A brief lesson on centerplates

For those of you not familiar with trailer sailboats, let me explain what the centerplate is and how it works.

As shown in the diagram below, the centerplate rotates on a bolt or pin. It's completely lowered during sailing and raised for trailering and beaching the boat. It's raised and lowered using a pulley system (also shown in the diagram) or winch. The centercase has an open slot through the bottom of the boat. Since this slot is basically a large hole in the bottom of the boat, the centercase will fill partially with water. It has to be well sealed and the sides have to be high above the waterline so the boat doesn't take on water. (Or on some boats, the top of the case is sealed somehow).



So why does my boat even have a centerplate? Well, I won't get too into all the physics, but it's because sailboats need a large underwater area to provide lateral resistance. This lateral resistance allows the sailboat to sail upwind. When sailing upwind, the force created by the airfoil of the sail on the boat is diagonal, trying to pull the boat forward, but also trying to push it sideways. The centerplate keeps the boat from slipping sideways across the water.



Smaller sailboats use daggerboards, which are the same as centerplates, except they are simply pulled out of the top when not in use, instead of rotating into a centercase. If the boat runs aground in shallow water, the centerplate rotates up and the boat is usually undamaged. The more rigid daggerboard can cause more damage to the hull when running aground.

Small sailing dingies without a centerplate or daggerboard cannot sail upwind! Well, actually, it's difficult to sail in any direction other than that of the wind. Most larger sailboats don't need a centerplate to sail upwind, because they have large fixed keels that provide the necessary lateral resistance. The Hartley plans actually show two large fixed keels on either side of the bottom of the hull as an alternative to the centerplate, but this would make trailering difficult. I've never seen a Hartley TS14 or TS16 with fixed keels.

The centerplate on my boat is made of heavy steel, thus serving another purpose: ballast to keep the boat from heeling too much and make it more stable. So if the centerplate is not lowered before raising sails, the boat is unstable, difficult to control and may capsize.

But here is safety issue to think about ... what if the boat does somehow tip over on its side during sailing? Then gravity may cause the centerplate to flip into the centercase, thus raising the center of gravity of the boat, making it more likely that the boat will flip completely upside down (or "turtle") and making it very difficult to right. To solve this problem, a lot of sailboat owners have a locking mechanism to keep the centerplate down at all times, except when in shallow water. This locking mechanism makes the boat much more "self-righting". If a small boat gets on its side and does not self-right, the wet sailors simply stand on the side of the protruding centerplate and pull the boat back upright. And if the boat turtles even with the centerplate extended, the centerplate can still be used as a lever arm to try to right it.