Re: [Paddlewise] Kayak speed vs length

The ability for longer boats to go faster has to do with the 
interaction of the waves generated by the moving boat and the hull 
shape. As boat speed increases, the length of the wave created also 
increases. When the length of the wave equals the length of the boat, 
it is hard to make the boat go faster because the wave starts getting 
bigger without getting any longer and the force required to make the 
boat go faster increases dramatically.

This interaction is called "hull-speed" but it is not a real speed 
limit. It is a theoretical idea based on the increase in drag as the 
wavelength and boat length become the same. Hull speed is 
proportional to the square root of the boat length.

However shorter boats can have less drag at a given speed because 
most of the time the primary source of drag is the skin friction of 
water molecules moving against the hull of the boat. This drag is 
related to the wetted surface area of the boat. More wet surface, 
more drag. Shorter boats typically have less wetted surface and are 
thus more efficient at speeds below their hull speed. It is just hard 
to push them beyond their hull speed. Since a longer boat has a 
higher hull speed it can be more easily pushed to go faster, even if 
it has more drag at lower speeds. It is kind of silly to say one boat 
could be "slower" at slow speeds, but since a long boat may have more 
drag at low speeds, it may be harder to move than a shorter boat.


At 12:53 PM -0700 5/12/01, Evan Dallas wrote:
>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.
-- 
Nick Schade
Guillemot Kayaks
824 Thompson St
Glastonbury, CT 06033
(860) 659-8847
***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************