Greetings Swift Builders,
There has been some email discussion lately, among some class members, about
the shortage of hybrid and carbon fabrics and the different forms of Kevlar and
weave patterns available. So I'd like to add a few comments and put the
discussion in the sunlight and in perspective.
I think most of us have realized that the Swift Solo hull design provides
over-engineered strength and rigidity by its monocoque hull, deck and bulkhead
arrangement. The design purposely relies heavily on the excellent properties of
the cedar core. The fibers of the core material (a natural pentose ring tubular
carbon fiber) provide excellent strength for'n'aft but the hull structure
must be reinforced athwartships. As Meade Gougeon has shown great increases in
strength and stiffness are obtained by adding carbon across the core grain of a
stripped hull but much lesser gains in stiffness are obtained by reinforcing in
the fore'n'aft direction. Because we use cedar cores in our class Meads
observations allow us to use carbon reinforcement in the one direction only
(athwartships) without much loss of stiffness in the hull.
We are using Kevlar as a reinforcement mixed with graphite in hybrid fabrics
inside the hull, specifically in our case, to protect the core and prevent the
compression of the core under the bulkheads (as well as to provide damage
resistance, and help prevent catastrophic failure modes on acute impact). Such
damage to the core under the bulkheads would lead to a noticeable loss of
stiffness and the ability to maintain shroud tensions. (Something that is
plaguing other foam core skiff classes). The lower modulus and resilient
stretch properties of Kevlar allows the material to disperse loads especially
well, particularly off-axis loads, which might otherwise fracture carbon fibers
that are particularly poor in off axis strength. It is clear that placing
carbon fibers under the compression load of the bulkheads is not the optimal
use of this material and that Kevlar to protect the carbon fibers would be the
material of choice for that function.
Since the fore'n'aft Kevlar fibers are not being used to develop really
significant stiffness in the hull either of the two common types of Kevlar,
Kevlar 29 or Kevlar 49 may be used in our application. John Sweet has been
supplying the higher modulus fiber (K49) in his hybrid fabrics (Style 94905).
Most of us have used this fabric since I arranged a Swift kit price with John
for the class. This style material is out of stock for the foreseeable future
and an alternative fabric and supplier will have to be sought.
As a practical confirmation for the skeptical, most of you know Bram has three
boats, two of these boats were built with the bidirectional 94905 hybrid and
the other with the crossed directional K29 fabric and there appears to be no
difference in the stiffness of either of these boats measured under stringent
"Bram conditions" over a period of at least a year. Bram did mention to me
however, that he believes there is more variation between his boats that have
the same style hybrid inside than the two boats that have the different styles
but that it's most likely a mast or shroud thing anyway....
In my boat, to hedge my bets, I have run the hybrid 94905 fabric at a 45 degree
bias inside the hull, and tabbed the bulkheads and case with hybrid tape. I'm
not sure this is necessary but I don't think it can hurt, I'm sure the boat
will be as stiff as all getout with paired Kevlar and carbon fiber in either of
the two axes.
Finally, Al Pritchard and I have found a source of Hybrid stocked in three 100
yard rolls at 60 inches wide. There are hefty cut and re-pack costs from the
distributor for this fabric but if there are 5 buyers in a group a whole bolt
of that material becomes a workable deal. NYC SKIFFS will cut and repack your
section of the roll, gratis. Let me know if you need a roll of hybrid we will
try to put a group buy together.
The table here shows some values I collated from several sources
Fiber
Density
g/cm^3
Tensile Modulus GPa
Tensile Strength GPa
Kevlar29
1.44
60--83
3.6
Kevlar49
1.44
131
3.6--4.1
S Glass
2.54
80
2.0 - 2.5
E Glass
2.60
22.0
3.4
Carbon
1.75
230.0
3.5
Steel
7.87
210.0
1.3
Note: The modulus of the Kevlar is ½ to 1/3 of the contribution of the carbon
and while the tensile modulus and strength of Kevlar 29 is roughly comparable
to that of S-glass, its density is almost half that of glass.
Greg
Swift Solo -- Rose
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