Mayday. (French 'm' aidez', 'Help me'.) The international spoken radiotelephony signal of distress.
Maximum Altitude. Greatest angular distance above horizon.
Maxwell's Rule. 'Corkscrew Rule.'
McIntyre System. Double-bottom construction involving the cutÂting of frames and reverse frames and the fitting of a continuous water-tight angle bar to take lower edge of tank margin plate.
McIntyre Tank. Water-ballast tank seated on top of ordinary floors. Preceded double-bottom tank.
Meade Great Circle Diagram. Graticule for solving great circle sailing problems from projection of course on a Mercator chart.
Mean Draught. Half sum of forward and after draughts of a vessel. It differs slightly from draught at half length.
Mean High Water. Mean of all high waters at a place when observed during a lunation of 29 days.
