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making the above corrections, is the allowance for the complete
erection. The deck erection allowance is then based on this difference
and according to the extent of the length of the ship they cover,
although the allowances are actually less than the proportion of the
difference between Tables A and C that the proportion of length
covered would give. For instance, a poop, bridge and forecastle
covering half length would obtain an allowance of 32 per cent, of
the difference, the reduction from 50 per cent, allowing for the breaks
in the strength caused by the three separate erections, other cor-
rections being made if required for " iron deck," extra strength,
round of beam, fall in sheer, non-compliance with rules regarding
freeing port area, satisfactory arrangements for crew getting back-
wards and forwards from their quarters, etc., deduction for summer
voyages, and height of statutory deck-line, as stipulated in the rules,
the freeboard is eventually obtained. In the case of raised quarter-
deck ships, corrections are to be made in cases where the height of
raised quarter-deck above main deck is less than that required by
the rules, and also when the engine and boiler openings are not
covered by a bridgehouse.

The present opportunity may be taken to make mention of the
1906 amendments to the tables of freeboard, which then caused no
small amount of controversy from various points of view. One or

152 The Theory and Design of British Shipbuilding.

two alterations were made to the rules governing the amounts of
the various corrections for co-efficient length and erections, but
the most important changes were made in the tabulated freeboards.
Fig. 73 is utilised to show these, the full lines representing the
present-day curves, and those dotted are according to the tables
existing previous to March, 1906. In the case of Table A, it will
be seen that a reduction has been made in the freeboards of vessels
from 330 ft. to 510 ft. in length, the curve being straightened out
to the extent of 3 in. at the widest part. Table B is unaltered.
The awning- deck curve C shows a great alteration throughout its
length, varying from 3J in. for ships 96 ft. long, to 13 in. in ships of
408 ft. In the sailing ship curve D it will be noticed that in the
larger vessels the freeboard has been reduced, the alteration com-
mencing from nil at 195 ft. long, and extends to 5 in. for vessels of
300 ft. The greatest alteration, it will be seen, was made in awning-
deck ships, and, while 13 in. of difference in the largest vessels of
this class appears at first sight to be a large amount, yet, when
compared with the freeboard taken to the awning deck, it is a small
proportion viz. :

At 408 ft. length, old tables give ... 14ft. 2J in.
At 408 ft. length, new tables give ... 13ft. If in.

Giving the difference of 1 ft. 1 in.

The awning 'tween decks are taken as being 7 ft. high. For the
vessel represented in Fig. 74, the dotted line C T shows the position
of the old load-line for Table C. In connection with these alterations
it should be noted that coincident additions were made to the scant-
lings of awning- decked vessels, further justifying the amendments.

The Theory and Design of British Shipbuilding. 153



Tonnage. In addition to using the dimensions it is usual to
express the size of a vessel as being of so many tons, or of so much
tonnage. Much confusion is caused by the use of these words,
except when the actual method of measurement is mentioned, since
there are a number of ways in which the tonnage may be given.
Displacement and deadweight are actual tons i.e., of 2,240 Ibs.
The tonnages given in the vessel's certificate of survey are of a
totally different nature ; they represent volume in cubic feet, which,
divided by 100, gives the " tonnage." There are three tonnages
measured by the Board of Trade viz., under deck, gross and net
register, their composition being briefly as follows : Under deck
tonnage is the total up to the tonnage deck ; gross tonnage is com-
posed of the under deck plus the tonnage of all permanently closed-in
spaces above the tonnage deck ; net register tonnage is the result
of making certain deductions from the gross. We, therefore, have
the following means of expressing the size of a vessel in tons or
tonnage, assuming figures for the sake of example :

550 tons ... Under deck tonnage.

980 ,, ... Gross tonnage.

360 ,, ... Net register tonnage.

1,700 ,, ... Load displacement.

700 ,, ... Light displacement.

Carries 1,000 ... Deadweight.

From the above it will be seen that the statement of a vessel being
of so many tons is most vague unless accompanied by the exact
mode of measurement.

