[Paddlewise] Speed and short boats

In the discussion about speed and short boats some people may find the
following interesting.

1. The waterline lengths of many of the shorter sea kayaks (Coaster,
Speedee, etc.) are actually quite long relative to the overall length (i.e.
short overhangs in the bow and stern) whereas many "long" sea kayaks  have
long overhangs. For example the 20' Looksha II has a waterline length of
16.16' so the overhangs are approximately 20% of it length. For comparison
the Coaster is at 13.44' is 12.76' on the waterline and its overhangs are
approximately 5% of its length.

So, when comparing lengths it pays to look at the waterline length rather
than the overall length.

2. If displacements are similar, the shorter boat will often have less
wetted surface than the longer boat. For this reason the shorter boat will
likely have similar resistance at lower speeds where wavemaking resistance
is a smaller portion of the total.  For example, at 4.0 knots the Coaster
has 11% more resistance than the Looksha II (Results from KAPER) but at 3.0
knots the Coaster has only 2% more.  (I chose the Looksha II only because
it is a much longer boat than the Coaster. No other reason)

3.  The studies I have read so far suggest that a just noticeable
difference in exertion (a difference that can be noticed 50% of the time)
tends to be +/- 10% so in the speed range between 3.0 and 4.0 knots the
paddlers may not notice much difference between the Looksha II and Coaster.
Once beyond 4.0 knots the difference climbs rapidly becomes more
noticeable.

I noticed the term "hull speed" rearing its ugly head again.

Keep in mind that "hull speed' is not a limit to speed. It is the speed at
which the length of the transverse wave produced by the boat is
approximately equal to the effective waterline length of the boat. This
quote from "Introduction to Naval Architecture" by Gilmer is a good one.

"Hull speed is somewhat analogous to the 'sound barrier' for aircraft,
which is not really a barrier but a practical limit for economic
transportation"

Gilmer was talking about ships an few ships have as low a
displacement/length ratio as kayaks. For heavy short boats it does
represent a practical limit to speed but for light, long boats like sea
kayaks it is just another point on the graph. Even old, worn out, unskilled
paddlers like me can paddle sea kayaks at speeds well above "hull speed".






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