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Guidance on Inspection, Repair, and Maintenance of Wooden Hulls
US Coast Guard NVIC 7-95
Specifically Chaper 5 (E)
BUTT JOINTS IN PLANKING
Planking butts should not terminate on frames in normal construction. They should be located between frames on proper butt blocks, though in light construction with narrow strakes, they may sometimes be found as glued scarf joints at the frames and in some construction with massive framing they may be found butted on the frames. As a rule
of thumb, butts in adjacent planks should be at least three frame spaces apart for transversely framed, longitudinally planked vessels.
Those butts which fall in the same frame bay should be separated by at least three solid strakes. This is not always possible, especially at the end of the vessel, but serves to illustrate the principle of keeping butts separated as much as possible. Where frame spacing is unusual the following rule
may be used as a guide.
Butts in adjacent strakes should be no closer together than 5 feet. If there is a solid strake between, they should be no closer than 4 feet. Butts should be shifted so that three or more do not fall on a diagonal line.
To be effective a butt block must have adequate size (See page C-12). If the frame spacing allows, its length should be at least 12 times the planking thickness. Its thickness should be one to one and a half times the planking thickness and its width at least 1" greater than the strake width. Prior to installation
it is recommended that the faying surface of the butt block and strakes be coated with a wood preservative. The top of the butt block should be curved or chamfered to allow for water run off. Avoid butting the block hard against the frames to minimize decay.
The fastenings of the strake to the butt block should be of equal strength to that of original butts. The fastening size should be equal or larger and no fewer number of fastenings should be allowed. Through bolts or machine screws are preferred fastenings in butt blocks because the joint will achieve maximum strength. Care should be exercised to avoid over tightening so as not to crush the planking or split the butt block.
butt blocks should be avoided because plywood
has somewhat less strength than the "along the grain" strength of the basic wood from which it is made. Plywood is also prone to delamination
and rot precipitation.