The Under Deck Tonnage- The under deck tonnage is the first
found in the computation of the Board of Trade tonnages. The
tonnage deck is the second deck from below in all ships with more
than one deck. The length for tonnage in modern ships is measured
along this deck, the extremities being the points where the inside
lines of the framing or cargo battens meet (see Figs. 75 and 76).
This length is divided into a number of equal parts, the number


The Theory and Design of British Shipbuilding.

being determined according to tonnage length. Fig. 75 shows a
vessel (length over 225 ft.) divided into 12 equal parts. The points
of division, 1, 2, 3, etc., are the positions at which the transverse
sectional areas are found. The depths of the areas are measured
in feet, from the top of floors or inner bottom plating to the underside
of the tonnage deck, deducting 2J in. for the thickness of ceiling,
when such is fitted, and also one-third of the camber of beam (see
Fig. 76). This depth being equally divided to suit Simpson's first
rule, the breadths in feet are taken to the inside of cargo battens or
to the inside of the frames where the battens are omitted. By
putting the breadths through the rule the sectional areas are found
at the various positions, and then by putting the so found areas
through the rule the under deck volume in cubic feet is obtained.
Dividing this by 100 the result is the Under Deck Tonnage. Owing
to the uneven line of the floor tops caused by variation in height,
or omission at certain parts of double-bottom ballast tanks in some
vessels, it is sometimes necessary to divide the ship into different
portions and to compute separately each volume. In the case of a
raised quarter-deck ship an imaginary main deck line is carried
right aft and the U.D. Tonnage measured below this line, the portion
contained above the line being afterwards added into the gross
tonnage and designated the tonnage of the " break."

Gross Tonnage. To the Under Deck Tonnage is next added the
tonnage of the " break " and all other closed-in spaces above the
tonnage deck, with the exception of spaces fitted with machinery,
wheelhouse, shelter for deck passengers, galleys and w.c.'s of reason-
able extent ; also a small portion of the tonnage of the cargo hatches.

Fig. 75.

When poops, bridges or forecastles are fitted with doors or other
permanently attached means of closing them, they must be measured
into the Gross Tonnage, but when otherwise they are exempt. The
following is a summary of the items composing the Gross Tonnage
of the vessel shown in Fig. 75 :

The Theory and Design of British Shipbuilding.


Under deck tonnage (total up to tonnage deck) ... ... 6,508-21

'Tween decks (A) 1,832-46

Poop (B), fitted with openings, therefore not measured...
Bridge (C), fitted with openings, only engineers' cabins

measured 41-58

Forecastle (D), fitted for crew space, etc. ... ... ... 52-61

House on Bridge (E). captain and officers' cabins ... 178-24

Chart room (F) 5-02

Light and air space (G), machinery casings ... ... 149-56

Excess of cargo hatchways, over J per cent, of gross tonnage 24-35

Gross tonnage 8,792-03

Having completed the total for gross tonnage the allowed deductions
are next to be estimated.

Net Register Tonnage Net register tonnage is the amount
remaining after making these deductions. A low net is most de-
sirable from the shipowner's point of view, therefore any means
adopted to make the amount of deductions as large as possible will
be in his favour. Deductions of tonnage are allowed for space
occupied by the propelling power, crew space, master, charts, boat-
swain's store and water ballast spaces complying with the regulations.

Fig. 76.

In ascertaining the amount of deductions for propelling power
space, the first thing necessary is to calculate the cubical contents
of the engine and boiler rooms clear of bunkers, store rooms or
cabins, the shaft tunnel being also included) ; the spaces allowed
being of reasonable extent. The light and air space contained in
the casings is included in the Gross at the owner's request, and, if
this has been done, the amount is now included in the propelling
power space. Having found the actual contents of the propelling
power space, the amount of deduction is determined as follows :

156 The Theory and Design of British Shipbuilding.

Actual Tonnage of P P Space

Allow deduction of

When between 13 per cent, and 2(H
per cent, of gross tonnage in screw -
steamers ... ... ... ...J

When between 20 per cent, and 30 'j
per cent, of gross tonnage in pad- I
die steamers

When 13 per cent, of gross tonnage
or under in screw steamers

When 20 per cent, of gross tonnage
or above in screw steamers

PWhen 20 per cent, of gross tonnage
or under in paddle steamers ...

When 30 per cent, of gross tonnage
or above in paddle steamers ...

32 per cent, of gross tonnage.

37 per cent, of gross tonnage.

|' Board of Trade have option of allow -
I ing 32 per cent, of gross tonnage,
j or If times the tonnage of actual
I P P space.

( Owners have option of allowing 32
I per cent, of gross tonnage, or If
j times the tonnage of actual P P
I space.

C Board of Trade have option of allow -
I ing 37 per cent, of gross tonnage,
j or 1 times the tonnage of actual
I P P space.

