ABOUT FIBREGLASS BOATS

By Paul Stock 27/02/2026

This article is one of a series of “in a nutshell” articles about boat constructions. It’s an opinion piece and hasn’t been fact checked. My version might not align with your version so although I’ve done my best to be true to the facts, it is a long time ago now (I was just starting at primary school when this all began) and I may not be 100% correct. Please feel free to get in touch if you have any feedback.

The article is intended to be helpful to students, buyers, sellers and anyone interested in fibre reinforced boats. The following is covered:

• The terminology and acronyms
• Brief background of the adoption of GRP production boatbuilding in New Zealand
• Production and materials of mainstream production boats (advanced composites will be covered in another article)
• Deterioration and comments about the various constructions

To stop you falling asleep (hopefully) this article is written as an approximate chronological account that introduces the concepts along the way.

It starts when “fibreglass boats” started to be built in the 1970s (yes I know that there were some built before that but we’re going to run with the 70s as that is when the boom began).

Terms and acronyms

To begin with we need to define a fibreglass boat . There are many different names used to describe the same or overlapping things; For example, the following all describe fibreglass in some form or other:

GRP (glass reinforced plastic)

FRP (fibre reinforced plastic)

FRC (fibre reinforced composite)

“Fibreglass” is possibly the most widely used descriptor used for glass fibre reinforced  resin blend. The fibres are either pre impregnated in resin using one or more methods, “wet-out” by hand on the mould, or sprayed with resin from a gun. The resin must be catalysed by adding catalyst or hardener (depending on the type of resin).

The resin/fibre blend or “laminate” can be applied or “processed” in a variety of ways that are discussed later in the article.

Acronyms and terminology used in the industry may include ambiguous terms such as “composite” and “matrix” which can have multiple meanings. So the context of how the words are used is important. The following definitions are some of the more common ones used in this article. If you aren’t an industry person, you’ll probably need to refer back to these terms in order to understand them.

