Coefficient D It is the part of vessel’s deviation which varies as the sine of twice the compass course. Deviation = D x Sine 2 ( comp course) Maximum deviation on inter cardinal headings NE, SE, SW, NW Zero deviation on cardinal headings N, E, S, W As deviation changes every 90°with change of heading, this pattern of deviation is called quadrennial. Sign convention

Coefficient D Cause of coefficient Induction by earth H component on 1) F/A soft iron- effective pole level with compass in F/A line (a rod) 2) Athwartship soft iron- effective pole level with compass in Athwartship direction (e rod) Quadrennial deviation due to a rod causes – D effect. Comparatively loss of DF on N & S headings than E & W headings.

Coefficient D Coefficient +D caused by e rod The athwartship e rod effect causes +D and F & A a rod would cause –D in most cases. Therefore the resultant coefficient D is positive as e rod poles are closer to the compass and more effective than a rod poles. Directive force Coefficient +D due to e rod( P – S) effect is predominant causing less DF on (E - W ) headings than (N – S) headings. Coefficient –D effect causing less DF on ( N – S) headings than (E- W) headings. Therefore components which cause coefficient D do affect the mean DF at compass position

Coefficient D Effect of magnetic latitude In high latitudes both H and F are small. In lower latitudes both H and F increase in same proportion and deviation remains same. The increase in the earth horizontal field H will proportionally increase in the induction in the ship’s horizontal soft iron hence in the disturbing field F. The deviation due to coefficient D is independent of The Latitude in both hemispheres

Coefficient D Correction of coefficient D This is done by attaching soft iron spheres(Kelvin’s balls) to the binnacle. Their centers should be in the same horizontal plane as the compass needle. Induction due to earth’s field in the soft iron balls create an equal and opposite effect to vessel’s coefficient D. It is usual for vessels to have + D due to dominant P –S Soft iron than F - A soft iron. Therefore spheres to be placed P – S direction on each side of the binnacle. Effect of spheres to increase the mean DF at compass Position. To correct + D error- move the spheres inward To correct + D error- move the spheres outward The correction of –D is made by attaching spheresi n F – A Line. But it is unusual for a merchant ship to have coefficient –D before the compass is corrected