Design For Series Production
heat is generated causing laminate
and gelcoat distortions. The opposite
situation can also exist, i.e. lack of
sufficient resin will result in a dry
laminate. Both of these conditions
produce inferior lay-ups exhibiting
physical properties that are less than
ideal and with dry laminate the addi-
tional possibility of leaks exist.
Laminate glass content is the basic
parameter to be used in evaluating
the layup. The relationship between
laminate properties and glass con-
tent is well established and glass
content data is readily obtained from
samples in the average shop.
The designer should keep in mind
that the strength of a laminate and
its stiffness is dependent on glass
content and any slight variation in
the resin to glass relationship will affect these values.
Generally, decreasing glass content reduces the mechanical properties but thickness per ply increases resulting in
greater inertia, area, and section modulus. Thus, to some degree, these two factors tend to offset one another.
However, this is only true within defined limits that are well documented by the fiberglass industry.*
Deep pockets also present problems to the fabricator in terms of obtaining a sound laminate, one devoid of air
pockets or voids. Should they exist, leaks are possible and the reduced section can produce higher stress levels
(stress raises) with the probability of laminate failure (Figure 1).
To eliminate or reduce these problems, the designer should consider the alternatives available in lieu of deep nar-
row sections. Outside ballast instead of inside is one solution, for example (Figure 2).
3. Split Molds, Inserts - Many of the fine details obtainable with FRP laminates are only possible utilizing split molds or
inserts. Such details as cove stripes, recessed windows normally require molds made up of several sub molds.
During the lamination process the gelcoat flows into the joints between the mating surfaces which then appears as
a flashing or small flange on the finished part. Often this flashing is cosmetically unacceptable and must be re-
moved by grinding, sanding, buffing, and finally polishing. The polishing attempts to blend the joint into the overall
finish and depending upon the care exercised will accomplish this objective with varying degrees of success.
Through repeated use, the joints between mold surfaces tend to break down making the flashing areas larger and
more difficult to finish. Patching of the finished part is often required with its inherent problem of color matching.
* Ref: Scott,
Fiberglass Boat Design and Construction
Frequently a good match appears in the shop but exposure to the elements on the water will frequently in time pre-
sent a different picture and one that is unsightly.
If split molds or inserts are required the designer should attempt to limit the number and extent of joints requiring
post finishing. Many times the joint can be covered with a flange (window) or trim piece (rubrail, guard, etc.) (Figure
4. Sharp Corners, Tight Radius - Sharp corners, very tight radius (less than 3/8") produce many of the problems associ-
ated with deep pocket laminations. In addition the reinforcing laminates behind the gelcoat will often pull away
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