Terms	Meaning
Bog	Thickened resin made by adding substance like industrial talc  and/or other powers (sometimes called “extenders”) to resin to make a filler
CAD	Acronym for computer aided design
Catalyst	Usually methyl ethyl ketone peroxide, used to activate curing in polyester resin (always follow the manufacturers spec sheet)
Chopped strand mat (CSM)	Matting of random cut glass fibres that are held together with a glue called a “binder” that dissolves in resin when it is “wet out”.   CSM can also be created from a chopper gun machine. The gun has a rotating head with sharp blades that cuts up glass fibre known as “gunstock”. A chopper gun machine looks like a spray gun connected to a long mechanical arm. The chopper gun simultaneously chops gunstock into short lengths and distributes them into a resin stream that blows fibres and resin into a mould. This creates CSM is much the same result as hand laying CSM (if the operator is good at what they do).
Composite	In the fibreglass context, it means any construction made up of a resin and fibres. That extends to include high tech fibres like carbon fibre and advanced epoxy resins. “Composite” as a term can be confusing as vessels built with steel framing and wooden planking is/was also commonly referred to as composite construction.
Core	Material used as a layer between an inner and outer resin/fibre layers (sometimes known as sandwich construction)
Core mat	A soft matting used as a low cost core. The matting is wet out and laid in between an inner and outer GRP skin. The wetted matting introduces bulk to a laminate to stiffen it.
Cure	Cause the resin to go hard. Most resins used in FRP boatbuilding whether polyester or epoxy require the addition of a calalyst or hardener to initiate the hardening(curing). Advanced composite constructions often require a secondary post curing process at an elevated and controlled temperature to fully cure the resin to realise the intended laminate properties.
Epoxy resin	More advanced resin than polyester using a different technology utilising a hardener rather than a catalyst. It is used in multiple applications and in different formulations, also used in paints and for adhesives. Applications include pre impregnated fibres and use with more advanced fibres than glass such as carbon fibres. Technically epoxy is: more resistant to degradation from flexingisn’t brittle like polyester can be modified with the use of different hardenerscan be formulated to be matched to the reinforcements used in a laminate has good waterproofing properties has good adhesion to most types of wood – as long as dry and suitably preparedHas gap filling properties if mixed with appropriate extenders (powders)
FRC	Fibre Reinforced Composite. This covers the whole basket of technologies that use fibres and resin. Tends to be used for more advanced resin/fibre/core matrixs’
Glassed	Has had glass fibre applied on or over, for example “a bulkhead is glassed in place”
GRP	Glass Reinforced Plastic. This generally means polyester reinforced glass fibres and may include “core” materials. This type of fibreglass accounts for almost all of the production boats that are the vast majority of the market. The resin (plastic) is normally polyester but could be vinyl ester or epoxy.
Hand laid	Reinforcement is laid by hand into a mould and then wet out with resin
Keel floors	Transverse beams that support the bottom structure on a sailing boat surrounding the keel
Laminate	The combined “stack” or “lay up” of fibre layers, core, core bonding layers of adhesive etc. that form a finished panel.   The most basic “laminate” (that is the combined glass fibres and resin that make up a boat hull or deck or a toilet compartment or a shower etc) that was used in the beginning (early 1970s) is known as a “monolithic” or “solid glass” laminate, because the laminate is made up of layers that start with CSM, then roving, then alternate between layers of roving and CSM until an adequate skin thickness has been built up. The laminate is “rolled out” using grooved steel rollers between layers to compact the laminate and remove air in the laminate, and the CSM is like a bedding and bulking layer that ties the rovings together. CSM is a pretty weak way of building up strength compared to directional fibres (like rovings). Solid laminate is a bit of a misnomer because a thin monolithic laminate might be flimsy. So I’m going to call it “monolithic” to avoid confusion such as could occur if describing monolithic as solid glass in a survey report.   On a boat, the laminate specification often varies in different areas of the boat. For example, under the waterline or under the chines the laminate may be heavier and may be monolithic, whereas above the chine may be cored using a core material.
MDF	Medium density fibreboard – non waterproof board used for plugs and moulds that is hard and easy to finish and shape
Monolithic	A fibre laminate without core, made up solely with multiple layers of fibre. Sometimes called “solid glass” but in thin layers it may not seem “solid”.
Mould/tool	Highly finished component that is the shape of a finished product. It is covered in laminate then the laminate that has become the product is cured and “released” from the mould
Plug	Object made to the shape of a finished product used to make/take a mould from
Polyester resin	The main resin used in production GRP boatbuilding. To make the resin cure it is usually catalysed using MEKP (methyl ethyl ketone peroxide) at between ¾ and 2% of the weight.
Promotion	Adding of additives to resin to increase the speed of cure when the catalyst is added. Typically used in cold weather or to otherwise accelerate cure time.
Release	Break the contact of a product and a mould in order to separate them
Sandwich construction	Cored fibreglass layup with an inner and outer skin like a sandwich. In a cheese sandwich the cheese is the core and the bread is the skins.
Secondary bonded	Gluing of resin components that have already initially cured.
Stack	Combination of fibres and core material layers making up a laminate. Laminate drawings tend to include thicknesses of each layer – may also be called “layup”.
Stitched fabrics	More advanced reinforcements than the traditional woven rovings and chopped strand matt, that have directional fibres stitched together to hold them in place.
Vinylester resin	A superior resin compared to polyester. In layman’s terms it is chemically halfway between polyester resin and epoxy resin. A catalyst is used to activate the cure in the same way as polyester resin. Vinylester resin is often used in below waterline laminates adjacent to the gelcoat as a barrier layer because of its excellent water resistance, reducing the chances of permeation through the laminate that can cause osmosis. It is also excellent for bonding to aged polyester resin such as in repairs and for its chemical compatibility with both polyester and epoxy resin.
Woven rovings	A basic non stitched weave of under and over matting of thick strands available in different weights

History

Fibreglass boats exploded on to the New Zealand market in the 1970s covering all sizes of craft from kayaks to trailer boats to larger launches, multihulls and keel boats.

