Hows that thing float?

General Discussion
1

I just read through a post from october-“Can we put the eps = extra float myth to rest once and for all?” which is an excellent discussion of bouyancy, flotation, helium, concrete and the Archimedes Principle. Good stuff.

I was wondering now that many of you are using eps what have you found as far as adjusting design/overall size and thickness to imitate pu shapes. I guess it’s a non issue really after all it’s just surfing in the end but I thought it would be nice to know what some of you were finding in your work.

I want to have a board made from eps shaped on an aps 3000 and I want to do the design but I was assuming I would need to redesign my favorite shape to compensate for bouyancy. According to the thread from october that may or may not be true.

What r you guys finding?

2

I wouldn’t change the shape (thickness) for bouyancy, but for flex.

EPS, epoxy resin, and particularly sandwich skins (if you get into that) conspire to make boards stiffer than the same shape would be in pu/pe. That stiffness feels ‘corky’ under your feet when surfing which is why people describe them as ‘floating’ higher. The stiffness also helps them paddle efficiently, another thing that people attribute to bouyancy.

So, if you like the paddling speed and don’t mind a board which doesn’t like its rail buried, or surf a lot of glassy water, then you can make it the same exact shape as your pu/pe.

If you want it to generate speed through bottom turns and ‘feel’ more like surfing your pu/pe, make it about 1/4-3/8" thinner than your old one.

3

Thanks for the reply benny. What your saying makes sense.

So going that much thinner-1/4-3/8"-the extra float of the eps foam will equalize taking out thickness? Or should I add length to make up for less foam?

4

based on my understanding of what Bert is saying… wider and thinner, that is what I am going todo on my comp/sand shorty

5

I’ve had good luck going about 1/4" thinner with eps/epoxy. -mv

6

Oh yeah wider…duh:)

7

Archimedes Principle: The buoyant force on a submerged object is equal to the weight of the liquid displaced by the object.

Buoyancy is a combination of weight and displacement.

EPS foam does not float better than polyurethane. It is the fact that most EPS surfboards are lighter that produces this feeling.

Two vessels will have the same buoyancy if they are the same displacement and weight. For example, if you were to place two identical Tupperware bowls in water and fill one with 6 oz. of lead and the other with 6 oz. of feathers they would displace the same amount of water. If you add 6 oz. more material to one of the bowls, the water line would then change. The water surrounding the bowls doesn’t care what’s inside the bowls. With two objects of exactly the same displacement, it is their difference in weight that affects the floatation.

Let’s say you were sitting in a boat in a pool with a large rock inside of the boat and you were to toss the rock out of the boat into the pool. The boat now sits higher on the water and the water level of the pool lowers. When placing the rock which has little displacement into the pool, it lowers the pool’s water line because the rock displaces only its volume. While the weight of the rock was in the boat the rock displaced its total weight, so the boat displaced much more water, raising the level of the pool’s water line.

• Buoyancy is a concept that can be difficult to comprehend at times. However, with a simple consideration of forces it can be made clear.

• The buoyant force is a consequence of the variation of pressure with depth.

• The pressure on the bottom of the object is larger than the pressure at the top.

• This leads to a net upward force on the object.

• If this upward buoyant force is larger than the weight, then the object floats!

• The difference in the weight of two vessels with identical displacements will determine the difference in how high or low they float.

• The difference in the displacement of two vessels weighing the same will determine the difference in how high or low each floats.

• If they have the same weight and displacement, they will float the same, no matter what material they are made of.

8

Rich,

If I had two boards about 8’ long with the same dimensions all around. One a EPS/Epoxy and one PU/PE with the same weight. If I put a 100 pound weight on them both, they would sink into the water the same while the weight stayed out of the water. Now take a 250 pound weight on them (me), they would be underwater to a point. Would they be equal to the same depth in the water? What do you think volume will have on them if that was to change? Like the EPS would have more volume to meet the same weight as PU boards. And how would that effect the float if the volume was a domed deck or from the rail up? I understand the displacement factor but I was wondering what you thought.

Timmmyyy

9

In your first sentence you say they have the same dimensions all around. Then later you say, “Like the EPS would have more volume to meet the same weight as PU boards”. Only one of these is possible - either they are the same dimensions or one has more volume. So you might want to clarify the set up of your hypothetical situation.

BTW, in reality, if the boards are going to be the same size and same weight, most likely the eps boards is going to need way more glass or a heavier stringer and so perhaps less actual foam than the clark foam board.

