Stress Cracks & Surface Irregularities
Surveying Fiberglass Power Boats
by David Pascoe
Stress cracks are a commonly occurring phenomenon with reinforced plastic boats.
A lot of boat owners get very upset when they see these cracks, sometimes needlessly, while at other times their fears are well founded.
Clients will often ask about cracks and the surveyor will be expected to provide answers.
This chapter is included not because stress cracks are unsightly and tend to upset people, but because they are often indicators of more serious, underlying problems.
After looking at many thousands of boats with stress cracks, I still have to say that evaluating the import of stress cracks, particularly in the bottom, is not always easy.
There are often times when the surveyor can find no basis or cause for their existence, and thus it is left without an explanation to give. It will be useful to have an understanding of how and why these cracks commonly occur.
The primary reason why stress cracks appear in gel coat is because gel coat is brittle and the laminate to which it is bonded is less so.
Gel coat is basically a resin to which a large amount of coloring pigment has been added, pigments that are oxides of metals like aluminum.
Because reinforced plastic and gel coat are two very different materials, they do neither expand and contract at the same rates when heating and cooling, nor do they have the same degree of elasticity and thus behave differently when flexing.
We can add to this another factor, and that is the intended cured hardness. Gel coats that are harder than others are more durable, but they also have the down side of being more brittle and prone to cracking.
Gel coat cracking is the result of some kind of induced stress. This stress can come from improper design, or it can be the result of imposed stresses from within, without, or both.
It can be the result of aging or faulty application such as excessive catalization or hardeners.
By far, the most common reason for stress cracking is due to improper structure design. Even the finish on a flat panel will crack when bent too much.
When it comes to large, flat panels, cracking usually results from excessive panel flexing or panting, such as on an unsupported hull side.
Cracking also occurs as a result of an engineering phenomenon called a stress riser. A stress riser occurs at point in a panel or hull skin where a normally somewhat flexible area is suddenly made rigid without a means of transferring the imposed stress gradually to the rest of the laminate.
A good example is a strut attachment to the bottom without any kind of doubler. Another example would be a bottom stringer butting hard against the laminate with no fillet to distribute the load. Yet another is a railing stanchion attached to a deck laminate with no fillet or doubler beneath. The end result is that the amount of stress suddenly rises at a certain point and so stress cracks are likely to appear.
(Additional spaces are added for easy screen reading.)
Table of Contents: Chapter 8
8. Stress Cracks & Surface Irregularities 195
Stress Cracking on Bottoms 196
Weak Panel Flexing 197
Hinge Effect and Hard Spots 198
Parallel Stress Cracking 198
Starburst Cracks 199
Gel Coat Break-Outs 200
Drawing Conclusions 200
Bottom Crazing 201
Cracking on Hull Sides 202
Cracking on Decks and Superstructures 203
Surface Irregularities 204
Print Through 204
Excessive Gel Coat Thickness 206
Gel Coat Voids 206
Mottling, Uneven Coloration 206
Surface Porosity 207
- 2nd Edition
by David H. Pascoe
Publisher: D. H. Pascoe & Co., Inc.
David Pascoe - Biography
David Pascoe is a second generation marine surveyor in his family who began his surveying career at age 16 as an apprentice in 1965 as the era of wooden boats was drawing to a close.
Certified by the National Association of Marine Surveyors in 1972, he has conducted over 5,000 pre purchase surveys in addition to having conducted hundreds of boating accident investigations, including fires, sinkings, hull failures and machinery failure analysis.
Over forty years of knowledge and experience are brought to bear in following books. David Pascoe is the author of:
- "Mid Size Power Boats" (2003)
- "Buyers’ Guide to Outboard Boats" (2002)
- "Surveying Fiberglass Power Boats" (2001, 2nd Edition - 2005)
- "Marine Investigations" (2004).
In addition to readers in the United States, boaters and boat industry professionals worldwide from over 70 countries have purchased David Pascoe's books, since introduction of his first book in 2001.
In 2012, David Pascoe has retired from marine surveying business at age 65.
On November 23rd, 2018, David Pascoe has passed away at age 71.