Waves 19 
“DEAD WATER’—WAVES WITHIN WAVES 
A variation on this theme is the phenomenon which arises 
when the ship happens to be passing through a portion of 
the ocean where there is a surface layer of water of subnor- 
mal density of a thickness about equal to the draft of the 
ship. The layer of water may simply be fresher water from 
melting ice, or from the mouth of a near-by river, and should 
the sea be moderately calm there may be a sharp boundary 
between the fresher water floating on top and the denser salt 
water below (much as oil floats on the surface of a body of 
———$—$< O_O 
| LIGHT LAYER 
HEAVY LAYER INTERNAL 
WAVES 
water ). Should conditions be favorable, internal waves may 
be set up between the two liquid layers, and grow to fairly 
large size, although they are not visible on the surface at all. 
Internal waves are waves between the two liquid layers in- 
stead of being on the surface between the water and air 
like ordinary waves. Such waves are a serious drain on the en- 
ergy of the ship and as a result the ship will make little head- 
way. It will seem to be “stuck” in the water. In the old days 
of sailing ships, sea captains often experienced this phenome- 
non—in a stiff breeze their ships barely crept along—and 
without divining the cause they called it “dead water.” 
