179. Which factor has the greatest effect on the amount of gain required to obtain a fathometer reading ? ____

At 0400 your vessel is dead in the water and in heavy fog. Your loran set fixes your position at LAT 4100b012.1’N，LONG 7200b043.5’W. Bottom samples are taken and indicate a composition of soft mud and shell. Your fathometer reads 40 feet. If the vessel draws 9 feet of water，which of the following is TRUE?（）