페이지 이미지
PDF
ePub
[merged small][graphic][merged small]
[merged small][graphic][merged small]

As the boom is to be topped up, we must add the resistance due to friction, in calculating the strength of fall required. Total resistance on topping-lift including friction,

=9+9=16.3 tons (nearly).

Maximum tension on fall of topping-lift=163

6

=2.75 tons.

Size of fall required for topping-lift=√15×2.75=61⁄2 (nearly).

(c) To find the tension on the guys.

The pull on the masthead is the load on the topping-lift (9 tons) plus the tension on the part, BC, of the main purchase fall leading from the boom-end to the masthead.

We have found the tension on the hauling part DE, to be 2.75 tons. The tension on BC is considerably less than this, being reduced by the friction of two intervening sheaves; but for convenience we may neglect this difference, and take 2.75 tons for the tension on BC. This gives, for the pull to be resisted by the guys, 9+2.75=11.75 tons. The tension on the guys is found from this as in other examples preceding.

Tension on guys=81⁄2 tons.

(d) To find the thrust on the boom.

This is the resolved component along the line.of the boom=6 tons.

(e) To determine what thrust the boom will safely stand. Diameter of spar 10 inches; length 17 feet.

[blocks in formation]

Hence the booin is strong enough to stand this thrust and also the increased thrust (about 20 per cent) which will result from topping up the boom to land the boiler on the deck.

(Nov. 25

CHAPTER VI.

THE COMPASS, LOG AND LEAD-SUBMARINE SIGNALS.

§ I. THE COMPASS.

The essential part of the compass is a magnetic needle which turns freely on a pivot, and which, if unaffected by local disturbing influences, would point due north and south (magnetic). The local influences which necessarily exist on ship-board and which on steel ships are very large, are more or less fully neutralized by the methods used to "compensate" the compass; and the needle does, in general, point approximately to the magnetic poles.

A recent experience in which a ship was stranded because a small piece of iron had been carelessly left near the compass suggests the necessity for a warning on this point. Iron or steel, even in masses as small as a pocketknife, if brought near the compass, may affect it. Helmsmen and quartermasters should understand this.

Stanchions, railings and other metal fittings. near a compass should preferably be of bronze or other non-magnetic substance. If made of iron, they should never be absent, while the compass is in use, from the positions occupied when observations for deviation are made.

Attached to the needle and moving with it, is a circular card marked around its circumference with two graduations; one of points, half-points and quarter-points, the other of degrees. (Plate 47.)

The card with the attached needle (or needles) is floated in a liquid composed of 45 per cent pure alcohol, and 55 per cent distilled water, within a chamber, where it is pivoted upon a jewelled bearing which keeps the card centered and supports a small fraction of its weight. On the inside rim of the card-chamber is a vertical mark known as the "lubber's point," which, in installing the compass, is carefully adjusted in the fore-and-aft line of the ship with reference to the center of the card. In steering, this mark is held in coincidence with the point of the card indicating the course to be steered.

The graduation of the card in points and half-points runs as follows, beginning at North:

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

The enumeration of the points and fractional parts of points of the compass in regular sequence is called "Boxing the Compass."

The delicacy with which steamers can be steered by means of the arrangements now employed for the purpose is leading to the disuse of points and quarter-points for compass work and the custom is becoming general of shaping courses and noting bearings, in degrees. The seemingly trifling error that may be

« 이전계속 »