Jökull - 01.12.1969, Blaðsíða 113

has been shown, that the salinity of the sea
water has no significant influence upon the
icing, compared with the temperature of air
and sea, and the wind force. The formation
of the sea spray and the height it reaches above
sea level, is mainly dependant upon the wincl
force and the state of the sea.
The second and third items, the ship ancl its
movement, are also very important. The size
and type of ship is important, but also the
way it is loaded and its course relative to the
movement of wind and waves. Irregular seas,
composed of several wave patterns, will cause
irregular rolling of the vessel so the vessel pro-
duces more sea spray at higher levels. Besides,
the irregular seas themselves will usually pro-
duce more sea spray higher over sea level. The
size of the ship has great influence on ieing,
rnainly the freeboard. Bigger ships have larger
freeboard. Usually the icing on the outside of
a smooth hull is not a problem, as the sea
breaks ice off again. Ships having a large free-
board have their decks, masts, rigging and
other super-structure at a higher level above
the sea, and as the sea spray reaches only a
limited height above sea level, the icing on such
ships is considerably reduced compared with
smaller ships with less freeboard. Even on
smaller ships, it is often seen that the icing
is most severe on the lower parts of the masts
and rigging etc., the sea spray not reaching
the highest parts.
The thickness of icing on different parts of
ships lias been measured on Japanese vessels
(T. Tabata, S. Jwata and N. Ono, 1963). They
carried out measurements of the rate of icing
at constant intervals by placing thin pieces of
paper on the ice. Thus it was possible to
measure the amount of icing at all the differ-
ent measuring spots during each time interval,
and so get a goocl general impression of the
ice-accumulation on the various parts of the
ship. The results showed, that the thickness of
icing is very different on the various parts of
the ship. Certain places on deck are consider-
able ice collectors, for example corners and
rigging. Since 1962 a special icing meter has
been used for measuring icing on ships. The
icing meter records the weight of the sea spray
that freezes on the cylinder of the icing meter,
relative to the total amount of sea spray at
that point.
However, such icing meter has so far only
been used on a few ships. Therefore the results
are still limited. Yet it is possible to obtain a
rough information on icing frorn crews of fish-
ing vessels and other small vessels in northern
waters. This lias been done and still is by
several nations. Although the material is still
limited, icing forecast with the weather fore-
cast can be of great importance for the safetv
of fishing vessels and other small crafts in
areas where icing can be expected. The Ice-
landic Meteorological Office started therefore
last winter to send out icing forecast for Ice-
landic waters.
ICING FORECAST
WITH THE WEATHER FORECAST
The icing forecast of the Icelandic Meteoro-
logical Office is based on diagrams which ex-
press ice accretion on ships with low speed, as
a function of the wind force and air and water
temperatures. These diagrams have been ob-
tained from Mertins (1967). The grade of icing
is divided into 5 degrees: No, low, 1—3 cm in
24 hrs., moclerate, 4—6 cm in 24 hrs., heavy,
7—14 cm in 24 hrs., and very heavy, 15 cm in
24 hrs. or niore.
As an example consider the following weath-
er forecast: Windforce 9—10 Bft, airtempera-
ture — 8° C, sea water temperature 3° C. Ex-
pected icing according to diagrams: heavy icing
7—14 cm in 24 hrs.
The diagrams are based on the data collect-
ecl and observecl by Mertins wlten on German
research vessels, altogether about 400 observa-
tions of icing on trawlers on the fishing banks
of Iceland, Greenland, Labrador and Barent-
sea. Although not based on measurements witlt
an icing meter, the 400 observations should
give a rather reliable result for the vessels con-
cerned. l’he icing of some Swedish vessels in
the Baltic Sea has been compared with the
diagrams and conformity appeared to be rea-
sonably good.
It was of interest to follow predictions of
icing of the Icelandic Meteorological Office and
compare it with icing on Icelanclic vessels. For
this purpose the Icelandic State Director of