Compared to glued and sealed wood construction, fibreglass was a different alternative and competitor, emerging at a similar time.    

Advantages included:

• Efficiency: Fibreglass boats were made from moulds. Once a mould for a boat hull and decks was made multiple vessels could be made in a fraction of the time required for a wood boat.
• Durability: Laminates have proven to be quite tough compared to timber because of the higher laminate density than a typical wood or sheathed wood core boat of the era.
• Finish: The gelcoat finish – from highly polished waxed moulds – was excellent, shiny and fair, compared to the single pack, often hand painted finishes of wooden boats that the market was used to. And the hard gelcoat surface has, over time, been found to have quite good abrasion resistance.
• Availability: Faster build and production set up of builders enabled much shorter delivery time than glued wooden boats.
• Skills needed: Many of the tasks were able to be standardised and done by relatively low skill workers and/or workers able to be trained quickly compared to the lengthy training periods of craftsperson wooden boatbuilders
• Price: Less expensive than the same design built in glued timber
• Standardisation: Potentially one of many sisterships with standardised build quality and better-known resale values

Many of the 1970s and 1980s fibreglass boats were ordered as “hull and decks”. Hull and decks packages usually had windows fitted, possibly the engine, and were a weathertight shell, enabling the DIY home builders to complete the interior, plumbing, and engineering themselves, potentially saving money.

Some readers may remember the hulls of large keel boats and launches parked on lawns or driveways of almost every second street. Those were the days when you could buy a hull and decks, finish it yourself using real intelligence and effort, and sell it at a tidy profit. And there was a boatbuilder in nearly every industrial area.

Moulds

Regardless of the laminate specification, a mould (sometimes known as a “tool” or “tooling”) will normally be required. I say normally because panel type GRP products that have foam or balsa core that can be used like sheets of plywood to fabricate a boat, but these are not used for mainstream fibreglass production boats.

A mould can be in male or female form hull or deck. Production boats are almost always made from female GRP moulds so the moulded surfaces are the reverse of the actual hull or deck.

To make a hull mould you make a plug the same shape as you want the boat. It was common to build a prototype model hull out of wood epoxy, finish the exterior to a high standard, then polish and carefully wax the surface to enable release of the cured mould. The hull mould is then laminated in heavy fibreglass (for stability) over the “plug”. The mould was then released from the hull,  leaving the hull to be finished and launched.

GRP moulds were/are made from the same type of laminate as actual boats, but  heavier and stiffened using a steel framework around the mould, and usually set up on gimballing rolling frames, that enable the mould to be swung side to side to enable access during lay up.

Tooling gelcoat is used on moulds. It is usually harder than normal gelcoat to withstand multiple releases of product from the mould.

When I was an apprentice in the mid 1980s we took the hull mould of a well-known design from the strip planked cedar core prototype hull. The wax on the hull was either inadequate, not allowed to cure or not compatible with the hull mould tooling gelcoat or hull paint that was applied on to it. This resulted in what is known in the industry as a “stick up”. The mould was intended to release from the plug silently and easily with the slightest of persuasion and air injection. Instead, after an agonising separation job of banging, wedging and injecting with compressed air it finally came away leaving massive sheets of what I believe was Epiglass Reaction Lacquer paint, stuck like shit to a blanket, another industry term as they say in the trade, to the inside of the mould and great patches of bare fibre on the hull. An army of us then spent days with acrylic scrapers. Chipping paint off the mould. And then wet sanding and polishing the mould.

One detail aspect of the hull and deck moulds that has to be designed is the stern and gunwale join details that will require a flange or overlap detail that enables strength in the hull/deck connection and minimal finishing, the detail of which is a bit beyond this article.

Deck moulds were very time consuming to make. Their plugs were typically made out of MDF as single use plugs with heavy GRP moulds take off. For shorter or single custom boats, a finished mould would often be made from MDF.

