steveJ wrote:
Glen and all, Would you agree that the Gougeon Brother's On Boat
construction book, when discussing wood strip composites, talks only
about small boats, canoes,etc? I think they mentioned that further
research was needed and you have apparently done some of that. Also,
MacNaughtons are the only people that I am aware who have done any
published work on scaling up the scantlings that the Gougeons
initially offered. I'm curious to know if there are any other
published testing data that you know of?
There is not a whole lot of published work out there. Baltek has done
several white papers on their Duracore strip/composite, PBB has done a
few articles and I found some other research done by universities. It
has been 5 years since I did all that research so It will take some
digging to get the references. Large scale strip composite is not in
the "experimental" stage though. Several mega yachts in the 120'+ range
have been built in the last few years.
When scaling up to thicker strips, edge nailing is frequently used to
hold things in place. I think MacNaughton mentions that the nails
are not a structural element in the finished monocoque shell. I'm not
disagreeing with what you are saying. But I just wonder how scaling
from a canoe with a 1/4 inch strip thickness translates to a larger
boat of, say, 3/4 inch thickness. It seems that most of the larger
strip built boats have structural bulkheads which may come into play.
Actually strip/composite requires less interior structure by a long way
than most other methods. My 45' hull has only a single structural
bulkhead at the mast to take the transverse load of the shrouds.
Scaling up is basically a balancing act between strength and weight
using glass and wood. In a 1/4" stripper canoe with a layer of 6 oz
cloth inside and out, the wood is the major structural component and the
glass acts more to hold it together and protect the wood. The wood
carries most of the load.
As you go up the the strips act more like a core (but not completely)
and glass gets more critical. Design work gets a lot more complex. The
strips on my hull are 1 1/8" thick. The hull was simulated to examine
the stresses and each side was divided into 5 areas. In each area the
unidirectional glass oriented to the lines of force and the number of
plys and type of fabric adjusted to local requirements. The wood
carries most of the fore and aft tension forces and virtually all of the
compressive forces in all directions.
Possibly bead and cove milling, rather than square edge strips might
add enough sheer load resistance to warrant using yellow glue. My
preference on small boats is to use epoxy with micro balloons to make
sanding easier. Without the microballons the epoxy is harder than
the cedar and is difficult to sand fair. But yellow glue works too
and, at least on small boats, it is faster then waiting for epoxy to
kick so you can lay more strips in a session. I've also tried
powdered Plastic Resin glue but never had much luck with it since it
does not really bridge gaps too well. Titebond II is definitely a
Royal PITA to sand.Too rubbery. On one small boat I built I used
epoxy to edge glue the strips. The hull seems very stiff and strong.
But I have cracked the hull when hitting a rock. On another small
boat I built I used yellow glue and the hull seems more flexible and
has a "dead" sound when tapped. But I have never cracked this hull
even though I've given it some rough use. It's also only 3/16 inch
thick. Mabey flexibility is a good thing.
Bead and cove definitely helps. Besides making alignment a lot easier
iy widens the glue surface. I have found that a mix of about 50/05
cabosil and microballons makes a good sound sandable epoxy joint that is
only slightly harder than the cedar. IF the hull will be in the water
for only a few hours at a time i suppse that Tightbond would work but if
it sits in the water for days at a time the MC is going to get high
enough to weaken it.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com