Operation of a GEM over Water
The operation of a GEM over water is similar to that over a solid surface with some small exceptions. At low forward speeds the higher base pressure displaces the water downward relative to the undisturbed surface. However, Eq. (12-5) still holds if h is measured in relation to the displaced surface. The magnitude of this displacement is, of course, equal to the base pressure expressed in inches or feet of water.
At low forward speeds, therefore, the GEM over water behaves like a displacement vessel. This means that in addition to the other sources of drag we must add a wave drag. As the velocity increases, a point is reached at which the displacement of the water surface is negligible. This results since the impulse applied to each fluid particle by the base pressure becomes less and less as the speed increases. This behavior is illustrated in Fig. 12-7. Here a drag breakdown is shown for the S. R.N. 1, an 8500-lb machine with 535 ft2 of base area. For this particular craft the wave drag reaches a maximum at about 12 knots.
Fig. 12-7. Drag breakdown of a GEM: W = 8500 lb, S = 535 ft2.