15 August 2002
Keel Blocks. Keel blocks are placed under the longitudinal centerline
keel of the vessel. The exact location of the blocks depends on a vessel's docking plan.
All keel block are interchangeable; therefore, each is designed for the maximum ship
load likely to be imposed upon it at any location. Compression is the primary stress, but
provision must be made to resist uplift, overturning, and horizontal movements induced
by eccentric loads, earthquakes, or accidental impacts. Reinforced concrete stresses
are not critical in design; grade of concrete and amount of reinforcement steel are
selected to resist rough handling and temperature variations. Standard composite keel
blocks (see Figure 8-1) were historically rated at 228610 kg (25 long tons), based on an
allowable stress for wet timber in compression perpendicular to the grain taken at 1724
kPa (250 psi) for soft caps. This allowed a reasonable safety factor. For the standard
1.8 M (6 ft) center-to-center keel block spacing, that rating represented a 34014 kg/m
(37.5 tons/ft) ship load. Now, the safe allowable timber compressive stress for
distributed loading, taken as the fiber stress at the proportional limit of Douglas Fir, is
2552 kPa (370 psi). This assumes a uniform pressure over the entire 1067 by 1219 mm
(42 by 48 inch) top of a docking block, resulting in a total load of about 335,295 kg (330
long tons). Most ships have narrower skegs and the allowable block loading is
decreased accordingly. For allowable block loadings for this condition, refer to Naval
Ships Technical Manual, Chapter 997, Docking Instructions and Routine Work in
Bilge or Side Blocks. Bilge or side blocks are composite or timber, built
up, shaped, and located according to dimensions indicated in the table of offsets of
docking plan of the vessel. These are designed for 1724kPa (250 psi) load applied
uniformly over the effective bearing area in contact with the hull of the ship. Batten
each block adequately for stability, and the resultant load reaction should fall within the
middle one-third of the base dimension of the block on the dock floor.
Type of Construction. Build composite blocks with wood top and bottom
layers, and concrete sandwiched in between. Use sufficient concrete to make the
blocks nonbuoyant. Secure the wood layers to the concrete with steel bolts embedded
in the concrete. U-bolts embedded in the sides of the concrete may be provided for
lifting, or pipe holes may be provided through the blocks to insert pipes for lifting by
forklift or crane rigging. All hardware (except dogs) should be zinc coated or cadmium
plated. For a typical block, see Figure 8-1. For heavy loads, these blocks may be used
double as indicated in Figure 8-1.
Industrial Shop Facilities. Shipbuilding graving docks and graving docks
used for extensive repair, alteration, and the rebuilding of vessels must be supported by
industrial shop facilities capable of manufacturing or otherwise supplying, installing, and
testing the large number of items required.
Transportation Facilities. A graving dock must be serviced by the
following transportation facilities.