Secondly, if you are willing to throw #FuelEfficiency to the wind, sort to speak, a powered vessel simply goes faster with less of a wetted surface. And, as you bring the bow up, you reduce the wetted (in the water) surface. Compare how much more of figure 1 is below the water line, versus figure 2.
Unless you are driving one of those “battlewagons” out there, or are involved in commercial navigation, you’ve probably never heard of “boat squat.” Even if you are in those situations, you still may not have heard of it – and it is critical to understanding why a boat with 4’ of draft hits the bottom in 5’ of water…
When any boat is making way through the water, she starts by pushing a large amount of water ahead of her. If she a planning vessel, she’ll climb up on that wave as she picks up sufficient speed. But if she is a “big ‘un”, she won’t be planning anytime in this lifetime. She is a displacement vessel. So, this water that is getting pushed ahead returns to the side and under the boat’s bottom. As she starts to put on some way (speed), imagine this cycle of water building up speed under the ship. This causes a drop in water pressure under the boat. This causes the ship to vertically drop in the water. This is “boat squat” and how a boat with 4’ of draft hits the bottom in 5’ of water. (Hint: go slow in shallow water, Big ‘Un.)
Now, for a displacement vessel, trim is different from squat. Trim is the difference of the forward and aft draft while the boat is stationary. As she gets underway and her aspect to her water lines changes, she is affecting “squat.” Naval architects justifiably worry about whether she has forward or aft “squat” (leans forward or aft as she builds speed.) This is largely determined by her center of gravity and her “block coefficient”, which is the volume of the hull (V) divided by the Length of her Water Line (LWL) times the (maximum) Beam of her Water Line (BWL) times her Draft. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship.
So, now, you can say that you do know squat…!
* a “slow bell” means making way at the minimum speed at which the boat can maintain steerage. Larger boats, with exposure to the wind, need more speed to maintain steerage than a smaller, low profile boat.