A topic that occasionally comes back: the building of fiberglass-sandwich one off boats without mold.
I have just finished responding to two home-builders on the subject, so I also report here my considerations.
(in the photo the hull structures of a semi-displacement engine 33, built on site in sandwich from a "disposable" mold)
--The short version is: forget it.
- the long version requires you to develop some more considerations:
- external finish: the hours of work required to arrive at a good external finish of the hull (bottom sides, etc) are ONE MOUNTAIN more than that of a boat of the same size in radius chine or in multichine plywood or in molded fiberglass ; as much as you can work the foam well and start from a nice smooth core, the lamination of the glass layers of the outer skin will give you a surface with several imperfections that you will have to fill in subsequent passes, if the boat starts to become a crispy 650 the hours of filler-smooth-filler become days of filling ...
- modeling the foam on a rounded shape is not trivial, the foam is rigid, it must be convinced to remain in the fold, the joints glued between foam panels must be done well and perfectly started without steps, in areas with high curvature (bow, of the knee on non-edged hulls) is quite a nuisance; in the mold the core of the sandwich is glued by pressing it into position with the vacuum in 99% of cases, out of the mold everything is more complex
- build the molded shape where to rest the core: an operation not complex but which requires a certain amount of material and hours of work because the mannequin must have very close sixths and currents to be able to model and fix the core effectively
- skin-core bonding: this is also a process that in the mold and with the vacuum is all in all ordinary but requires some attention, done outside the mold by laminating by hand on the core is a process that requires precision, experience and attention and is THE process essential to ensure that the hull is structurally solid
-times: having built a few boats also in GRP sandwich (from mold) I venture to quantify that building a crispy 650 in fiberglass sandwich one off takes away about double the hours of work than to build the same hull in plywood and epoxy.
- costs: we often get enticed by the fact that we can use the cheaper polyester resin instead of the expensive epoxies, but in reality considering that we need much more resin and that airex or other SAN or PVC cores do not they give, indeed, and that we really need so many kilos of glass fabrics, in the end the bill goes even with a plywood and epoxy construction
-finished product: for me from a structural point of view, leaving aside the "emotional" aspects, the two hulls that emerged from these two processes are essentially equivalent.
Cristian Pilo on facebook Nautikit Group
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