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 TB 55-1900-201-45/1
little flexibility along its length and breath and tends to develop "tension cracks" which destroy watertightness and
strength.
b. Before allowing any wooden structure to be repaired using reinforced resin, an evaluation shall be made con-
sidering the following items in addition to those noted before.
(1)
In the hull, even a hairline crack can allow undetected entry of marine borers.
(2)
With an old structure, which has been painted or preserved, a good bond is very difficult to attain.
(3)
Any rot present may continue to grow worse under the glass if the proper conditions of moisture and heat
develop.
(4) It is difficult to acquire enough strength from a reinforced resin coating to make up that lost from the un-
sound substructure.
(5)
It is difficult to check the soundness of the sub- structure once the glass has been applied.
c. Full sheathing of an existing wooden hull with fiberglass reinforced plastic as a method of restoring strength
and water- tightness to a deteriorated vessel is not normally acceptable.
2-18. Mechanical Fastenings. Mechanical fastenings should be of material suitable for the service intended. Ferrous
fastenings should be hot dipped galvanized. Among usual non-ferrous types, brass is not acceptable where it will be ex-
posed to salt water attack. Caution should be used in selecting fastening material because of the problem of galvanic
action which can arise if dissimilar metals are used close to one another, ie, a bronze washer used with a steel bolt will
result in the eating away of the steel. The number, size and spacing of fastenings shall be equivalent to or better than
those replaced.
2-19. Glues and Gluing.
a. Glues of the resorcinol and phenol-resorcinol resin types are the most satisfactory for severe service use, as
for marine re- pair. Other glues have lower resistance to moisture and should be avoided where this is an important fac-
tor.
b. Not all wood is easily glued. Wet wood (above about 15% moisture content) is difficult to glue as is very dry
wood. Normal seasoned wood of most species can be glued. Strong glued joints are possible only using the side grain
of wood. These joints can be very nearly as strong as the wood itself. It is impossible to join end grain with glue and get
joints which are even 20% as strong as wood.
c. Curing temperature and surface condition are important. The temperature must be about 700F or higher for a
full cure of resorcinol resin glue and the surface should be smooth. In the past, surfaces have been intentionally rough-
ened in the mistaken belief that slightly
2-16
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