(no subject)

The theoretical speed of a vessel in knots through water can be
generally calculated as 1.34 x SqRt of the length of the water line. 
Therefore, if a 17 foot (length on deck) kayak has a water line length
of 16 feet, then the theoretical maximum speed is 5.36 knots.  This
assumes a displacement hull condition, that is that the vessel is not
planing on top of the water, and that there is enough forward thrust
available to achieve "maximum hull speed".  Here's a very simple
explanation of why this is so.  As the kayak moves through the water it
creates a bow wave.  The faster it goes, the higher the bow wave.  The
higher the bow wave, the longer the wave length (the distance from one
wave top to the next wave top.  Behind that bow wave peak is a trough. 
When the vessel achieves a speed such that the trough of the bow wave
occurs at or near the stern of the vessel drag increases dramatically. 
I know that others with a better grounding in the theory will probably
disagree with this interpretation of the effect, but it has always
seemed to me that when the stern sinks into the trough of the bow wave,
the vessel is, in effect, constantly going up hill.  The only way to
overcome this effect is to increase forward thrust sufficiently to cause
the vessel to plane.  That's why a 30 foot sailboat is able to achieve
hull speed with a modest 15 to 25 horsepower engine.  And, why a ski
boat of half that size is equipped with a monster v-8 engine.

Now others can jump in with the exactly correct information.

Incidentally, according to the formula above, your mile long kayak would
be capable of achieving a speed of 97.4 knots given sufficient thrust. 
I leave it to you to determine the correct paddle length and the size
and physical condition of the paddler.

Steve King

PaddleWise wrote:
> 
> PaddleWise            Sunday, May 13 2001            Volume 01 : Number 1509

-------------
> 
> Date: Sat, 12 May 2001 12:53:05 -0700
> From: "Evan Dallas" <Evan_Dallas_at_notes.ntrs.com>
> Subject: [Paddlewise] Kayak speed vs length
> 
> I've often heard (and intuitively accept) that all other factors being equal, a
> longer kayak will be faster than a shorter one.  I've got a friend who disagrees
> with this, but I've had a hard time explaining why this would be true.  I would
> speculate (fancy word for "guess") the following possible reasons:
> 
> 1.  Paddling on the ocean, there are generally little (or big, for that matter)
> waves and chop that push against the kayak from different angles.  The longer
> the kayak, the more these "local" disturbances would tend to cancel out and thus
> interfere less with the boat's movement through the water.  If this is true,
> then would this advantage disappear when paddling in very smooth, flat water?
> Or would the longer kayak continue to be faster?
> 
> 2.  A longer kayak has more floatation than a shorter boat (again assuming other
> factors are equal), so it would float a little higher in the water, thus
> reducing drag.  I assume that most of the friction comes from the movement of
> the bow cutting through the water and that the additional length contributes
> very little additional drag.  (I expect I'm going out on a limb on this one,
> having once gazed at Matt's speadsheet on this subject (phew!)) I know enough to
> know it's ultimately more complicated than that).
> 
> My friend countered my comments by saying if that were true, then paddling a
> boat a mile long would be faster than a 17 footer.  Obviously a REALLY long boat
> like that would weigh a lot and thus have a lot of inertia to overcome getting
> it moving, but once up to speed -- would it be faster than the 17 footer?  Or
> perhaps another way to ask it:  would it require less paddle effort to maintain
> a given speed than a 17 footer??  What if it were so loaded with weight that it
> floated the same height in the water as the shorter kayak -- would it still be
> faster?
> 
> Lastly, is there a simplified formula for estimating the speed, given a certain
> beam and length (holding other factors constant)?
> 
> Evan Dallas
> Woodinville, WA.
>

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