lamination basics as I understand them. Note: I am not in the boat building buisness so this is just my take on the whole deal.
Most beach cat hulls are composed of a paint/gel coat layer then outer skin then a lower density core then an inner skin.
The outer layer is gel coat (thickened polyester or vinal ester resin with UV inhibitors and coloring added. Or it is a catalyzed linear polyurethane paint (Awlgrip for example)
The skins must be able to withstand tension, compression, offer impact resistance, and adhere to the outer paint/gel coat. They are typically made of a woven fiber (fiberglass/ Kevlar/carbon) held in a matrix of resin either epoxy, polyester or vinyl ester resin.
The core is a low density material that must resist shear loads, not dent too easily (high crush strength), bond to the skins, and not weigh too much. Not absorb water Common core materials are foams in various densities and types, end gain balsa, expanded cell nomex (honeycomb).
Layup methods in order of decreasing expense/quality
1. Prepreg/autoclave cure
2. Prepreg hot vacuum bag cure
3. Resin infusion
4. Wet layup vacuum bagged
5. Wet layup not vacuum bagged
1. The cloth comes preimpregnated (usually with epoxy) with the exact fiber to resin ratio for optimum strength. Material is shipped and stored cold and then gets flexible at room temp. After the part is layed up in the mold, it is placed in a vacuum bag and placed in an autoclave generally cures at 100+ °C at 5-6 atmospheres of pressure. Yields very good compression strength which is why this is used for making carbon masts and beams.
2. Same as above, just heated with only 1 atmosphere of pressure on the bag. Slight loss of compression strength.
3. All the glass, core, etc is layed up dry without resin. It is placed in a vacuum bag with multiple inlets for resin and be pulled by vacuum through the fiberglass towards the vacuum source. The advantage is low waste, the ability to work slowly on getting the fibers into place ahead of time. It requires a resin that does not cure before it is fully wet out lower viscosity, and if you are not careful there can be dry spots that never get wet out. This may not be visible if you sprayed gel coat into the mold first.
4. Wet out the glass by hand with a brush and squeegee, place in a vacuum bag and cure at room temp. The breather absorbs most of the excess resin
5. Wet out the glass by hand with a brush and squeegee
Cloths in order of expense and quality costs are per ~6 oz 50” wide cloth
Carbon fiber cloth Highest stiffness (Tensile strength) (comes in different grades of stiffness) 40-60 $ yard
Kevlar Best impact resistance good Tensile strength 30-40 $ yard
S glass fiberglass very good impact resistance Moderate Tensile strength 30% stronger 15 % stiffer then E glass. 17$ yard
E glass fiberglass Lowest Tensile strength very inexpensive 5.50$ yard
Resin in order of performance
Epoxy 40-75$ gallon
Epoxy resin is known in the marine industry for its incredible toughness and bonding strength. Quality epoxy resins stick to other materials with 2,000-p.s.i. vs. only 500-p.s.i. for vinyl ester resins and even less for polyesters. In areas that must be able to flex and strain WITH the fibers without micro-fracturing, epoxy resins offer much greater capability. Cured epoxy tends to be very resistant to moisture absorption. Epoxy resin will bond dissimilar or already-cured materials which makes repair work that is very reliable and strong. Epoxy actually bonds to all sorts of fibers very well and also offers excellent results in repair-ability when it is used to bond two different materials together. Initally, epoxy resin is much more difficult to work with and requires additional skill by the technicians who handle it.
Vinylester resin
Vinylester resins are stronger than polyester resins and cheaper than epoxy resins. Vinylester resins utilize a polyester resin type of cross-linking molecules in the bonding process. Vinylester is a hybrid form of polyester resin which has been toughened with epoxy molecules within the main moleculer structure. Vinyester resins offer better resistance to moisture absorption than polyester resins but it's downside is in the use of liquid styrene to thin it out (not good to breath that stuff) and its sensitivity to atmospheric moisture and temperature. Sometimes it won't cure if the atmospheric conditions are not right. It also has difficulty in bonding dissimilar and already-cured materials. It is also known that vinylester resins bond very well to fiberglass, but offer a poor bond to kevlar and carbon fibers due to the nature of those two more exotic fibers. Due to the touchy nature of vinylester resin, careful surface preparation is necessary if reasonable adhesion is desired for any repair work.
Poly ester is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC's. Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive.
Outer coverings. There are 2 choices gel coat, and paint.
Good paint is tougher and lighter but not as thick. Awlgrip is an excellent product for usage but it is nasty stuff. Harder to match repairs.
Gel coat is cheaper and thicker so it can be scraped a bit without exposing the skin. Easier to repair small patches, hard to refinish a whole boat.
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