There are probably many people wondering about the same sorts of questions so we thought we'd write a detailed answer. (Sorry about the length)
First and foremost: You are certainly able to build your own boat any way you want.
However, let us give you some things to consider.
The cold-mold process is a proven process based on the ability to create a relatively light and strong core by:
- laminating multiple layers of that core (in this case plywood)
- in alternating directions
- into a monocoque structure.
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-load-bearing skin. Monocoque construction was first widely used in aircraft in the 1930s. Structural skin or stressed skin are other terms for the same concept.
By the way, the primary purpose of the fiberglass on a cold-mold boat is not structural. It is for waterproofing. It encapsulates the core, inside and out. It also provides some amount of abrasion resistance.
Back to your question... If you build a boat from one layer of plywood, quite obviously you are not achieving a monocoque structure. So what exactly do you end up with? The answer is quite simply --- a fiberglass boat. Let me explain. A single layer of plywood, made up of strips, essentially provides uni-directional reinforcement. This is very much like C-Flex.
What is then required to create a sound boat is to develop a fiberglass laminating schedule that assumes that any
unidirectional reinforcement like plywood (or C-Flex) 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.
Seriously. Ask Seemans Fiberglass, the makers of C-Flex, which, by the way has been in production since the early '70s.
Wait, you say. 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... 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 as correctly stated above by Tarbaby and quoted here:.
"If the boat is small enough ( less than 18') you can get by with one layer but there is a trade off for everything. You will spend more time fairing the hull once it is glassed."
Now, the fundamental basis of your question was 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.
There is another construction method that we haven't yet discussed. That is a fiberglass "cored" boat where the strength is derived from two
structural layers of fiberglass separated by a core. This is the same method by which an I-Beam derives its strength. This method employs a different fiberglass lamination schedule specifically designed for this method. This is certainly fine... except... with plywood, you are using a core that is much heavier than that typically used for this method. You should use a closed cell foam designed for this method and you will achieve your weight savings but... probably only if you vacuum bag or possibly even infuse. These two processes carefully control the resin to fiberglass ratio; the most critical factor in strength / weight ratios. The only drawback will be cost. Check the prices for structural closed-cell foam and you will understand.
Now we really must apologize for the length and answer a question that comes up frequently but was not yet specifically asked here. The "chopper gun" question above is related however.
Question:
Why do I have to use more expensive epoxy resins in cold-mold construction when polyester-based resins are significantly cheaper?
Answer:
Epoxy resins have much better properties of elasticity and can handle and "match" those properties of the core material much better than the relatively brittle polyester-based resins. For this reason, they are very well suited to cold-mold. If you choose 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.
Hope this helps.
