Warren and others,
Let us know how your blades turned out?
I've been reluctant to get into this discussion because it's hard to know
how to address this issue since I simply don't have enough experience to say
with certainty what is the right and wrong way to do this lay-up.
Let me start by telling you that I've only built three blades using foam
cores so far. I destroyed one of those by overheating it after applying a
thin
epoxy filler coat (I thought the heater was on low). The other two are the
rudder and centerboard that I've been using exclusively for over a year.
Those blades show no signs of problems. I would expect them to soften where
they
exit the bottom of the rudder and centerboard cassettes if they were going
to fail and neither have done so. They were built using the lay-up and the
method explained in the PDF.
A couple of things need to be pointed out with regard to the new thinner
daggerboard blanks. I'm going from memory but I believe that the old blanks
were 1 3/16" thick and produced a blade that was 1 1/4" thick. The new ones
are
15/16" thick and will produce a blade that is 1" thick. Generally speaking,
If you use the same lay up on both boards, the new thinner blade will be 64%
as strong as the thicker blade. I have no idea if this is strong enough but
is suspect that we need to add two additional layers of hybrid to
centerboards boards built with the thin blanks. Remember this, the strength
of the
blade changes with the square of the distance between the two skins.
Regarding internal beams: My personal belief is that looking for strength
from an internal beam is somewhat futile unless specifically engineered to
mate with a more elastic outer skin. The current skins are capable of
handling
loads far in excess of what they'll likely experience (both compression and
tension) Failure will likely occur only if the skin is penetrated by a point
load or if the skins are allowed to come out of column (get closer or further
from the skin on the opposite side of the blade).
Internal ribs may well be a great idea in that they can attach the two skins
with a fixed spacer which will both keep them in column and maintain a fixed
distance from one another.
On the other hand, the skins on an internal beam will, by definition, be
closer together than the otter skins of the blade. Because of the lack of
elasticity of the outer carbon skins on the blade, it is very doubtful that
inner
carbon skin (on a beam) with the same modulus will ever see much tension or
compression until the outer skin fails completely (the square of the distance
rule). To make an internal beam work we'd need to engineer a complex
structure with perhaps s-glass on the blade skins and carbon on the beam skins
(please don't think I'm suggesting this because it is much more complex than
this
simple example).
The good news is that ribs are much easier to deal with than an internal
beam anyway since they only require a saw cut in the foam to facilitate
insertion. I will attempt to get to a new blade and pdf this winter showing
this
technique but the following is the idea:
I suspect that the three ribs will be made of hybrid strips cut from a flat
sheet of perhaps 6 layers cut to the appropriate width with one in the
fattest point and one 1 1/2" on each side. These three strips will stop
short of
the top of the blade by 2" and short of the bottom by perhaps 12". Assuming
that the center strip is 15/16" thick, I'll cut a "V" out of both ends that
go in about twice that distance in order to reduce point loading of the skin
at the ends of the strips. By leaving the ragged Kevlar fibers sticking out
on the edges of the strip (rib), my hope if to get a great bond between the
skin lay-up and the ribs.
Let me know if this doesn't make sense.
Best regards,
Bram
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