Planing

Thought I read an interesting definition in my Tasar manual years ago which was based on the position of the bow wave relative to the hull where (and this is where it gets a bit hazy) fully planing is when you overtake the bow wave. Sounds wrong when I re-read it.

Hi,

If you remember (?)  - a "displacement" hull has to push the water aside as it sails. There is a formula - max speed = 1.3 times square root of the waterline length. Note speed in knots, length in feet and 1.3 is a factor that changes depending on the hull shape in question.

So if you have a 16 ft boat (say a Wayfarer - for easy math!), max displacement speed is 1.3 x 4 = 5.2 knots.

In a heavy yacht, there is no way that you can go faster (unless surfing) so you would adjust the sail area to sail at 5.2 knots. A saining dinghy, however, can "climb" out of the water and skin on the water. At this point, the hull isn't trying to push water aside, so the drag force reduces significantly, and the boat will plane.

In a small dinghy, the transition can be dramatic (bow lifts out of the water, accelerates, "clean" wake). Just before planing, the dinghy will be making large bow & stern waves.

Often there will be a high bow wave - with spray - not solid. The exact details will vary depending on the hull sections.

HTH
Colin