What constituent provides the most flotation per unit of volume…Styrene? Urethane? Cork? Air? Compressed air? Or, what have you?

Because I’m a lay-down for logical argument, Bert B. has inspired me (within the constraints of diminishing returns) to return to shorter + wider and thinner in this, the late afternoon of my surfing “day”. Answers will be applied to configuring my next custom epoxy board.

Your wisdom is appreciated!

I hate to say it, but in the words of my old philosophy professor - " That depends" . You have to go with lbs/cu ft. or kilos/cu meter of the whole thing, core and structure. Vaccum ( zero pounds/cu ft ) is the absolute lightest, but the skin required to contain it against ambient air pressure can get mighty heavy. A hollow structure containing air inside a lightweight but stiff skin is mebbe second, but such a structure needs to be reinforced here and there to take compression loads, which can add weight.

My own SWAG ( Scientific Wild Assed Guess) is that a very low density core foam with a higher density foam skin ( vacuum bagged onto it) , covered with a vacuum bagged composite shell (epoxy resin plus your choice of fiber, with cost/benefit of that fiber choice being a factor) would be the best reasonable choice within reasonable economical construction constraints. I hope Bert comes in on this, as he knows a helluva sight more than I do about this and he’s done the field work.

Fat lot of help that was…


I assume, in a surfboard application.

Then lightest weight per volume is the floatiest.

Styro epoxy sandwich, for sure.

I might agree with Bert on some things, but his idea of a easy paddlin, easy wave catchin small wave board is not my idea of reality…I need float, hard rails, flat rocker, WP aft.

But maybe we can’t all go surfing 4+ days a week.

I surf about once a month, regardless of whether I need to or not.

SWAG is what I’m chasing, it’s mind expanding. It’s been noted that I tend to “walk this way” and SWAGger. I, too, progressed from air to helium to vacuum…at which time my mind imploded at the cost of structural implications. Practicability is key. Occam’s Razor/KISS principles must rule.

Somebody’s been smoking a little too much greene lately, eh?

I could be wrong, but I think the lightest solid is a substance called Aerogel.

Bada bing, man. As the German cat said, " perfection is the mortal enemy of good enough".

If you don’t have the financial and engineering resources of NASA behind you, you gotta work with what’s available, easy and economical. You can fake vacuum bag tech with a shop vac and some smarts and ingenuity ( or a refrigerator compressor, a bleed valve and etc. ) and do the best reasonably, repeat reasonably achievable, but lets not go nuts. You’re not sending this board to Mars.



Agreed, LeeDD…not many get to surf, or have surfable waves, as a regular way of life. Sounds great, though. As you, I get to surf an average of 1-2X/month, exclusive of my 2 annual pilgrimages which can constitute half of my annual surf hours. Obligations are problematic, indeed.

My experience with speedy boats back in the 60’s (on parental dole) leads me to believe that there is iron in Bert’s philosophy…within limits. It relates to aspect ratio of about 3 point low-something to 1 (length:width). A (approx) 7’x14x23+x17+x3, medium rockered, flat, or flat to slight V bottom, with a deck concaved between 1/4-3/8, wide point slightly south, boxy-mid to hard-tail railed tri-fin should, in my experience/judgment, work very well for a 180-85 lb., in shape, but age and abuse wounded, elder like me. I don’t disregard “flotation”, I simply want to go optimally short for smaller, crappy waves. I’ve tried the thicker/narrower route and find over-flotation; time now to expand my experience.

'Tis but a rare thing these days, LeeDD.

I know we’ve been over this several times… in fact, I know I’ve taken some pretty hot responses from people who know nothing of fluid mechanics. The only reason why I bother drudging it up again, is because I believe firmly in understanding the basic principles behind something… once that’s understood the rest will all come.

Materials don’t float better than others, providing that they’re all air tight. A cubic foot of air and a cubic foot of brick, underwater, will have the same bouyant force pushing up on it. It’s just that the brick’s weight, pushing down, is greater than the bouyant force pushing up, so it sinks. To sum it up, the bouyant force is equal to the specific weight of the fluid displaced by the object. Whether or not it floats is the difference between the bouyant force and the weight of the object. So two boards, same shape, different materials, will have the same bouyant force pushing up (if totally submerged). How much better one floats you is a factor of that bouyant force and the weight of the board - ignoring the fact that you’re partially submerged while on the board.

