These hulls include most regular work boats, general fishing boats and the pleasure boats where speeds of 1.34 times the square root of the water line length (or less) are sufficient to fulfill their operating requirements. For instance let us consider a 40 ft / 12.19 M, LOA. motor cruiser with a waterline length of 36 ft / 10.97 M, the square root of the waterline is 6 so multiply this by 1.34 and you arrive at a potential speed of just over 8 knots / 14.8 kmh. This is an economical speed for this vessel taking into account power required and fuel used to drive the vessel at "hull speed". Medium displacement vessels can only exceed the 1.34 rule by adding excessive amounts of power. If you already own an engine that has more horsepower than required to fall within the 1.34 calculation, then consider building a longer hull or one that employs semi-displacement hull characteristics. In a medium displacement hull, the V at the transom is usually fairly flat with anything from 3 to 7 degrees being the norm.
Once the most economical speed is achieved, it takes a considerable amount of power to make a displacement hull go faster. When this type of hull is over driven then the stern will drag in the water and usually create a large stern and bow wave. The boat may reach such an extreme bow high, stern down angle, where water could come in over the stern and swamp the vessel.
Displacement hulls should not be driven much in excess of their "hull speed". For vessels ranging in size from 30 ft / 9.1 M to 60 ft / 18.3 M waterline length, you should consider displacement hulls if your speed requirement is around the 6 to 12 knot mark respectively. For higher speeds consider Semi-displacement or Planing hulls.
HEAVY DISPLACEMENT | MEDIUM DISPLACEMENT | SEMI-DISPLACEMENT or SEMI-PLANING HULLS | PLANING HULLS
PLANING FLATS AND STRAKES | ROUND BILGE | RADIUS CHINE BUILDING TECHNIQUES | POWER CATAMARANS