Most of the sailing boats or yachts now have auxiliary engines installed in their hulls. Why yachts that are mostly propelled by sails still need engines? It is easy to answer this question. Wind is not always available when yachtsmen need it to sail at sea or to return home. Or if it is available, it is too light to sail. Sometimes when facing bad weather or heavy seas an engine is essentially needed to move the boat out of such dangerous condition. 
In addition, certain port authorities do not allow yacht owners to hoist their sails due to safety reasons. The waters in most of the harbors may be full of mooring ships and yachts. Maneuvering with sails is not recommended in the area that is crowded with other vessels.
Now, how many horsepower does a sailing yacht or boat need per its metric ton of displacement? Actually, there is no standard rule for this as every yacht owner will have his or her own power criteria. In the past, as a simple guide, 2 horsepower per one metric ton of displacement might be enough. Now, many sailing yacht owners prefer 4 hp per ton of displacement. They might need higher power because they need to sail home faster to go to work. To better determine the power requirement of a yacht or boat, it is necessary to carry out resistance calculation in various speeds during the design phases of the yacht. This can be carried out manually or with the help of computer program such as Delftship. When the Effective Power and Speed Curve has been obtained from the calculation, a naval architect or yacht designer will analyze it and determine the most suitable engine rate for the boat. The calculation can then be continued to propeller design to assess the most efficient dimensions for the boat or yacht. by Charles Roring
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