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On 4/13/11 6:42 PM, Apache Runner wrote:

 > In  historical accounts, of varying accuracy, I've read about one
 > technique called 'pacing', where the mariner drops some garbage in at
 > the hull and walks toward the stern, keeping abreast of the garbage.

I definitely use the technique when racing dinghies, especially during
"floaters."  I also enjoy it when cruising on low wind/wave days to
check out the boat impeller speed readout and GPS speeds--and kill time.

It is simply the Speed = Distance/Time equation using seconds and feet
and reading out in knots. Once you know the length of your craft in
feet, you have a constant that can be used with the time and calculate
speed in knots.

The formula is: Speed (knots) = (boat length in feet x 0.6) divided by
time to pass an object in seconds.

Boat length in feet x 0.6 is your boat's constant.

For example my Hobie 16 measures 16' 8". 16.67 x .6 = 9.99. 10 is easy
to remember so I use that as my constant. So speed = 10/time in seconds
to pass the object.

10 seconds to pass = 1 knot. 1 second to pass = 10 knots.

Having read your posts, you seem quite bright and probably have the
concept well in hand by now, and can switch it to meters if desired.

For my fellow liberal arts majors who can read the "20 Items or Less"
sign at the market Express Checkout Lane but cannot count:
--Distance in feet/time in seconds = feet per second
--Feet per second x 3600 seconds per hour = feet per hour
--Feet per hour / 6076 feet per nautical mile = Speed in knots

How is the 0.6 factor derived?

Short version:
Feet per hour/6076 ft per nm = distance in feet / (Time in seconds x
3600...3600/6076 = 0.5925

Actually 0.5925 is closer than 0.6--but on the water?

To work in meters use measured distance in meters over time in seconds
and convert feet per nm to meters per nautical mile. Perhaps 3600
seconds per hour / 1852 meters per nm = 1.94 as a boat length multiplier?

Hope that helps.  I'm back to trying to figure out whether a case of
beer counts as 1 or 24 items on the express lane ;-)

Bill B


> Here's a question about the historical practice of reckoning a boat's
> speed on the water.   The use of log-lines is pretty much a given.
>
> In  historical accounts, of varying accuracy, I've read about one
> technique called 'pacing', where the mariner drops some garbage in at
> the hull and walks toward the stern, keeping abreast of the garbage.
> I've also read an assertion by Samuel Eliot Morrison that Magellan's
> captain has a  quote that corresponds to finding boat speed by counting
> the time it takes a piece of flotsam to move along the hull.
>
> I've also heard that it's more of an intuitive feel.
>
> I'm a sea kayaker and not a sailor, so I estimate the speed from my
> paddling and the wind conditions - I'd probably vote for the more
> intuitive school if you were to ask me, but I'm curious whether anyone
> in the past practiced some technique using the timing of flotsam passing by.
>
>

   

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