10

I have two identical 8’2" surfboards, one is epoxy, the other is a Poly board. The epoxy board is a XTR board and is very light…so light that I really don’t like it much. The two boards are glassed with 6,4 - 6, both with regular surfboard glass, nothing special.

it’s just like Rich says. The different feeling comes from swing weight or total board weight. Both boards float the same, but they don’t paddle the same. They paddle different, the difference is the epoxy board feels squirly. why squirly? squirly because it’s lighter, and doesn’t “feel like” its displacing water on the take off. It also feels strange on the bottom turn, once again wobbly, why too light. Cut backs…you’d better stay on top of that rascal.

The boards are identical, but surf completely different. Once you get used to the epoxy thing the board would be fine, and you will probably get more performance out of your surfing.

If you add glass for weight, then you add stiffness. To combat the stiffness you thin the board out , but you loose floatation. To add floatation you add width…then you have a completely different board. Weight make a huge difference in a board.

I’m glad Ken & Greg now have 3-4 lb EPS. Thats the next project, and should make a lot more sense in the Poly=Epoxy quest.

11

One more thing: The XTR board is now in the San Diego Landfill. It lasted a full 3 months of service, delam’d and had full honors during the burial service.

12
Quote:

hey paddle different, the difference is the epoxy board feels squirly. why squirly? squirly because it’s lighter, and doesn’t “feel like” its displacing water on the take off. It also feels strange on the bottom turn, once again wobbly, why too light. Cut backs…you’d better stay on top of that rascal

.

I’ve noticed the same thing (but I didn’t call it squirly). What I finally attributed it to was inertia. With a heavier board it takes more to knock it around, but on a light board the side chop hits you and you and you’re feeling like a kook. This is one of the reasons that I think its important to make 1 lb boards thinner so it sits a little lower in the water (until you get up to planning speed).

13

You’re right about Archimede’s Principle and bouyant force - the bouyant force is only in play with the portion of the board that is below water. and that it does not determine if an object floats.

Quote:

It is the fact that most EPS surfboards are lighter that produces this feeling.

What your talking about is density (an eps board is less dense) which is determined by the mass and volume of an object. If an object has a density greater that water it sinks and less than water it floats. However, for all practical purposes the density of the board is also a product of the surfer sitting on the board (its our added mass that pushes the density above 1g/cm3) which pushes it under water. To get the float out of the board you can change two variables - the mass by going with a heavier foam or the volume by changing (shrinking) the shape of the board. Either of these will get you floating where you want to be. The advantage that is see with sticking with the lighter foam is that you can change the volume of the board by making it thinner (you gain more sensitivity). If you keep going thinner you have the freedom to add back some of the volume by going wider (to get better planning). The price you pay for lighter is that you get kicked around by the ocean a little more on choppy days. Its why its important to pick the right material for the right conditions. On a super glassy day light has not down sides, but once theres some chop on the water I can see an advantage of a heavier board. This is why I think that there no right ways to build a board, but instead right techniques for the right conditions. There’s so much negative talk about this being “epoxy land” and “not fun” but theres a reason for the talk (and a reason it keeps coming up). I’ve never been against the old poly boards. They did meet a lot of peoples needs and they were easy. Bouyance, density or how to cover your board never needed to be discussed because all of the choices were made for you. In regards to longboards, if you make the right material choices you can create super high performance that lasts (like Bert’s) or you can make a classic cruiser that has amazing glide. Neither is right or wrong - its about surfing the way you like (and that is fun!). If you don’t take into acount materials you are fighting to live in a one size fits all world.

14

You guys are lucky to have waves enough with all these varying conditions to be able to tell what works and what doesnt, what feels tight and what feels squirmish… when we get waves, they always suck,unless its from a hurricane. So most of the time its hard to tell whether its the board or its me… but Id have to disagree on the chop part, I think the lighter smaller board is easier to meanuver through/in the chop. My 6’6 with hardly any tail rocker I took out for the hurricane clean stuff worked great, but it sucks trying to get through the chop, and I usually use my 6’3 with more rocker for that. The feel of EPS on my friends 6 '2 was good when traveling through and riding the chop I thought, the light weight made it easy to get quick bursts in speed to travel between breaks so it seemed. Sorry just venting and adding my 02 cents.

15

Moraboutslim

Sorry, I didn’t mean to be confusing. I should have put the questions in separate paragraphs.

First, if we had two boards made in the same dimensions, shape, and rocker. The only difference is one is Styro/Epoxy and the other is PU/PE. Both the same weight regardless of how they were made to be equal weight. Be it more resin in one and less in the other. Both weighing 14 pounds, if I put a 100 lb. weight on both and that weight would not sink them below the surface of the water. They should both float at the same height.