( Owners have option of allowing 37
I per cent, of gross tonnage, or 1
j times the tonnage of actual P P
I space.

According to a recent Act, the allowance for P P space must not
however, exceed 55 per cent, of the remaining gross tonnage after
deducting the other allowed deductions for crew space, ballast
tanks, etc. This applies to all steamships, except tugs exclusively
used in towing.

The following is a summary of deductions for the vessel previously
dealt with :

Actual P P space, including light and air space (H + G) = 1,783*50 tons,
which is over 20 per cent, of the gross tonnage ; therefore, being a screw
steamer, the owner may have an allowance of If times the actual measured
tonnage of P P space, or 32 per cent, of gross tonnage at his option. Pro-
pelling power space (H + G). 1,783-50 x If = 3,121-12, deduction allowed.
32 per cent, of gross would give only 2,813-44 allowance, so the former is
taken. Other deductions are as follows :

Crew spaces in forecastle 49*22, in bridge 41-58, in house on)
bridge. 173-34 ,

Master's berth 4'9, chart-room 5-02, boatswain's store 42-51,1
forepeak ballast tank 34-30, aftpeak ballast tank 16-32... )

Total deductions excluding P P space 367-19



8,792-03 gross 367-19 = 8,424-84 remaining gross

8,424-84 x 55 per cent. = 4,633'66 maximum allowance for P P space,
according to Act.

The Theory and Design of British Shipbuilding. 157

Seeing that the above figures for P P deductions (3, 121 '12) do not exceed
the latter amount, the estimated deduction is allowed. Had the actual
space x If been in excess of 55 per cent, of remaining gross, nothing further
than 4,633 '66 would have been allowed. The total deductions allowed is:
therefore :

P P space ... ... 3,121-12

Other deductions ... ... 367-19

Total deductions 3,488-31

The net registered tonnage is now obtained by :

Gross tonnage 8,792-03

Total deductions 3,488-31

Net Registered Tonnage ... 5,303-72

To show the effect of the Act, which now limits the P P deduction f
let us assume the actual P P space, as measured, to be 2,950 tons
instead of 1,783-5. The deduction under the old regulations would

have been :

2,950 x If - 5,162-50

Other deductions = 367-19

Total deductions = 5,529-69

8,792-03 gross 5,529-69 = 3,262-34 old net registered tonnage.

Under the present Act, where the P P deduction is limited to 55
per cent, of the remaining gross, the maximum deduction was seen
to be 4,633-66, while, under previous laws the deduction would have
been 5,162-50, this alteration causing a difference of 528-84 tons in
the vessel's net register, which would be 3,791-18 under existing
law. An amendment in the computation may be made here, however.
Under ordinary circumstances, where the If deduction is taken, it
will be seen that by adding the light and air space into the gross
tonnage, and then deducting its amount If times with the P P space
allowance, a reduced net register is the result. When engine-room
(actual tonnage X If) gives a figure exceeding the new maximum
allowance, it will be seen that it is unnecessary to include the light
and air, as its amount will then only increase the tonnages. This
occurring in the above case, where we have assumed an increased
engineroom, it will be better to omit the light and air tonnage,
which will consequently reduce the gross to 8,642-47 tons.

8,642-47 - 367-19 = 8,275-28 remaining gross.

Taking 55 per cent, of the remaining gross, we find the maximum
allowed deduction for P P space to be 4,551-40 tons. Actual P P
space without L and A = 2,800-44, and multiplying this by If we

158 The Theory and Design of British Shipbuilding.

have 4,900-77, which is still in excess of the maximum P P allowance.
Taking the maximum P P allowance, 4,551-40, and the other de-
duction, 367-19, from the Gross gives :

Gross 8,642-47

Total deductions 4,918-59

Netreg 3,723-88

'The net register is seen to be smaller with the light and air omitted,
although it is still 461-54 tons in excess of that under the old regu-
lations. The present Act certainly tends to produce a more fair
measurement, as with the old unlimited If deduction it was possible
in quite large vessels to reduce the net register to an absurdly low
amount in cases of high-powered ships with large enginerooms,
and even in some of the smaller craft it was possible to reduce the
net to a negative quantity on account of the deductions being
greater than the gross. For instance, s.s. - , 185-62 gross

.actual P P space x If == 93-31 X If == 163-29
other deductions ... 25-31

Total deductions 188-60

Gross 185-62

Total deductions 188-60

Net. reg nil.

