I suspect the answer is already in the Swaylocks archive, I did a quick search and was overwhelmed by the response, so I aborted… Anyway, the following addresses the benefits of using ‘the least amount of resin possible’, and to that end touches on the possible strength differences (which are likely to be small) between gloss and sanded finishes. (Writing this also made me feel better, given the line I’ve been using on my customers about the gloss vs sanded controversy, that is it being purely a matter of cosmetics.) Recently, while attempting to move some of my product, I was asked, ‘Are glossy surfboards stronger than sanded finish surfboards?’ Since the board I was trying to move had a sanded finish, I of course said, “No, where did you hear that? Its all just cosmetics.’ Well, I didn’t make the sale, and now after some research, I realize that I didn’t deserve to. A general rule of thumb is ‘the deeper the crack the higher the concentration of stress at the crack’s tip’, see illustration (Inglis, 1913.) Apparently, the effect can be quite dramatic, especially with non-composites – i.e. the gel and gloss coats! In a nutshell, it would appear that if you over-do the gel coat, or gloss coat, or even over saturate your laminate, you can weaken the board. That extra resin (free of glass) literally amplifies the stress at the tip of any crack which manages to form in the gel/gloss coat! (The excess resin in the laminate gets squeezed to the surface and pools because of the shrinkage of the curing glass/resin.) … no wonder stress cracks happen on the bottom with such frequency. Sure, the bottom is usually the brunt of a lot of tensile stress, but there’s that big puddle of resin between the laps, which surely can’t help. (By the way, once a stress crack makes its way to the glass layer, other mechanisms determining the mechanics of fracture come into play… which is interesting stuff, but not really the point here.) Curiously then, if you had two blanks similarly shaped and used the same amount of glass on both, but in the end one was heavier then the other, by the above reasoning there’s a good chance that the heavier board will actually be structurally weaker – that is heavy does not necessarily mean stronger! So an unfinished, or sanded finish board may actually say something about its strength, assuming in taking the board to a gloss you take off that much more resin. Its not always just cosmetics, you have got to know more about the board’s construction to come down one way or the other. Is this a big deal? It probably can be, but if your doing you glassing right, keeping the resin usage to a minimum, its not likely to be a ‘really’ big deal. So the customer had a point and in my haste to sell I lied? Well, maybe not lied, but spewed ‘ignorance’, … its not the same, I don’t think you go to Hell for spewing ignorance, otherwise I’ve got my bus ticket and seat reserved… still, I didn’t deserve the sale. If all this is old news, my apologies. I actually did think it was just a matter of cosmetics (… and the fact that I’m apparently only capable of sanding for about 2 hours before I’m ready to blow my brains out, and will therefore seek out any justification for not bringing the board to a full gloss.) Kevin 
Kevin: Not sure I agree with the conclusion. I think the stress diagram explains why cracks propagate. Regardless of the thickness of the “gel” the crack will reach the laminate. The thickness may define the time it takes to get to the laminate not if or whether it will get to the laminate. By the way…did your fin experiment work? Newbs
Kevin: Not sure I agree with the conclusion. I think the stress diagram > explains why cracks propagate. Regardless of the thickness of the > “gel” the crack will reach the laminate. The thickness may > define the time it takes to get to the laminate not if or whether it will > get to the laminate.>>> By the way…did your fin experiment work?>>> Newbs Here’s Inglis formula, (it is crude but handy) st = s ( 1 + 2 (L/r)^1/2 ) where, st is the stress in the region of the crack tip s is the stress in the material far from the crack L is the depth of the crack r is the radius of the tip of the crack and (L/r)^1/2 is the square root of L/r So lets consider two (hypothetical) cases, where only the depth of the crack changes, that is the s and r really don’t change. st1 = s ( 1 + 2 (L1/r)^1/2 ) and st2 = s ( 1 + 2 (L2/r)^1/2 ) then, after a little algebra st1/st2 = ( (r^1/2)/2 + (L1)^1/2 )/( (r^1/2)/2 + (L2)^1/2 ) But r is very small compared to L (the square roots may not be, but what the hell… ) So roughly, we have st1/st2 ~ (L1/L2)^1/2 Lets say, I really slop on the gel coat, say it winds up being 1/32 in thick on one board (L1 = 1/32 in), and on my next board (same shape etc…) it winds up being only 1/64 in thick (L2 = 1/64 in). Then, st1/st2 ~ ((1/32)/(1/64))^1/2 = (64/32)^1/2 = (2)^1/2 = 1.4 or the stress at the tip of a crack in board 1 is possibly 40% greater than in board 2. (These are crude ball park number to show the kind of differences.) Now the ‘thin’ gel coat board crack will hit the laminate much sooner than the thick gel coat board and stop, having at its tip a given stress. (The mechanics of fracture change at the laminate layer.) But the thick one progresses and eventually reaches the laminate, but when it does it will have a much greater stress concentration at the tip, and is more likely, or at least you increase the likelyhood of the crack progressing right into the laminate. … About