Questions/Observations RE: Board Weight

General Discussion
1

For the past year I have been riding EPS boards. The first EPS board I got was a Beatty Rocket Fish that I rode while trying to rehab an injury. The board is massively thick and has a ton of volume (7 ft. long, 21" wide and 3-3.125" almost all the way to the rail) but it is incredibly responsive for a board that size. In fact, I was so blown away by the responsiveness that I had an EPS version of my “standard shortboard” made.

I took the EPS shortboard out last week on a dialed-in rock boil type of break in the shoulder to 1-2ft OH range. It was interesting to surf the EPS shortboard because I could compare it to an almost identically sized PU board.

A few observations (probably none of which are revolutionary) and then 1 question for the EPS wizards on the forum:

Observation #1 - the EPS board was much easier to get into waves than the PU board. My question (below) relates to why this is.

Observation #2 - at first I was kooking on the EPS board b/c it was a lot more sensitive/responsive than I was used to. Part of that is b/c I had been riding the tanker Beatty board for a while but I don’t think that explained it all. The EPS board was so sensitive, in fact, that I finally had to make a conscious effort to reduce the energy I was putting into snaps, etc. Basically, it was like I could use 75% of the effort to get the same result that I would use 100% effort to get on my PU shortboard but if I tried to use full effort, I just blew it. Probably just a matter of getting used to the EPS board but it was definitely interesting.

Now for the question -

Question #1 - I spoke to the guy who shaped the board (a well known Orange County shaper) and asked him why the EPS boards seem to paddle into waves better. His response was basically that it is purely a matter of total floatation of the board vs. total weight the board has to float and, because the EPS board is lighter than the PU board, the EPS board has less weight to float if all other things are held equal. Thus, in his view the EPS boards paddle into waves better than PU boards simply b/c the EPS board itself weighs less and therefore the float-to-weight ratio is higher.

I don’t completely buy that explanation. If my EPS board weighs, say, 5 lbs less than my PU board, under his theory it would seem that I should be able to paddle the PU board into waves just as easily if I lost 5 lbs? Conversely, under his theory if I went out and ate a big steak dinner with some brews so that I put on 5 lbs then my EPS board would paddle no better than my PU board before dinner.

From experience, that simply isn’t the case. The EPS board, at say 5 lbs lighter, paddles WAY better than its lighter weight suggsts under the shaper’s theory. I suspect that planing (i.e., the ease with which a lighter board can be up and planing) factors into this but I don’t have the theoretical background to confirm that suspicion.

Can anyone help out?

Thanks in advance.

2

I got a B in my physics classes, but think there is something to the float to weight ratio…

With the EPS being lighter/more buoyant… I’m guessing that if you got a huge block of EPS foam and laminated it (let’s say 10ft W x 10ft L x 5in thick and stuck a 200lb person on it, it would sink by 1/16th of an inch (hypothetically). Whereas if you got the same size PU/PE block and stuck 200lbs on it, it would perhaps sink by 2/16th (hypothetically). Additionally, because the PU/PE is heavier when compared against a block of EPS, it would probably be 1/32nd lower in the water from the beginning. (again hypothetically). I don’t know maybe I’m just rephrasing what your shaper said… :slight_smile:

Looking forward to hearing from the experts though…

3

I used to think that way about lighter boards being more buyant, and hence easier to stand up on. I bought an EPS board a few years ago and was imediately impressed. It seemed to be a big improvement over my similarly sized PU board. later, I made a chambered 6’2" board from balsa and redwood. This board weighs 15 lbs., and is very easy to paddle into waves. Easier, I would say than my 6’10" EPS board.

I had Bill Thailkill consult me on shaping that board, and he gave me some good advice on how to make it paddle better. One of the things that he told me about was adding bellly to the nose. This helps paddling because water is pushed to the side as you paddle forward. Having a flatter rocker, and a wider tail also helps a lot.

After my positive experience with the 15 lb. wooden board I have changed my mind about lighter being better. A good board will paddle well mainly because of the way it is shaped. Once you are up and going the extra weight is nice for cutting through choppy sections. The extra weight is also good for dropping into waves with strong off shores blowing.

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T-Roy,

Welcome to the darkside and it’s magic.

-Resinhead

5

http://en.wikipedia.org/wiki/Buoyancy -Carl

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My popout 9’6" LB (soon sold) sucks to paddle through serious windchop–the overall rocker is kinda crap.

My total sinker but pretty low rocker 6’10" semi-gun PUpe (also soon sold) is so sleek through the water and easy to duckdive, I can get outside (200 yards, 300) in no time, easy. Once there though, the board doesnt have enough float or plane especially in the tail to catch anything but the steepest, and the steep drops inside here have been scarce lately.

