, not in, the water. It can be rounded ( the so-called displacement hulls) or vee-shaped or flat or combinations of all that. The shape of the planing area contributes to how fast it can go more by skin friction changes than by the shape itself. Though they have other things going on.

Vees, for instance, tend to be nice in chop 'cos when the boat (let’s use boats as our examples here) jams into a chop the planing area doesn’t increase radically and throw the boat towards the sky. Flat bottoms plane easily and economically, though in chop or large seas life gets interesting. Rounded bottoms are kinda in between, with no fast transitions.

But for our purposes, lets say that all surfcraft are flat bottoms, which they are, at least compared to round bottom or vee bottom boats.

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While I would love to see some in depth flow tank experiments, realistically this probably won’t happen unless Quicksilver or Billabong start investing in such research and development. So I will go back to my humble “floatation/buoyancy/volume/displacement/weight” measurements.

Well, the industry is happily selling the bejeebers out of boards-as-they-are, so why would they want to make a science out of it? Especially when only the damn kneeboarders will be able to understand the numbers anyhow. Bear in mind that mainstream industry is, in essence, a con job, hype masquerading as knowledge and helpfulness to peddle more cr@p . They could care less, unless it’d let them write off a bunch of surf toys , trips and wild parties where major surf company executives can get drunk as owls and chase sweet young things.

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Doc, I believe that displacement and volume are different (unless the board is totally submerged). For example a wafer board will be totally submerged when sitting on it, thus the displacement and volume would be equal. However on a big mal that does not fully submerge when sat upon, the displacement is a measure of the portion of the board that is submerged. Do I have this correct? I guess planing displacement is important in surfboards. The point about displacement hulls versus planing hulls was enlightening. I guess we know this stuff on some subliminal level, but need an expert like you to point it out.

Awright - a few things here too.

Displacement and volume ( total volume, that is) are indeed different. but the amount it can float and the displacement are one and the same. If, lets say, you have a board that’s floating 80 kilos, then it is displacing

(80/[the density of local salt water])liters

The amount of the board that is out of the water is irrelevant. It doesn’t figure into the equation. A humongo board that isn’t all immersed won’t be any different , paddling wise, than the same board with the un-immersed part hotwired off.

Now, there is no such thing as ‘planing displacement’ - it’s not just apples and oranges, it’s more like apples and bricks. Two very different things. In planing, it’s all about the area and the weight and the power needed.

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You would have to use your eyes to allow for shape. Maybe one day you could get a computer program to analyse the shape data (like the data a CNC shaping machine uses) that would spit out a drag factor for that shape?

Actually, there are already software setups for such stuff, but they run on Crays and cost zillions to develop. If you have an insane budget, like America’s Cup boats, then you can do such stuff. But for somebody futzing around with a sufboard, it’s not gonna happen. Best we are gonna do is maybe model testing.

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OK here are the examples (don’t forget I am ignoring shape, except in the last example);

And here we go…

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  1. Two boards of equal volume, one is slightly heavier. It won’t float as well and therefore not plane as readily or paddle as easily, but it will have more momentum once moving.

Okay, if ‘slightly heavier’ isn’t a whole lot, then lets throw some numbers in. Lets say they are 5 and 6 kg boards, respectively, not out of line for weights of an 8’ board. Lets say the rider/paddler is around 70 kg in both cases. The difference is tiny, overall, as is the difference in momentum. Furthermore, if the heavier board won’t go as fast, it won’t develop the same speed as the lighter board so the momentum ( mass x velocity ) may well be less.

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  1. Two boards of equal density. One is a 6 foot thruster and the other a 6 foot 10 inch thruster. The 6 foot 10 inch will paddle and plane easier.

But not that much better. Lets throw in a 20’ paddleboard of the same density, which will paddle like a bandit. See, then density kinda becomes one of thhose ‘that’s neat, but…’ deals. Density could indicate a stronger board, but that’s about it. Overall volume ( floatation) and planing area are what’s gonna drive paddling and planing.

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  1. Two boards of equal weight. One has more volume. It will float, paddle and plane easier.

However, it’ll probably be weaker. It will float more, may or may not paddle easier ( overall shape will govern this more ) and as to how it’ll plane, that will be determined by the shape of the planing area.

Let me throw a pet theory of mine into the mix. Two boards, lets call 'em a thruster shortboard and a longboard. Both planing and travelling at equal speeds, trim and angle of attack on the wave, the weights of rider and board combined are the same in both cases. Is the area of board that is actually in contact with water the same on both?

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  1. Two boards with the same bottom shape and outline, one thicker than the other. The thicker one has 200cc more volume and weighs 150g more. A 60kg bloke rides the thinner one and a 70kg bloke the thicker one. Assuming the same paddling power, which one will paddle faster? Which one will go faster across the water at surfing speeds? Being able to reach an accurate answer to this question quickly and easily would be groovy baby.

Okay- this one sounds like fun.

The 50cc additional buoyancy of the thicker board ( just a little over 50 g additional float) isn’t gonna matter much, especially when you consider it’s gonna be an overall volume on the order of several liters. The way they surf will be more related to thicker rails and the planing of the boards with more weight per unit of planing area.

However, lets play with this example some. Lets say Mister 60 KG is on a board that’s 240 cm long and Mister 70 KG is on a board that’s the exact same shape but scaled up ( in all dimensions) so that it’s 280 cm long, also 7/6 as wide, as thick, etc. Whats it gonna do now? Will they behave the same?

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Like I mentioned in my opening post, a 40kg kid won’t notice as much difference in paddle-ability or planing speed between a 5 foot 10 inch and a 6 foot 4 inch board. But an 80kg man would notice a huge difference between these boards. This is why I wondered whether the curve of “total weight of board + surfer” versus “board volume” would be linear. Buggered if I know

Well, lets think about that. Would the 80 kilo guy notice as much as the 40 kilo kid? Especially as the overall areas and volumes versus weights are gonna be relatively small for him. Lets say the boards have bottom areas of 1.0 and and 1.25 square meters, respectively, volumes of 7 and 8 liters likewise. Pulling these numbers out of my hat, but they are probably something like right.

Ok, playing with numbers, you got planing area loads of

Kid - 40 kg/sq meter and 40/1.25 kg/sq meter - about 32.

Guy- 80 and 64 respectively. both of these are relatively high.

It would be useful to find just what planing area loadings can be used, what’s too high and what’s so low as to be silly.

and on that note, off to work.

doc…

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You rule doc. Thanks a heap, I’ve learnt a lot from your great posts. I think you really hit the nail on the head when you said that extra R&D probably wouldn’t affect board sales, people buy them as they are now. Therefore bigtime R&D won’t happen. America’s Cup contenders we are not and never will be. The science fascinates me but I guess realistically it will be art that continues to rule surfboard design, and maybe that’s not such a bad thing.

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Hi DOC

Quote:

Let me throw a pet theory of mine into the mix. Two boards, lets call 'em a thruster shortboard and a longboard. Both planing and travelling at equal speeds, trim and angle of attack on the wave, the weights of rider and board combined are the same in both cases. Is the area of board that is actually in contact with water the same on both?

Surely the longboard would have greater surface area in contact with the water, and won’t rocker have an effect. What am I missing?