TABLE II.--PASSENGER STEAMERS FITTED WITH TWIN SCREWS._Scientific American Supplement, No. 821, September 26, 1891

-----------------+-----------+-----+-----------+-------+--------+-------+

| diculars. | |-----------+-------| square |power. |

-----------------+-----------+-----+-----------+-------+--------+-------+

City of Paris. |\ | | | | | |

| } 525 | 63¼ |45, 71, 113| 60 | 150 |20,000 |

City of New York.|/ | | | | | |

-----------------+-----------+-----+-----------+-------+--------+-------+

Teutonic. |\ | | | | | |

| } 565 | 58 |43, 68, 110| 60 | 180 |18,000 |

Majestic. |/ | | | | | |

-----------------+-----------+-----+-----------+-------+--------+-------+

Normannia. | 500 | 57½ |40, 67, 106| 66 | 160 |11,500 |

-----------------+-----------+-----+-----------+-------+--------+-------+

Columbia. | 463½ | 55½ |41, 66, 101| 66 | 160 |12,500 |

-----------------+-----------+-----+-----------+-------+--------+-------+

Empress of India.|\ | | | | | |

Empress of Japan.| } 440 | 51 |32, 51, 82 | 54 | 160 |10,125 |

Empress of China.|/ | | | | | |

-----------------+-----------+-----+-----------+-------+--------+-------+

Orel. | 415 | 48 |34, 54, 85 | 51 | 160 |10,000 |

-----------------+-----------+-----+-----------+-------+--------+-------+

_Weight of Machinery Relatively to Power_.--It is interesting to compare

the weight of machinery relatively to the power developed; for this

comparison has sometimes been adopted as the standard of excellence in

design, in respect of economy in the use of material. The principle,

however, on which this has generally been done is open to some

objections. It has been usual to compare the weight directly with the

indicated horse-power, and to express the comparison in pounds per

horse-power. So long as the machinery thus compared is for vessels of

the same class and working at about the same speed of revolution, no

great fault can be found; but as speed of revolution is a great factor

in the development of power, and as it is often dependent on

circumstances altogether external to the engine and concerning rather

the speed of the ship, the engines fitted to high speed ships will thus

generally appear to greater advantage than is their due. Leaving the

condenser out of the question, the weight of an engine would be much

better referred to cylinder capacity and working pressures, where these

are materially different, than directly to the indicated power. The

advantages of saving weight of machinery, so long as it can be done with

efficiency, are well known and acknowledged. If weight is to be reduced,

it must be done by care in design, not by reduction of strength, because

safety and saving of repairs are much more important than the mere

capability of carrying a few tons more of paying load. It must also be

done with economy; but this is a matter which generally settles itself

aright, as no shipowner will pay more for a saving in weight than will

bring in a remunerative interest on his outlay. In his paper on the

weight of machinery in the mercantile marine,[3] Mr. William Boyd

discussed this question at some length, and proposed to attain the end

of reducing the weight of machinery by the legitimate method of

augmenting the speed of revolution and so developing the required power

with smaller engines. This method, while promising, is limited by the

efficiency of the screw, but may be adopted with advantage so long as

the increase in speed of revolution involves no such change in the screw

as to reduce its efficiency as a propeller. But when the point is

reached beyond which a further change involves loss of propelling

efficiency, it is time to stop; and the writer ventures to say that in

many cargo vessels now at work the limit has been reached, while in many

others it has certainly been passed.

[Footnote 3: Transactions Northeast Coast Institution of Engineers and

Shipbuilders, vol. 6, 1889-90, p. 253.]

