208
N. A. HOLME
to the release arm for working in the North Sea, and a small vane was
used for t’he same purpose by Stetson (1938) and by Holme (1949).
Thorson (1957a) relates that on the GaEathea expedition the resistance
of the long arms of the van Veen grab caused mid-water tripping, 80
that the Petersen grab was used in preference. Any device which is
actuated solely by a slackening cable may be prematurely tripped in
rough weather.
A messenger-operated closing gear is not possible as t,here will
usually be a bight of slack wire lying on the bottom when the grab
has been lowered. The most reliable type of release for open-sea work
is actuated by a device triggered by actual contact with the
bottom. In the Aberdeen grab (Smith and McTntyre, 1954), which was
designed specifically for work under difficult sea conditions, contact
with the sea floor by two trigger plates, one on each side of the apparatus, is necessary before the release is actuated (not one plate as shown
by Barnes (1959b)). An alternative method is to suspend a cylindrical
counterweight from the release arm, as in the Okean grab (Lisitsin
and Udintsev, 1965). This will keep the release locked until the weight
strikes the bottom, and .can be used if required to trip the release
before the main part of the sampler reaches bottom, as in Hvorslev
and Stetson’s (1946) free-fall corer.
Drift of the ship after the sampler has been lowered may cause
considerable difficulties in sampling due to oblique wire angles, and
this has often been considered one of the major causes of inefficient
working of grabs (Davis, 1926). The van Veen grab, with its long
arms, would seem to bc particularly susceptible to an oblique pull
while closing. The time taken to operate the Knudsen sampler on the
bottom may later render raising it difficult (Johaneen, 1927; Ursin,
1956).
There has been a tendency in recent years for benthos sampling
gear to be supported on a horizontal frame or ring to give it stability
on the bottom and so allow it to operate even when the upward pull
is oblique. Examples include Holme’s (1949) scoop-sampler, and the
Aberdeen grab (Smith and McIntyre, 1954). Dr. P. R. 0. Barnett of
Millport informs me that he is successfully using a modified Knudsen
sampler supported on the sea floor by a framework.
Such frames may make the samplers less suitable for deeper water
work, as water-resistance and therefore lowering time would be
increased ; but their main advantage would, in any case, be in shallowwater operation where tidal streams are stronger and where even a
short delay while the sampler is on the bottom might result in a very
oblique cable angle on hauling.
N. A. HOLME
to the release arm for working in the North Sea, and a small vane was
used for t’he same purpose by Stetson (1938) and by Holme (1949).
Thorson (1957a) relates that on the GaEathea expedition the resistance
of the long arms of the van Veen grab caused mid-water tripping, 80
that the Petersen grab was used in preference. Any device which is
actuated solely by a slackening cable may be prematurely tripped in
rough weather.
A messenger-operated closing gear is not possible as t,here will
usually be a bight of slack wire lying on the bottom when the grab
has been lowered. The most reliable type of release for open-sea work
is actuated by a device triggered by actual contact with the
bottom. In the Aberdeen grab (Smith and McTntyre, 1954), which was
designed specifically for work under difficult sea conditions, contact
with the sea floor by two trigger plates, one on each side of the apparatus, is necessary before the release is actuated (not one plate as shown
by Barnes (1959b)). An alternative method is to suspend a cylindrical
counterweight from the release arm, as in the Okean grab (Lisitsin
and Udintsev, 1965). This will keep the release locked until the weight
strikes the bottom, and .can be used if required to trip the release
before the main part of the sampler reaches bottom, as in Hvorslev
and Stetson’s (1946) free-fall corer.
Drift of the ship after the sampler has been lowered may cause
considerable difficulties in sampling due to oblique wire angles, and
this has often been considered one of the major causes of inefficient
working of grabs (Davis, 1926). The van Veen grab, with its long
arms, would seem to bc particularly susceptible to an oblique pull
while closing. The time taken to operate the Knudsen sampler on the
bottom may later render raising it difficult (Johaneen, 1927; Ursin,
1956).
There has been a tendency in recent years for benthos sampling
gear to be supported on a horizontal frame or ring to give it stability
on the bottom and so allow it to operate even when the upward pull
is oblique. Examples include Holme’s (1949) scoop-sampler, and the
Aberdeen grab (Smith and McIntyre, 1954). Dr. P. R. 0. Barnett of
Millport informs me that he is successfully using a modified Knudsen
sampler supported on the sea floor by a framework.
Such frames may make the samplers less suitable for deeper water
work, as water-resistance and therefore lowering time would be
increased ; but their main advantage would, in any case, be in shallowwater operation where tidal streams are stronger and where even a
short delay while the sampler is on the bottom might result in a very
oblique cable angle on hauling.