Going back to our original example it will be seen that the If de-
duction gave an allowance of 3,121-12 and the 32 per cent, of gross
gave only 2,813*44, and the owner here, having the option, obviously
takes the former. The actual P P space was 1,783-50, composed
of 1,633-94 engineroom and 149-56 light and air, and, without the
light and air, the actual P P space would have been under the 20
per cent, of the gross, thereby only obtaining the 32 per cent, de-
duction. The advantage of including the light and air space, in
this case, is therefore seen to enable the owner to obtain the option of
the larger deduction, in addition to it making a further reduction by
its being afterwards deducted If times. A similar occurrence takes
place if the actual P P space is under 13 per cent, of gross in screw
steamers, when the inclusion of light and air space, or enlargement
of engine-room, may enable the 32 per cent, deduction to be
obtained instead of the If, the former method being, in these cases,
generally the larger deduction.

The Theory and Design of British Shipbuilding.




Types of Ships. In the article dealing with freeboard it was
seen that three distinct types of steam vessels were provided for in
Tables A, B and C, i.e., ships of the heaviest scantlings, spar-decked
ships, and awning-decked ships respectively, the second being of
lighter construction than the first, and the third of a still lighter
build. To outward appearances these three vessels may be exactly

Fig. 77.

similar, except, when laden, the amount of freeboard is seen to vary
in the three types, the vessel of the lightest construction having the
largest amount, therefore reducing the weight carried. The re-
striction of loading in the two lighter types tends to make the stress
upon the structure somewhat equal in all three cases ; therefore,
while one may be built much lighter than another, they are all of
about the same standard of strength. In Fig. 77 the relative
positions of sections of the various types are shown alongside of
each other, and in Fig. 78 the relation is shown in profile. A is
the vessel of heaviest build, A, is the same vessel with the addition
of erections such as poop, bridge and forecastle (shown dotted),
which, having provided a further reserve of buoyancy, consequently
allows of the vessel being more deeply immersed. B is the spar-
decked vessel, and C the awning-decked type. In the cases of A,
B and C, the vessels being of the same dimensions, the internal
capacity will be equal, but, on account of the shallower depth of
loading, the latter two are most suitable for carrying light cargoes
requiring large volume.

100 The Theory and Design of British Shipbuilding.

The following comparison shows this point :

A 3,000 tons
B 2,700
C 2,100

Hold Capacity
in cubic feet.

200,000 ...

200,000 ...


'Capacity per ton

of Cargo.

67 cubic ft. per ton.

Since this article was originally written, the type B,
spar-deck, has almost become extinct in new designs,
and to a smaller extent this can be said of type C, the
awning-deck type. Both have been largely superseded
by the shelter-decker, which type is refererd to below. The
spar and awning-deck types were generally adopted in
designs where the principal feature required was cubic
capacity, but this requirement is now economically pro-
vided by the shelter-decker on a relatively low tonnage

Minor Types. By the addition of erections to vessels
of the full scantling class we have other types, as follows :
The three-island type, where a poop, bridge and forecastle
are added, as shown by A T in Fig. 78. The shelter-deck
type, which has a complete erection extending all fore
and aft, being practically a three-island ship with the
wells filled in (see Fig. 80). A tonnage opening is fitted
in the shelter deck either at the fore or after ends,
qualifying the vessel for the exemption of measurement
of the shelter 'tw r een decks for tonnage. To all
appearances this type seems to be the same as a vessel
of awning-deck type, as was shown by C in Fig. 78, the
upper and shelter decks corresponding with the main
and awning-decks respectively.

In Fig. 79 is shown a comparison of awning-deck
and shelter-deck types of same dimensions. It will be
observed that the " main " deck in the former corres-
ponds to the " upper " deck in the latter type. Owing
to the tonnage opening in the shelter-deck vessel the
'tween deck has a less relative value for freeboard
purposes than in the case of the awning-decker where
the reserve buoyancy contained in the 'tween decks is
intact all fore and aft. The awning- decker now
Fig. 78. referred to is of modern construction, the topsides being

The Theory and Design of British Shipbuilding.


of heavy scantling as required to confirm to the new Freeboard
Tables of 1906 (see reference at end of Chapter XV).

Fig. 79.

Midway between the three -island and shelter- deck types we have
the combined poop and bridge, as shown in Fig. 81.

Fig. 80.