_Economy of Fuel_.--Coming to the highly important question of economy

of fuel, the average consumption of coal per indicated horse-power is

1.522 lb. per hour. The average working pressure is 158.5 lb. per square

inch. Comparing this working pressure with 77.4 lb. in 1881, a superior

economy of 19 per cent. might be expected now, on account of the higher

pressure, or taking the 1.828 lb. of coal per hour per indicated

horse-power in 1881, the present performance under similar conditions

should be 1.48 lb. per hour per indicated horse-power. It appears that

the working pressures have been increased twice in the last ten years,

and nearly three times in the last nineteen. The coal consumptions have

been reduced 16.7 per cent. in the last ten years and 27.9 per cent. in

the last nineteen. The revolutions per minute have increased in the

ratios of 100, 105, 114; and the piston speeds as 100, 124, 140.

Although it is quite possible that the further investigations of the

Research Committee on Marine Engine Trials may show that the present

actual consumption of coal per indicated horse-power is understated, yet

it is hardly probable that the relative results will be affected

thereby.

_Dimensions_.--In the matter of the power put into individual vessels,

considerable strides have been made. In 1881, probably the greatest

power which has been put into one vessel was in the case of the Arizona,

whose machinery indicated about 6,360 horse-power. The following table

gives an idea of the dimensions and power of the larger machinery in the

later passenger vessels:

TABLE III.--DIMENSIONS AND POWER OF MACHINERY IN LATER PASSENGER

VESSELS.

-----+----------------+-----------------------+-------+-----------+

-----+----------------+-----------------------+-------+-----------+

1881 |Alaska | 68, 100, 100 | 72 | 10,686 |

-----+----------------+-----------------------+-------+-----------+

1881 |City of Rome | 46, 86; 46, 86; 46, 86| 72 | 11,800 |

-----+----------------+-----------------------+-------+-----------+

1881 |Servia | 72, 100, 100 | 78 | 10,300 |

-----+----------------+-----------------------+-------+-----------+

1881 |Livadia yacht | 60, 78, 78; 60, 78, \| 39 | 12,500 |

| | 78; 60, 78, 78 /| | |

-----+----------------+-----------------------+-------+-----------+

1883 |Oregon | 70, 104, 104 | 72 | 13,300 |

-----+----------------+-----------------------+-------+-----------+

1884 |Umbria |\ 71, 105, 105 | 72 | 14,320 |

1884 |Etruria |/ | | |

-----+----------------+-----------------------+-------+-----------+

1888 |City of New York|\ 45, 71, 113; \| 60 | 20,000 |

1889 |City of Paris |/ 45, 71, 113 /| | about |

-----+----------------+-----------------------+-------+-----------+

1889 |Majestic |\ 43, 68, 110; \| 60 | 18,000 |

1889 |Teutonic |/ 43, 68, 110 /| | |

-----+----------------+-----------------------+-------+-----------+

In war vessels the increase has been equally marked. In 1881 the maximum

power seems to have been in the Inflexible, namely, 8,485 indicated

horse-power. The following will give an idea of the recent advance made:

Howe (Admiral class), 11,600 indicated horse-power; Italia and Lepanto,

19,000 indicated horse-power; Re Umberto, 19,000 indicated horse-power;

Blake and Blenheim (building), 18,000 indicated horse-power; Sardegna

(building), 22,800 indicated horse-power. It is thus evident that there

are vessels at work to-day having about three times the maximum power of

any before 1881.

_General Conclusions_.--The progress made during the last ten years

having been sketched out, however roughly, the general conclusions may

be stated briefly as follows: First, the working pressure has been about

doubled. Second, the increase of working pressure and other improvements

have brought with them their equivalent in economy of coal, which is

about 20 per cent. Third, marked progress has been made in the direction

of dimension, more than twice the power having been put into individual

vessels. Fourth, substantial advance has been made in the scientific

principles of engineering. It only remains for the writer to thank the

various friends who have so kindly furnished him with data for some of

the tables which have been given; and to express the hope that the next

ten years may be marked by such progress as has been witnessed in the

past. But it must be remembered that, if future progress be equal in

merit or ratio, it may well be less in quantity, because advance becomes

more difficult of achievement as perfection is more nearly approached.

* * * * *