Non skid, hatch detailing and many other details are usually required to be incorporated into the plug/mould. Back in the 1980s details would be tweaked and sometimes decided on the job, whereas it is more likely these days for a 3D CAD team to draw the deck in software, to enable mechanised cutting of detailed components. And the plug material is more likely to be a synthetic board. Slight tweaks and modifications are still to be expected on the plug before the final moulding is taken from the plug.

Moulds were maintained by fine polishing and waxing to ensure reliable release. Periodically moulds would be stripped of all wax, re polished and sealed and waxed.

Gelcoats

Gelcoat is the first layer that is applied into a mould when making a product. It is brushed or sprayed thickly into the mould. After curing for a limited time (so that a chemical bond of the glass fibres to the gelcoat is maintained), glass fibres are laid up progressively in stages to build up the shell thickness to the desired specification. The gelcoat used to make products from female hull and deck moulds is unwaxed gelcoat. When applied to a mould surface the gelcoat inherits some wax from the mould surface which results in a shiny, glossy, waxy finish of the product when released from the mould.

Gelcoat is essentially a pigmented polyester resin. Early boats were painted as polyester resin is quite porous, but it was found accidentally that pigmented resin (gelcoat) gave good service as it was hard.

Gelcoat must be applied thickly in manufacture to prevent styrene attack/alligators. It also shrinks a lot when curing that leaves the surface in a sort of tensioned state. That is why spider/crazy cracking occurs easily on gelcoat.

For surveys the spider cracking is an indicator of flex which may or may not be indicative of a damage event. If there is repetitive flex in polyester laminates “micro cracking” tends to occur that results in softening of the laminate. Cracked gelcoat or microcracked laminates are likely to absorb moisture in exposed laminates.

The porosity of gelcoat means that:

• It’s a good idea to keep it waxed for protective reasons
• If a paint on gelcoat repair is done it is not going to stay matched for long as the paint and gelcoat are totally different materials (will fade differently, maintain different gloss etc)
• A GRP vessel with faded gelcoat will have a superior protective coating if repainted with a modern 2 pack paint system than when the boat was new – though possibly not as hard as the original gelcoat

It is also worth noting that if spider/crazy cracking in gelcoat is painted over, the cracks will come show through the paint.

Flowcoat

Gelcoat that is brushed-on like a paint and allowed to dry used in lockers or to finish internal structure must either have wax added to it, to prevent it from remaining sticky on the surface, or be pre waxed. Pre waxed gelcoat is often known as flow coat. If you were to lay up laminate onto it without grinding the gelcoat the fibres would probably not stick. 

Hull and deck skins

The New Zealand boats of all sizes were/are generally strongly made. The skins of the very early ones were mainly monolithic (without core). Preparation for “lay up” included pre cutting the fibres of chopped strand mat and woven rovings, then laying fibres into the mould, wetting out with resin and consolidating the fibres to remove air by rolling out using a grooved metal roller. This type of lay up is often described as “hand laid” probably the most basic type and still commonly used. Other options include chopper gun sprayed chopped fibres that looks like CSM. Fibres that are chopped up and spray applicated by a chopper gun machine use reels of fibre called “gun stock”.

Many traditional yards viewed the sticky and fumy fibreglass materials almost with disdain others had a go at transitioning to GRP, some successfully. One of the new things to discover was mixing resins and glues. Polyester, vinylester and epoxies, despite their differing chemistries, all develop heat when they cure. This is due to the curing reaction (exotherm). The larger the volume of mixed resin/glue, the more heat and bubbling. And the more pungent probably poisonous fumes and probable fire. For that reason, you should only mix just enough that you can spread out or use in a short space of time, or you should spread it out in a shallow wide container, or use a slow hardener, less promoted resin, or reduce catalyst to a minimum.

Don’t do what one yard did; mixed their first batch following all mixing ratios, a good quantity. Then it was smoko. Imagine the effects of the exotherm when they returned!

