partly on it and partly on the metal containing it, and also that efficient courses for the distribution of the lubricant are provided.

But as shafts must be strong enough to resist the maximum twisting strain, it is necessary always to base calculations on it instead of on the mean twisting moment.

LINE SHAFTING.

Diameter of the line shafts

=

3 I.H.P.
R

X F.

Compound engines, cranks at right angles—

Boiler pressure 70 lbs., rate of expansion 6 to 7, F = 70.

Triple expansion, three-cranks at 120°—.

Boiler pressure 150 lbs., rate of expansion 10 to 12, F = 62.

Expansive engines, cranks at right angles, and the rate of expansion 5, boiler pressure, 60 lbs., F = 90.

Single crank compound engines, pressure 80 lbs., F = 96.

SCREW PROPELLERS.

The area of blades given by the following rule is such as is generally found to give good results, and may be used by those who have no good experience to guide them:

Total area of screw-blades =

I. H. P.

KV revolutions

The value of K, for four-bladed propellers, is 15.

In practice the thickness of each collar (D-d).

= 0.4

(1) Space between the collars, if rings are of solid brass = 0.4 (D-d).

(2) Space between the collars, if rings are of cast iron faced with brass or white metal = 0.75 (D-d).

(3.) Space between the collars, if rings are of hollow brass for water to circulate through D-d.

=

The number of collars depends very much on the size of the engine and the prejudice of the designer. If there are many collars, they are of necessity somewhat small, and although the chances are in favor of the majority of them acting efficiently, allowance must be made for the

contingency of the whole thrust coming only on one of them, and the larger the number of collars, the less able is each one separately to resist the whole thrust. The chief objection to a few collars is that they are of necessity of comparatively large diameter, and have, therefore, a higher speed of rubbing surface; there is also the consideration of cost of forging against large collars.

When there are a few large collars, a better de

sign of thrust-block is possible, and the rings can be made adjusiable without removal.

The number of collars should vary with the size of the shaft, and a very good rule is that there should be one collar for shafts up to 6 inches diameter, and then an additional collar for every 2 inches of diameter beyond this.

The common plan of thrust-block with many collars is shown in Fig. 33, and a modification of the same is made by having the rings in one

casting. These plans are cheap, and do very well for small engines. So long as no heating is allowed to take place in the bearing, it will work very well; but when once it gets out of order it is difficult to deal with, and impossible to adjust at sea. It suffers from being enclosed, and from the rings lacking means of independent adjustment. Fig. 34 shows a plan of thrustblock which is most suitable when there are a few large collars. Here the thrust is taken by

FIG. 34.

horse-shoe shaped pieces of metal faced with brass or white metal, and fitted sometimes carefully into recesses on either side of the main block. When faced with brass, each may be adjusted very simply by putting thin tin liners behind the facings, which are hung on steady pins.

Figs. 34 and 35 are an elaboration of the form of block. Here the horse-shoes fit over two screwed bars, one on either side of the block; nuts are fitted to these bars, so that each collar may be adjusted by its own nuts, or the whole of them by the nuts at the end.

Both these plans are most successful in practice, in great measure due to the fact that the collars are open and exposed at the top, so as to be easily lubricated and cooled by the air, and to their running in oil, or in a mixture of oil and soapy water contained in the trough below them. It is most important that a bearing be placed close to the thrust, so that the shaft cannot vibrate and cause uneven pressure over the surface of the collars. The function of the thrust

FIG. 35.

bearing is to take only end pressure. This is particularly the case when designed with horseshoe rings.

THE CONDENSER.

The function of the condenser is to so cool down the exhaust steam as to reduce its pressure to a minimum, and in doing so the steam is converted into water.

Condensing engines require from 20 to 30 gallons of water to condense the steam represented