METHODS O F SAMPLINU THE BENTHOS
223
use, but i t should be at least as efficient as any other grab of comparable size and weight.
VII. WORKING SAMPLING GEAR AT SEA
A. Dredging
On conventional side-trawlers, dredges are usually worked through
the after trawl gallows (Fig. 6), being hauled inboard, if required, by
additional hoisting gear attached to a boom or to the mast. If a cod-end
can bo fitted, this greatly facilitates emptying the sample.
If deck Rpacc aft iu restrictcd, it may be more convenient to work
the dredge from the forward gallows. In this instance it may be
possible to go astern while dredging, and this is the manner in which
we work the anchor-dredge on R.V. Sarsia (mainly because it assists
subsequent manoeuvres in recovering the dredge). Alternatively, the
forward warp might be brought in by a messenger and clamped into
the towing block a t the stern just as it is when trawling. This would
enable the ship to manoeuvre freely, steaming ahead.
On large vessels not primarily equipped aa trawlers it has been
customary to work over the stern, and this may become the regular
practice when stern-trawlers become general, as it offers several
advantages.
When working from a large ship it is advisable to guard against
the warp parting should the dredge foul an obstruction. Apart
from a weak link on the dredgc: itself (p. 183), some precautions may
be taken on the ship. It is advisable to have a strain gauge, which can
easily be contrived with a snatch-block attached to a strong springbalance. When the wire has been paid out the snatch-block and
spring-balance are attached and strain put on the balance so as to
deflect the towing warp from a straight line between two sheaves.
Strains on the warp can be read off on the balance, and converted
into absolute figures from the deflected wire angle. On larger ships it
may be convenient$to have a gauge installed which records the strain
on a dial fitted on the bridge (as in the new R.R.S. Discovery).
When trawling or drddging it is customary not to tighten the
winch brakes hard down, so that any unusually great strain will result
in wire being paid'out.' This could no doubt be contrived by eome
automatic device, or alternatively a " nipper " could be used. This is
a strong pincer arrangement (Fig. 28) which grips the towing wire
and which is itself made fast to the ship by a length of rope of some
2-3 in circumference (16-24 mm diameter) (Fowler and Allen, 1928;
Kemp et al., 1929). The winch brakes are not applied and the entire
r.
223
use, but i t should be at least as efficient as any other grab of comparable size and weight.
VII. WORKING SAMPLING GEAR AT SEA
A. Dredging
On conventional side-trawlers, dredges are usually worked through
the after trawl gallows (Fig. 6), being hauled inboard, if required, by
additional hoisting gear attached to a boom or to the mast. If a cod-end
can bo fitted, this greatly facilitates emptying the sample.
If deck Rpacc aft iu restrictcd, it may be more convenient to work
the dredge from the forward gallows. In this instance it may be
possible to go astern while dredging, and this is the manner in which
we work the anchor-dredge on R.V. Sarsia (mainly because it assists
subsequent manoeuvres in recovering the dredge). Alternatively, the
forward warp might be brought in by a messenger and clamped into
the towing block a t the stern just as it is when trawling. This would
enable the ship to manoeuvre freely, steaming ahead.
On large vessels not primarily equipped aa trawlers it has been
customary to work over the stern, and this may become the regular
practice when stern-trawlers become general, as it offers several
advantages.
When working from a large ship it is advisable to guard against
the warp parting should the dredge foul an obstruction. Apart
from a weak link on the dredgc: itself (p. 183), some precautions may
be taken on the ship. It is advisable to have a strain gauge, which can
easily be contrived with a snatch-block attached to a strong springbalance. When the wire has been paid out the snatch-block and
spring-balance are attached and strain put on the balance so as to
deflect the towing warp from a straight line between two sheaves.
Strains on the warp can be read off on the balance, and converted
into absolute figures from the deflected wire angle. On larger ships it
may be convenient$to have a gauge installed which records the strain
on a dial fitted on the bridge (as in the new R.R.S. Discovery).
When trawling or drddging it is customary not to tighten the
winch brakes hard down, so that any unusually great strain will result
in wire being paid'out.' This could no doubt be contrived by eome
automatic device, or alternatively a " nipper " could be used. This is
a strong pincer arrangement (Fig. 28) which grips the towing wire
and which is itself made fast to the ship by a length of rope of some
2-3 in circumference (16-24 mm diameter) (Fowler and Allen, 1928;
Kemp et al., 1929). The winch brakes are not applied and the entire
r.