Re: [Paddlewise] skeg ups

Dan wrote;

>This may or may not be important for
> sea kayaks. In principle it seems as though it may be important, but
> when paddling with others I have not noticed any obvious changes in
> relative speed among boats with various designs (some with skegs) when
> encountering conditions that produce leeway. I suspect that this is the
> sort of thing that could only be detected with very carefully controlled
> experiments, which is another way of stating that the effects are not
> large. But maybe I would be surprised.

This problem of "noticeability" pops up a lot. I have found people rather
poor at noticing changes in effort etc. Many, many kilometers of paddling
hooked up to a B&G Speed Boss just depress me about my ability to  detect
differences in effort etc. According to some fairly extensive studies on
perceived effort done by G.T. Fechner the "Just Noticeable Difference" is
one that a person can notice 50% of the time and tends to run about +/- 10%.
Not very good if true. Many paddlers believe they can sense smaller
differences. I will gladly pass on a rather sneaky test I have used  to test
paddler abilities to sense differences in effort if anyone has an interest.

In any case, I suppose one could calculate the lift etc. of rudders and
skegs (maybe I will do this on  cold day this winter) since the lift data
comes prepackaged in Abbott and Doenhoff "The Theory of Wing Sections".
calculating the lift of hulls presents a bigger problem since they have
aspect ratios so low as to slither off the scale. In short, they probably
don't produce much lift but they can produce measurable lateral drag
forces.  This would lend itself to easy investigation for anyone interested
but I think one can do it mathematically with a bit of thought by adapting
existing drag formulas. In fact, I think I can do it with my boat drag
formula now that I have an improved low speed transom method. HMMMM.


>
> One major difference between skegs and rudders that I have not seen
> mentioned (in this recent discussion) is the tendency for rudders to
> lift free in steep seas. I suppose that this is also something that can
> be attributed to poor design, since it is a function of rudder
> placement.

Not sure this constitute poor design but designers and builders certainly
need to address it. Since the water in following waves travels roughly in
the same direction as the boat the rudder has less effect on direction
anyway. Of course, this applies to any part of the hull and it would seem a
mistake to rely upon the rudder as the sole source of directional control in
large following waves. It may actually occur that the water in the wave
crest moves from stern to bow thus reversing the effect of the rudder.



>This can be avoided by mounting the rudder under the hull,
> and yes (in anticipation of the standard reaction) it *is* possible to
> mount a *retractable* rudder under the hull. I recently saw one such
> design that fully retracts. In addition to avoiding "lift out" and
> surface piercing effects, I was told by the designer that there is
> another advantage to this type of rudder. He claims that a foil-shaped
> rudder can create lift more efficiently if it is in line with the boat's
> keel. The standard stern-mounted rudder does not pivot about its center,
> but instead swings out to the side. Does this really make a difference?

I cannot imagine why.

Woody asked about "lift" and skegs and I think that got answered rather
nicely. I would add that every time the stern wiggles due to a stroke the
skeg develops lift or drag to offset the turning moment.

Alex mentioned some variations on rudders. North Americans rarely see these
variations and tend to think that the flat plate type typifies all rudders
and draw their conclusions based on this assumption. Rather a shame because
a "good" rudder  really does make a huge difference in controllability.

Dave asked about the best airfoil and , yes, we do have lots of information
on this. I see the NACA 0012 section most often on sailboats. I have a bit
of partiality to the NACA 2006 as providing a better lift to drag ratio but
I know some who would argue for laminar flow blades. The section one chooses
depends upon objectives.  For aggressive steering the 0012 delays stall best
while for subtle steering techniques a laminar flow section (NACA 16-006)
might work better. The laminar flow section should work better on skegs.

Dan also wrote;

>A suggestion: Pour yourself a glass of Merlot, put your feet up, and
>think about this a bit...

A good suggestion even if you don't feel like thinking. :-)



Cheers,

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





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