The role of buoyancy during paddle-in take-offs

General Discussion
1

The following are some observations on the role of buoyancy during the initial stages of paddle-in take-offs. (See reference material below.)

Here’s an interesting assertion: shortboards (a surfer on a shortboard), generally sink a little as they catch a wave. Here’s the other side of that coin, longboards (a surfer on a longboard) generally don’t.

I can be more precise, the more buoyant you are the less you’ll sink while catching a wave. And by sink, I mean drop vertically with respect to the direction of gravity, which is not necessarily perpendicular to the water surface. Essentially, I’m referring to the apparent change in waterline that will occur, or the change in the percentage of the submerged region of a partially submerged body that occurs as it interacts with a wave. Its almost as if you magically gained some weight (became more dense) while trying to take-off. (My guess is that language is going to be a problem here, hopefully the reference below will help.)

Is this important in design? I’m inclined to think so, on a number of levels, but right now I’m more interested if anybody else sees this as an issue.

I think a reasonable argument could be made that the first wave of commercially available closed cell/epoxy boards tended to be significantly more buoyant than prior boards, as all other design features tended to remain the same. Some builders are now adjusting board dimensions to take better advantage of the new materials, but the ‘added’ float still seems to be a selling point.

In general, you may see added buoyancy as a minor concern during take-off. But I’d argue that it becomes far more critical the later and steeper the take-off. The question is why? In part I tend to see it as matter of what collectively amounts to as an increase in stability during the early moments of take-off. Anyway, I’d be curious to know what others think.

Also, if you do actually sink a little, then you actually suffer a little more drag in those early moments of take-off, which is also kind of interesting. I mean how could that be a plus?

kc

Some reference material:

A feel for the changes in hydrostatic pressure on a buoyant partially submerged object during the passing of wave can be found at

NEW!!! Buoyancy Demo download (11/17/09) kc

It’s not a complete picture, and the author’s model doesn’t account for all the dynamics, but it is close enough – not to mention a nice toy. You can download the application and run some fairly interesting experiments. You can edit/create your own *.bob files. I suggest you just start hacking and see what happens. And by the way, try keeping the difference between bob and liquid density to something that reflects the density difference between surfer/surfboard and sea water, which isn’t much.

2

Kevin , I don’t think I have the intellectual firepower to contribute much to this discussion but I sure as hell think it is highly relevant and pertinent to surfboard design. Hopefully some of the Einsteins on this forum will weigh in. Thanks for bringing it up (so to speak).

3

my 2¢ as I head out the door for some R&D…there are a lot of factors in wave-catching, beyond pure paddle-power: rocker, length, width of tail, did I mention rocker?

As far as pure float goes, imagine trying to paddle and catch waves on a really buoyant yoga ball…

4

just watch the classic wave tank demo with a cork and you can see why.

the higher you sit on top of the water’s surface the more likely the force of the wave will just pass you by. All my surfing experiences in these super light and floaty EPS boards only confirms the hydrodynamics I should know better…

Super bouyancy also has a preference to bounce up out of the water to sit on the surface versus getting caught in the circular rolling energy flow that’s happening under the surface. So the deeper you sit in the energy flow the more chance you have of making use of it like propeling forward or down the face.

That’s why you get more energy to speed conversion staying close to the pocket versus being way out on the shoulder where the energy circle is less defined and less vertical. Your ongoing gravity argument I would guess.

You can compensate for the bouyance problem through design but too much bouyancy is still an issue in riding waves. That’s why the bigger the wave the better the pay off having a little more weight.

For comparisons you can do the following test:

catch a wave on an air mattress, then on one of those cooler styrofoam boards from the discount store, then a surfboard board, then a boogieboard with fins then just body surf with big fins and see which ones have the easiest time catching reasonably sized waves .

Pictures of the boys catching waves at pipe in an inflatable raft/swimming pool is another example…

I’m not a board designer just an old fart rider with some common sense and little physical oceanography schooling to conflict with my surfer mindset.

5

Thought about this very topic today (in a lennox76 kinda way – me engineer, not hydrodynamicist), and may have unwittingly done some experimentation.

We had some good offshores this am, and my friend and I were out on almost identical boards (he had the original, I had the copy of his board that I had made).

The copy is as close as I could make it to the original, minus a chunk of volume, I’d guess 4-5 liters. After a few waves, we switched, and suddenly it was significantly harder to catch waves – I completely missed the first one I tried for, wrote it off, and then had to up the catching effort by a few notches to get the second. When I did catch the wave, I found myself dropping in from much closer to the lip – made for a fun but sketchier drop.

Switched back, and caught the next wave easily.

I figured it was just a “I’m not used to that board” thing, or maybe a “more buoyancy exposed me to more offshores” thing. What you and oneula posted really makes a lot of sense to me, though, so I guess I’m going to believe that…

6

The thing I’ve noticed is that if I’m on a more buoyant board, and the waves are more hollow, the board tends to ride up the face of the wave on critical take-offs and the risk of getting pitched with the lip or getting stuck in the lip increases. With less floaty boards, the board tends to stay lower on steeper faces and you don’t pop up while getting sucked up the face of the wave or pitched. On waves that aren’t as hollow or critical, the boards with more volume seem easier to get into waves with due to increased paddle speed.