my fin… Its coming along fine. Regretably, the fellow who owns my shaping house (basically a shack with two rooms) is in the process of selling the place. Actually, the shack is worthless, its the land that has value. And the value of land in my little corner of the world has gone through the roof. The locals are all taking part in the windfall (I think out of the fear that it might be temporary) - its a real feeding frenzy. Virtually all of the towns in the area seem to be in the process of re-zoning commercial to residential, and want no part of me… ‘waste land’ is now ‘rustic’. There won’t be an industry left, just a lot of police, mega-homes and fences. So at the moment all extra projects have been slowed down a bit, as I am trying to put together some alternatives. My main thrust has been finishing and selling the blanks which I have… and there’s just one left now. Nevertheless, the fin was laid up, cut and sanded, and I hope to pour the silicone sometime next week. My final board will have priority though. Still this hasn’t stoped me from consider another solution… next post.
Pterodactyl 1: A need to be restrained? Chinese Junks. One of the reasons why these curious crafts are beautiful is in the way they handle stress, in particular the stress in their sails. Under a load their sails bellow out, decreasing their radius. The circumferential stress in the wall of a cylinder is given by, s = rp/2t where r is the radius p the load (on the concave side) t the thickness of the container The way this works is see in the illustration. In effect, you have a sort of feedback system which will maintain a relatively constant level of stress over some range. That is with increasing p, r decreases. Of course the effect is accompanied with a change in shape, but that’s ok. So, consider the weird fin in the illustration, Pterodactyl 1 (Named after pterodactyls which apparently beat the Chinese to the solution. Bats also use it.) As the load increases, the fin bellows out, in the process maintaining a relatively constant load on the fins structure. In the process the profile presented to the flow is reduced. Now (forgetting for the moment the larger question, “Will it work?”) for those really powerful turns one tends to rely on the rails and less so on the fins, as a matter of fact, if all you did was power turn you could probably get away without fins (consider skimboard surfing, that is when they actually make it onto a wave). If nothing else, small stiff fins would likely be the order of the day during power turns. The Pterodactyl will, during heavy loading diminish in size. Not only that, it will not unload with a vengeance (especially with my yet to be tested shock absorber.) (Forgive me, I seem to be talking as if this is all true, obviously it all has yet to be tested.) During those ‘trim’ kind of turns (adjustments) the profile of the fin will remain large and function (hopefully) as your average fin. Here’s the kick, the stiff bits I make out of carbon fiber, the floppy center membrane I make out of an old wind breaker. I figure it will be half the weight of a regular fin, but the weight to size ratio will be almost constant. And then there’s the additional possibility of adding a little control (ability to adjust the bellows.) At the trailing edge of the fin, or somewhere a little foreword of the trailing edge, extending from the bottom across the bellows, you put a piece of piano wire, or some really tough nylon cord, with some sort of way to adjust the tension, and hence the size of the bellows. So if you like a little more fin with your turn, tighten, a little less loosen… stupid or what? Ok, sure it looks hoaky, and the possibility of getting the construction right (so it doesn’t just flex out of the way with the smallest of loads) is touchy, but still you have to admit its kind of wild. Especially if you reduce the whole application down to the size of a standard shortboard fin… super light fins which adjust there profile with the load. Anyway, I still have the extended pliable tip fin to finish… hopefully, I’ve got a few more weeks before they level my little shack. Kevin 
The mechanism governing fracture in composites varies, but the following is believed to describe what is happening in fiberglass. It should be pointed out that this mechanism is in the glass/resin layer. (The gel and gloss coat, if sufficiently thick, fractures via the mechanism as illustrated in the prior post.) Basically the crack progresses through the material until it is in the vicinity of a fiber. The stress concentration ahead of the crack is very high, see prior post, and the weak interface between glass-fiber and resin breaks prior to the arrival of the crack. When the crack finally hits the fractured area, it stops (or has an increased likelihood of stopping) because of the geometry of the fractured area, i.e. you might say that the radius of the tip of the crack has been sufficiently increased to reduce the stress concentration, see prior posts for Inglis equation. In fact what appears to happen in fiberglass is that the fracture, once it hits a fiber, changes direction and travels along the fiber until it hits another fiber headed in another direction. All this fracturing is why dings appear to be white, or whitish. Curiously, and this may be hard to believe, the strength difference of cracked fiberglass is not much different from the strength of uncracked fiberglass, … uncracked fiberglass is more resilient or pliable though. 
