Re:[Paddlewise] Planing etc.

Robert wrote:
> In Derek Hutchinson's "complete book of Sea Kayaking",
> he describes different hull designs, and their characteristics.
> On the subject of hard chined hull shapes, the description has
> me a bit puzzled, because,  it is contrary to what a lot of
> kayak owners say about their hard chined kayaks.
> (From Derek Hutchinsons book "Complete book of Sea Kayaking")
> "The V-shaped or chine hull, the traditional shape of many
> Eskimo kayaks, gives directional stability as long as the amount
> of rocker is not excessive. However, with such a hull it is
> unfortunately almost impossible to maintain fast forward speeds
> because as the speed increases the boat tends to plane on the
> flat chines. The kayak thus retards itself on its own bow wave".

First I strongly recommend you tear out Derek's pages on boat design. He has
packed an enormous amount of inaccurate nformation in a few pages.

Next you need a useful definition of planing. Most naval architects use
Savitsky's definition who defines it as the point at which wave making
resistance begins to diminish with increasing speed. I am not sure if this
definition is published some where. Mr Savitsky gave it to me at a meeting
of the Society of Naval Architects and Marine Engineers. Why do planing
boats achieve such high speeds? They displace so little water and generally
have lower wetted surface.

 Do not confuse a change in trim as planing. Planing is a function of the
vertical rise of the center of gravity not a change in trim. Many people
think they are planing or approaching planing when the trim changes. Sorry,
just ain't so. As the dynamic forces increase the CG rises vertically out of
the water. This requires a lot of horsepower since initially the boat sinks
due to the increase in kinetic energy across the bottom. Experts argue over
the onset of semi-planing. Purists say it begins once the CG rises above the
static position. This definition has the virtue of precision but probably
overly restricts the advertising department's freedom.

Do not confuse surfing with planing. A surfing boat makes use of the power
in the wave to reach higher speeds. A planing boat can plane without help
from waves. This may seem like a trivial matter but it is important to
people who design planing boats.

Do not make the mistake of thinking that the boat climbs the bow wave or is
obstructed by the bow wave. The boat makes the bow wave and the bow wave's
size indicates the energy expended in its formation. Thus a large bow wave
indicates large resistance but it does not cause more resistance. This
problem with cause and effect often gets confusing.

Despite many claims no one has ever proven that they can plane a boat using
paddles. The test is simple. Set up a surveyor's level on shore aimed at a
vertical measuring tape affixed over the CG. Now observe the change in the
CG's vertical position at top speed. If the boat has risen you can then
calculate the displacement and, from that, calculate the resistance using
KAPER (or other formulas) to see if the wave making resistance has
diminished. This is not precise but if you detect a significant change you
can then use more sophisticated methods to confirm it (I.E. tank testing
using pulsed power).

In my research I could find very little difference between round bilged and
chined boats so long as all other parameters were kept equal (hard to do).
Most hard chined boats suffer form higher wetted surface and lower prismatic
coefficients (less efficiency at higher speeds) which result from the
restrictions imposed by the hard chine configuration. I did discover that
chine immersion in the fore body caused an increase in resistance so it
seems to pay to keep chines nicely sloped and high forward. People should
not draw too many conclusions from limited tests of a type unless you know
that the boats have significant similarities in form coefficients.

For displacement hulls naval architects generally prefer round bilge boats
for lowest resistance and will not use chines unless they have a specific
reason for doing so like ease of constitution, spray shedding, etc. etc.

Michael mentioned Marchaj's comments on sailing canoes. Unlike kayaks they
are true planing hulls and canoes only by virtue of having a more or less
rounded stern. They use chines aft to promote planing.






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