7

Remember when the style of board eg eightys truster twin or single would allow a surfer to pop onto a wave. just dig it in and jump on.

8

Interesting, Ron. I don’t find that effect due to buoyancy. I have the same rocker and rails on 2 6’2" boards that have 4-5 liter volume difference and they are comparable in those situations. The buoyant one gets me to the lineup quicker, but I think it’s a tad more difficult to paddle into hollow waves (but a dream when in mush). I’l concentrate more, nest time I surf, to see whether I’ve missed something.

I think a bit more volume in the tail helps paddling in (domed tail profile). The tail lifts up as you get sucked backwards up the face when paddling in. This seems to help with sliding down the face earlier (getting onto a plane?). Keeping the rails fine means that there is control for the bottom turn.

But it’s hell to design migrating deck roll (flat mid to rolled tail) and still get the rail foil right.

9

I am no great surfer or shaper by any means, but i definately notice what you’re saying ron, and others. When i surf a certain hollow beach break ( bodyboarders love the place ), i get stuck in the lip, and go over the falls a lot. It’s worth it though if you can get into the waves. I have noticed though, that since surfing my chunky, over volumed twin keel consistently, that i have adapted/got better at making these late take offs in steeper waves. But i do notice that feeling of floating up ( skward ) in the last few seconds on a hollow late take off. If its really hollow, i try to take off a bit more to the shoulder, or angle my board like crazy and paddle hard.

Interesting topic, very valid to me as i like boards with more volme than i need. Dont know why, i just like easy paddlers.

10

the concept to whether flotation is good or bad has so many variables ,that there is no completely objective answer.Otherwise we would all be riding the same boards displacement is a word that should be replacing flotation when you can formulate the proper displacement lets say for one criteria fast or slow moving waves given one surfers ability then you can start trying to come up with a solution.But then the variables that can be looked at, rocker ,rails, flex ,templates, fins ect… start to add new variables into the equation so the trial and error begins.No simple answers been using styros since the early 80"s and lots of poly before that, after a while for both styles of construction you know right away what the nuances are and then if its a keeper or to let it go some were too thick some too thin wrong rockers ect… some years i was surfing all the time some years not enough.when we start to look at boards more realistically for one true abilities and not want the latest greatest then you begin a path of discovery.The MCcoy zaps are a flotation anomaly compared to some boards in thickness but i think their is a displacement formula there that might be overlooked compared to a thin let say John Carper but surfers good ones swear by both can anyone say Horan.Styro/epoxy comes in many different flavors and as the R/D grows a new feel will start to develop that will have its own words of definition rather than this current try to make it feel like poly that prevails ,find a shaper or your self and pick a materials to build in and stay with it for a while and remember if it works for you who cares what other people say they laughed at Simon Anderson he got the last laugh. aloha…

11

I don’t normally chime in on these discussions. But you have definitely raised a good issue to discuss. If I am understanding you correctly, you are talking about sitting on a board with the water level say, between your hips and nip, and a swell comes and while you may rise a little due to floatation, you also sink a little due to your enertia. And perhaps that is a good thing as it may help you catch a wave easier due to the subsurface rolling energy.

I see guys catch waves who are sitting mostly submerged and then with 1 or 2 strokes they are in it. I think this is part of your thesis.

This phenomenon may explain why many mid-length fun boards are such dogs.

Keep talking. I’m reading. The older I get, the more I am interested in this subject. I foresee the day when I’ll be struggling to catch anything.

12

Red Boards, I think you are in agreement with my impression. Boards with more volume are better in waves that aren’t steep, the mushier waves. Those boards with much less volume don’t float up the face on steep and hollow waves, which generally have more energy, making the takeoff less hairy and seem to give me a small bit more time to pop up. I can’t imagine a 4 liter volume difference in two 6’2" boards with the same rails, however. The thickness factor must be substantial on the floatier one. I was comparing my experience with a standard 6’8" thruster and a 7’8" EPS/epoxy quad. Both great boards under the right conditions, but when it’s steep and hollow, the thruster is much easier to get into waves with.

13

float doesn’t have a ton to due with catching waves, I have tried catching them on beach balls and inflated and sealed trash bags with more volume and float than a little tiny 38x 20.5"x 2.25 bodyboard, but I could paddle in the bodyboard and get to my feet just fine

14

Statics and dynamics. Bouyancy is static. Takeoffs are dynamics. You engineers run with this one, I’m going to bed…

15

Mike, I agree.

But what I was hoping to get at was the transition between the two – transitioning from the static to dynamic, which is what happens when you catch a wave. Another way of putting it is when the surfer/surfboard – water interaction changes from pure displacement effects (hydrostatic lift) to planing effects (dynamic lift).

I also agree, sleep is good…

But since I seem to be awake (… could be a dream?… but maybe that’s best left to another thread) so …

If you read through the posts there does seem to be a sort of consensus, at least buoyant boards may not the best tools to catch certain kinds of waves. But, of course is possible to adapt –i.e. develop your own techniques which overcome any problems a little extra float may cause. Lets call this the ‘skill’ solution, which cannot be undervalued.