Some definitions, *** Stress *** Stress is the load per unit area. s = P/A (where s = stress, P = load, A = area) Stress has dimensions of force per unit area, like pressure. Example A fellow weighing 185 lbs. manages to stand on a brick with a surface area of 12 Sq inches, the stress in the brick is s = 185 lbf/12 Sq in = 15.4 lbf per in, or 15.4 psi. *** Strain *** Strain is the amount of stretch under load per unit length e = l/L (where e = strain, l = total amount of stretch, L = original length) Often, e is given as a percentage, that is as if L was 100 units. Strain has no dimensions. Example A rod, 125 inches long, stretches 1 inch, then the strain is e= 1/125 = .008 or .8% *** Young’s Modulus, or Elastic Modulus *** It is found for many (if not most commonly used engineering materials) that the ratio of stress to strain is constant over some range (of stress or strain), this constant is Young’s Modulus and has the dimensions of stress. E = s/e (where E = Young’s Modulus, s = stress, e = strain) Young’s Modulus is often referred to as stiffness. Example Material… E (roughly) Unreinforced plastic … 200,000 psi Wood (along grain)… 2,000,000 psi Glasses…10,000,000 psi So to have strain of 1% or e = .01 in plastic, that is to stretch a rod initially 100 inches long to 101 inches, would take a stress of s = Ee s = 200,000 (.01) psi = 2000 psi In wood, s = 2,000,000 (.01) psi = 20,000 psi In glass, s = 10,000,000 (.01) = 100,000 psi Hefty numbers huh? *** Stiffness *** See Young’s Modulus. *** Strength *** Strength is the force, in this case stress, needed to break a thing. Strength has units of stress (when referring to materials, however when referring to structures, often strength is given in units of force.) Example Material… E (roughly)…Strength Unreinforced plastic … 200,000 psi… 50,000 to 15,000 psi Wood (along grain)… 2,000,000 psi… around 1,500 psi Glasses…10,000,000 psi… 5,000 to 25,000 psi As you can see, though there a lot of variation in the numbers, however one thing is clear, strength and Young’s Modulus differ dramatically - Young’s Modulus is not an indication of strength. *** Resilience *** The ability to return to its original form when unloaded, that is it more or less is unchanged by loading and unloading. *** Elastic *** Materials or the science of materials that behave in a manner as described in the definition of Young’s Modulus. *** Plastic *** Materials or the science of materials which do not behave in a manner as described in the definition of Young’s Modulus, but in tend to irreversibly change shape when loaded. *** Toughness *** The ability to handle fractures, or cracks. *** Structure *** A structure is basically materials arranged with a certain geometry. The point being that the ultimate strength of the structure is influenced by the materials used, but that how the load is distributed is basically determined by the structure’s geometry. For example a tube or a pipe can be expected to behave a given way, that is distribute their respective loads in fixed way. But a tube made out of sheet of paper, and one made out of cement, will have differing abilities when it comes to handling a given load. (See Material definition.) *** Material *** Materials don’t (at least theoretically) have any particular geometry, but have properties which are specific to that material, e.g. tensile strength, Young’s Modulus, etc. That is, the geometry of the structure in which the material is used will not change the tensile or compressive strength of the material. For example, a beam made of cement and a tube made of cement can be expected to fail if the stress reaches the ultimate tensile or compressive strength of cement, the two structures may appear to behave differently however. Please Note: It is of course possible to describe a structure using these terms, but the terms will then refer to the whole, as opposed to any given material. For example, to say a given surfboard is strong, possibley implies that not only are the materials strong, but the shape, and construction, that is it’s structure is also possibly strong, or the combination is strong. (See Structure definition.)