The second question was if you put a 250 lb. weight (lead) on the same boards and they sank under the water. Would one sink or settle to a lower height or level in the water?

Now put the same 250 pound weight a person on the same board. Would one board sink lower then the other?

I added the variables to see what other surfers and engineers thought and if you and everybody else had an opinion on the effect on the laws of displacement plus volume.

So, if you added more volume to Styro/Epoxy board to equal the weight of the PU/PE board. Based on the same length, outline shape with same rocker and rails to a point half way up the rails from the bottom to the deck. Would the board sink more based on weight on the board be it 100 or 250 pounds? At what point does the buoyancy of the board’s volume make a difference. What if one board was sealed during a high-pressure day and the other was Vacuum sealed as a lot of you guys do? Does that have an effect on the buoyancy?

The reason I am asking this is because I have been building two boards. Both 8’-1" long, 23" wide with the same outline as my 8’-1" long 21-1/4" wide McCoy Nugget Surftech. I love the board but it doesn’t work well in less then head high surf. The board could not be ducked dive under the bigger waves because of its buoyancy. Therefore, I made 2 boards to counter these shortcomings. One was Styro (EPS#2) and other was PU (Clark ultra lite). Both were laminated with 4-oz S-glass with Epoxy on the bottoms, 5.7-oz Carbon/Poly hybrid deck using Epoxy resin. Both were hot coated and sanded as per the wording on the System Three SB-112 literature on their website. Then the bottoms were glassed with 2-oz E-glass using PE resin. The decks were glassed with 4-oz E-glass using PE resin. Both boards were hot coated with PE resin. The same fin setup will be installed in the same locations on each board. The major change between the two boards is thickness (volume). Since the Styro is supposed to be lighter I made it the thicker board. It is 3-1/2" thick. The PU board is 2-3/4 " thick. The intent of the Styro board was to have a buoyant board that would paddle and plane better so I can ride smaller waves. The PU board would paddle and plane better while being able to duck dive on bigger days. The building of these two boards was based on the boards’ ability to displace enough water and hold up my weight on the water while planing.

I will be bringing the two boards and the Nugget to Costa Rica for the month of February. I plan on bringing two boards in the water at the same time every morning early. I will paddle one out and drag the other behind me. I will carry a bag of rock in a net bag with a rope attached to the board. I will switch off to test the two boards on the same surf with the same fin set to see if they do as I planned. What are the differences? Will I be able to duck dive the PU board? I will let you know when I get back or if I get access to the Internet.

I have learned a few things along the way with the use of the PE, Epoxy, PU, and Styro. Both boards weighed the same or with in a 1/4 pound after shaping. The Styro being the lighter board was spackled and put in the oven for three hours at 150° F The PU board was also put into the oven for three hours. The Styro board cam out 1/4 pound lighter, making it 1/2” pound lighter then the Ultra Lite Clark blank before we started glassing. The PU board also added 1/4 pound more through the glassing process. It is presently 3/4 pounds heavier then the Styro board through the glassing process. I will weigh the boards again tomorrow before the FCS plugs and the Fins Unlimited boxes are put in.

A Note to the newbies using Epoxy. Choose your glassing stand material carefully if you do not want to spend time cleaning the material off the laminate. If you wanted to hold a board in place with sandpaper so it doesn’t slip while you cut your laps as could be done with PE resin. You cannot with Epoxy. We wasted two hours cleaning sand paper from the deck of a post-cured laminate that could be handles with our hands. The stuff is ruthless when it has something in its grip. Plastic seems to work best but it will slide around on the stands. The worries of the PE not adhering to the Epoxy are not true if you do as required by the manufacture. I had drops of PE on the sanded Epoxy deck and scraped them off with a razor blade. The drip shaved down to a thin film and it did not let go.

In ending I have to admire all the board builders out there for the ability to build surfboards as we want them made and as cheaply as they have. It has been a long learning process to make these boards. We surfers who have never understood the process of building boards should be more then thankful to all the shapers and builders who make them for us. It is hard work and there is very little room for error. There are so many variables to deal with. Creating a controlled environment is key and then you still have to work the resins before it sets. Will the set early or will they take to long and run off the boards. I have a new appreciation of the builders of the boards I have ordered.

Timmmyyy

16

Hi BobMarley,

I see your point about the chop thing. I reckon it’s a matter about what you consider chop, and how big it is during the surf session.

