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The Principles:

First of all...
There is no ONE correct way to build a boat. Within the cold-molding technique, there is no ONE answer to any boatbuilding question. The methods described in this section will allow the amateur to create a strong and seaworthy craft. If something isn't clear in these descriptions, just ask. We are here for you. If something doesn't seem right, please, please, say something! We are human. These descriptions might not be as clear as we think they are or maybe we left out an important step. Could happen. Or, quite possibly, you know or have discovered tricks we don't. We are also happy to learn!
With that said...
The cold-mold process is a proven process based on the ability to create a light and strong core by:
  1. Laminating multiple layers of that core (in this case plywood)
  2. In alternating directions
  3. Forming a monocoque structure (key point)
It is called cold-mold because the binder (glue / resin) is designed to fully cure at room temperature. 
From Wikipedia: Monocoque, from Greek for single (mono) and French for shell (coque), is a construction technique that supports structural load by using an object's external skin as opposed to using an internal frame or truss that is then covered with a non-loadbearing skin. Monocoque construction was first widely used in aircraft in the 1930s. Structural skin or stressed skin are other terms for the same concept.
The primary purpose of the fiberglass on a cold-mold boat is not structural.  Although it certainly helps.  Fiberglass, when used in a cold-mold boat provides two primary benefits:
  1. It is for waterproofing as it encapsulates the core, inside and out.
  2. It also provides some degree of abrasion  / penetration resistance.
We have frequently heard the questions: "Why do you need two layers of plywood? Why can you just use one layer and fiberglass it."
There are two basic reasons for two layers. The first is to create the monocoque structure described above.
One layer of plywood only creates a unidirectional reinforcement. 
Wait. Plywood by nature has the grain running in alternating directions. Doesn't that, therefore, provide omni-directional reinforcement? Well, not exactly. While that is certainly true within each plywood strip, the strip itself forms a unidirectional reinforcing system. Only gluing the strips themselves in alternating directions in successive layers provides omni-directional reinforcement.
So, if you consider that one layer of plywood strips provides only a unidirectional reinforcement, what is desired is to develop a fiberglass laminating schedule that assumes that any unidirectional reinforcement like one layer of plywood isn't there at all. (This is no problem. There are many books on this topic.) Consider it as a male mold that just happens to end up glued to the boat.
So... What then are the implications? If you are not an expert in hand-lamination, and most amateur builders wouldn't qualify, then the build-up of fiberglass (on a male mold) required for a "fiberglass" boat will require significantly more time fairing.
The question of "single layer" typically comes out of concern for weight savings. Let's address that. It is well known that a cold-mold boat is one of the lightest construction methods. If you build a boat with a hybrid construction method - using one layer of plywood - and do it in a structurally sound manner by adding sufficient fiberglass - you actually end up with a heavier boat. Remember, you end up with a fiberglass boat with the mold glued to it. Virtually any commercially produced solid fiberglass boat is already heavier than any similar cold-mold boat.  
If building the lightest possible boat is your primary objective, then give us a call and we would be happy to discuss techniques such as using marine quality structural foams as the core.  Bear in mind however that for every significant decrease in weight comes a similar increas in cost.
Another question we often hear:
"Why do I have to use more expensive epoxy resins in cold-mold construction when polyester-based resins are significantly cheaper?"
Epoxy resins have much better properties of elasticity and can handle and "match" those properties of the core material much better than the comparatively brittle polyester-based resins. For this reason, they are very well suited to cold-mold. If you mistakenly chose to use polyester resins (which are well suited for solid fiberglass lay-ups) rather than epoxies, the boat will likely suffer from delamination problems in relatively short order.
Obviously, this overview only scratches the surface of the cold-mold process but it does begin to answer some of the high-level questions we hear most frequently. We are here to help. When you purchase one of our jigs / kits, we offer technical support during the build at no additional charge. Our success heavily depends on your success. Our customers openly (and positively) discuss our level of service on many of the boatbuilding forums on the Internet.
Last modified on 08-12-13
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