Ship Superstructure Icing:
Crystalline and Physical Properties
CHARLES C. RYERSON AND ANTHONY J. GOW
INTRODUCTION
trawler (Fig. 2). Maximum accretion is likely to occur
Background
higher on the superstructure in forward areas, as spray
Ice grows on ship superstructure components as a
flux normally decreases with distance aft of the bow,
result of precipitation of saline spray from the ocean
and with height above the deck.
surface in winter seas. A potentially serious problem,
Thermally limited accretion (Fig. 1) takes place
superstructure icing reduces ship sea-keeping ability and
where the spray water delivered exceeds the
hinders deck operations. Resulting largely from spray
atmosphere's ability to remove its sensible and latent
lofted from the sea surface by bowwave collisions,
heat. Thus, ice accretion rates are smaller than water
spray drops are carried by the relative wind, the result-
delivery suggests. Large spray fluxes, and thus ther-
ant of true wind speed and direction and ship speed and
mally limited accretion, are normally only found on the
direction, over the ship. The rate of spray freezing and
bow areas of large ships, even though large volumes of
subsequent ice growth is a function of both the spray
spray can reach farther aft on smaller ships. Figure 2
rate at every location on the ship, and the rate of latent
illustrates a situation where thermally limited accretion
and sensible heat removal at these locations.
is restricted primarily to portions of the bow, the fore-
Spray ice accretion rates vary considerably with
castle deck, and the forward bulkhead of the trawler.
location on a ship (Ackley 1985). Ice accretion rates
Mass limited accretion generally occurs aft and
are determined by the balance of heat delivery by spray,
above the maximum accretion zone because spray gen-
both sensible and latent, and atmospheric heat removal
erally decreases with distance aft of the bow and above
processes. Figure 1 illustrates three icing zones that
the main deck (Fig. 1). The mass limited accretion zone
often occur on all sizes of ships. The maximum accre-
is characterized by water delivery rates, and, thus, sen-
tion zone is where spray delivery matches the
sible and latent heat delivery rates, that are smaller than
atmosphere's ability to remove sensible and latent heat
the atmosphere's ability to remove the heat. Thus, all
from impinged water at a sufficient rate for all spray to
of the spray (except brine concentrated within brine
freeze (although some spray remains trapped as brine
pockets) freezes and ice accretion is limited by avail-
within the ice). Maximum accretion may take place at
able spray. It does not reach its potential thickness as
bow locations maximally exposed to the wind, such as
determined by the atmosphere's ability to remove heat.
at the top of the bow and windlass located on the fore-
Mass limited accretion is most dramatically illustrated
castle of the fishing trawler shown in Figure 2. How-
by the upper portions of the twin masts on the trawler's
ever, during heavy spraying, the most ice is likely to
fantail, where ice thickness decreases with height
accrete amidships, where the spray flux is smaller and
(Fig. 2).
the rate of heat removal by the atmosphere is still large
The three superstructure icing zones are dynamic,
(Fig. 1). This is demonstrated by the wheelhouse roof
with the amount of superstructure covered by each
and areas immediately aft of the wheelhouse on the
changing as spray delivery rates and patterns, and
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