My take is about those days when there is a lot of water moving around, lot’s of foam and current. It’s another reason why I won’t surf a sub 7’6" board when it gets thumpy. You give up the ability to do super deep duck dives, but you make up for it in sheer speed paddling out to the line up. ie, I get drilled at the same point a guy on a 6’1" gets drilled. he does a clean duck dive, i get pushed back 20 ft. I come up get realigned and in 20 strokes I’m 20 ft in front of him.

same holds for the weight issue, a heavier board streamlines through the water better (in my opinion) I want to cut through the chop, and not manuver around in it, I need positive stable lines…shortest distance is a straight , blab, blab stuff. But were talking about head high plus waves. If it’s a small day, I guess it really wouldn’t matter, because there’s not that much water moving around.

My idea of thumpy,in the picture.

superlight boards have there place, I just haven’t figured out where they are in my life at 43?

Good luck,

Jay

17

Timmy,

A fellow named Archimedes figured this out in the first century BC. He was commissioned to find out if a goldsmith had cheated on a supposedly solid gold crown by slipping in some silver. The crown was a holy object, so he could not to disturb it.

The solution which occurred when he stepped into his bath and caused it to overflow was to put a weight of gold equal to the crown, and known to be pure, into a bowl which was filled with water to the brim. Then the gold would be removed and the king’s crown put in, in its place. An alloy of lighter silver would increase the bulk of the crown and cause the bowl to overflow.

While this is probably more folk tale than fact, the principal remains.

Two items of exactly the same displacement and weight will have the same flotation, no matter what they are made of. Changing either the weight or the displacement will cause a difference in the amount of water displaced in the bowl.

Thinking that one will have to increase displacement of an EPS to match weight of a PU may be incorrect. Wider stringers is one solution, and T-Bands and multiple stringers can really add weight. One can also add deck glass, as the bottom is almost always the surface that stretches. And flexibility is a major concern when getting away from the Clark “standard”. Also, Resin Research resin comes in several different flex standards, and as time marches on many of us will become experts on this as it is applied to various lengths and thickness of boards.

18

Hey Timmmyyyyyyyy!

Just some thoughts on your post.

Pay attention to Rich. Displacement is displacement and not a solution to most surfing/shaping problems.

I don’t know of anyone who can duck dive an 8’ board that is not an NFL linebacker in big waves. A thick 8’ EPS board will be almost impossible to duck dive so you should concentrate in shaping a board that is fast paddling instead. Watch some old surf flicks to see how the oldies knee paddle up the face of a wave!

There are basically two types of bottom shapes: an old school displacement hull (like a boat hull) and a planing shape (modern short board). There is a lot of bottom roll like a displacement hull in surftech boards that doesn’t give them a lot of performance in small surf. A planing hull is a better bet. Make sure your board is flat rail to rail and the rocker has no sudden changes and is smooth throughout the length. If you want a speedier bottom in shorter lengths, add concave at the very end but don’t go to the edge of the rail.

Flex has a lot to do with how a board surfs as does rail shape. A PE board with “A” rails surf a lot different than standard down rails all other things being different. Wonder why modern fish are so much fun?

Anyway, I hope this helps, keep the planer runnin!

Brian

19

What r you guys finding?

ah yes this subject again.

im going on a limb and make a conclusion about shorties:

if youre building lighter eps boards, for every 0.5-0.6lb weight reduction from your ‘normal’ pu board,

drop 1/8" thickness. So if your normal poopee board weighs 7lb, and you build a similar eps but at 5.5lb,

then drop 1/4" to 3/8" thickness. If the weight diff is minimal, you still would want to drop your thickness a bit, say 1/8", just to be safe. Rail volume is critical as well.

HTH

Cheers

20

Hello Rich,

Thanks for posting a simple way to understand the theory. I stated in one of my posts. I wanted to shape two boards with the same outline, bottom rocker, V rolled bottom and semi tucked rails of the McCoy Nugget with some modifications for my weight of 250 pounds. I widened both boards by 1-3/4" to make them plane better on the wave because the Nugget didn’t in smaller waves with my weight. This is what I was shooting for and I think I came close. I will test them in Costa Rica at the end of the month. I was interested in how things float so I can understand if my planning will do what I want the boards to do. The PU board, which is thinner by 3/4”, is heaver then the Styro by a pound. Both were glassed the same way. Since I can almost duck dive the McCoy Nugget I should be able to do a better job on the thinner and heavier PU board.

I’m also happy you mentioned something about Resin Research’s flex standard. I have not read anything about that on this site before now. Good information to know. Thanks again for the information. I will try to use it wisely.

Timmmyyy