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Old 22-05-2004, 06:41   #1
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The following is neither intensive (deep) nor exhaustive ( broad), and is offered to illustrate the complexity of the gelcoat - laminate synergy, and the significance of understanding the importance of the gelcoat to your boats structural integrity, and the possible consequences of ignoring it.

Boat hulls are usually made from the outside-in, by applying the gel coat to a waxed mold and then adding the layers of glass reinforcement and polyester to complete the hull. The gel coat, the pigmented and filled polyester, is used to hide' the underlying glass composite structure, to color the hull, to produce a flexible surface which acts as a shock absorber and to help keep water from diffusing into the composite. Blisters and de-lamination are caused by water diffusing into the hull and reacting with water soluble material to form a droplet of solution which, because of osmotic pressure, grows in volume and creates a force which results in a blister or a de-lamination.

The integrity of the hull surface - ie: gelcoat - is essential in preventing this disastrous water diffusion into the laminate. The Gelcoat is basically a resin-rich surface (much like a person’s skin), which is designed to:

1. protect the laminate from the environment
2. reduce fibre pattern
3. provide a smooth aesthetic finish
4. eliminate the need for painting

Hairline cracks in a gelcoat surface (hereafter crazing) are often (mistakenly) considered merely a cosmetic problem. However, on occasion, gelcoat cracking may be an indication of underlying structural problems, or a result of manufacturing defects, environmental, or operating conditions.

Allowing cracks to remain “open” would be a serious misjudgment, likely resulting in future serious structural & cosmetic problems, including de-lamination and osmotic blistering.

Whatever the underlying causes (or source); the mechanisms causing gelcoat crazing are always STRESS and MOVEMENT.

Gelcoat, by nature of being on the outer surface of a structure, is subject to the highest strain of the entire laminate. The tensile or compressive strain in a loaded laminate increases with distance from the neutral axis of the load. Under a flexural load, the highest tensile strain is recorded at the top surface, while the highest compressive strain is at the bottom surface. There is no strain at the interior of the laminate, at the neutral axis. Because of the critical positioning of the gelcoat film in a laminate structure, both the laminate and the supporting structure must take into account the strain imposed by anticipated operating loads.

There are a number of sources of localized stress in a boat hull, all of which could first appear as crazing or hairline cracks, and may ultimately lead to structural de-lamination, and/or blister formation and growth.

1. Stresses are produced by polymer shrinkage during curing. As the laminating resin cures it bonds to the solid gel coat and then shrinks on curing producing a tensile stress in the laminate near the gel coat interface. After the gel coats are cured on a mold, the resin is applied. It bonds to the gel coat before it cures. The resin near the gel coat interface goes into tension as the resin away from the interface cures and shrinks. Undercure, resulting from under-catalization, low shop temperature or too thin a film, will usually produce a flexible gel coat. While this flexible gelcoat is not prone to cracking, it may be inclined to premature color degradation, loss of gloss, chalking or chemical attack. On the other hand, over-catalization can easily lead to a brittle gel coat which cracks with little provocation.

2. Stresses are produced by swelling of the resin due to water diffusion. The amount of water present causes swelling of the polymer. The resin can swell as much as 10 percent by volume, and this is greatly affected by the degree of cross linking. Stresses are generated by differential swelling. If the entire hull swells uniformly, no differential stress will result. However, if one layer swells and the adjacent layer does not, the adjacent layer will be pulled apart (put in tension) by the swelled layer. The level of differential stress generated will be determined by the water gradient and discontinuities in the gradient and not by the absolute amount of water present.

The stress is transient. The maximum tension will move inward and decrease in magnitude as water diffuses. If the resin has high strength, that is, it is well cured, highly cross-linked, and reinforced with glass, it can survive the passing stress field and not crack. If a disk crack forms, it constitutes a vacuum. Any local WSM units will be drawn toward the crack to increase the pressure. This is a mechanism for concentration of WSM units in the vicinity of the crack. Stress cracks can create blister centers.

3. Stresses are produced during boat use. Peak stress is produced by wave action, rigging stresses, impact stresses and buoyancy stress.

4. Internal cracks produce stress concentration sites at the crack tips which can lead to further cracking or accelerated chemical attack. Strictly speaking, the crack does not produce a new stress but intensifies one of the above three stresses. Cracks can magnify a stress by hundreds of times.

5. Thermal shock or direct sunlight can heat darker colored composites to beyond the heat distortion temperature of the resin causing warpage, creeping of built in stresses, over expansion of trapped air or moisture - causing laminate separation (de-lamination), blistering, or even catastrophic collapse of entire structure.

