Planing hulls are designed to ride on top of the water, regardless of the weight of the boat. The flatter the bottom, the easier it is to get on plane. Also, less power is needed to attain high speeds with a flatter hull. The trade off is in handling. Flat hulls do not do well in rough water. Many planing boats use a shallow "V" shape to ride better in rough waters.
This Fountain 42RC shows several design elements commonly found in modern boats-- a V-bottom for better handling in rough water that uses a "stepped" hull to give additional lift.
Displacement hulls typically have a rounded bottom with a tear drop shape
running bow to stern. Displacement hulls "displace" or move, an amount of water
equal to the weight of the boat. Displacement hulls are very efficient-- most
long range cruising boats such as trawlers and many sailboats use this type of
hull. But because of their design, displacement hulls are restricted in their
speed to the square root of their waterline length times 1.34. Therefore, a 64
foot boat can realistically only expect a top end speed of a little over 10
knots. The Cape Horn 65 pictured below shows several design elements typically found in
displacement hulled vessels such as a rounded hull form and a bulbous bow.