Buoyancy Thread

General Discussion
1

This horse has been beat pretty good but here goes....

From Yellow/Marko thread:

Surfding wrote while quoting his customer:

EPS on Steep Waves too much float

PU on Steep Waves Magic (Genie in a Bottle)

Need to go thinner by 1/8" on the EPS however for high tide days at Huntington might be fine?

Crafty wrote:

OFF TOPIC: Btw, there are lots of people on this forum, including the real 'sciency' types that disagree with the notion of "more float", that buoyancy is the same no matter the material and it should not be a factor. I couldnt disagree more, but that's another thread. In fact the archive is loaded with arguments for and against. Sorry to bring this up? 

BenT wrote:

Crafty or someone else,

Could you point me to an old post(that you referred to) that has a good argument about how different materials of the same density may have different buoyancy?  Or if that's not what you are saying...how boards that have 5oz difference will have a significant effect on how the board rides.

Crafty Wrote:

Is that a trick question? You know the buoyancy is the same no matter the material or density. 

 

           BenjaminThompso wrote:

           Or if that's not what you are saying...how boards that have 5oz difference will have a significant effect on how the board rides.

 

Probably doesnt. An appreciable percentage difference in board weight affects how the board rises from beneath the water, whether its real or perceived is the real question. Strange thing is, the perception is widespread, even with identical shapes, as with surfding's customers, most call it 'corky'.

 

I dont do searches anymore, not as user friendly as it used to be.

 

I think maybe you or someone else should start a thread about the subject. They sell wood balls at the craft store if you can find one that is same diameter as a ping pong ball that could make a nice video experiment, see which one gets to the surface the quickest and shoots out of the water's surface the highest. Good for another thread.

DISCUSS....

2

It seems to me that if density is the same (or close), then float shouldn’t be the cause for a different feel all else being equal.

I reckon the ride feels different, maybe the EPS doesn’t bend the same, or as much, as PU giving it the “corky” feel on hollower waves.

3

The density can be the same, meaning they float the same. So far all things equal.

 

But there can be a weight distribution factor which will affect the ride.

4

Ahmmmm-  a coupla things -

First off,buoyancy, density and volume; the buoyancy of a particular volume is affected by the density of the material in question. Say you get a wooden ball of a fairly dense wood, like oak. It will barely float, buoyancy low. Take a ball of the same volume but make it out of the low density bark of a particular oak tree. It'll float like a cork, 'cos that's what it is. Buoyancy high.Volume the same, density different.  

Two boards of equal weight, identical shape and thus equal volume will float the same. Their buoyancy will be the same. They displace the same amount of water in the same way.

And- while buoyancy/displacement will have some effect on how a board floats you, or how it paddles, but buoyancy itself won't have any direct effect on how the board works on a wave. Howcome?

Well, on a wave, a board isn't floating, it's planing . It ceased to be floating once it got going fast enough to ride the wave.  

What will affect how that planing happens is the shape of the planing surface and the combined weight of board and rider. All other things being equal (volume, shape, angle of attack, trim, the wave itself, mechanical properties of the board, weight distribution of the board) two boards of equal weight will surf the same for the same rider.

But, as was mentioned ( thanks, surfthis) , the mechanical properties of two different materials will make different boards. Differing flex, twist, rebound, etc. The planing area will be different in shape in all three dimensions. Make the rails a different shape, they'll plane ( not float) out of the water differently.

Although the perceived way they surf may well be different. But that's the problem with a lot of this, it's all 'how it felt' and not any accurate measurements.

One of the real problems surfboard design has is that the terms used are the same terms as those used in, say, boat and ship design, But they are used differently. For instance, 'displacement hulls' -in boat and ship work, these are relatively slow hulls that don't ride/hydroplane on the surface of the water, but in surfboards, well, they all do in fact ride on top of the water/hydroplane. What is called a 'displacement hull' in surfing terms  is more accurately a planing hull with a little curve in the bottom of  it.

 doc...

 

5

First, I’ll say that bouyancy and float are not the same, but most board builders use the terms interchangably. Bouyancy is a function of volume and the density of the medium. Boyancy is a force. Foat is a function of both volume and density of the object. The oak ball and the cork ball, if the same size, have the same density, but float differently. This is, as I’ve said here before, semantic. Still, I feel there needs to be some clarification on this. We should forget the term bouyancy because of it’s constant misuse. Just say float. That’s what we’re talking about, right?