The raised quarter-deck ship, Fig. 82, is greatly popular among
the smaller classes, especially those engaged in the coal trade. The
vessel shown in Fig. 82 has a topgallant forecastle and bridge upon

Fig. 81.

the main deck, this deck terminating at the after end of the bridge,
where the deck is lifted up to about half -height of the bridge, and
from this point to the aft end it is known as the raised quarter-deck.
By lifting up the deck in this way the vessel is provided with extra
reserve of buoyancy, which is appreciated by means of a reduction


The Theory and Design of British Shipbuilding.

in the freeboard. The cubic capacity is also increased and com-
pensates for the space taken up by the shaft tunnel, and further

Fig. 82.

tends to give better sea-going trims in small vessels, where, by
reason of the fineness of the vessel's form, less sheer than at the fore
end, and space taken up by the shaft tunnel, the amount of cargo
carried in an after hold would be very small if the main deck was
carried aft in the same line, probably resulting in such vessels trim-
ming by the head when carrying cargoes of light density.

Fig. 83.

Fig. 83 shows a vessel with raised fore deck. She is exactly similar
to the raised quarter-deck type, except that the deck in the forward
well has been lifted up also.

A partial awning-deck vessel is shown in Fig. 84, where it will be
seen a raised quarter-deck is fitted aft. The combination of the
bridge and forecastle provides the partial awning-deck.

Fig. 84.

The shade-deck type is illustrated by Fig. 85, where we have a
lightly constructed deck fitted between the poop and forecastle.
It is supported by angle frames or round stanchions, and is usually

Fig. 85.

open at the sides, as shown, although it is sometimes closed by means
of light plating. The structure is only built strong enough for
providing a passenger promenade or shelter for cattle on the upper



Admiralty Co -efficient 123

Admiralty Co-efficient, average

values ... ... ... 125

Air Resistance ... ... ... 1 1 5

Apparent Slip ... ... ... 120

Areas, Rules for Finding ... 4

Area Wetted Surface 36

Attwood's Formulae for Statical

Stability 93

Augment of Resistance 120

Awning Deck Vessel ... ... 159

Beam, Camber of ... ... 3

Bending Moments, Curve of ... 142
Block Co-efficient, Formulas for 26
Block Co -efficient, of Displace-
ment 22

B. M. Longitudinal 104

B. M. Transverse ..." ... 46
Board of Trade Rules for Free-
board 148

Breadth Extreme ... ... 2

Moulded 2

,, Registered ... ... 3

Buoyancy, Centre of ... ... 15

,, Curve of ... ... 141

, , Curve of L ongitudinal

Centres of ... 43
,, Curves of Vertical

Centres of ... 43

Reserve of ... ... 147

Calculating the Displacement,

etc 35

Camber of Beam ... ... 3

Centre of Buoyancy ... ... 15

Centre of Flotation ... ... 41

Centre of Gravity of Curvilinear

Figures ... ... ... 4


Centres of Gravity of Areas of

Curved Surfaces
Centres of Gravity of Weights
Centre, Tipping ...
Change of Trim
Coal Consumption on Trial
Co-efficient, Admiralty
Co-efficient, Admiralty, Average



,, Average Values in

Different Types


,, 'Midship Section


,, for Heights of Cen-

tre of Gravity ...

,, Prismatic


,, Relation to each


, , Waterplane Area . . .

Combined Poop and Bridge

Type Vessel
Comparison and Types of


Comparison, Extended Law of
Composition of Deadweight ...
Composition of I.H.P. ...
Conditions of Equilibrium
Construction of Dynamical Sta-
bility Curve

Construction of Lines ...
Corrections for Freeboard
Corresponding Speeds . . .
Cross Curves of Stability
Curve of Bending Moments . . .

,, Buoyancy

,, Loads ...
Longitudinal Centres
of Buoyancy




























INDEX Continued.


Curve of Shearing Forces ... 142

Stability 94

Vertical Centres of

Buoyancy 43

Weight 141

Curves of Freeboard . 149

Deadweight ... ... ... 16

Composition of ... 17

Scale 17

Depth 3

Moulded 3

Determination of Dimensions ... 27

Diagram, the Metacentric ... 51
Difference in Draught Sea and

River Water 19

Dimensions ... ... ... 1

Displacement ... ... ... 14

Co-efficient ... 22

Mean Draught ... 38
,, Scale and Various

1 2 3 4 5 6 7 8 9 10 12 14

Online LibraryAmos Lowrey AyreThe theory and design of British shipbuilding → online text (page 12 of 14)