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 TB 55-1900-232-10
dling functions and act as attachment points for the towing hawser. Other equipment on the tug generally serves only
line-handling functions. These are discussed in the following paragraphs
2-5.9.1 Towing Bows. Towing bows are transversely installed beams or pipe that bridge the caprails on the afterdeck of
the tug. Their function is to keep the towline clear of all deck fittings and to provide a protected area below the sweeping
tow hawser where personnel can pass safely. See Figure 2-33.
2-5.9.2 Caprails.
CAUTION
Whenever the surface of a caprail becomes rough, steps should be taken to repair or replace it in order to
protect the hawser. Caprails should be kept clear of all appurtenances.
The caprail is the riding surface on top of the bulwark. The tow hawser bears on the stern caprail as it passes astern of
the tug and enters the water. Caprails are installed in several configurations. They are fabricated from pipe or plate. On
the newer tugs, they have large-radius surfaces which are contoured to the tug's deck layout. It is important to keep the
caprail smooth and free of nicks and burrs which damage both synthetic and wire hawsers. In current design practice,
the bearing surface of the caprail is hardened to a minimum Rockwell C hardness of 40 to 50. See Figure 2-34.
2-5.9.3 Stern Rollers (Horizontal). The essential function of the stern rollers is to provide a minimum chafe point for the
tow wire during heave-in and payout. It is a large-diameter roller, set in the stern bulwarks on the centerline and faired to
the caprail. As it rotates with the movement of the wire, it provides a constantly changing point of tangency, thus
spreading the wear from the wire. As it is also hardened, it resists scoring and thus provides a smooth surface on which
the wire rides. See Figure 2-35.
2-5.10 SWEEP LIMITING DEVICES. These items restrict the horizontal sweeping of the wire across the fantail
WARNING
The Stern Rollers and Norman Pins onboard the ARS 50 Class ships will drop when a load of 50,000 pounds or
more is applied to mid-barrel height. The resulting uncontrolled sweeping of the towline may injure personnel
or damage equipment.
2-5.10.1 Stern Rollers (Vertical). Stern rollers are an assembly of rollers used to tend the towline both during heave-m
and payout and during long-distance straight towing. Stern rollers prevent a wire from sweeping across the deck and rail.
In the newer ships, the stern rollers or pins are normally operated hydrauhcally from a remote location. See Figures 2-35
through 2-38. Onboard the T-ATFs the hook-shaped items on either side, just outboard of each vertical roller, are
hydraulically-operated "capture hooks, " often used instead of the vertical rollers to provide lateral restraint for the
towline. On the ARS 50 Class, the vertical stern rollers drop (but do not fully retract) when the side force at mid-barrel
height exceeds 50,000 pounds.
2-5.10.2 Norman Pins. Ocean tugs generally are provided with sockets along their aft bulwarks into which fixed or
movable Norman Pins are fitted. These pins limit the sweep of the towline forward of the pins. Current design practice
requires that the wire bearing surface of the Norman Pins be hardened to a minimum Rockwell C hardness of 40 to 50.
The limiting of towline sweep may be based on a desire to keep the hawser out of the screws during slack-wire conditions
or to prevent the tow wire from sweeping beyond a specified position.
The newer tugs and salvage ships have remote-controlled, hydraulically-operated Norman Pins
2-44
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