rgnmstr wrote:
I'm going out for the evening and hope to get an answer by the time I
return. Can a fellow sailor explain what semi planing is on a SAILBOAT
and how one knows when it's happening? Thanks.


Sure. Think for a second about what planing is... when the boat's weight
is supported by the force of the water going under her hull, rather than
by sitting down in the water, making a hole big enough to displace her
weight.

Displacement sailing, limited to "hull speed" by wave-making resistance,
is easy to see. As the boat speeds up, bow and stern wave build and get
further apart until the stern waave is right at the aftmost part of the
waterline. The hole in the water, caused by the boat's displacement, is
closing up behind the boat as it goes along.

Now make it go just a little bit faster. Still making big waves, that
hole is still closing up behind... but the crest of the stern wave is
now a little bit aft of the aftmost point on the LWL. If the power is
available to push just a little faster, you will notice that the crest
of the stern wave may be two or three feet aft. At this point, the hole
in the water is not really closing up behind and the boat is partially
supported by hydrodynamic force.

If there is yet more power available, the stern wave will get further
back and start to diminish until the boat is fully planing and there is
only a very small stern wave or perhaps none. The Johnson 18 when
planing fast leaves a wide smooth track of bubbles.

Somewhere along the way, you can say you are "semi-planing" but few
sailors of planing boats bother to talk about it. Sailors of heavy boats
that will not lift never notice the stern wave because their boats are
getting really squirrelly about that time.

Fresh Breezes- Doug King