but 40% of a very small number is still a very small number. In the big picture, I’m not sure that the gel and/or gloss was intended to provide that much additional strength to the laminate. It’s more a filler to smooth out the irregularities of the lam and to make it shine…
but 40% of a very small number is still a very small number. In the big > picture, I’m not sure that the gel and/or gloss was intended to provide > that much additional strength to the laminate. It’s more a filler to > smooth out the irregularities of the lam and to make it shine… I agree about gel/gloss adding little strength to the structure. But a 40% increase in stress is kind of hefty (if its true) and will definately make an existing crack migrate that much farther. Once the crack does hit the laminate layer things do change however, see Composite post in this thread. But at the laminate layer you still have to deal with the increased stress concentration. The point is that by using excess resin you are amplifying the stress concentrations at the laminate layer unnecessarily. The resin doesn’t seem to play any great role in strengthening the structure, but it does play an important role in toughing the composite (See Composite and Definitions post. Tough and strong are not the same thing; tough refering to the ability of the material to handle fractures, strong refering to the ultimate breaking stress.) All this doesn’t make your point any less significant however, the gel coat definately works the way you mention. But if what I have said is true, then using as little resin as possible (in general) will make for a stronger (and lighter!) board. So heavy does not necessarily equate with strength in surfboards… less is more. Kevin
I agree about gel/gloss adding little strength to the structure. But a 40% > increase in stress is kind of hefty (if its true) and will definately make > an existing crack migrate that much farther. Once the crack does hit the > laminate layer things do change however, see Composite post in this > thread. But at the laminate layer you still have to deal with the > increased stress concentration. The point is that by using excess resin > you are amplifying the stress concentrations at the laminate layer > unnecessarily.>>> The resin doesn’t seem to play any great role in strengthening the > structure, but it does play an important role in toughing the composite > (See Composite and Definitions post. Tough and strong are not the same > thing; tough refering to the ability of the material to handle fractures, > strong refering to the ultimate breaking stress.)>>> All this doesn’t make your point any less significant however, the gel > coat definately works the way you mention. But if what I have said is > true, then using as little resin as possible (in general) will make for a > stronger (and lighter!) board.>>> So heavy does not necessarily equate with strength in surfboards… less > is more.>>> Kevin That`s right, Kevin! For instance, instead of a thick, double six ounce, think a lean resin lamination using 3, four ounce layers… higher glass content, less resin.
That`s right, Kevin! For instance, instead of a thick, double six ounce, > think a lean resin lamination using 3, four ounce layers… higher glass > content, less resin. This IS the next logical step! … and it shall be so on my next board. Thank you, Kevin
Forgive me for continuing this thread, but I think what I present here will be of some interest to those considering how they might incorporate additional ‘flex’ into there surfboards. Before considering the laminate and gel-coat layers, consider a much simpler example. The purpose of which will be to introduce some of the concepts. Consider a rectangular sheet made of any (elastic) material you like (see Definitions post, this thread for a definition of elastic.) Under simple loading, see illustration Example 1 - General Treatment, for the two cases; Sheet 2 being twice as thick as Sheet 1, all else being the same, including the applied load. We have the following results from engineering Beam Theory, For the same load, we have the following rough results, Max Stress in Sheet 2 = (1/4) Max Stress in Sheet 1 Max Deflection in Sheet 2 = (1/8) Max Deflection in Sheet 1 Slope at End in Sheet 2 = (1/8) Slope at End in Sheet 1 Not surprisingly, Sheet 2 would likely feel ‘stiffer.’ Not only that, but as a structure, appear to be ‘stronger.’ (Remember, that both are made of the same material so what I’m refering to is more of perception, or an structures operational behavior.) What is also curious, is the slope at the ends of structure, that of sheet 1 being significantly greater than in sheet 2. So, if these were surfboards (which for me is not that far a stretch as I have been accused of making the occasional plank), the difference amounts to something like 12% more rocker in the tail or nose for a given load,… but they’re not surfboards!,… still the principle is likely too apply in very general terms – thicker means less ‘flexible’, that is what we (surfers) refer to as flexible. These results shouldn’t surprise anyone as they tend to be in the realm of common experience (maybe not the numbers, but the experience.) Now however, consider sheets made in the same way we make the glass/resin shell of a surfboard; that is, a laminate layer, followed by a