What's a chopper in fiberglass terms?
Here is some interesting information that I found that discusses the difference between Hand laid boats, chopper boats, layed up boats and cored hull boats.
Hand laid fiberglass boats (such as Malibu) are also referred to as Fiberglass Reinforced Plastic (FRP) boats. This is the full name of what, for over forty years, has been known as the fiberglass boat. It consists of a basic standard of 65% continuous glass fibers, in the form of fabrics, and 35% plastic resin. The fiberglass consists of fabrics of woven mat made of continuous fibers. The length of some of these woven fibers run the length of the boat. These fibers, much like the huge cables that hold up suspension bridges, rely upon the continuous lengths and orientation of the fibers for their strength. Today, there is a large variety of weaves available, but they are all essentially weaves of continuous fibers. You can see this if you have ever cut out a chunk of your BU. It is all matting and roughly 3/8" to 1/2" thick. You can also see the fibergalss mat being laid the entire length of the boat.
In the early years of small FRP boat building, a few companies tried making boats from chopped strands of fibers, mixed with polyester resin and blown through a gun into a mold. The length of these fibers was about 3-4 inches and were usually curled like cut hair when viewed in the mold. Very quickly we learned just how weak laminates made with short fibers are. Those "blow-molded" boats tended to break up all to soon. The chopper-gun boats soon disappeared from the scene. Today, things like shower stalls, truck fenders and the Corvette automobile body are made with chopper guns because they don't require great strength like a boat hull. For this reason, chopped strand is not considered as a structural fiber.
That does not mean that chopped strand mat and chopper guns have disappeared from boat shops. Chopped strand mat (CSM) is still used on all boats to prevent the weave pattern of fabrics like roving from showing on the gel coat surface. A very thin layer of mat is also used between heavy fabrics to prevent concentrations of resin between the heavy fabrics. And for other uses where very high strength is not required. One of our complaints about Taiwan boats has always been that they make use of the chopper gun too much.
In modern fiberglass boat building a new boat is "layed up" (built) in a female mold which has been polished smooth and waxed with a specially formulated release wax to allow the finished boat to be removed. With the mold prepared, the gelcoat is sprayed over the entire surface of the mold to a uniform thickness. Next a layer of chopped, stranded glass mixed with polyester resin is shot into the mold completely covering the gelcoat layer, again evenly. This operation is accomplished with what is called a chopper gun which mixes the fiberglass mat and polyester resin in the proper proportions.
Next the whole works is rolled along it's entire surface to insure a good bond between the two. This process while simple in theory must be done thoroughly or voids (areas where the gelcoat and mat are not chemically bonded) will be left.
Next, resin inpregnated woven mat is layered on and again rolled and depending on the construction alternated with chopped fibers and woven mat until the "lay-up" is finished. The woven mat may only be small sections and may not run the entire lenght of the hull. This layed up process what boat companies like MC or even my old Glastron uses and is the most common procedure in manufacturing boats. I had to do some repairs on my Glastron hull and you could see this type of layering. Also, it did have some voids. Right above the bow ring was lip/line that ran the length of the boat. One time we got the boat a little low on the trailer and the bow hit the winch area lightly. The gel coat just broke away. Underneath it was an area about 1" wide and aobut 4" long that was just a void or air bubble. Also, when I had to drill a hole in it to add an accessory, the glass broke away as I was drilling. Basically because the layering was mat, chop than mat.
Lastly there is a process called cored hull. Somebody in the manufacturing end of the racing sailboat business (where saving weight means winning races) figured out that if you put something lightweight between two thin layers of fiberglass, the hull would be really lightweight! Thus came the introduction of Balsa Wood, Foam and other High Tech (read: mystery) materials appearing between ultra thin layers of fiberglass. This type of construction is called a cored hull.
A solid fiberglass (FRP) hull consists of approximately 65% matting & cloth, and 35% resin. Today’s cored hull boat has only about a 10% to 20% fiberglass content (the expensive stuff) with the other 80% to 90% of the boat’s hull consisting of some kind of strange, inexpensive concoction of materials hidden under its fiberglass skin. The main disadvantage of the cored hull system is that if water penetrates that thin “skin” and gets into the core (for example, a small crack, or a hole drilled for a new part), that inner core will start to degrade. As the core starts to fail, it delaminates from the fiberglass skin and the boat literally begins to fall apart.
Below are some pictures of various boats that have been "layed up" or built with a core hull
Edited by stewart, June 28, 2007 - 08:53 AM.