The Mystery of Sealing Things

It Is Not Rocket Science….

On our boat we have a lot of things that need sealing, some of these are hard and some of these are easy. Today we are going to talk about the ones that are actually easy, but people make really hard becasue they do not understand how the correct kind of gaskets work

Refrigerator lids, and deck hatches frequently are sealed with a hollow “D” shaped gasket of made of various types of rubber. They typically look like this:

Some are round, like this, some are half rounds, and a variety of other shapes. There is a trick to using gaskets like this that most people do not appreciate: If they are squashed flat, they don’t work! The typical specification for the maximum compression of this kind of gasket is NO MORE THAN 25 to 33%. If you squeeze them flat, the gasket deforms and wrinkles. It also takes a permenent “set” and loses its “squishiness”. All these things result in a gasket that doesn’t last very long, if it ever really works correctly at all. This is typically not true of solid rubber gaskets, where more compression force (to a point!) yields a better seal.

There are two ways of making sure that this type gasket is not over-compressed. The more obvious one is to support the closing lid so the gasket is not required to support its weight. If the gasket is 1/2 inch high, for example, it would have a 3/8 inch or 5/16 inch support around it that would actually carry the weight of the closed locker. In the case of the lids that close our fridge and freezer, we found inexpensive stick-on furniture glides at the local hardware store that are the perfect thickness to support the lid and give us the exactly correct amount of compression to keep the heat out and the cold in.

The other approach is a bit more clever, and subtle. It involves putting a length of hard .plastic rod inside parts of the hollow interior of the gasket. The rods actually carry the weight of the lid, and prevent the over compression of the gasket. This is a great solution for lids that need to be clamped down tight (like deck hatches!) but where we still want the gasket compression within limits.

It is entertaining reading comments from “experts” who don’t understand the original purpose for the solid plastic rods set into the gaskets when they try to explain why replacement gasket installations never seem to work as well as the original. They fuss about exactly which brand of gasket is the “right one.” Should it be neoprene? or EDPM? If you get the size right, and install it correctly, either will work just fine for keeping water out of a boat locker. The exact shape isn’t critical. My favorite bit of horse-hooey is “they just don’t make them like they used to.” Nonsense. Properly installed with lid supports–either inside the gasket or outside–this gasket type will give a long lived and reliable seal. Today, just like 20 years ago.