On Mon, 22 Nov 2004 22:38:45 -0500, Wayne.B
wrote:
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".
That makes sense, but I'm more inclined to describe "plane" as the
least amount of hull in the water at cruise. I'm also not sure you
can apply the word to boats like yours and Mr/Mrs E's as they are not
true planing hulls.
To my mind, which is a strange and curious place I'll admit, the act
of planing on water is similar to how an airplane works. You use a
lot of power to get the boat up on plane, but once there, you throttle
back to maintain speed and attitude. To do that on the water, more
hull has to be out than in. Yes/No?
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.
I can't get into this one as I've been on big sport fisher's that
clearly weren't planing, but plowing, although easily and without much
bow lift. When you apply a lot of power to a small area, you will get
voids and that is essentially what is happening in this circumstance.
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.
I need to give that one some thought. My initial reaction is that
power can't replace the concept of the least amount to do the most
work which would imply that brute power is not a factor in planing.
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?
Oh - I like it.
The problem is that you'd have to build a set of universal variables
to account for weight, length, beam, deadrise, lift strakes, power,
blade pitch or perhaps solve for the individual variables, then use
those as plug in's for an equation for different speeds and power
settings....
~~ mutter - naval architecture - grumble ~~
Later,
Tom
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