Influences Due to Roll and Pitch of the Ship

The Magnetic Compass

Another error, called heel­ing error, is caused by the change in the dis­pos­i­tion of the material of the ship with ref­er­ence to the com­pass. It is brought about when the ship rolls. For example, a ship heading on a northerly course would, if rolled to port, place all magnetic material of the ship to the east­ward of the compass. This pulls the north end of the com­pass to the east­ward. The action and effect would be just opposite to this on a roll to the star­board. The result is that the needle is caused to oscil­late in either direc­tion. The helms­man in his attempt to keep “on” will cause the ship to traverse a sinuous course.

The card and needle of the mag­netic compass are placed in a bowl filled with a liquid. The purpose in so doing is to make the action of the card some­what slug­gish, so that it will not follow very slight magnetic dis­trac­tions or ship move­ments. Every time the course of the ship is changed the slug­gish action, due to adhesion between the bowl, liquid and card, pulls the com­pass off the mer­i­di­an. Of­ficial test has shown that from three to four minutes are re­quired for the com­pass to over­come this “lag.” The “lag” is some­what less in the dry card compass.

The Sperry Gyro-Compass

Not only is the Sperry Gy­ro-Com­pass unaf­fec­ted by mag­ne­tic con­di­tions, re­sult­ing from the heel­ing error, but before being placed upon the ship it is tested for days under con­di­tions sim­u­lat­ing the mo­tion of the ship in the most severe storm.

A ship steered by the Gyro-Compass trav­erses a straight line course; the Gyro-Com­pass does not os­cil­late with the rol­ling of the ship. It is not nec­es­sary for the helms­man to use as much helm to keep the ship on her course. A great saving is made in the use of the steering engine.

There is no “lag” in the Sper­ry Gy­ro-Com­pass, be­cause it does not leave the mer­i­dian, no mat­ter which way or how quick­ly the ship may turn or zig-zag. Ex­haus­tive tests have been con­duct­ed on com­pas­ses installed on torpedo boat des­troy­ers. Even when zig-zag­ging at top speed in heavy seas the Gy­ro-Com­pass shows no “lag.”

Trav­e­ling the straight line course instead of the sin­u­ous course, ships equipped with the Sper­ry Gy­ro-Com­pass have saved from one to ten per cent in time over the average sched­ule time re­quired to cov­er their courses when steering by the mag­ne­tic com­pass.

The Savannah, first steamship to cross the ocean.