ssc-367 - Ship Structure Committee

Shrouds
Shrouds consist of an outer shell, separated from the tubular by a
gap of about 0.10 diameter, with many small rectangular holes. The
limited data available indicates that shrouds may not always be
effective. The advantage of shrouds over strakes is that their drag
penalty is not as great; for all Reynold’s numbers, CD= 0.9 based on
the inner tubular diameter. Like strakes, shrouds can eliminate the
in-line motion of the two low-speed peaks without covering the
complete length of the tubular. However, any design that requires
shrouds (or strakes) to prevent cross-flow motion should be
considered with great caution. Their effectiveness can be minimized
by marine growth.
Offset Dorsal Fins
This is the simplest device for the prevention of oscillations. It
is probably the only device that can be relied upon to continue to
work in the marine environment over a long period of time without
being affected adversely by marine growth: It has some drag penalty,
but this is not likely to be significant for most designs.
The offset dorsal fin is limited to tubular structures that are
subject to in-line motion due to flow from one direction only (or one
direction and its reversal, as in tidal flow).
This patented device comprises a small fin running down the length of
the tubular. Along with the small drag increase there is a steady
side force. This may be eliminated in the case of the total force on
multi-tubular design by placing the fin alternately on opposite sides
of the tubulars.
D-26