Wood Epoxy Boat Hull Construction

I guess that before we move on from our discussion of fiberglass and composite boat construction that we should include a segment on a technique known as “Wood / Epoxy”. This is a building method that became popular in the late 1960’s with the introduction of epoxy based resins. This was at about the same time that production boat builders were making the switch from wood to fiberglass, utilizing mainly polyester resins as a building material. Until now most boats both production and home / custom built were constructed mainly of plywood, both in sheet form for hard chined hulls and various diagonal strip planking methods for curved hulls.

I remember that as a kid growing up on here in the north shore of Lake Ontario that wood boats were everywhere with sheet plywood and cedar strip being the most popular. Fiberglass was this wonderful new material that made for bright shinny curved hulls and sleek looking boats that almost overnight seemed to take over the lakes and rivers. For while wood was still popular with custom and home builders. It was at about this time that someone decided that it would be a good idea to cover these wooden hulls with lightweight spun glass cloth saturated with polyester resins. It seemed like a great idea at the time effectively turning the exterior surface of a wood hull into a hard piece of plastic. For a few years fiber glassing your wooden boat was the thing to do. It wasn’t a difficult process although a little messy and somewhat smelly.        This went on for a few years until after some time boaters realized that the fiberglass cloth was delaminating from the plywood hull material. It was later determined that polyester resins, in damp conditions, over time will separate from plywood. So much for all the hard work.      At about the same time Epoxy resins, even though they have been around since the 1930’s were beginning to show up as a boat construction material. Their increased cost was prohibiting them from seeing considerable use in production boats however with the custom and home builders who’s predominate material was still wood then began to utilize epoxy resins as a hull overcoat, both with and without light glass cloth re-enforcement. This was possible because of epoxy’s superior bonding strength to wood, it’s superior film strength and it’s superior hygroscopic properties. Remember a few posts back we learned about polyester resin’s hygroscopic properties, well epoxy is far superior in this regard. It does however have a couple of disadvantages. It degrades rapidly when exposed to UV and it also doesn’t like high temperature situations. This means that it makes a poor gel coat in that it must be over coated, preferably with a light colored paint or coating.

Lets’ now skip forward to a few months ago when I was contacted by a potential client to perform a survey inspection on his 26 foot wood epoxy Thunderbird sailboat. For those of you that aren’t familiar with the Thunderbird it is a 26 foot sailboat, designed by a naval architect named Ben Seaborn in 1958. It became incredibility popular with home builders in both North America and Australia as both a racing and cruising sailboat with one design racing fleets and an international class association. They were quite popular in my area here however the numbers have dwindled in recent years. As with many marine surveyors these days I generally shy away from wooden boats for obvious reasons but after a short discussion with the owner a few things came to light. The boat was constructed by the owner in 1977. The hard chined hull construction is GIS fir plywood sheeting over mahogany framing and the exterior hull surface is covered in light glass cloth saturated with epoxy resin. The hull interior is coated out in epoxy resin only.

I then decided to accept the assignment. At the inspection my findings were, that after 37 years of constant use, electronic moisture readings were not far off what I would normally see in dry plywood and percussive tests showed no structural issues. I found this very interesting that the plywood was in very close to new condition. I often inspect fiberglass boats that haven’t fared nearly this well. It looks like this epoxy is pretty good stuff. I guess that it’s just one of those things that just makes you want to go HMMMMMM now isn’t it?????

Fiberglass Boat Construction Part 3 Cores

Back to our discussion of fiberglass boat construction. Last time we looked at the various materials used in cored hull and deck construction. We learned that the most common materials are End Grain Balsa Wood and various types of Closed Cell Foam. The next question is, that since these materials are rigid and do not bend, how do boat builders manage to shape them into the compound curves necessary for hull and deck construction. Core materials are supplied in various configurations other than standard sheets. It can be supplied in narrow strips and small blocks attached to a scrim which is a thin flexible material that hold the blocks together and the small gaps between the blocks, called Kerfs allow the material to be shaped into both single and compound curves. Below is a diagram of core material showing the blocks, scrim and kerfs.

Core Close up1_Layout2

Here we can see a Balsa core material laminated into a boat hull. The kerfs are easily visible between the blocks.

This is a side view diagram of sandwich core construction as used in boat hull.

Cored GRP Hull2_Layout4

When performed correctly sandwich core construction can provide strong light structures but the trick here is to ensure that core materials are installed correctly. As we’ll see in future posts this has been somewhat of a problem for boat builders over the years.

Surveying Fiberglass Boats in Sub-Freezing Temperatures

Every winter I get request to survey boats when the ambient temperatures are below freezing. Surveying fiberglass boats in sub-freezing temperatures just doesn’t work and here’s why.

As we have previously discussed fiberglass as a material is hygroscopic (it absorbs moisture over time) and as such, during the winter months when the ambient temperatures are below freezing any moisture contained in fiberglass structures, such as boat hulls and decks freezes as well. During the freezing process crystallization of the substance occurs, adding small air pockets which make electronic moisture detection virtually impossible. Also because the substance is frozen it becomes very hard rendering percussive sounding tests useless as well. It is for these reasons that I do not recommend surveying of fiberglass boats when the ambient temperatures are below freezing. Any surveyor who claims that their inspections are valid under these circumstances is, in my opinion simply not being truthful and should be seriously questioned.

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

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