They are all mistakenly considered to be the same, because they simply look like inflated pieces of rubber. In fact, since their job is to protect your boat, you don’t have to be superficial in choosing the right model for you. Basically, there are two different types. The classic ones, tubular or spherical, inflated by air, and the full ones, made of foam and rectangular in shape. The former are the most common; their sizing must be related to the height of the boat on the water, its size (the bigger the boat, the more pressure it exerts) and the type of port in which you normally moor. Keep in mind that if the movements are large, the cylindrical fender can be raised above the area to be protected from the movement of the hull itself.
Consider at least three per wall and at least one every 2 meters. Inflatable models should not be too inflated and their pressure should be checked at least once a year. The rubber must be resistant to ultraviolet rays and must not release the colour, otherwise you risk finding the side dirty. Fabric fender covers are available to overcome this.
The foam models are more resistant over time, which is why they are often adopted on large boats. Their greatest advantage, however, is that they can be easily stowed on board, as they take up less space and can therefore easily be stored in many lockers. Keep in mind that in case of strong wind, this can flutter on the wall. In addition to the classic sideboards, use those from the stern that protect the boat from any impacts on the dock during maneuvers. If you do not want to use them on board you can always leave them fixed at the dock, of course, depends on what type of mooring you have.
Fenders: inflatable ball and cylindrical, or foamed and flat?
The first ones are more bulky and difficult to stow, but offer more protection for the same volume: they keep the two walls (or the wall and the quay) further away. The latter are easier to place, in a locker or in the special spaces in the guardrail. And they are suitable for all types of boats. The full, flat and generally rectangular models can be easily stowed and find different uses, even for work on board. You should know that their purchase price is a bit higher than the respective pneumatic models.