On Mon, 22 Nov 2004 08:29:29 -0500, Harry Krause
wrote:

When, exactly, is a boat on plane and how can you tell, precisely, when
this happens?

==================================

Good question. We all know it when we see it, but the formal
definition is tougher.

I think it relates to how much of the hull is riding above the water
as opposed to plowing through it. That implies that planing could be
defined as a percentage of hull lift versus displacement, for example
50% or more of the hull normally below the water, rises above water
level when "on plane". This jibes reasonably well with the popular
phrase "out of the hole", meaning "on plane".

Another definition might involve flow separation at the transom. Most
times a boat "on plane" will have little or no transom surface
directly touching the water even though the bottom of the transom is
still below the water line.

It is also tempting to define planing as a ratio of actual speed to so
called "hull speed" but there are plenty of heavy displacement boats
that exceed theoretical hull speed by wide margins without actually
giving the appearance of being on plane. Motor yachts and fishing
trawlers come readily to mind as examples. They do it through sheer
power, pushing aside a lot of water as opposed to actually rising
above it. By the time a boat is going over two or three times its
theoretical hull speed however, it usually gives the appearance of
being on plane regardless of weight.

There might be another mathematical approach worth considering. As a
boat begins to exceed theoretical hull speed it takes huge increases
in power to increase speed by relatively small percentages. Typically
power required varies as the cube of speed. As a boat begins to plane
however, the power required still increases exponentially with speed,
but at a slower rate, more like the square of speed opposed to the
cube. Using that approach, planing would be defined as the
mathematical inflection point where the speed/power exponent begins to
decrease.

Comments?