(18) Bilge diagonal coefficient (see diagram) = .82.

(19) Dimensions as determined = 460 × 58′ 6′′ × 33′ 6′′. (20) Displacement "D"

Should it be found, however, that the weights calculated for the dimensions as worked out are lighter than anticipated when we started with the 64 per cent of the dead weight, the length should be reduced accordingly. On the other hand, if the weights be excessive, the length must be increased. The length is the only dimension that should be adjusted, as it is the one factor which has no vital relationship to the element coefficients, as it will have been noticed that the primary quality aimed at was the G.M. as a measure of the ship's initial stability; and as the center of gravity varies with the depth, so the metacentric height is dependent on the breadth and draught.

For the preliminary design it will be sufficiently close to estimate the machinery weights on the I.H.P. required, and for ordinary merchant practice the power may be calculated fairly accurately by the Admiralty constant with the formula: —

D3 x V3 *
I.H.P. =
C

We then have for the present example, with constant=267, speed 12 knots, and displacement 16,400, an indicated horse-power=4000. By referring to the table given elsewhere, it will be found that for twin screw freight steamers with this speed that the I.H.P. per ton of engine boilers and water equals about 5.5, so that we get for a total machinery weight

4000
= 730 tons.
5.5

The displacement and coefficients should, in all cases of steel steamers, be calculated to the moulded line of frames, the excess water displaced by the shell plating, amounting to about 1%, being retained in hand as a margin against contingencies. In this case its value is 164 tons, representing 3 inches of draught.

* See Table of Constants, and chapter on Resistance.

Three-deck freighters, with poop, bridge,

All the elements insuring the qualities that embody a wellshaped boat of the particular type contemplated and at the same time a stable ship having been thus determined, the lines may be commenced with the certainty that no unnecessary alterations will be required.

The freeboard will be calculated from the legal tables given and explained herein, but in any case the limiting draught consistent with the block coefficient determined on as the maximum available for the required speed should be taken advantage of.

After carefully drawing the center and other construction lines, and marking off the ten or twenty ordinates that it is proposed to

use, it will be well to have a definite routine or method in which to draw down the various views comprising what are embraced under the general term "lines."

To this end the following will prove a good sequence:

1. The "dead flat" section on body view.

2. Rail sheer line.

3. Contour of stem and stern in profile.

4. Rail half-breadth.

5. Load water line half-breadth.

6. Bilge diagonal.

7. Transfer L.W.L. and B.D. -breadths to body plan.
8. Draw freehand the sections to foregoing.

9. Trial displacement by planimeter.

10. Sheer heights from profile to body plan.

Taking this routine in order:

1st. The dead flat or midship section should present no difficulties, as the area of this section is pre-determined from the coefficient B. This being so, the height of rise of floor construction line is assigned by giving the easiest bilge consistent with the area of section demanded. In no case should the bilge be 66 squarer " than the demands of this area require, as in full vessels sufficient difficulty is encountered in setting the bilge strake plates and bending the frames without adding further to it.

2d. In most vessels, except yachts and launches, it will be found advisable to make the lowest part of sheer at the half-length amidships, as otherwise correction would have to be made for freeboard and the classification societies' numerals. It is best, then, after fixing the height of bulwark or sheer strake above upper deck to underside of moulding, to run a pencil line parallel to L. W.L. from A.P. to F.P., at which points and above this line the sheer forward and aft should be set up. The amount of sheer will of course depend on the type of vessel, i.e. whether intended for sea or river. In the latter case it is evident the same amount of "spring" would not be required as for over-sea voyages. The standard sheer prescribed by the British freeboard tables will be, however, a good guide, and where this is deemed insufficient or where special cases suggest a departure from these, as in passenger steamers and first class ocean liners, a handy rule and one that gives a very symmetrical sheer is to take one-fifth of the vessel's length in feet, calling the quotient inches which will equal the amount of sheer forward. One-third of this will be the sheer aft, as :Length in feet

Sheer forward in inches,

= Sheer aft in inches.

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