Solid and Sandwich Core Fiberglass Boat Hull and Deck Construction. Part 2

Now that we have discussed some of the terminology and nomenclature used in fiberglass boat building we can take a closer look at some of the materials. OK, let me get this off my chest right now. I severely dislike the use of the word “OSMOSIS” in any discussion  pertaining to fiberglass boats. It has become one of the most misused generic terms to describe almost any type of moisture related malady related to fiberglass boats. Let me explain further. Webster’s Dictionary describes osmosis as: Movement of a solvent (as water) through a semipermeable membrane (as a fiberglass structure) into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane

So taking the above into consideration, in order for Osmosis to occur in a boat hull we would need to have water present on both sides of the hull and in reality we have water only on the outside of the hull (hopefully). In addition, when taking into consideration the thickness of most fiberglass hull structures this should rarely, if ever occur.

What we do see with fiberglass resin is that it does do is exhibit hygroscopic properties, with different resin formulations showing this to varying degrees. Back to Webster’s Dictionary again:

Hygroscopic – readily taking up and retaining moisture. Taking it further, the ability of a substance to attract and hold water molecules from the surrounding environment.

This is what resin can do with the condition sometimes manifesting itself as gel coat blisters and softening of core materials. We’ll talk more on this later.

Next are the Spun Glass Re-enforcements utilized in fiberglass construction. The two most common configurations are:

Glass Matt                                                     Woven Roving

Fiberglass Mat                              Fiberglass Woven Roving

They are usually alternated in layers and saturated with resin to make up the required thickness of the laminate.

Now lets look at the Core Materials utilized in Sandwich Core Construction.

1-Endgrain Balsa Wood: Probably the most common if all the core materials in use today. Its open cell make up gives it good bonding strength will all the resins. It is light in weight and when used correctly it makes strong structures. Balsa has been used almost universally in deck and cabin superstructure construction. It has one serious weakness in that it softens and decomposes rapidly when exposed to moisture. It has however been used successfully for many years and I might add, un-successfully as well.

2-Closed Cell Foam: Foams have been around for some time now and are marketed under various brand names such as Airex, Divinycell, Corecell etc. They are more water resistant than Balsa and due to their closed cell make up bonding strength has been brought into question. They are also light and strong.

3-Plywood: Has seen limited use as a core material. Its tendency to delaminate when used with polyester resin in damp conditions has been an issue and since it is rigid and stiff its use has been somewhat limited to flat surfaces.

Next time we’ll look at core configuration, how it gets formed into compound curves when used in hull layups and the issues that this can cause

Solid and Sandwich Core Fiberglass Boat Hull and Deck Construction

One of the topics that I’m sure we will be doubt be visiting ad re-visiting in the future are issues surrounding fiberglass (FRP) construction and repair as it pertains to hulls, decks, rudders etc. In that vein I think that its prudent to discuss some of the basics and get some of the terminology straight so that we are all on the same page so to speak. I know that some of this will be old hat for some but for others I hope that it proves interesting and informative.

I have even done numerous seminar presentations on all of the above topics so in the next few posts I’m going to present some of that information here.

Relatively speaking fiberglass re-enforced plastic (FRP) as a method of boat construction is a fairly new technique, dating back to the 1950’s and really not becoming the dominant method of production boat construction until the late 1960’s and into the1970’s. I’m sure that these dates could be debated but in my re-search they are close.

Next let’s look at some the main components of FRP laminates and some of the common terms used:

Resin: The plastic or “glue” so to speak that is used to saturate the glass cloth or mat utilized in the hull or deck construction. The three most common types in use today are Isolathic Polyester, Vinylester and Epoxy. They all must be catalyzed by the addition of a hardener chemical.

Before we discuss resins further I want to define one important term:

Hygroscopic:  Readily taking up and retaining moisture. Taking it further, the ability of a substance to attract and hold water molecules from the surrounding environment.

This is what resin can do in varying degrees with the condition sometimes manifesting itself as gel coat blisters and softening of core materials. .

1-Isolthalic Polyester: The mainstay of fiberglass boat building. It’s been around since the beginning and when utilized correctly it can produce a quality product and is still in use today It is however the most hygroscopic of all the commonly used resins, has a weaker molecular chain and is the least expensive.

2-Vinylester: This resin is seeing more use recently in boat construction particularly in the outer hull laminate layers. It is stronger, has reduced hygroscopic properties and shows superior bonding and molecular strength.

3-Epoxy: The highest quality of the resins discussed here. It is used on almost all Carbon Fiber and Kevlar laminates, shows the best (almost no) hygroscopic properties, the highest strength in its chemical molecular chain and in bonding. Its high cost has limited its use in production boatbuilding but it is beginning to show up on higher end boats and in composite spar construction. Epoxy resins are often used both with glass re-enforcement and on their own to strengthen and waterproof wooden hulls and structures. Its superior bonding strength and reduced hygroscopic prosperities make it well suited for these purposes as well.

Next we’ll discuss the different types of spun glass re-enforcing materials.



Welcome to, a place to share, discuss and learn about most all issues pertaining to the maintenance, repair, refit and upgrade of  pleasure craft boats both power and sail.

I’m just getting this set up so be patient, check back often and I’ll have some interesting stuff posted very soon.

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