Hulls below waterline/osmosis

Early hulls were often affected by water permeating the below waterline skin of the boat and reacting with pockets of uncured resin in the laminate. The reaction of these areas caused pressure to build up and force the products of the reaction – a vinegary “osmotic” fluid of brown colour – though the hull skin causing blisters (osmosis). The blisters may  burst. Raised early-stage pimples or humps in the skin may be signs of pending osmosis but aren’t always. Other relatively harmless defects or issues may also look like osmosis like solvent entrapment in antifouling. Scraping or wet sanding is recommended to verify depth of origin to diagnose osmosis (unless chronically obvious).

The cause of osmosis has been attributed to many factors and in my opinion all of these are valid but may vary from case to case:

• “Binder” in hand laid chopped stand matt (a soluble adhesive that holds the fibres together and dissolves when wet out with polyester resin)
• Excessive air in the laminate
• Dry laminate – not enough resin
• Elevated moisture in the laminate
• Incorrectly catalysed polyester resin
• Low specification resin
• Lack of an epoxy water barrier paint over the gelcoat prior to application of antifouling paint

To repair affected laminates the hulls are usually planed (about 3 mm removed), dried and then the 3mm reinstated using vinylester resin reinforced laminate.

To prevent osmosis, the following options are available. Most modern production boats use more than one of these options. But this may not always be the case especially if made with low quality control:

• Barrier laminate of vinylester resin below the waterline or vinylester resin throughout
• Epoxy primer/undercoat sealer before antifouling
• Resin ratio correct and laminate rolled out well
• Isophthalic resin instead of orthophthalic resin
• Materials of known quality/specification (resins/fibres)

Osmosis is much less prevalent in the 2020s than what it was 1970s/80s although still not uncommon, especially on older vessels.

Hull and deck structural members (scantlings)

Earlier GRP vessels often had internal scantlings and a backbone like a timber boat but internal kelson/hogs are seldom fitted on modern GRP boats.

To strengthen and brace the hulls from deformation, plywood bulkheads are fitted on almost all boats. Early GRP boat bulkheads were typically attached with hand laid fibreglass CSM (chopped strand matt) tapes lapped half onto the bulkheads and half onto the hull skin usually using tapes on both sides of the bulkheads. Other significant structural members were often made of wood, glass sheathed and taped onto the surrounding hull.

On yachts transverse timber keel floors and longitudinal blocking would be fitted in the vicinity of the keel and a mast step region which would be heavily glassed over and onto the hull skin (hand laid). Keel bolting would be either through massively reinforced monolithic skin between the floors or through the timber floors. On some yachts the keel floors would be hollow though or only have timber core through the mid section and considerable variation between yards existed.

Keel floors hand built on a well built foam core FRP boat typical of early GRP and custom FRP boats

On launches an external wooden keel for directional stability was often fitted and to enable slipping or drying out on a tidal grid, bolted through an internal timber encapsulated hog/keelson (these keels are often cut right down these days such as on boats like the Salthouse Corsairs). Timber engine bearers were usually sturdy, extending well fwd and sometimes to the stern. Like keel floors they were well glassed over but if surveying you may find some unbonded glass as polyester resin doesn’t adhere well to wood. The launches tended to be efficient, low or moderate deadrise hull configuration, some with warped bottoms that ran much more level and pushing far less ocean than the deep V keel less configurations now seen on most imports.

To stiffen the monolithic hull skins on early boats “top hat stiffening” was usually used in conjunction with the floor grid/engine bearers. The idea was to make a non-structural shape (former) out of cheap non waterproof material like open cell foam (like surfboard foam) or cardboard tube cut in half. “Bog” (thickened polyester resin) was used to hold the foam or tube in place to form longitudinal stringers or ring frames. Then the formers were glassed over to create effective hollow fibreglass stiffeners.

Hull/deck joins on the New Zealand boats tended to be very good with good sized bonded faying surfaces, fastenings and often glassed internally.

Fibreglass opened up new opportunities for both industrious entrepreneurs, and wooden boatbuilders embracing the new material.

