A post in boat design forums
from 2005 by Eric W. Sponberg
I did a technical paper some years ago that was published in Marine
Technology of SNAME, Vol. 23, #2, April, 1986, pp. 165-174 called "Carbon Fiber Sailboat Hulls: How to Optimize the Use of an Expensive Material". It was also referenced by Larsson and Eliasson in Principles of Yacht Design. One of my references
for this paper was another paper that I did before that for the Southeast Section of SNAME, Sept 1983, called "Fundamentals and Practicalities of Carbon Fiber Composites for Marine
Applications." It became quite a bible for a lot of people in the marine industry.
I did physical tests and engineering analyses on 5 different laminates using unidirectional materials over foam core
(4 samples) and one with balsa core
(1 sample). In all the samples, the fibers were always running in the same direction, that is, I did not have any laminates with non-zero orientations.
#1: 3-ply S-2 glass, equal thickness skins both sides of 1/2" Airex core.
#2: 1 x S-2 glass + 2 x Carbon + 1 x S-2 glass, equal thickness skins both sides of 1/2" Airex core.
#3: 3-ply S-2 glass skins one side, 4-ply Carbon other side of 1/2" thick Airex core.
#4: 3-ply Carbon, equal thickness skins both sides of 1/2" Airex core.
#5: 3-ply Carbon, equal thickness skins both sides of 1/2" balsa core.
a. computer predictions for strength and stiffness
b. test results for strength and stiffness
c. weight per unit area
d. cost per unit area
e. specific strength vs. specific stiffness (i.e. strength and stiffness divided by weight)
f. a desirability factor
g. Impact tests.
The results were:
Laminate #2 was the overall winner when considering all of the factors above. The all-carbon laminate scored very high for strength and stiffness, but at the time, a very high cost. The cost is still pretty high.
Laminate #3, where the S-2 glass is one side of the core, and carbon is the other side, was the worst laminate. It is not strong or stiff, and it wastes the mechanical advantages of the carbon, particularly considering its price
In my practice, I always advocate for the balanced laminate. When mixing materials of different strengths and stiffnesses, it is always beneficial to make the skins mirror images
of each other either side of the core. You get the most efficient use of the materials. This mirror image technique is quite fundamental to general composite design, and has been proven innumerble times in engineering analyses and tests.
I don't always mix materials, and every design seeks to satisfy different needs. As you are aware, the science is complex, so you have to pick and choose the best combinations for any particular design.
I hope that helps.
Eric W. Sponberg
St. Augustine, FL