Re: [Paddlewise] Excessive Weather Cocking (or is it Fairy gliding with the wind)

Nick wrote:
<SNIP>>>>>>The reason I don't like the climbing-out-of-the-hole model is it
implies that the slope drag is the primary source of drag at hull
speed. There is an expenditure of energy associated with the fact that
the waves create a slope that the boat would like to slide down, but
the slope is created by the waves which are a result of the boat
pushing the water. It takes a lot of energy to push the water, and not
that much to stay on the slope.<<<<<SNIP>

The more energy the boat applies to the water to go faster the higher the
crest, the deeper the trough, and the longer the wave-length of the wave the
boat is making. The wave barrier is being made larger in because of the
process used in trying to climb out of the hole or over the wave. It is a
vicious circle. Just climbing up against a force that wants to accelerate
you down at 32 ft/sec./sec. for every second of every day takes a hell of a
lot of energy. Especially when you have only a very inefficient and low
power propulsion system with which to do it. To then also have to push
increasing amounts of water up (why water is so hard to push aside/up is
also due to gravity "pulling" it down) against that same 32 ft/sec./sec
force just so your speed can increases to the point your hull can use the
waters density as a ramp to climb up out of the trough also take a hell of a
lot of energy. The heavier you , your boat and your load is the harder it
will be to do.

>>>>>>>>At hull speed the water is moving at its natural frequency.  The
waves
rise up and drop down over the length of the boat when the boat is
going the natural velocity of a wave with that wavelength. When you try
to go faster, than the natural velocity of the waves the boat produces,
it is like trying to force a clock pendulum to swing at a different
frequency. It takes a lot of power. You are over driving the pendulum.
The natural response is for the waves to just get bigger without
actually changing their frequency or speed.<<<<<<<<<<

A strange but interesting analogy. Both frequencies are the result of the
acceleration due to gravity and the mass of the objects. Since you can't
easily get a grip on a wave unlike on a pendulum waves are probably going to
propagate at their natural frequency no matter what you do short of
overpowering huge masses of water. The water is always in the way of a hull
and the more of it you have to lift (displacement weight) and the less time
there is to lift it (as your speed increases) the more energy must be
applied in a given time to push it aside (and thus displace it upwards
against gravity). Gravity is "pulling" things (including the water) back
down at a constant 32 ft./sec/sec. and this rate is what is determining the
constant speed to wave-length ratio of a wave (speed in knots is equal to
1.34 times the square of the wavelength in feet). Using the right terms you
will find the acceleration due to gravity somewhere in the formula used to
arrive at the 1.34 etc. Once the wave has gotten so long that the boat is no
longer level but on the slope of that lengthened wave. The way to go faster
is to take on gravity in a more direct way by climbing out of the hole (in
addition to the energy to keep pushing up waves that is also still rapidly
increasing as you climb the wave slope). It is the fact that the wave gets
longer with increasing speed that eventually leaves the boat with no way to
go but up (or some say through the wave with a very narrow hull like a
catamaran-but I think there are those who dispute this).

 Niels Blaauw niels.blaauw_at_wanadoo.nl wrote:
Subject: Re: [Paddlewise] Excessive Weather Cocking / Energy, force and work

>>>>>>>>I got the feeling we got about all the theories we need. What I'd
very
much like would be some graphs, some figures, some statistics, on the
force needed to move a displacement hull, to make the transition to a
planing hull, and to keep the hull planing.

Matt, I've seen your homepage, you've done a lot of testing on
hullshapes. Any measurement on planing hulls there? Anyone else?
Something on jetskis maybe? Powerboats? Surfboards?<<<<<<

If you go to the downloads page of our website you can call up (the one
ending in XLS or download the ones ending in ZIP) the drag prediction
spreadsheet (MS Excel 2000 or 5.0) that I put together to automate John
Winters method of trying to mimic my method for calculating drag (used for
the other Sea Kayaker magazine drag prediction method) by using formulas and
data taken from the graphs (rather than plotting parameters directly on the
graphs like I do). Both methods use Gertler's "Reanalysis of the Original
Taylor Standard Series" graphs to predict the drag (derived from tank
testing of ship models which were varied a little bit at a time in
consistent ways about a century ago). Although they use a few different
parameters both methods are similar and have been tweaked slightly to bring
their predictions closer to Sea Kayaker magazines 1986 towing tank test
results for several sea kayaks. I plot from Gertler's graphs and John used a
formula and data from Gertler's graphs to try to get the same results. We
are usually reasonably consistent out to 4 knots but if they differ
significantly it is most likely due to the limitations of the "worm curve
formula" in the spreadsheet (by not being wormy enough). The data is
calculated out to 8 knots (where possible) so that usually brings them over
the wave hump and up to at least semi-planning speeds. You will notice on
the drag graph that at no point does it get easier to go faster, only the
rate of the increase with speed is reduces (the drag curve starts to shallow
in slope not turn down). This is also true for the residual (mostly
wave-making) drag component of the total drag. The drag curves are further
broken down into frictional drag and residual drag (anything other than what
friction would be on a smooth thin flat plate of equal wetted
surface-wave-making is by far the biggest component with normal hull
shapes). The new Excel 2000 spreadsheets I just uploaded a few days ago also
have all the Sea Kayaker magazine results (for the 75 kayaks reviewed so far
with this method) archived below in the spreadsheet in a way they can be
copied and pasted on to row 9 of the spreadsheet to also look at their
results in graphic form. You can enlarge the graph size for more detail by
pulling the corners or sides.
I'm sure there are some methods that are optimized for predicting drag on
planning hulls and John Winters might know which ones to look for or where
to find them. Are you out there John?

Matt Broze
http://www.marinerkayaks.com


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