Cores

The problem with monolithic GRP is that it needs to be quite substantial and heavy to resist flex. And it takes lots of time and material to lay up thick monolithic layers. Flex tends to cause micro cracking and gelcoat star/crazy cracking.

Cores solve the problem by providing a spacer to separate an inner and outer skin from each other. The increased bulk increases stiffness without adding much weight.

Cores used on New Zealand production boats are typically either:

• core mat which is a semi saturated matting that bulks up the thickness of the skin
• end grain balsa or
• closed cell PVC foam like Divinecell, Kledgecell, or Airex

End grain balsa, although a wood core, is very common in GRP construction. End grain refers to the core being oriented so the grain runs from skin to skin when a core in a laminate. Think of a tree being sliced across the tree at 20mm intervals, then it is cut into blocks of about 40mm x 40mm (contoured) and held from falling apart by a light fabric scrim on one side. The core has next to no directional strength but has reasonable crushing resistance and resin penetrates well into the contours so it bonds okay even with polyester resin that doesn’t stick well to wood.

End grain balsa has proven to be good in panels that have no penetrations like hull sides and flat deck panels. Unfortunately, the end grain was promoted in the 1980s as an advantage, partly because any water ingress would be isolated to the immediate surrounding area rather than wicking along the grain. In fact the water does manage to migrate sideways and rot the end grain balsa readily. So, it is really important not to have any penetrations into balsa cored panels such as screw holes for awnings and deck fittings or the balsa must be replaced with monolithic GRP or other durable core replacement like epoxy filler.

Most modern production builders do a good job of replacing core with monolithic laminate such as in deck areas where hinges for deck hatches are fitted and anywhere where fittings are attached. But there are exceptions including one modern manufacturer that screws hinges and fittings onto thin skin foam core laminates which isn’t adequate.  

Structural hull liners (matrix)

For production efficiency, some New Zealand builders, in the 1980s began to use internal hull liners (known internationally as hull matrixes). These replace hand fitted timber framing and top hat stiffening and incorporate the structure in GRP mouldings. The mouldings have flanges that are glued onto the hull. Now, many if not most of the big international volume production builders use these matrix/liner mouldings.

The problem in the 1980s was that the adhesives used did not always adhere well enough to the surface and faying surfaces were too small. Later adhesives have proven to be variable. Often the adhesive is thick and brittle and more like a bog than the type of adhesive you need in a mission critical area like the keel structure.  Issues with these matrix structures can include:

• Slamming causing flange adhesion to fail
• Vessel suffers grounding, matrix is overloaded and fractures/breaks away from hull. This is very common on fin keel boats where grounding forces the aft end of the keel upwards.
• Vibration/misalignment on engine bearers causes glued in liner to come away

With either of the above problems – that are regular on the imported production boats – it is very difficult to strongly re adhere the liner/matrix faying surfaces to the hull shell if they get damaged, especially if the surfaces have been exposed to oily bilge water or similar. Access is usually very difficult.

The most common remedy is to grind away the loose shit, re bog it up with polyester and glass tape over the flanges onto the hull. This is a less-than-ideal option compared with – in the first place:

• strongly gluing in separate keel floors and longitudinal   spacers, and
• hand laying stitched double bias taping to additionally join the timber floor or glass web junction to the hull.

In the event of a grounding the latter method is far more easily and reliably repaired.

Some of the higher priced production vessels do not use liners/matrix, instead hand laying GRP webs/floors which are also more readily accessible for inspection.

The 1990s and beyond

In the 1990s, as the DIY boom tapered off, we started to see imported boats in New Zealand. It became less profitable to build yourself. By the 2000s many of the larger kiwi GRP boatbuilders had wound up their operations, or their businesses had failed due to increased competition from imported boats.

An exception was trailer boats. I think that to this day there are more New  Zealand trailer boat builders than importers which is a credit to that industry and the suitability and quality of the boats for New Zealand conditions. Second hand resale values have been good – partly due to the NZ Marine CPC scheme. There are also, I believe, more boats per head of capita in New Zealand than any other county in the word.  