A cubic foot of saltwater weighs about 64 lbs… probably about the amount of water displaced by a shortboard. So your EPS/Epoxy board weighs 4 lbs. and your PU/PE board weighs 6. That means the E/E is giving you 60 lbs of float and PU/PE is giving you 58. Foam doesn’t weigh much, so even if you could make a hollow board of air, same glassing schedule, you’d get maybe 62 lbs of float? Compressed air… more molecules, suprise! …although it’d hardly be measurable, it’d weigh more. 61.99999 lbs of float.

All in all, it’s pretty similar… a difference that could easily be spoiled by a rookie glass job. Not that I don’t support the advances of science… and if you offer me a material with same or better structure/strength and similar cost, I’ll take it, even just to drop a pound or two. But if you think that you’re going to find some foam that makes this insane floating beast, that needs to be 1.5" thick so that it’s not to corky, think again…

Go look up Archimedes principle. If you look into it enough you’ll probably even get a nifty, albeit fictional, story of Archimedes, the King, and a crown, that all our physics teachers probably told us.

I have an awful lot of respect for Bert’s opinions, but I still disagree on short, wide and thin. I simply feel that the surfboard should support the weight of the surfer to get up to speed paddling. That can be done in a variety of ways. I always liked shorter, thicker and wider because I needed the maximum float, but wanted to take out the lenth so I could do shorter turns. I am a big guy and own a 7’2’’ thin and wide surfboard. I like the way it rides, but it simply does not paddle as well as the shorter thicker one I own because it does not support my weight as well. I could go longer, but that would defeat the purpose of shorter turning. It’s all a trade. I agree with whoever said there is only so much float your going to get from any of it.

Gee - rKelly is right. Science is science, hydrostatics is hydrostatics. The average longboard is approximately 2.5 cubic feet (I actually calculated this by analyzing the board in sections); equivalent to approximately 160 lbs of floatation based on 64 lbs per cubic foot of seawater. The board weighs about 18 lbs. So, if you (the surfer) weigh more than 142 lbs, you’re going to sink. The only way you are going to prevent yourself from sinking is 1) get a lighter board, 2) lose some weight 3) get a more voluminuos board that weighs 18 lbs.

Now, if you actually catch a wave, then you are into hydrodynamics and that’s a whole other animal…

I’d have to agree with mr. rKelly, board flotation is a simple function of volume displaced minus weight, and since the weight of different core materias changes insignificantly compared to the force of the volume displaced, the volume of the shape is the only pertinent factor in floatation.

My guess is perceived corkiness in a board might arise from other factors such as flex, that are hard to measure, but influence the feel of a board. Lack of flex may make moulded boards harder put on a rail, because that initial pressure might bend a trad. board into a tighter arc, ferinstance. Also, volume itself is hard to measure, boards can look deceptively thin or thick, and no assumptions can be made about total volume without a displacement tank. (I suppose the nipple to bellybutton scale is ok, but kinda influenced by things like water temp and whether or not you “backed one out” that morning)

That being said, lighter boards would be good for a completely different reason, that momentum would be lessened, and you could throw it around more easily.

But that is a whole other topic…

Careful, careful… don’t twist those words around too much, or maybe I said it porely. And don’t forget that when you’re sitting on your board, or laying, your body is in the water too - adding to the floatation.

My focus was on the science, as I’m amazed at how many people don’t really understand floatation. Unfare I guess, coming from a scientist, but I am what I am. It seems most people think the material makes something float, and a spongy, airy material should float more. I think this was illustrated when the previous poster mentioned compressed air. I believe the though here was, “Air makes things float better. A hollow surfboard with air inside should float really well. Wait… if I compress a bunch of air into the board, it’ll float super-duper well” And that’s obviously just wrong. Not to capitalize on the mistake, my appologies, but I think it illustrates many peoples thoughts on bouyancy.

