On older long keel boats was usual to cut an aperture in the aft end of the keel to accommodate the stern bearing and propeller.   Today most long keel boats are designed with a metal or timber heel or shoe attached to the aft end of the keel so apertures are seldom cut into the keel itself.   In cases where there is not enough room for the propeller aft of the keel and ahead of the rudder, it will still be necessary to cut an aperture.

   As it may be desirable to place the engine in your hull before the installation of the deck and superstructure, it is probably a good time to consider forming the aperture or arranging the heel on your hull. If the engine is to be installed at a later date it is still possible to prepare for the installation using a cardboard or plywood mock-up of the engine and using measurements from the manufacturer’s brochure.

   The size and location of the propeller will go some way towards governing the angle of the engine (maximum angle 10 deg) and the size and location of the aperture.   Next you need to figure out the shaft line.   This can be obtained using the simple plywood mock-up of the engine.   This mock-up should show the shaft line in relation to the engine and gear box and position of the engine feet.   It is possible to buy an angled gearbox but, even though the engine remains level or near the level in the hull, the gearbox will still have to be lined up with the shaft.

   The pre-prepared profile mock-up can be arranged so that it is in the proper position relative to the engine beds.   Once the beds are installed, the profile can be used, in conjunction with a string line from the centre of the drive shaft to a hole in the aperture to obtain the correct shaft line. In fibreglass boats, once this shaft line is established, you can make a fibreglass shaft tube over a mandrel, with the cutlass bearing in place, and glass the tube in to the boat making any final adjustments through the engine mounts.

   There are three types of aperture.  One is where the aperture is cut out of the aft end of the keel.   The second is where there is a skeg and the aperture is cut out of the skeg or, better still, the propeller is just in front of it and the third, and more desirable arrangement, is where the bottom of the keel is extended in the form of a shoe (heel) which is used to take the lower rudder bearing.  The first two types are usually found on sailboats.  The third is found on both sailboats and displacement hulled power boats.   Where the aperture is cut out of the back of the keel, it should be cut out so that it will provide room to remove the propeller without having to remove the rudder and the shaft could be slightly offset so that it can also be removed without removing the rudder.   The aperture should be of sufficient depth to allow at least 2” [50mm] propeller tip clearance both top and bottom.   Once the aperture is cut then it must be reconstructed from foam or timber and shaped to allow the fitting of the shaft tube with fluting above and below the tube to facilitate a clean flow of water around the rudder and propeller.