Hulls: Displacement and planing

[Newboy]

I have very often heard boats are described planing or displacement, so how do they differ?

 

 

[duncan]

displacement hulls are designed to move through the water, planning hulls to ride 'over/on top' of it. semi displacement are able to generate some lift and are better described as semi planning (IMHO)

 

if you do not create this lift then your craft is limited to a speed (related to the square root of the water line length). Above that speed you will literally get sunk by your own stern wave - planning craft avoid this by rising up over this stern wave. there are generally accepted formulae for the power required to get a particular weight of boat over this stern wave (often referred to as the hump or resistance hump).

 

planing hulls have some horizontal surfaces to assist with lift or are generally flatter in profile and avoid the drag of a keel (many modern ones have a bit of one to assist with steering at slow speed but they are generally avoided) - displacement ones are more likely to be smooth, rounded, nice entry at bows and have a keel for stability.

[Afishionado]

To find the maximum speed of a displacement hull ..........

 

Multiply the square root of the waterline length in feet by 1.5.........

 

eg Waterline length 16ft. Sq root is 4. Multiply by 1.5 = maximum hull speed. Which in this case will be 6 knts.

 

A totally flat hull is a planing hull. but it will slam into waves and skid in turns. So to give a flat hull some 'bite' and lessent the slaming the flat shape was developed into two flats joining as a shallow V shape. This is a compramise between stability at rest and efficiency at speed. Take the V form a bit futher and one gets a Deep V hull which given enough power slices through the water beautifully but wallows from side to side like a pregnant sow when at anchor.

 

For comfort and stability go for Displacement.....But they are slow

 

For a bit of speed but retaining stability go for a moderate V hull

 

For speed and manouverability on the go then a deep V is the thing.......But it is uncomfortable at anchor and on the drift.

 

Mad Mike