OzBungy wrote:I have had two kinds of failures over three boards.
Two boards appear to have leaked around one of the thruster fin boxes (both Futures boxes, one heel side, one toe side). There are no visible cracks around the fin boxes but the water stains can be seen in this area.
easy problem solid divinycell H80ish jammed into the core from top skin to bottom skin. In the area in and around the fin boxes. put it in before shaping. once shaped, it adds very little weight, is closed cell (waterproof) and takes a beating. I'll see if I can find a pic of my build. will have to shape with a router or planer though, sanding will not work well.
Also gives you mad dent resistance in the rear foot area.
I ALWAYS bash my fins on crap when I ride. Was always breaking fins so I started making my own solid G10 probox fins.... Now I just get ejected....... or rip the set screws clean out of the plastic fin box .....(can be repaired epoxy milled glass and a few pieces of carbon tow) regardless infinitely fewer failures and NO LEAKS.
S A wrote:
The problem is that we are all after the magic combination of lightest weight, highest strength and great flex characteristics and they are all (almost) polar opposites, especially when you ad pricing into the mix
S glass has the best flex, (lowest young's modulus) almost identical strength (Ultimate tensile strength) AND the best toughness(drop test results....most relevant to kiteboarding) as well as being the lowest priced of carbon or Kevlar.....
It is a far better material choice for surfboards
As for core crush. This has to be designed into the sandwhich, The EPS core is pretty useless against a point load. So the sandwhich structural foam is there in combination with glass to create a beam to protect it.
I really wanna work out the calcs and weights of a hollow board I suspect if you took the weight of the core and put it into something more structural you'd have a superior board
In general you want to make sure the shape of your heel gets distributed over the largest area possible on the foam beneath... 0-90 and 45/45 plies in combination make for better load distribution while the 45's don't have that much impact on the overall tip to tail board flex
In addition to the higher density core SA mentioned
some other lighter weight solutions are
More plies (both above and below the sandwich) will protect the EPS.
thicker sandwich material makes the sandwich stiffer
Deck pad. (best bang for the buck) distributes the point load of your heel to a larger area of the top deck.
you should be able to get more bang per gram out of the above than you would using wood veneers
Your particular failure "a crack" sounds like a full failure of the sandwhich an extra ply or two on top along with a deck pad can go a long way. but if you're really pounding these things a core with some real compression toughness may be the lightest way to do this.
Some materials notes.
Combinations of cabon, + Kevlar with glass should generally be avoided. especially in a patch . they have much higher stiffness than glass meaning that when the assembly flexes the carbon or Kevlar takes virtually all the load and will fail first while the glass is just along for the ride.
Most people assume Kevlar has high impact toughness.....it doesn't. It will fail in impact (drop testing) before glass. The fibers tend to stay intact however...giving you a nice aramid woven basket to take home your broken surfboard in.
Works great for ballistics as it doesn't penetrate....but still breaks. Might have an advantage in a tumble on the rocks. but when jumped upon Kevlar will behave worse than traditional fiberglass.