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by
David Pascoe |
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Hull
Design Defects
Part II
Photos
by David H. Pascoe, Marine Surveyor
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Example of hard spot caused
by improper stringer design and installation. Bottom
hinges around hard edge of stringer wood core. |
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Wood core is elevated by a soft material
so that it does not touch the hull skin and the
load is borne by the more flexible tabbing. |
| Stress cracking is one of
the visible effects of panel deflection.
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In this case, the panel was
dimpled or "oil canning" as the elliptical
array of the stress cracks indicate. The number
of cracks tends to indicate the severity of the
deflection or bending. |
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Natural hinge points such as
chine flats and other angular surfaces require the
build up of extra laminations called fillets which
add extra strength. Lacking these, bending and stress
cracking is likely to occur. |
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The hinge effect, or stress cracking initiated
by hinging off of an internal structural such as
a stringer, produces parallel cracks such as these.
The deposits made by weepage of styrene based fluids
indicates that the cracks penetrate well beyond
the gelcoat. |
| Stress cracking appearing
transversely across a bottom strake.
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In this case, it was caused by
a 27' boat having only one structural bulkhead,
located in the wrong place and no transverse framing.
Torsional twisting of the hull caused the cracking
which is on the verge catastrophic failure. |
| These stress crack appearing on the interior
bottom under an engine were nicely shown up by black diesel
oil via the capillary effect.
The 11 parallel cracks indicate that the degree
of panel bending is severe and the possibility of
failure must be considered. In this case, there
were an insufficient number of transverse frames. |
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To determine whether a hull is cored or
not, look for those areas on the interior hull where
the core terminates. In this photo, the core can
clearly be seen standing out around the bow of this
yacht. |
Go to article
Related Photos: Hull
Design Defects Part I
First posted 6/16/97 at David Pascoe's site www.yachtsurvey.com.
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