Why is there slits on a hovercraft's skirt?

there are 3 types of skirts on hovercrafts: The bag skirt, the finger or segment skirt and a combo, the bag and finger skirt.

You are talking about the finger skirt.

The finger skirt is comprised of a large number of separate segments which are able to slide and bellow individually to conform with the shape of the water surface or terrain over which the craft is traversing.

The fingers should not be sewn or connected together in any way or else the design of the skirt which is to form a very flexible air seal between the hovercraft and terrain, will be defeated. To achieve this, the fingers should only be attached to the craft by a rigid strip along the upper edge and by some from of corner tie at the two innermost apexes. The straight finger serves well as a bow skirt due to the advantageous receding angle of attached which helps in riding over waves and ground undulations and at keeping frontal spray and dust to a minimum. The straight finger however, is very unforgiving to being dragged backwards over obstacles. The extended finger is best used at the side and rear of the hovercraft as it has a better obstacle clearing characteristic and also its design helps to maximize the effective cushion area because the ground contact point is much further out towards the hull perimeter than for that of the straight finger.

The inner fact of the finger skirt is completely open which means that apart from a very marginal increase in pressure due to the peripheral jet stream, is a very soft and flexible with only actual cushion pressure holding it firm. This is vastly different to the bag skirt which has a peripheral pressure much greater than cushion pressure. The finger skirts at the rear of the hovercraft must be equipped with either a single anti-scoop flap or for improved flexibility, individual strips may be sewn to the inner edges of each finger which extend out belong the ground contact point. This reduces their efficiency in retraining cushion air from escaping, but unfortunately is quite essential if the hovercraft is to be used anywhere than over smooth terrain.

CHARACTERISTICS OF THE FINGER SKIRT

The straight finger skirt is easy to design and construct and repair work is also easy because of the small size of each segment. The extended finger however is a little more difficult to design.

The finger skirt, straight or extended, gives a very smooth ride and has a low friction characteristic and so long as the cushion height is adequate, is ideal for traveling at high speed over waves and rough ground. Apart from the rear fingers which should have an anti-scoop flap, a hovercraft with a finger skirt does not trap water like a bag skirt at lift off. An extended finger uses a great deal more material than a straight finger or bag skirt and for this reason, it has become common practice for the expensive low friction neoprene coated fabric to be used only on the knuckle which constantly rubs across the on-coming waves and for less expensive non-coated fabric to be used on the bellow and sides of the finger.