HEAVY DISPLACEMENT:
These heavy displacement hulls include such craft as tugs and deep sea trawlers. If you study these boats in profile you will notice that the stern rises above waterline. The mid section of the hull is very full and the midsection is deep in the water. The chine and buttock lines will reveal the full bellied shape usually present in this type of hull. The heavy displacement hull has to be able to carry great loads and in the case of tugs, be able to get a great grip on the water in order to do its job properly. The "hull speed" of this type of vessel is generally less than that of other types.

MEDIUM DISPLACEMENT:
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 fulfil 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 km/h. 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.

 This PCF 40 is a typical full displacement Trawler type hull. This hull is also used as a working trawler.

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 only if your speed requirement is around the 6 to 12 knot mark respectively. For higher speeds consider Semi-displacement or Planing hulls.

One important factor is that Displacement and Semi-displacement hulls are generally considered better ‘Sea Boats’ and are more suitable for serious offshore cruising than the planing hull type. As with heavy displacement hulls, medium displacement hulls are not so affected by weight as the semi-displacement and planing hull types