Vertical (up/down) acceleration is a measure on how hard a boat rides in waves. The phenomenon is called “pounding” or “slamming”. The acceleration is usually measured in terms of G’s, this is why it is also referred to as G-force. Because waves are not alike on any two occasions, it is very difficult to say that a boat has exactly X G’s in sea state Y. Measurements of vertical acceleration are therefore usually relative and comparative.
All planing boats create a certain amount of lift that works to counteract the weight of the boat and make it rise out of the water. Running in calm water, this lift is dependent on the boat’s speed, trim angle (angle of attack), deadrise angle and the boat’s lifting (pressure) surface area.
When a wave hits the boat, the load is suddenly increased significantly for a fraction of a second, the panel structure of the boat is strained and the boat pounds. When the lift is twice the weight of the boat, the vertical acceleration is 1 G, when lift is three times the weight of the boat, the vertical acceleration is 2 G, and so on.