I’m talking sliding onto waves here…

Light weight and volume work together with correct rocker to catch em cuz your tail is closer to the surface already and the sooner that comes up to plane, youre in, but whether you can ride and turn that board very well after that…

Planing surface and low rocker is good if you dont use a ton of float–a lot of guys swear by width as (diametrically) opposed to thickness, on fish, say.

The right weight can help get you sliding down a slopey face, and be easier to keep your rail in during turns.

The ideal is a combination of correct weight, plane, and float, and slick hydro design, even with logs.

Better waves are easier to catch and ride.

7

Thanks - all of this is interesting but I am still not sure that I have my mind fully wrapped around the explanation. The Wikipedia link was generally helpful re: buoyancy and I definitely agree that a board’s shape plays a major role in how it paddles.

But…

The EPS board and the PU board I am comparing are both identical in virtually every respect except the EPS board is slightly thicker (= slightly more volume) and lighter by virtue of the material used in its construction.

I suppose that it could get into waves better through the combination of slightly more volume and its much lighter weight. These two factors would certainly increase buoyancy. But I am still not completely satisfied with that explanation because the EPS board is not THAT much lighter than the nearly identically shaped PU board. Were I to get the exact weight difference, I would bet that it is no more than 5-7 lbs max. I would also bet that if I put on my weight belt that I use for diving (and which weighs much more than 5-7 lbs) the EPS board would still get into waves much better than the PU board.

So, after much typing I guess I am saying that I still don’t get it. The volume of the PU and EPS boards are very close and the EPS board is about 5-7 lbs lighter but it gets into waves much better than the weight difference would have made me expect.

Any other thoughts would be welcomed.

8

Janklow posted while I was typing my 2nd post on this thread.

“Light weight and volume work together with correct rocker to catch em cuz your tail is closer to the surface already and the sooner that comes up to plane, youre in…”

Now we are getting somewhere. Intuitively, that makes more sense to me than “it gets waves better b/c it is lighter and therefore more buoyant”. I think that strictly speaking, what Janklow said is a FUNCTION of the board being more buoyant but it somehow seems intuitively more of a satisfying explanation.

Thanks - any other thoughts would be welcome.

9

How slight is slightly thicker? Small changes in displaced volume can make big differences. Think about this example…

You are floating on your back in the water, lungs fully inflated. You let some of your breath out, and you start to sink. Compared to the rest of your body, the amount of displacement/volume change that came with you letting air out and the subsequent rib cage compression is not much, but it makes the difference between you floating and sinking.

So, that small change in volume may be the difference maker, and where that extra volume is on the board as well…

Just a thought…

JSS

10

Great visual/example maxmercy. Like it!

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Quote:

Planing surface and low rocker is good if you dont use a ton of float–a lot of guys swear by width as (diametrically) opposed to thickness, on fish, say.

I’, going to throw another confounding factor in here.

I ride (knee)boards that are 23.5" wide, 6’2" long. They are hell to paddle - I pump my volumes to 40 liters for my 75kg (160lb?), 6’2" frame - compare this to the 26 liters used by pros and probably 30-35 liters for average surfers.

With these volumes (2 3/4" thick) I am competitive without flippers paddling into waves.

All the kneeboarders I know have similar issues - they all go for max thickness from blanks.

But many of them jump onto regular standup boards and paddle them without any issues.

I’m guessing that the width is having a hugely detrimental effect on how they paddle. Big piggy outline just ain’t streamlined so we’re using volume to overcome drag.

I like the idea above that tail close to surface may have bearing on wave catching ability. With 2.5" of tail rocker (but basically the rocker through the middle 2/3 is a standard standup 6’ rocker) it’s quite possible that the “belly” effect is combining with the outline to make boards that are hard to paddle.

I think that the “planing area” argument fails unless the area is fairly flat so that it can get onto a plane easily.

12

I like the side by side testing. Let’s get more information. You never gave us the real weight of your boards. Just a 5 pound guess. What are the true deminsions of your poly short board and your EPS short board. I’m interested in length and thickness. Rocker and other stuff make a difference too, blah ,blah, blah…

We can talk theory all day long. I enjoy hearing observations from side by side tests like yours…Did you read about the testing that Resinhead did. Epoxy board rode better but got snapped and then repaired and then snapped again…heard its still a great board…

Here’s a few Stingray observations :

Magic boards can be made out of many different materials…

Sometimes light is good , sometimes light is bad…

A magic board for me may not be a magic board for you…

Fins matter

Rocker is complex and very important…

My spelling sucks…

Stoke is more important than any of this stuff that we talk about…

Share the stoke…

Ray…

13

This lighter board = increased buoyancy business is touted alot by epoxy exponents.

