70

From calculations based on sections of Nelson
Harbour, I find that fully 37 1/2 millions of tons of
water ought to flow in and out every spring tide
(rising 15 feet), in order to make the levels inside
and outside agree, but the cross section at the en-
trance is such that, even allowing a current of five
knots to run for six hours, only some 31 millions of tons
can pass in and out through the channel; and though
no doubt the "overflow channels" will allow the
high-water level at the lower part of the Harbour to
be raised to sensibly the same height as it is outside,
I should consequently expect that such is not the
case over the upper portions of the mudflat, and
that the whole of the proposed reclamation, and also on
any tidal water.

Even should this not be the case, however, as the
whole of the reclamation shown on the plan, both
along Haven-road and on either side of the proposed
basin, will exclude very little more than one and a-
half millions of tons, or one-twenty-fifth of
the whole tidal flow, there seems no reason to fear
that such a moderate change should have any seri-
ously injurious action, either at the entrance or on
the bar, and that in consequence any special mea-
sures to counteract such action will be required.

The whole of the reclamation, shown along Haven-
road and on either side of the proposed basin, would
amount to about 126 acres, and would require in all,
over three millions of cubic yards of filling, or, de-
ducting the earthwork measured with and included
in the estimate for the wharfage, training-wall, and
dock, over three-quarters of a million of cubic yards.
Of course it is not contemplated that such an ex-
tensive work should be undertaken at once, as a very
modified amount of accommodation would amply
suffice for the requirements of the port for a long
time to come; the design is merely shown on so ex-
tensive a scale to enable an opinion to be formed as
to its convenience when completed, and to facilitate
the adoption of some general plan, of which all
works undertaken from time to time, as required,
would form an integral part.

PATENT SLIP.

It is much to be regretted that the land at Green-
point has been leased, as otherwise an excellent site
for a Patent Slip could have been erected there; but,
without interfering with the leased sections, it would
now be almost impossible to select a good place on
the main land for such a work, which would not
entail considerable expenditure either in excavating
at the outer end of the ways, or in reclamation at
the landward end; and on any such site the slip
would be very liable to interfere with future works,
and ought not to be constructed without very care-
ful consideration of all the bearings of the case—at
least should the general idea of a basin for shipping,
either present or future, be entertained. To avoid
these difficulties I have drawn a slip on Haul-ashore
Island. At the same time, that site is in some re-
spects very inconvenient—workshops, for instance,
and all work, unless large workshops were erected
on the island, would require to be carried backwards
and forwards in boats at considerable cost and great
loss of time. Moreover, it seems probable that so
considerable a swell must trouble the low parts of the
Boulder-bank during certain winds, as to render it
at times impossible either to raise or lower vessels
with safety. The absence of fresh water on the
island is also an objection to the site. On the other
hand, a slip could be constructed on Haul-ashore
Island more economically than anywhere else in the
harbour.

HYDROSTATIC GRAVING DOCK.

As many people object to the use of patent slips,
especially for large vessels, I have examined several
other systems of docking vessels for repair, and on
the outer end of the proposed reclamation, and also on
a large scale in figures 4, 5, and 6, have shown a modi-
fication of the graving dock which seems to possess
qualities which should recommend it for very general
adoption in the colonies, and to be well adapted for
Nelson.

In the patent slip a vessel is grounded on a cradle,
which is then hauled up an inclined railway till it
is at a high level; in the ordinary graving dock
a vessel is floated into a chamber, grounded on
blocks, and either left dry by the ebbing tide or the
water pumped out by power—in either case the water
being excluded by gates till the repairs are com-
pleted; but in this system the vessel is lifted by the
flotation of water until it is above high-water level,
and then grounded on proper blocks; the general
principle is well established, and was carried out on
a very magnificent scale at Sebastopol.

This arrangement has many qualities which recom-
mend it for adoption in this country, as simple
earthwork may, to a very great extent, be substi-
tuted for fine masonry, and as the dock gates and
other works which are required do not need to be so
absolutely watertight as in an ordinary graving
dock, in which the whole work is exposed to the
searching test of a constant pressure from without,
due to a head of water varying from perhaps eight
feet at low water, to twenty feet or more at high water;
while the hydrostatic graving dock is only required
to retain the water until the less has been floated
over the graving blocks, and, if there be a good sup-
ply of water, a very considerable amount of leakage
would be of trifling consequence. As soon as the
vessel is in position in such a dock the water is run
off, and as the vessel is above the influence of the
tides, no annoyance from leakage can be experienced,
and no steam pumping gear is required.

The best site for such a dock is shown in close
proximity to deep water, and it is pointed so that
vessels hauling in or out will not be athwart the
stream of tide. To save expenses, the floor of the
upper chamber is proposed to be six feet below high
water of spring tides, a very light second pair of
gates being fitted to exclude the water, and the flo-
tation power is proposed to be obtained by fluming
from the Maitai.

ESTIMATES.

In the absence of more detailed information, and
working drawings, of course only roughly approxi-
mate estimates can be submitted, but I have little
doubt that the works could be completed for the
several sums mentioned below, if not for a smaller
amount:—

1. A Patent Slip complete, capable
   of taking ships of 1,200
   tons register, should cost com-
   plete not more than ... £28,000 0 0
2. Hydrostatic Graving
   Dock—Entrance gates
   with masonry ... £8,000
   Inner gates ... 1,000
   Ten shoring stages, at
   £300 ... 3,000
   Dock floor, graving
   blocks, &c. ... 5,000
   200,000 cubic yards
   earthworks at 2s. 6d. 25,000
   Flume, &c. &c. ... 3,000
   45,000 0 0