Re: [Paddlewise] Pro's and Con's of the "Swede Form"

Matt Broze wrote:

<snip A whole bunch of stuff>

> A wave begins to "feel" the bottom in water about 1/2 as deep as the
> wavelength(the waves orbital motion below the surface is touching the bottom
> which slows it down--compressing its energy into a shorter space thereby
> making the bow wave higher, steeper and harder to climb). At 5.4 knots the
> wavelength created in deep water is about 16 feet long--which means it
> begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is
> about 5 feet long so the wave-drag effect starts (but is probably not
> noticible yet) in 2.5 foot deep water]. In four feet of water the 16 foot
> wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to
> about 4.4 knots and that sure slows your racing speed. In 1 foot deep water
> that wave's speed is down to 3.3 knots and so is the kayaks hull speed unless
> it can break the wave barrier and start to plane. In 1/2 foot deep water
> wave speed is slowed to 2.37 knots. Remember that in water less than about a
> foot deep bottom drag due to turbulence drags one down even more than this.

Thanks for this, Matt -- never had anything quantitative before, though I sure
noticed the "shallow water" effect.  Feels like paddling in mud.

> Water does speed up in a constriction--the Venturi effect (a special case of
> the Bernoulli effect)[snip]
> I should point out here that when two ships pass near each
> other or a ship passes near a wall the same effect moves them closer
> together (a real danger for ships).

Local examples:  

1. There is a very narrow place in the shipping channel where it is jammed up
against the WA side of the Columbia River -- about 3 to 4 miles downriver from
Skamokawa.  It is notorious for this "Venturi" effect on freighters passing in
opposition, and the River pilots slow way down there.  

2. There was a glancing bow-to-bow collision a year or so ago off Hammond, OR
(close to the River mouth) in which a downbound freighter tagged an
upriver-bound Coast Guard buoy tender (150 foot ship?) pretty good.  No
serious injuries, but probably a lot of soiled underwear <G>.  Night time, but
good visibility.  Scuttlebutt is that neither skipper thought they were "too
close."  Guess they were.

-- 
Dave Kruger
Astoria, OR
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