The reality is that an epoxy board will only be maximum 1 - 2 lbs lighter than a pupe…so if you’re 200lbs that’s a maximum difference of less than 1% overall weight…not very significant.

Remember buoyancy is a function of your weight plus your boards weight.

My experience of lighter boards is that they don’t catch waves quite as well as slightly heavier boards…I think heavier boards give you added momentum (especially when paddling into offshores).

I suspect the difference you found lies in the thickness…an inrease of just 1/8" gives a large increase in overall volume.

However ----- stiffness also comes into it…the stiffer the board, the less energy is lost when paddling into waves…Epoxys are generally stiffer…

14

mr. T, a very good shaper from the canarian islands said that in windsurfing 1kg board weight equals appr. 15kg body weight.

take for what its woth

salu2

uzzi

15

So, after much typing I guess I am saying that I still don’t get it.

Unfortunately, there is no easy answer on this issue/debate…it has been brought up many times before…heated arguments and all.

Basically, board surfing is way too complex and dynamic. By dynamic I mean lighter buoyant objects float quicker than heavier buoyant objects. You can test this yourself in a pool with similar sized spheres of different weights. Submerged, the lighter one will rise to the surface faster than the heavier one, even tho both are buoyant.

Board weight is design variable. Very often, lighter boards go better, other times, a heavier board goes better. Depends on the surfer, wave and board design.

16

“Remember buoyancy is a function of your weight plus your boards weight.”

Yes! Thanks for that. Also remember that the magnitude of the buoyant force has only one variable. The buoyant force is ONLY due to the volume of water displaced, and it is equal to the weight of that volume of water. That’s it. Has nothing to do with board weight.

That’s why in craftee’s example, the lighter sphere will rise faster. Both spheres will have the same buoyant force upward, but the heavier one will move more slowly upwards, due to a smaller upward resultant force and f=ma. In other words, not only is the heavier sphere harder to move upwards becasue it has more inertia, but since it is pulled downwards by gravity more than the lighter sphere, less force is available for it to go upwards. Same thing with a surfboard. Equal volume surfboards, if both are totally submerged, will have the same buoyant force upward. Period.

When you place a human on the board, and that human is not totally submerged, the equilibrium problem becomes more complex. If the board isn’t totally submerged either, it becomes even more complex. That’s why the debate goes on, due to the complexity of the problem. The buoyant force depends on the volume of water displaced by board+surfer, not easily measured due to one or both only being partially underwater.

When you have a board+human combo that is only partially submerged, the buoyant force is again ONLY due to the volume of board + volume of human that is submerged, but that volume is difficult to calculate, due to the complex shapes of board and human.

The above is ONLY the static example, no movement. So, if you have a board that is 5lb lighter (5 lb lighter board+surfer combo), the volume of water that needs to be displaced weighs 5lb less, and you float a little higher when not moving. How much higher you float depends on how much of you and board (near the waterline) weighs 5lb.

Start paddling and everything changes, as you now have lift and drag forces to think about, so craftee is right on the mark. Way too many variables that are difficult if not impossible to measure, and you have said heated arguments.

JSS

17
Quote:

Equal volume surfboards, if both are totally submerged, will have the same buoyant force upward. Period.

Nuh-uhhh.

Wait, um, okay, college boy ; ) – explain how a shell filled with buckshot is the same boouyaunt force as a hollow shell!

18

Youre joking right?

Buoyancy provides an upward force on the object. The magnitude of this force is equal to the weight of the displaced fluid.

Period, like MM said. The scientific definition of buoyancy is different than what lay people like us define as surfboard buoyancy. There is mega confusion in this. Its way too dynamic and complex for anyone or any supercomputer to figure it out. Textbook explanations make simplifying (erroneous) assumptions to explain it. Free body static diagrams give a tiny snapshot of time…ahhh forget it!! Lets talk about hot chicks…

19

Hey janklow,

No problem, this was a common question when I was a physics tutor. The empty shell and shell filled with buckshot both have the same bouyant force upwards when submerged. But the shell filled with buckshot has more gravitational force acting on it downwards than the empty shell. So, the resultant force (sum of forces) for the empty shell is up, and it floats. The resultant force for the buckshot shell is downwards, and now you have to go fish it out of the bottom of the pool…

You just forgot to include gravity. Remember, the buoyant force is only upwards.

Make sense?

JSS

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Roger that, John–YESofcourseIWASJOKINGDAVE!–you guys don’t mind if I delete that post, do ya? lessee here…

wtf, I dint haffa go physicks class