As for how EPS and PU cores feel, I agree it’s all about the mechanical properties of flex and springback. When we first started building EPS boards of low density, we started glassing them heavier, which shot the flex we were after. So, we started building them thinner to start get the flex back. It’s easy to do, you don’t need to order custom stringers, use different materials, etc. From there, surfers, being surfers, noticed that the new EPS boards were thinner (and lighter) thought it had something to do with the superior float of EPS, which it did not.

In surfding’s test, the glassing is the same, so I my guess is that it has to do with flex, springback and weight, not bouyancy.

6

Haven’t looked at the B equation in a long while if there is a “W” variable I will need to retract my earlier comments. From memory its Fb - W?

Problem with it is the static nature of the analysis.

7

This topic always gets mucked up cuz the terminology always gets mixed up.

Bouyancy force is equal to the volume of the object (surfboard) times the density of the medium (water) in which it sits.

First off,buoyancy, density and volume; the buoyancy of a particular volume is affected by the density of the material in question. Say you get a wooden ball of a fairly dense wood, like oak. It will barely float, buoyancy low. Take a ball of the same volume but make it out of the low density bark of a particular oak tree. It'll float like a cork, 'cos that's what it is. Buoyancy high.Volume the same, density different.   If you completely submerge both balls of same volume, Bouyancy Force will be equal because the force is dependant of volume of the ball and the density of the water.  The buoyancy force on the floating cork ball is actually less than the floating oal ball because less of the ball is in the water!  The oak ball sits lower in the water because the the weight of the ball is closer to the weight of the volume of water that is displaces (when compared to the cork) aka higher density.  Doc I know you know all this but the terminalogy used is confusing.

Bouyancy is a function of volume and the density of the medium (RIGHT!) Boyancy is a force. Float is a function of both volume and density of the object. The oak ball and the cork ball, if the same size, have the same density, but float differently (NOT EXACTLY - the cork and oak ARE deferent densities and that is WHY they float differently.)  I think you are trying to say that float is the force that is equal to the buoyancy force minus the gravity force (weight).  Again terminology gets confusing.

I actually broke out the old fluid dynamics book so I could have a reference to make sure I was not using the terminology uncorrectly (I probably still made a mistake anyways).  Something that I came across was a section on STABILITY, and it has to do with the center of gravity of the floating object versus the centroid of displaced volume and the relationships of those points with respect to each other and the water line.  Wildy was hitting on this point by saying that an equal volume and weight surfboard can be more or less stable based on its weight distribution!!!!  This may be a big factor (as well as flex responce, etc) on why an EPS board 'feels corky'.  Even if the PU and the EPS truly are the same density so they have same center of gravity, they will still have different riding characteristics based on the material properties.  This would be really interesting to see if surfding really could get equal weight cores with a pro surfer analyzing the ride characteristics. Hard to keep track of all the variables - just shape it glass it ride it make another!

wow this post turned too long....no one is going to read it...oh well.

 

 

 

8

[quote="$1"]

This topic always gets mucked up cuz the terminology always gets mixed up.

[/quote]

Definitely - I'd love to see surf terminology more in line with the stuff that's used in the rest of the world, but that'd make it too easy. The same words, different meanings. However, let me have a shot at this, and forgive me if I misattribute here.

[quote="$1"]

Bouyancy force is equal to the volume of the object (surfboard) times the density of the medium (water) in which it sits.

[/quote]

Close

Now to really piss off surf terminology: I'm gonna make an equation of it.

Lets call the buoyancy B. Volume is V, density of the object is Do and density of the medium is Dm

Thus

B= V x ( Dm-Do)

Or, in words, the buoyancy of an object with volume V is that volume multiplied by the difference between the density of the medium and the density of the object. If the object is less dense than the medium ( say wood, or a duck, or a witch for you Python fans, versus water ) it'll float. More dense, say like a brick in water, negative buoyancy: it sinks. Careful adjustment of density ( as on a submarine in water or a balloon in air) and you can have neutral buoyancy.

meanwhile, back at the ranch

[quote="$1"]

First off,buoyancy, density and volume; the buoyancy of a particular volume is affected by the density of the material in question. Say you get a wooden ball of a fairly dense wood, like oak. It will barely float, buoyancy low. Take a ball of the same volume but make it out of the low density bark of a particular oak tree. It'll float like a cork, 'cos that's what it is. Buoyancy high.Volume the same, density different.   If you completely submerge both balls of same volume, Bouyancy Force will be equal because the force is dependant of volume of the ball and the density of the water. 

[/quote]

Ahmmm - no. What if one ball was, say, a bowling ball, or solid lead? Buoyancy would be negative, it'd sink, no? The buoyancy is dependant on not only the volume of the ball but the densities of the water and of the ball.

