Re: [Paddlewise] Ballast: do you need it?

At 03:19 PM 8/19/2002 -0400, Keith Rodgers wrote:
>Having got that of my chest, my ballast consists of a flat-sided plastic
>gallon container (used to hold anti-freeze) positioned on the keelson in the
>day-hatch and held firmly in place with two straps rigged with "D" rings and
>Velcro.  because of the leverage due to its on-the-keel placement, will 
>provide
>a useful ballasting effect.
>Keith Rodgers


Thanks all for the very informative and helpful posts (and offline 
messages) from which I extract the following not entirely orthogonal 
principles:

First, the waterline (how low the boat rides in the water) is mostly a 
function of weight.  Adding 25 lbs of ballast won't make much difference 
here, but loading the boat with 200+ pounds of camping gear will have a big 
effect.

Second, front-to-back trim is a function of the placement of weight (and 
the paddler's fore/aft seat position) relative to the CG as much as it is 
the amount of weight. Depending on its placement, 25 pounds of ballast can 
have a big effect on trim, but the general sense of the posts is that 25lbs 
of ballast anywhere around the seat (day hatch to immediately in front of 
seat) should be OK. Time to test this hypothesis in my boat with a level!

Third, side-to-side trim will vary with the placement of weight relative to 
the keelson.  Off center will make the boat unstable.  Placed low and 
on-center will improve stability by influencing the righting moment, e.g. 
by tending to pull the boat back over the keel line.  This is true for 25 
lbs of weight or 100 pounds of weight, depending more on centering the 
weight on the midline as low as possible, than on the absolute amount of 
weight, although there must be some interaction with how low the boat sits 
in the water here as well.

So, taking all this into consideration, I'm going to try 25-30 pounds of 
lead shot in a dry bag just behind the seat. I'm going to tie it down with 
strips of velco attached to the dry bag and the peri-keel line area and 
also by velcro straps laid across the top. Because the dry bag will be 
partly wedged under the seat, this should be secure enough, but I'll 
probably go the D-ring route, which will be simpler to use, once I get it 
the parameters fixed.

Note hardened lead shot (steel coated, so it won't oxidize; essential in 
the SE) is approximately 20 dollars/25 pounds here in North Carolina.  I 
found mine at a gun shop--good laugh from the good old boys when they found 
out why I wanted it.  Seemed the easiest and most compact route to go, 
especially given the low, keel line centered imperative in point three above.

Now for the hard part, trying to escape work during the day so I can test 
all this stuff out.

Pragmatically,

John







*********************************************************
John S. March, MD, MPH
Professor of Psychiatry and Behavioral Sciences
Department of Psychiatry and Behavioral Sciences
Duke Child and Family Study Center
718 Rutherford Street
Durham, NC 27705
919/416-2404 (P); 919/416-2420 (F)
Email: jsmarch_at_acpub.duke.edu
Website: http://www2.mc.duke.edu/pcaad


"I maintain there is much more wonder in science than in pseudoscience. And 
in addition, to whatever measure this term has any meaning, science has the 
additional virtue, and it is not an inconsiderable one, of being true."
--Carl Sagan

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