Re: [Paddlewise] Paddler's weight

Dave wrote;

(SNIP)

>One of the first
> things I look at is the weight of paddler the kayak was designed for. I
> know about buoyancy and displacement and why a boat floats but don't
> understand some kayak capacity specs. Take the Northwest Synergy RM or
> others like it...a 16'+ boat with a load capacity of 340lbs but their
specs
> say it's ideal for a 120-200lb paddler. I would assume a good kayak would
> have the paddler sitting at or very near it's CG. So except for all the
> effects that any weight would have on it...like riding lower in the water
> and maybe handling sluggishly...why would the boat care if the weight is
> the paddler or 100lbs extra pounds of gear?...assuming it's distributed
> evenly. Should I not even consider a boat like this and look only at those
> rated to carry a 240lb paddler? There are not many that do...I don't want
> to paddle a 20' double by myself.

I won't discuss that part of boat buying that involves fitting the boat to
your frame. It should be fairly obvious that you should look elsewhere if
you can't get in the boat.

First a definition.

Displacement = the total weight of the boat, gear and paddler.

In a better world every boat would be designed to a specific displacement
range and would perform best at that displacement range. My research shows
that some paddlers can detect differences in performance within a range of
+10 / - 20% of the designed displacement (nice round numbers for simplicity
so do not claim that I said they were absolute. Testing human perceptions is
not science it is statistics). In other words, overload a boat by 10% and
you might begin to notice that it performs more sluggishly (turns slowly and
feels "slow", etc.) . Under load by 20% and you might notice that it handles
differently (blows around too much. has marginal stability etc. )

Why?

Because the form factors (Prismatic coefficient, block coefficient,
waterline length, waterline beam, etc. etc.)  that affect performance change
with increased or decreased immersion. I have not yet seen a sea kayak that
had consistent form factors and immersed dimensions regardless of load.

Unfortunately we do not have a better world. Some designers do not design to
a specific displacement. Most salespeople (and a lot of builders) don't even
understand the concept.

Dave recognizes an important the point that the boat doesn't care if the
weight comes in the form of gear or paddler. Displacement includes
everything. A fifty pound boat with a 200 pound paddler has the same
displacement as  a 200 pound boat with a fifty pound paddler. Remember the
old trick question from school, "Which is heavier, fifty pounds of lead or
fifty pounds of feathers?"
It really doesn't matter how you distribute it from a resistance standpoint
(it does matter from a stability and pitching moment standpoint).

 In the absence of designer information how do you select a boat that has
suitable displacement for you? Given a normal configuration (no extreme or
unusual hollows or bumps in the hull) most recreational kayaks will perform
reasonably well if their fatness ratio falls between 0.95 and 1.6.

The fatness ratio = Volume/ (0.1 * Waterline Length) ^3

This does not mean that  a boat instantly becomes a water pig above or below
the figures given, it just means some people might begin to notice a
difference. Noticing the varies between people. Some psychokineticists (easy
for you to say :-)) claim a "just noticeable difference"  is +/- 10% of
effort
50% of the time. For the typical recreational paddler that seems fair to me.
I know that some paddlers claim the ability to detect very subtle
differences in performance (usually much smaller and with greater accuracy
than a test tank) but I remain unconvinced.

Cheers,

John Winters
Redwing Designs
Web site address, http://home.ican.net/~735769





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