What is the difference between semi-displacement hulls and full displacement hulls?

Question

I see the term used on big yatchs but in looking at them I can't discern the difference.

Answer 1

It's the underwater hull and the amount of power that the engine(s) provide that determines if a boat can "plane"  Meaning use the lift that the hull provides as the boats speed increases.  Most runabouts lift on to plane at about 22-25 MPH and only a small area in the stern remaines in the water.  A Tug boat or oil tanker, can never plane because the hull can't provide enough lift to even begin to get it on plane, it simply displaces to much water.  Then there are boats that are`sortof -in-between.  No amount of power will get the boat to plane, but as the boat gains speed the hull does provide lift.  This  semi-displaces some of the water.  I've seen 50' yachts that cruise at 22 mph. but there burning 28-30 gal. per hour.  much larger than that and it's just not practical.

Answer 2

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 fulfill their operating requirements. displacement hull, the V at the transom is usually fairly flat with anything from 3 to 7 degrees being the norm.

SEMI-DISPLACEMENT :

The stern of the Semi-displacement hull is lower and designed to be always below the water. The hull can be round bilge form but is generally of the 'Hard chine' type. These hulls have less fullness than a full displacement hull. The chine line runs aft with a small curve from where it enters the water and on back to the transom. The hull sections are moderately Veed. The semi-displacement hull will out perform the displacement ‘Hull Speed’ rules and will accept additional power and convert it to additional speed however there are limits to this benefit. You should only consider Semi-displacement hulls if your speed requirements do not exceed 12 to 18 knots.

Answer 3

Planing hull >>>not planing> displacement> under 15 knot.