As the imports, new and second hand flowed in, yacht and launch buyers and New Zealand boatbuilders have been exposed to larger international markets and the associated economies of scale.

Vessel designs reflect their target markets and the conditions of those areas, the charter market being one of the main ones. Unsurprisingly, vessels from more rugged climates seem to be built more strongly than mass market vessels aimed at mainly fair weather areas. But of course, the prices for more rugged vessels or for smaller niche yards are higher now as they were prior to globalisation. 

Another changing trend more recently is buyers acquiring large new vessels with no prior boating experience. Their expectations of comfort and luxury is not impacted by preconceived ideas of boating from being bought up in boats. And the luxury and comparably low price of imports have won over many experienced boaties embracing the new status quo.

Modern imports and advanced composite constructions and processing

Laminate strength can be varied in the laminate design by using directionally oriented stitched reinforcements (uni directional/biaxial/triaxial etc) and varying weight of fibre according to the differing strength requirements in different parts of the boat. Modern production boats often use these materials and appear to have been engineered, usually they are designed to meet standards such as ISO and are quality controlled to an extent, at least manufacturer declared as complaint. This enables their sale in their respective target communities such as EU, America or Australasia for example.

Possible combinations of reinforcements, cores, resins and their processing methods are almost infinite. The range in cost and performance is wide, from fibreglass production boats, hand laid and sprayed up with chopper guns in low tech materials on the one hand as in this article; to Americas Cup boats, engineered, processed and crafted from advanced composites on the other.

As with low tech GRP, the materials may be partly made up in preformed panels to avoid using moulds, or “laid up” on moulds from dry reinforcements that are then wet out. Moulds are generally required to provide the shape of the items being moulded – female or male. However the more advanced materials tend to use vacuum and pre impregnation to achieve higher fibre to resin ratios and better processing as appropriate to the higher material specifications and cost.

The below table divides the range of construction types, from GRP to advanced composites the most common groupings. Note that the type of processing varies according to the materials used.

Construction type	Description	Type of boats
Basic GRP	low cost, polyester resin, chopped strand matt laid or sprayed, woven fabric such as woven rovings, “bulking” cores such as core mat. End grain balsa cores, some foam cores. Laminate specifications may be based on experience rather than formally engineered. Rolled out manually to consolidate fibres. Made in open moulds	Production boats of all kinds – hulls/decks typically at the value end of the market Most trailer boats Lower cost or non weight sensitive custom boats
Slightly advanced GRP –	Engineered laminates, possibly to ISO standards to meet EU or survey requirements, low cost polyester resin, chopped strand mat, use of quality assured cores such as closed cell PVC like divinecell or airex, or end grain balsa wood, use of quality assured directional woven structural “E glass” reinforcements such as double bias cloth, biaxial, triaxial etc. May use viny ester resin below waterline for moored boats. Sometimes models may be available with epoxy resin as an option rather than polyester. May be infused. May be hand rolled out in open moulds.	Production boats of higher specification/cost Vessels required to meet engineered standards. Custom boats
FRC	More likely to be referred to as FRC than GRP Laminates are engineered, likely to a known standard, epoxy or vinylester laminating resin throughout. Engineered fibres such as S glass, Kevlar used, no chopped strand matt. Some use of carbon fibre in high load areas is likely. Likely to be vacuum bagged to consolidate the laminate or vacuum bagged and infused. Likely to use closed cell PVC foam cores. Elevated cure/post cure likely to be required.	Hi performance weight sensitive production and custom boats   Possibly rudder stocks and spars
Advanced FRC	Not appropriate to be described as GRP. Advanced processing skills required. Likely to use epoxy laminating resins Likely to be all carbon fibre laminates Likely to use honeycomb cores like nomex or close cell PVC Fibres and core bonding are likely to be pre impregnated with resin to precise low resin/fibre ratios. Moulds likely to be carbon Advanced processing techniques used Laminates vacuum bagged or autoclave consolidated. Laminates likely to be heat cured and post cured at elevated temperatures	Highest performance and weight sensitivity, racing sail and power boats.   Masts/spars, rudder stocks, Structural rigging Americas Cup and Grand Prix performance sailing craft. Foiling craft

Deterioration and aging of GRP structures

Here is a list of just some of the common issues that we see in GRP structures.