gel-coat. See Example 2 - Surfboard Shell Sheets. (Here I merge the gel and gloss coat into one.) In Sheet 2, both the laminate layer and the gel-coat layer have the same thickness (sure that is unlikely, but here they do) and its compared to Sheet 1, which is just the laminate layer. Before things start to crack or break, roughly the same results apply that are described above for the general treatment. And therefore, on this basis alone, one might be tempted to argue that the gel-coat surely adds ‘strength’ to such a structure - it would appear to do so by thickening it. At least, it will require a greater load to break the thing (structure)… using the kind of setup in the illustration. Of course the structure, Sheet 2, will be a little less ‘flexible’, that is you won’t see as much of a deflection or slope change at the ends for a given load (it will be ‘stiffer’, at least the stiffer in the way most surfers use the term.) But still in the end, you’re likely to conclude (given the above) that you’ll wind up with a ‘stiffer’ and apparently ‘stronger’, albeit heavier surfboard, if you lay on the gel-coat!>>>> This conclusion is the exact opposite of what I concluded in my prior post, well, at least given what I described so far. (I love this stuff.) For those of you who were curious enough to read the Pterodactyl post, and I wouldn’t blame you if you didn’t. I mentioned a neat thing about Chinese Junk sails, in particular that as the load on their sails grew, the radius of their sails diminished. But because the circumferential stress was directly proportional to both the load and the radius, that the behavior of the sails (load increases means radius decreases) the final stress over a large operating range tended to be somewhat constant, or fairly flat. You could apply the same principle (cautiously and in a somewhat convoluted manner) to flexing surfboards, that is as they ‘flex’ with an increased load, the stress in their decks and bottoms, etc. remains relatively flat or flatter (over some operating region.) A flexible board, would then be more able to handle greater loads – it would do this by literally flexing out of the way. The contrary for a stiffer board. Back to the sheets of surfboard shell structure, the same argument applies here, the sheet with a greater ability to ‘flex’ will be able to handle a greater load, a stiffer sheet reaching its breaking point sooner.>>>> So we have the first bit of the paradox, ‘stronger sure, but yet potentially weaker, operationally.’ But it gets better. The gel-coat, as described in prior posts, acts like an stress amplifier, when it finally does crack, (and such brittle materials will potentially do so explosively!) So, with a thick gel-coat under increasing load, unable to flex out of the way, a little crack starts in the gel-coat, and given the Inglis crack mechanism, which now amounts to a positive feedback, the crack rapidly migrates to the laminate layer. Now surely once it hits the laminate layer, it must start to behave as sheet 1? This is where things get interesting, for as mentioned in my prior post, the increased area of stress concentration precedes the crack tip, it literally runs ahead of it. So what you get is a hell of a lot more stress hitting the laminate layer than as you might expect from simply stressing Sheet 1 – the net result being a busted laminate, that is, its enough to bust the glass and a lot more.>>>> Any crack forming in the gel-coat has the potential to ultimately amplify the stress concentration at the laminate layer. The net result, a heavier, operationally weaker structure. (At least that’s the argument.) Am making all this up? Well, sort of, in the sense that I’m piecing bits together the best I can. I haven’t been able to find any information on the plus or minuses of gel-coat thickness in any industrial application. Could I be way off base? Definitely! Its all pretty much theoretical, backed up by vague subjective hearsay and just as subjective observation. Not to mention how far I’m stretching simply Beam Theory. Still its at least consistent with comments from some of the masters in our industry. (Like Jim Phillips regarding glassing – regarding the resin content in the glass layer, that less is better. Jim, if you are actually reading this, my apologies if I’m miss-stating you. Please correct if this is the case.) For those who seek to get themselves in similar ‘theoretical’ trouble, see Roak’s Formulas for Stress and Strain, and for some general problem solving and introduction to Beam Theory, try Schaum’s, Engineering Mechanics - Statics and Dynamics. It should be noted that I do not wish to imply that the way I’ve loaded the beams, as illustrated, is the way beams are loaded in a fluid. But, I do believe the general conclusions are correct. … and though I did not go into it here, Beam Theory has some interesting formulas that can be (dangerously) applied to surfboard in general… and I definately recommend doing so. Hydrodynamic loading is of course diferent than the fixed point loading as in the diagrams, but still its not hard to imagine how the parameters which appear in the formulas will determine or bear on deflections of the structure. The formulas are not that difficult, the problem is one’s willingness to make certain assumptions… dangerous? sure, but fun. 