You don’t strictly need the jib on a sloop rig (single masted vessel whether gaff, gunter or bermudian). You just move the mast forward so that the centre of effort is in the right place, and you have a catboat rig. Please see this article on the sail plan. This is the naval architect’s basic design of the boat, which determines the balance between the weather helm and lee helm. A boat should have more weather helm than lee helm, so that you feel the vessel luffing to the wind rather than having to be actively steered into the wind. This is determined by the length of the hull under the waterline, the position of the keel and the centre of effort of each sail determined by the size and shape of the sails and position of the mast. When the boat has more than one sail, the centres of effort on the different sails (typically a mainsail and a jib on a sloop or cutter) compensate for each other. Thus, naval architects can play with various permutations according to the use the boat will be put to. A fishing or freight vessel needs to have free access to the hold, so this will affect the position of the mast – you move the mast forwards and compensate by having a spanker sail in a yawl or ketch rig. On the other hand, a boat built for speed, such as a pilot cutter, has to have a very large mainsail and two jibs to compensate, one of which is rigged on a bowsprit. Some of the old clipper ships had as many as four of five jibs and sometimes a jib between the foremast and the mainmast and another between the mainmast and the mizzen mast.
Briefly, the jib is all about balancing the boat, and setting it well makes all the difference for a sensitive helm, just as much as properly setting the mainsail. At one time, I thought sailing was so much simpler with a single sail, but that is not so. I have had hell with so-called catboats (though the Laser is a much more pleasant boat than, for example, the Topaz Uno) and its weather helm, having to hike right out just to balance the helm by keeping the hull flat in the water and to avoid ripping the rudder off! The jib makes all the difference and makes the handling of the boat so much more sensitive and easier on the sailor.
I got out on the sea today – not too cold but too little wind. In November it’s all or nothing, no half measures. After an anxious moment of rowing against the current in the absence of wind and getting swept below the port, I got a little 7-knot breeze, which solved my problem and took me upstream from the port. Sailing days are becoming rarer and rarer, and the weather forecast promises us some real cold for next week, with temperatures going down to 0 or -1°C. Brrr! Looking on the bright side, one can sometimes get out on the water in December before the real winter sets in. Who dares wins!