When I teach seamanship classes, inevitably somebody raises their hand and asks about how “flat” the boat should be. I ask, “By ‘flat,’ I am guessing that you mean relative to her water-line. But do you mean when she is sitting at the dock, going slowly forward, or making all deliberate speed?” As their eyes glaze over, I know that we will have to take it by the numbers. This column is about that.
Understanding boat trim and boat squat is all about control and avoiding running aground. For a “planing boat,” i.e., those boats we’re most familiar with that buzz around the bays and creeks, usually with an outboard engine on the stern, that “climb up” on to the water as they go faster, trim is synonymous with every aspect of the boat. Whether it be at the dock, barely making way or operating at speed, how ‘flat’ she is to the water-line is largely under the control of the skipper, and he or she should be constantly aware of what trim they are assuming. This trim is best controlled by what angle you place the outboard engine relative to the transom.
Usually in the throttle, there is a thumb control that when you press it “down,” brings the propeller in closer to the transom.
By bringing the propeller in closer to the transom, you force the bow down from its manufactured water-line. When would you want to do that? How about if you were heading into strong wave action? If your bow was trimmed “up,” the force of the waves would accentuate that, possibly making it more difficult to see — and to control the boat.
Commensurately, if you press the thumb control to bring the engine “up,” it moves the propeller away from the transom, forcing the bow up from its manufactured water-line.
Why would you do that? Well, there are a number of reasons. One reason is that a powered vessel’s fuel consumption improves as you reduce its wetted surface. So, as you are cruising down the bay, you can trim the engine up and save fuel at a given rate of speed. Secondly, if you are willing to throw fuel efficiency to the wind, so 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.
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 four feet of draft hits the bottom in five feet 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 is a planing 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 planing 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 four feet of draft hits the bottom in five feet 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!
BTW, if you are interested in being part of USCG Forces, email me at [email protected] or go directly to the D1SR Human Resources Department, which is in charge of new members matters, at DSO-HR and we will help you “get in this thing . . .”