[quote="$1"]

The buoyancy force on the floating cork ball is actually less than the floating oak ball because less of the ball is in the water!  The oak ball sits lower in the water because the the weight of the ball is closer to the weight of the volume of water that is displaces (when compared to the cork) aka higher density.  Doc I know you know all this but the terminalogy used is confusing.

[/quote]

Okay, right idea, but not quite there.

The buoyancy of the submerged part of a floating object is precisely the same as the weight of the object. That which isn't required to be submerged to float the thing will be out of the water. Think of, say, an iceberg. Or a boat.

[quote="$1"]

Bouyancy is a function of volume and the density of the medium (RIGHT!) Buoyancy is a force. Float is a function of both volume and density of the object. The oak ball and the cork ball, if the same size, have the same density, but float differently (NOT EXACTLY - the cork and oak ARE deferent densities and that is WHY they float differently.)  I think you are trying to say that float is the force that is equal to the buoyancy force minus the gravity force (weight).  Again terminology gets confusing.

[/quote]

It does indeed. Buoyancy is a function of volume and density differences. 'Float' - look, lets get rid of 'float', okay? It's horribly misused, it can mean buoyancy, it can mean a lot of things, but it's not used consistently enough so that it has a standard definable meaning that everybody uses the same. If we want to ask how much a surfboard will float, okay, calculate the buoyancy of the entire surfboard. That's how much weight you can put on it before you start heading for the bottom.

See how 'float' is a useless term? Right. lets use 'buoyancy' instead. Lets save 'float' for an either-or situation, like 'will it float or sink?'.

[quote="$1"]

I actually broke out the old fluid dynamics book so I could have a reference to make sure I was not using the terminology uncorrectly (I probably still made a mistake anyways).  Something that I came across was a section on STABILITY, and it has to do with the center of gravity of the floating object versus the centroid of displaced volume and the relationships of those points with respect to each other and the water line.  Wildy was hitting on this point by saying that an equal volume and weight surfboard can be more or less stable based on its weight distribution!!!!  This may be a big factor (as well as flex responce, etc) on why an EPS board 'feels corky'.  

[/quote]

Ah- see, this is where surfboard 'science' has its problems. Lots of stuff like stability calculations ( metacenters and centroids of areas of inclined hulls, etc) get misapplied.

You see, stability calculations are for floating hulls. Say, a freighter or a barge or a rowboat. If your center of gravity moves towards the side the stability goes down, the way it rolls changes and so on.

But, note that I said 'floating hull'. The only times that applies to a surfcraft are when the thing is sitting still or moving slowly, paddling out or waiting for a wave, lets say.

A surfcraft on a wave  isn't floating, it is planing so that the buoyancy becomes irrelevant. Think of, say, a water ski. Buoyancy is just about zip, the things barely have enough buoyancy to float themselves at rest, let alone the happy water skier. But when up and planing , they do just fine. Planing area, aspect ratio, shape of the planing hull, those come to the fore. But poor ol' buoyancy has to wait 'til the wave is over and it's time to paddle out again.

[quote="$1"]

Even if the PU and the EPS truly are the same density so they have same center of gravity, they will still have different riding characteristics based on the material properties.  This would be really interesting to see if surfding really could get equal weight cores with a pro surfer analyzing the ride characteristics. Hard to keep track of all the variables - just shape it glass it ride it make another!

wow this post turned too long....no one is going to read it...oh well.

[/quote]

Ah, but I did: be careful what you ask for, you might just get it. And there I think you're onto something, though rather than playing with the densities, I'd mebbe wander over to the 'flex and mechanical properties' end of it. As we've seen, the density of the board doesn't have a lot to do with how it works on the wave as applied to buoyancy, but as applied to how it flexes and so on, there youre onto something.

And, most definitely, I'd lose the pro surfer as part of the testing regimen. Instruments, like stuff to measure flex, deformation, , accelleration, speed, turning force and the accellerations in a turn, those are relatively cheap and will give real information, not 'duuude, this thing feels gnarley'. Which is all you're gonna get with just somebody on it trying to remember how it went. Without comparative numbers, it's all hand-waving.

doc...

 

 

 

 

 

9

doc - you are using the term 'buoyancy' the same as njsurfer is using the term 'float' ie the difference between the buoyancy force and the force due to the weight of the object.  the term 'bouancy force' that i am refering to is soley the force exerted on a submerged or floating object by the fluid it is in. different things and again due to terminology confusion, i think you misread what i was saying.  If you completely submerge both balls of same volume, Bouyancy Force will be equal because the force is dependant of volume of the ball and the density of the water.  This is a true statement wether the ball is made of lead or of air.  Where the diference comes in is when you add the force due to the weight of the ball.  Lead ball - force of gravity on ball is much greater than buoyance force - sinks.  Air ball - force of gravity on ball is much less than buoyancy force - it floats!  Remember your old free body diagrams! Dont make me scan this text book.......

