Fibre Glass Yacht Construction

What’s fibreglass made out of?
For the purposes of this article it is beneficial to understand a few basic facts concerning how fibreglass boats are constructed, their typical strengths and weaknesses and most importantly what are the visible signs that manifest themselves to the owner, hoodpay such as stress cracks, de-lamination, osmosis and so forth.

How does it cure?
Briefly, most polyester resins are made up with glycol, organic acid and reactive diluents (styrene being one). When the catalyst is added, [MEKP usually] a chain reaction is initiated. The mixture forms a series of “cross-linking” reactions, which allows the styrene to create “bridges” which links together all the chemicals. The chain reactions run faster and faster until the glycol/acid chains begin to gel into a solid mass.

Eventually, all these “cross-linked” bridges redribbonlive form a solid plastic mass holding the fibreglass cloth (or matrix) firmly in place. Heat is given off in this reaction as the chemicals cross-link together (Exothermic Reaction). Amazing isn’t it?

Basic Construction
Fibreglass boat hulls are generally composed of several laminations (or layers) of glass fibre cloths, impregnated with polyester, vinylester or epoxy resin. This is usually done by building a “female” mould and creating the fibreglass hull within by a series of subsequent stages:

1. The “female” mould is built to the required hull shape.

2. A waxy release agent is applied to the mould surface.

3. A “gel coat”, which contains pigment (colour) of polyester resin, ristomanager is first applied to the mould (10-25 mls thick). This provides the smooth coloured finish to the hull.

4. The “gel coat” is then backed by a thinner fibreglass cloth then several layers of heavier cloths are added to it to form the basic hull.

The hull is then usually re-inforced with more layers of glass and resin onto areas that are under stress and the whole hull sealed with a final layer of clear resin. The rest of the internal fittings such as roof, decks, bulkheads and keel are added when the finished hull is released from the mould. (This does not always apply! Different builders vary this).

Wooden components
Often in fibreglass hulls, wooden components have been used to reinforce areas, such as galleys and so on. Often, the wood is subject to water exposure and swells, MATRIX CRACK eventually causing rot and decomposition.

Many modern boats have been constructed using internal cores together with resins. These can be polyurethane foam, end grain balsa cores and many lightweight racing hulls are using various lightweight “honeycomb pattern” materials.

These materials decrease the weight of the hull, often with very little strength loss. Also, the use of “closed cell” foam cores combined with epoxy resins has safe guarded many of these “composite constructions” from early failures but all must be subject to high quality and standards, especially where deck installations and fittings are concerned, due to repeated high loading.

Just because the hull is underwater does not necessarily mean it will degrade any faster but in the case of poor maintenance, hidden factors may be at work. Lack of anti-fouling procedures allows marine growths to proliferate. Barnacles are a sure-fire gel coat killer if they are allowed to remain undisturbed at work!

Naturally, a weed covered hull will hide the dreaded “osmosis blisters and underwater metal fittings will be subject to damage by galvanic electrical corrosion if the right conditions exist. Rudders and props, shafts too, are often overlooked when a hurried slipping takes place, usually for a quick anti-foul.

The dreaded monsters
A rudder repair I did recently involved complete decimation of the soft inner core by the dreaded ‘teredo” worm. The rudder was sheathed in fibreglass and the worm had entered via a pinprick and chewed the living hell out of the core! Take nothing for granted!

A word of warning!
If you are contemplating the purchase of an older style fibreglass yacht, use a qualified marine surveyor. They, unlike you are fully trained and experienced to spot any areas that are defective or likely to cause trouble in the near future.

If you skimp on these dollars, you’ve only got yourself to blame!

There are many and varied forms of glassfibre cloths available from the simple “chopped strand mat” to the more exotic (and more expensive) Kevlar Aramids and Carbon Fibres. All these fibres offer different characteristics such as stiffness, strength and can be combined in use. Examples of these cloths are woven cloth; chopped strand mat (CSM) uni-directional, bi- and tri-axial stitched cloths. E-glass is probably the most commonly used for general repair work.

How does it work?
Most of us are familiar with the way basic glass fibre and resins work. Separately, the glass cloth is soft, pliable and can be formed to almost any shape. The polyester resin (or any other, for that matter) is a clear sticky liquid that once mixed with the catalyst, (peroxide catalyst, usually MEKP) creates heat (an exothermic reaction) and eventually sets solid. Individually, the uses of these tow components are limited but when used together form a formidable alliance and produce a fibre re-inforced plastic (FRP).

How does it do this?
This incredible physical partnership enables huge stressed and loads to be transferred through the “cured” plastic and allow shells of immense load-bearing capacity to be constructed, i.e. boat hulls.

Sadly, there is no such thing as a free lunch and although “fibreglass boats” have heralded a huge revolution of long lasting boat constructions, time has shown that fibreglass boats are not absolutely maintenance free. With years of use, boat hulls incur much wear and tear in the form of bending, flexing, fatigue, sudden impacts etc. The fatigue cycle can cause breakdown in the cross-linked “constructional chains” of the hull causing weakness, cracking and de-lamination of the glass impregnated cloths from the internal components.


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