My main point was that bouyancy comes from the displacement of a fluid, and the pressure that that fluid exerts on the object that did the displacement. So for surfboards it helps to think of volume, not foams and air…

Now I’m kinda middle of the road on the surfboard lightness/floatation issue. It wouldn’t be hard to look back in the archives at some of my posts where I was adamant that a few pounds meant nothing. This hit me the other day while I was sitting in the water, in a lull:

Lets talk shortboards, 6 lbs epoxy eps and 8 lb PU/PE (a little heavy maybe… I’m still working on my glassing technique) Now were talking a 25% reduction in weight - significant when you’re talking about doing snaps and feel and what not, rotational kinetic energy… blah blah blah. But does this affect paddling, drop ins, etc? As I said, I used to be a adamant “No”. Now I’m a “well…”

Most people will be quick to agree that adding an inch or two to a board (length, or maybe 1/2" width) will help paddling. We’re not talking monumental, but more than a little weight subtraction. Well, what about that 2 lb difference. In terms of volume that’s about 3% of a cubic foot (3% of 64 lbs/ft^3), which we said earlier (back of the envelope) for a shortboard is about 1 cubic foot. So that reduction of 2 pounds is the equivilent of adding abour 3% to the volume of your board, or, for a 6’3, adding about 2 3/8" in length (yes, I know, adding percentage lenth will increase volume on all sides, and probably give you a lot more than a 3% increase in volume, but it’s back of the envelope, for argument sake). Suddenly it seems more substantial, but I’m still far from convinced. Yet, I’d be fully willing to admit that I was wrong.

What - what? A scientist admitting he/she is wrong! Can’t be, scientists are never wrong; only engineers are wrong.

Naw, as an engineer, I agree with you. The biggest influence on the system is the human. At 160 lbs of human to 8 lbs of board, I don’t see how anyone can tell the difference in flotation. More likely, surfers are noticing differences due to shape and finish for a given size board. With the exact same volume and weight boards, but one with 3 inches of nose rocker vs. 5 inches of rocker, I would argue that you can notice a significant difference in paddling. Similarly, a sanded finish board will paddle significantly different than a glossed board. I bet these differences have been mistakenly attributed to “float”. Because, as you previously noted (excluding factors for surfer in the water, blah-blah-blah), the flotation for these two boards would be exactly the same.


It seems most people think the material makes something float, and a spongy, airy material should float more. I think this was illustrated when the previous poster mentioned compressed air. I believe the though here was, “Air makes things float better. A hollow surfboard with air inside should float really well. Wait… if I compress a bunch of air into the board, it’ll float super-duper well” And that’s obviously just wrong.

I’ve always wondered how divers could get all that compressed air down with them and always assumed that’s what all those lead weights was for… :wink:




I’ve always wondered how divers could get all that compressed air down with them and always assumed that’s what all those lead weights was for… :wink:

Heh… nice And Billybob. I lied, I’m an engineer (engineers lie too). But I enjoy reading and discussing the sciences… especially quannum physics… so I guess that kinda makes me… well, actually… a dork :slight_smile:

interesting topic… you might also consider the fact that the relationship between displacement and paddling ease is non-linear; as flotation/displacement increaase, at some point you stop swimming and start paddling. (Once you have enough lift under you to get your body out of the water, there is far less drag; so a larger displacement board should reach planing speed faster, cp…)

EUREKA! Now that science fiction, magic and absurdity have been eliminated from the buoyancy/flotation question, what remains is how best to configure the optimal volume of material in shorter form, say 7-0, that would paddle adequately and plane early. Are the principles involved in hydroplane boat hulls applicable to surfboards as Bert B.'s experience suggests? I need some feedback here-I intensely dislike the lack of responsiveness obtained from longer boards. SWAG is as welcome as “harder” information.

As an aside, Bert’s concave deck would appear to assist in gaining backside rail leverage for those who can’t touch their shins with their toes.

You can start with flat deck, thus thicker rails, leading to harder turned down edges to control the thickness.

Lower the nose rocker, keep the same tail rocker, move WP back to just behind center, keep the hips wide and curvy.

Don’t need wide nose, as that just forces you to paddle from forwards, but doesn’t penetrate steep drops or offshore winds.

Bottom shape…KISS…as concaves or channels tend to use up some wave power…Flat to V

You need just enough planing surface for your weight and wave size. Too little, it won’t catch waves well…to much, it will feel like a boat…

Hydroplanes have finite amount of power, much more than wave power.

If you used little stage winglets/channels, you might gain directional stability and power, but loose out in quick, snappy turns and cutbacks.

However, theory is not always the same as application. Channel bottom surfboards are the loosest surfboards on the water…so much so they are best for glassy water surf conditions only.