Doc I see your point on the planing nature of surfboards but you cannot completely disregard the buoyancy forces while surfing - you are not always in trim while surfing - think how much foam is plowed under the water during a powerful bottom turn - depending on the speed and the precision of the surfer, buoyancy forces can make quite a difference (think full rail vesus thin rail - it is not solely geometry at work here). Professionals can push their equipment so much harder than the average joe and put their boards into many more diffirent situations where all the attributes of a surfing craft can show their true colors. They are also in tune enough to notice the differences.

Thanks for reading and thanks for the rebuttal to give a chance to clarify my point.

10

Ahmmmm- you're right, I'm used to calculating the buoyancy of submerged stuff, such as, say, stuff for growing shellfish underwater. And I've been using the force I have to overcome ( using ballast ) to keep it on the bottom. 

It's been a long, long time since school.

In any event - I would discount buoyancy  as a major component on, say, the rail of a turning board. Dynamic forces versus static forces, as it were.  If you're thinking of, say, a thick-railed board versus a thin-railed board, both turning hard, all other conditions being equal including rail contour/shape, you would get differing sections of the board in the water, yes. But those would be sections of similar area, generating similar lift.

The thing is, you'd have similar planing areas in play, but that would mean more or less of the board's width. Depending on the degree to which the thickness differed, yeah, you'd maybe get some CG differences too, but how much, good question.

Calculating the areas.... would be a bear. Differing angles of attack along the section  that's making lift, differing areas as you go along - hooboy. Makes model testing sound like a real good idea. On something like a standing wave, a mini-flowrider setup. That also removes the problems you get trying to derive results from waves, which will differ one to the next.

And again, I'd stress instrumentation/sensors over the 'feel' a pro surfer might bring to the table. 'Feel' ain't quantifiable, you can't get real numbers from it. And to learn something that could be applied, you need to be able to get numbers out of it. Consider aircraft design, for instance....

doc...

11

Oops… good catch. I meant to say the two materials have different density, not the same denisity… obviosly.

12

agreed upon definition of terms:

doc and grasshopper have both said it, but have yet to stick to the same list ;

buoynacy:

net buoyant force:

bouyant force:

gravitational force:

displacement:

weight:

density:

fluid density:

submerged volume:

?:

pick the ones you want

add any defintion that you all can agree on for the remainder of the thread.

after that tthe math gets pretty simple.

i'll be eager to see how it all relates to marko vs yellow at 2lbs each

regards,

-bill

 

 

 

13

yep - i am just adding the the confusion like normal....what i should have written was buoyant force not buoyancy force - that should clear it up.....damn terminology

14

Have we reached the point yet where someone has scientifically quantified why some of my (low density) EPS boards like to float up and out the back of waves that I am attempting to catch?

 

15

 

That’s likely not a bouyancy issue.

16

[quote="$1"]

yep - i am just adding the the confusion like normal....

[/quote]

(chuckling)  - me too, man, me too

[quote="$1"]

what i should have written was buoyant force not buoyancy force - that should clear it up.....damn terminology

[/quote]

Well put - and it does.

Best regards

doc...

17

what about inertia?

18

I agree with this.  Change the weight distribution and you change the ride dramatically.  Why are Taj’s boards weighted? Hynson proved this back in the 60’s.

19

I use Hulls and Delftship to model forms because they do a pretty good job at a fair price - free is good.

Coupled with a bit of imagination, these programs offer up a fair idea of what the distribution of displacement - another way of saying the volume estimated by area at different stations* of the hull - does to floatation giving an idea of how the form sits in the water.

Is there a cheap (free?) program designed specifically for surfboards in common use that quantifies displacement and couples it to the form which produces a more accurate model of how the board sits and moves in the water?

 

  • a hull station is basically a cross section.  There are typically 10 evenly spaced stations per form.  Find the area of each station and apply Simpson’s Rule to calculate the displacement.
20

Uhmmmm- therein lies the fun, 'cos I don't think displacement hull modelling will be all that much use in figuring out how a planing hull goes along. As I vaguely recall, the MIT open courseware site may have had something, the only planing hull modelling stuff I could find elsewhere was at http://www.well.com/user/pk/SPApower.html - haven't played with it, but you never know...

 

hope that's of some use

 

doc....