Microcracking	Small cracks in polyester resin caused by flexing, not really visible to the eye, often accompanied by or evidenced by: noticeable flex, spider/crazy cracks in the gelcoat and or softness in the laminate. Typical locations are: Around stanchion bases, boarding platform edges on launches, non cored areas on decks, flex in monolithic hull shells. Areas where cores have become disbonded from the skins leading to flex. May be found on hull skins of small craft, and likely on light racing dinghies – not noticeably visible.
Spider/crazy cracks in gelcoat	Indicator of flex which may or may not be indicative of a damage event. Cracked gelcoat or microcracked laminates are likely to absorb moisture in exposed laminates.
Delamination from core, from wood (polyester)	Anywhere where polyester resin is used to bond to wood. Often bulkhead taping. Bulkheads may be bolted on older craft to prevent disbonding
Wet core	Occurs when there is water ingress into cored laminates such as: around anchor winches/bow lockers, where aftermarket fittings have been bolted through decks such as upholstery bases and clips, rod holders etc. Also screwed teak decks is a headline item for this problem. Wet core is more significant when the core is wood like balsa or plywoodWorse on decks Worse when core is open cell foam which can become saturated and/or turn to powderClosed cell PVC foam does not absorb moisture, but water can still be present in the contours and joins and can travel through foam core structures, especially where air pathways may remain from vacuuming or vacuum infusion
Hull deck joins	Cracking usually from collision and often not visibly obvious, causes leaks behind internal linings, very common on imported vessels that have hull and decks bonded without internal taping or fastenings. Cracking from impact damage is also common around  the deck/stern join
Delamination	Can occur from impacts, incorrectly manufactured laminates such as skins not properly bonded (especially from infusion or vacuum faults), from deformation of deck panels such as adjacent to berthing collisions at deck level, and boarding platforms. In hulls from logs in the water, slamming, fastened items such as tender frames, davits, helm chairs, major foot traffic causing flex in cored decks, chainplates causing lifting/flex in decks. Also delamination can be indicative of wet core causing the core to fail alongside the skin (soggy). In inspections/surveys, core delamination is usually easy to detect as the skins will be noticeably loose where tapped. Inter skin laminate delamination can be harder to pinpoint as tap responses normally vary around bulkheads and framing etc. Odd responses and skin flex is sometimes manufacturing issue with voids being possible in vacuumed/infused structures that can occur from leaks in the mould/bag leading to uneven and/or insufficient clamping/consolidation onto the mould surface.
Bad laminate design	Impractical laminate design such as hulls with cored structure along the keel, very thin skins in core construction, low density cores, inadequate provision for attachment of skin/deck fittings
Cracking in hull skin	Impacts, faulty manufacturing
Flow coat flaking internally	Not a fault but looks dramatic, Flow coat does not adhere well and it flakes off on most older boats.
Deck cracking	May be found around railing bases, anchor winches, davits, beam connections on catamarans, areas where stress concentrates such as cut out areas/hatches on deck mouldings, mast steps, chainplates (lifting chainplates).
Cracked bulkheads	Can be caused by water damage, movement around stress areas such as cut outs in bulkheads/ mast steps, and may be related to hull damage.
Cracked keel floor grid (matrix)	Most typically from grounding, the flanges bonded to the hull skin crack in or alongside the adhesive when the yacht grounds. The back of the keel is typically the most visibly cracked area, the front also likely to be torn away.
Failing gelcoat	Faded, failing (cracking/crazing), worn down (high traffic areas). The best fix is usually to paint with a 2 pack marine paint system.