Greg’s interpretation, together with your (bedtime) complaint, is sort of on spot. As mentioned there is a transition, a transient phase, between moving from a condition dominated by hydrostatic effects to one dominated by hydrodynamic effects – as mentioned, from displacement effects to planing effects.

Hydrostatic effects are handy during paddling, but surfing is all about hydrodynamics – planing. Density plays the big role in hydrostatic effects, flow and planing surface play the big roles in hydrodynamics.

For a given mass, density is all about volume and planing surface is about area.

Back to Greg’s comments, my guess is that all would agree that floating in the ocean is not the same as standing on a solid floor. If the floor moves, the effect is communicated very quickly. Not so for fluids, at least not as rapidly. Fluids, by definition cannot handle a stress – they flow. If you’re bobbing around waiting for a wave, as one passes under you, you will bob, but with a little lag relative to the rise and fall of the wave. The fluid moves first and as it does buoyant forces do change, but your waterline won’t keep pace, there is a lag – it’s that inertia thing, or as Newton sort of put it, a resistance to motion –i.e. mass.

Dangerous Beside (but I couldn’t resist…)

I know we all think of mass as something pretty palpable, but mass is not what is being felt, its resistance to motion. Does this mean mass doesn’t exist, or maybe that I actually am dreaming? No, mass is the proportionality constant in Newton’s first law expressing the relationship between force and acceleration. In order to measure mass, you have to apply a force.

So. for a few moments you find yourself a little lower in the water than before, or maybe a little higher, but a few moments later all returns to as it was before.

But there’s more (which is why I included the reference material.) Hydrostatic forces are out of whack on the rising face of a wave. The water surface, no longer horizontal it’s now angled. From a purely vertical view, there is less holding you up during the rising phase. (You may not appreciate the math/physics in the reference, but there are plenty of pictures, and the application is pretty cool.) The net effect is that you literally start to cut through the face, vertically, as oppose to slide down, but you also slide a little too. And how much you slide or cut depends on buoyancy –i.e. the difference in density between surfer/surfboard and sea-water.

Summarizing (it’s 6:30 AM and I’ve got to go to work soon…) is that being able to cut rather than slide when catching some waves is pretty handy, which does seem to be the consensus, at least among those that posted.

But of course there is more, and it about the changes in drag, or ‘shape-drag’ that will accompany changes in waterline, and the impact that has on the transition from hydrostatic effects to hydrodynamic effects. You know when you feel that release, that rush that tells you – ‘I’m on it… time to surf.’

Sadly, time to work.

kc

16

I agree that flotation is an issue in wave catching. Generally, more boyant for smaller/weaker waves, slightly less boyant for bigger/more powerful waves. And by boyant I am referring to volume, as boyancy is a force determined by the volume of the object and the density of the fluid.

More importantly, however, is where and how the volume is distributed. You can achieve the “sinking” effect of reduced volume on takeoff without much reduction in overall volume if you foil down the rails and the tail, but keep the volume in the middle. This adds volume for flotation, but reduces volume for control and wave catching.

17

This is really good stuff. Because the physics of surfing is so elusive, we all love to think about it.

That moment of transition when we ‘‘catch’’ the wave (or is it the wave catching us?) is different on every

takeoff, and as the takeoff gets later or more critical it becomes increasingly difficult to model. For example,

a longboard catching an unbroken swell simply gains speed until it rides up on it’s bow wave and planes,

while a one-stroke free-fall on a low-volume shorty has dynamics I’m afraid to tackle. Any time you’re talking

about the lip or pitching portion of the wave you can throw most of the models out the window. We’re in

uncharted territory here (which is fun).

What you’re talking about also goes a long way towards explaining why low-volume boards release

so much better off the top, something we all know is true. But I like having an inkling of knowing WHY it’s

true. It might help me to make better boards.

Mike

18

Great stuff Kevin.

I have always thought of the take off being related to a fulcrum point, somewhere around your shoulders.

dropping your head or flicking you heels up can help getting into a wave.

When thinking about it in this way it becomes not just an issue of low volume Vs High volume, but the relationship between Volume/planing area in the nose Vs Volume in the tail. What I think I am saying is the planshape and foil are more important than overall volume during the takeoffs.

That’s not even considering rocker and bottom contours.

19

“What I think I am saying is the planshape and foil are more important than overall volume during the takeoffs.”

BINGO!

20

Greg Tate brings up the phenomenon of " mid length fun boards being such dogs ". I have heard this mentioned before and never quite understood. My understanding is that fun boards are wide, thick, longer and forgiving and are easy to catch waves on. Hence the “fun” in the name. What would you consider mid length and how could they be anything but easy ( not that I am disagreeing w/ GT, I honestly don’t get it)?

My boards have always been thick and floatie but most of my surfing is on East Coast mush. Just had a 6’1" x 19.5 x 2.85 I made out in powerful Puerto Rico waves and had a hard time getting into them without a last minute drop. Very frustrating. Seems backward but maybe a potato chip would have worked better there.