I've pulled the peel ply off my board, and started shaping it. There
are a couple observations:
1) the outer skin (at least) is resin starved. I'm pretty amazed by
this - I was quite careful to ensure that each application of epoxy
saturated the level, all the way down to the trailling edge, and I
squeegeed everything slightly to ensure even application. I think
what happened is the absorbent layer was too absorbent - I used cotton
similar to t-shirt weight, and it was just too thirsty. I'll use
muslin on the rudder.
2) On the rudder, I think i'm going to clamp the fabric exiting the
trailing edge with two strips of cedar on either side. Given that
there's just point contact on the trailing edges with the saddles, and
the saddles may not align perfectly, this seems like a reasonable
precaution.
3) I'm thinking of putting a strip or two of cedar through the length
of the rudder. Not sure on this yet.
I'm using the thick (v1) version of the foam cores - the ones you
built with, Bram. I'll keep shaping this weekend.
Warren
On 22/11/06, BDally6107@xxxxxxx <BDally6107@xxxxxxx> wrote:
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
--
warren stevens - catzooks.com - 299f3c94-d7c1-11da-95c0-00e08161165f
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