Two or more of the above five types of stress can interact at a particular point in time and space. For example, if a modest shrinkage stress combines with a small water swelling stress and at the same time, severe wave impact flexes the hull, localized disk cracking can take place. Furthermore, the reaction of the polyester resin to the stresses applied is dependent on the flexibility and toughness, i.e. resistance to cracking, of the resin. If the resin is brittle cracking will occur. A flexible resin can deform under peak stress loads without cracking. Resin flexibility depends on the type and number of links in the polyester chain and, very importantly, on the number of cross-links between the chains.

To reiterate: the mechanisms causing gelcoat crazing are always STRESS and MOVEMENT. Movement in one form or another can have a number of causes. Many times the cause of the movement can be determined from the pattern of cracking.

There are a number of types of cracks that are evidenced in gelcoat, and each type may signify a particular problem or set of problems. Various crack configurations may indicate the underlying causes, and are vital in troubleshooting the problem. In some cases the root problem has nothing to do with the gelcoat, and is a manifestation of a structural problem or unanticipated movement of the substrate.

Radial Cracks:
Usually associated with impact, radial cracks are a good indicator of the direction of the impact. The classic "spider" crack is a result of a reverse impact or sharp, localized stress riser. a frontal impact is indicated by a concentric circle pattern, with the diameter of the inner circle having a relationship to the size of the impacting object.

Linear Cracks:
There are two groups of linear cracks: stress field patterns and parallel stress cracks. The primary cause of these cracks is flexural strain. However, in the case of stress field cracking, either structural elements or local stress risers modify the parallel pattern into a more complex structure.

Parallel stress cracks indicate flexural movement perpendicular to the direction of the cracks. Parallel curvillinear cracks often indicate a distribution of stress over a supported panel surface. If the surface is restrained in two 90-degree planes, the flexural strain will "fan out," creating a "palm leaf" effect.

Parallel stress cracks radiate from a localized nucleation. The main effect is the deflection of the laminate inward toward the restraining member. The parallel stress crack is interrupted by a stress concentration around a point

Convergent stress field cracks may result when flexural strain is interrupted by a structural member.
Divergent stress fields occur when the laminate is deflected away from the supporting member and the crack propagation is consolidated through a localized lack of movement.

Thermal fatigue Cracks:
Thermal fatigue cracks are a result of repetitious expansion and contraction of the gelcoat film. Whether in a parallel pattern or an isotropic (nondirectional) configuration, thermally induced cracks are characterized by short discontinuous sections, and are usually grouped in forming in a dominate stress field.

Isotropic thermal cracks are a result of the surface expanding and exerting a tensile strain within the gelcoat film in a unidirectional fashion.

Parallel thermal fatigue cracks usually are propagated by expansion of the surface in conjunction with localized flexural stress.

Form stress risers:
This type of crack is a result of an intervening shape, usually a cutout, in the surface of a panel. The form or shape serves to concentrate strain into a localized area.

In the case of a hard point riser, a low-level strain may result in cracking due to high-level stress concentration in a very small area. A square shape with sharp corners is a prime candidate for creation of a hard point riser.

A radial riser may have a different origin. In this case, often a bolt or hardware fitting exerts a tensile force in the area around a hole. The edge of the hole distends causing a tensile failure of the gelcoat in the surrounding area.

Repairing Gelcoat Crazing and Hairline Stress-Cracks

Some TEMPORARY quick Stop-Gap Measures [pun intended ]

First: Roughen the damaged area and clean it with acetone. As with all sealing, painting (and the like), PREPARATION is everything.

“Captain Tolley's Creeping Crack Cure” is a one part water based acrylic polymer low viscosity penetrating sealant that uses capillary action to find its way inside a fine crack and sets by water loss forming a rubbery mass. It usually takes 24 hours to cure (air drying). It is recommended to apply a fill every 20 minutes till no product is absorbed into the crack. Because it has poor bonding strength, and can only accommodate a small amount of movement of the structure, I would only use it as an immediate TEMPORARY measure.

“Cyanoacrylates” ( like “Krazy Glue”) are acrylic resins (C5H5NO2) that cure almost instantly. The only trigger it requires is the hydroxyl ions in water. Super Glues may help to stop a crack from spreading, but will not fill nor seal a crack. Drill a small (about twice the diameter of the crack’s width) at each end of the crack, then dribble in some Super Glue. Again, only a TEMPORARY stop-gap.

to be continued ...

Hope this helps,
Gord May

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"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 23-05-2004, 00:47   #2
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If I may add a note to the subject!

For cracks that are not attended to before winters sets in, can lead to not only a larger crack but also delamination.

Here in the PNW we have many periods where it rains durning the day and freezes at night. The problem being, a crack absorbing water durning the day and freezing at night. This opens up the crack even wider. Then it absorbs even more water and freezes again. If the process continues long enough this actually lifts the gelcoat away from the fiberglass and starts a delamination.

It would have been smart of me to take pictures of this event but the repairs have already been made with some urgentancy.

Just a warning to those who live in the colder regions..............._/)
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Old 23-05-2004, 23:02   #3
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Actually I do have a picture, but it doesn't really show 3 dimentional. When I received the vessel I took a few pictures of the ole girls age marks. This is where the DPO made an attempt to seal the cracks and voids with what looks like epoxy and didn't finish the job. The other spots were untouched but no pictures. You could see where the gelcoat had lifted up and started breaking away.

This is the aft corner of the cockpit and it is 3/4" ply with 3 layers of glass/polyester and a gelcoat over the glass. The deck flexes a bit when steping down from the deck. So this was probably the start of the cracks. I've glassed in the corners to a larger raduis and painted. I plan to put in a removable teak deck grate to ad to the strength of the deck as well as puting in supports on the under side of the cockpit deck.

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Old 24-05-2004, 01:33   #4
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More cracks

Me too.

I have some of them gel coat cracks on meh ship, but they are all on the inside of the cabin, not outside.

Been told that they sprayed the inside gelcoat on and if too much, or too thick, then it will crack with any movement.
It did, mostly in corners.

Been eyeballing the cracks, they are not getting any bigger or smaller.
Also seen the same cracks in the same locations on other ships of same type. (1979 to 1981 CSY 33)

Did surgery on one crack recently as an experiment:
Tooth brush with acetone to get the mold and dirt out, then West System fast cure epoxy applied with tooth picks. (The cracks are in the curve from vertical cabin side to ceiling, had to apply upside down)

Not sure if I did much or not. The former cracks sre still very visible as the shiny epoxy sort of high lights 'em.
Been hesitating to paint over as I don't have the exact matching paint or gelcoat....Try to avoid looking at it instead.
Poor matching paint could make the situation worse or force me to paint the whole interior. Not in the cards.

Had a "gelcoat guy" come in and give estimate to fix it for good.
He was going to root or dremble the cracks, then fill em and color code and match and all that.

Not sure if it is worth the money if these here cracks are just cosmetic........Or are they?
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Old 24-05-2004, 02:04   #5
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Cockpit Sole Joint Failure


Thanks for the important additional information on moisture & freezing. The prodigious hydraulic force of freezing water is often forgotten.

Your photo seems to be representing a horizontal joint failure, complicated by vertical stress-cracking at the inside corners. While freezing water (inside the laminate or joint) may have exacerbated the problem, I'm not certain that it caused it. This might just as easily have occured in the tropics.

Stresses are concentrated at corners (& other directional changes), so I’m not surprised to see the vertical crazing pictured. However the separation of the cockpit floor from the well liner indicates an ill-conceived & executed connection (from the factory). Was the 3/4" plywood encapsulated /w 3 laminations on both sides, or merely surfaced /w glass’ & gelcoat on the upper surface. If merely surfaced, the assembly will not form a good structural “sandwich” (I-beam like assembly), and would be further evidence of poor engineering (at least in this instance).

It’s difficult to tell whether the cockpit sole flexure was a symptom of cause or a cause of effect (which came first?).

It (sole flex) may have been a result of the joint failure, which permitted the entry of water.
If moisture first got in through the pre-existing vertical corner stress-cracks, then that could have caused the sole softening and joint failure (in either order). I cannot tell (from the photo) whether an attempt was made (by the PDO) to seal the vertical corner cracks, or merely to cover over the horizontal joint separation.

I’d be interested in hearing more (& seeing photos) about your repairs. I’m almost certain that you didn’t just add some interior filleting (”...I've glassed in the corners to a larger raduis and painted...” ), and cover it up.

In any case, you present an interesting & illustrative practical demonstration of the effects of small surface damage being more than cosmetic.


PS: “WEST Systems” has an excellent booklet on “Fiberglass Boat Repair & Maintenance”, Cat. No. 002-550.
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Old 24-05-2004, 16:55   #6
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Yeah! The 3/4" ply is encapsulated with glass on both sides but the floor of the cockpit was just flat with no bracing. You figure after 20+ years of a raceboat crew jumping in and out of the cockpit there's bound to be some major stresses.

I'm not sure the freezing effect was part of the problem HERE but I have seen it on one of the other boats left rotting in the yard here. There's on old bayliner sitting (& forgotten since 96) down the way here and during the freeze while checking on my vessel I happen to see ice on the fore deck. So I went over to see how thick it was and you could see it lifting up the gelcoat where an anchor had been dropped. Looked like a pimple.

Anyway, I have a multiple of projects but I'll try to sneak in some more pictures here soon!

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