RE: [Paddlewise] Leaning in Surf vs Rivers

On Aug 27, 2009, at 11:48 AM, Craig Jungers wrote:

> This is what's confusing for me. The dynamics of a standing wave
> seem far
> simpler (to me) than ocean surf and I'm having a hard time picturing
> all the
> motions.

 Nick responded:
>>>>........Typically we use the river/ocean bottom as our frame of reference,
but
to the water both surf and river waves are traveling through the water.

Instead of thinking of approaching a wave in a river, think of the
wave approaching you and you will see you deal with exactly as you
would in surf.<<<<<<
Thanks Nick, you just saved me a long explanation. I'm glad I read all the
digests (as mixed up and out of order they remain) before jumping in here
again.

Relative to your boat, a standing wave is identical to a moving wave. In one
the water is moving through the wave and in the other the wave is moving
through the water but given they are the same size and the same kind of water
density and viscosity about the only difference you might detect is the wind
in your face (and a fan could probably remove that clue from the test in a
double blind study.) I might add that they act the same even when the waves
are both breaking. So for controlling the kayak when surfing either kind of
wave it makes no difference at that point in time when everything is the same.
The beach break wave does change as it approaches shore and a boat wake will
change along with your position in relation to the wave's source (or changing
the speed of the source). Standing waves can change too but the surges in a
turbulent stream are less predictable.

I think on closer examination Duane will find it isn't usually the chine in
the fore body that is helping him turn on a wave using a tilt to the outside
of the turn. When surfing a sea kayak the bow is usually not even in the water
at the point you can easily turn it. It is the stern chine and/or the shape of
the stern which is most important. The combination of the hull shape (and
mostly the chines, tubes, keels, V-bottom, skegs, fins, or rudder of the
kayak--and their location) with the viscous fluid and the side (skidding)
motion of the kayak down the face of the wave (powered by gravity) that what
will determine how the boat handles. The placement of the kayak on, or in, the
wave and where it is located on the wave (and the wave size) will also be
factors. The "trim" of the symmetrical object at the time will also be a
factor. The tendency of any long symmetrical object in waves is to rotate
until it is sideways to the wave direction (even if it is perfectly round in
all ways except for length). Even just floating free, the end of the
symmetrical object the wave contacts first gets moved more by the wave that
the other end. There may be more to it, but the combination of gravity and the
orbital motion of the water molecules in a wave (forward at the crest and
backwards in the trough) is enough to cause this.

Once the object starts skidding on the water (surfing) a lot of variations in
the shape of the object (that effect how easily different parts skid--fins,
chines keels etc.) will have a big effect on what the long object does. Since
the tendency is to broach and since one has to fight a tendency constantly the
more you can make the kayak neutral going in the direction you want it to go
the more control of the kayak will shift to the paddler and away from the
tendencies and the environment. If your boat broaches you wouldnt want to put
a fin or rudder up near the bow (even turned completely in the direction of
sideways motion it is still adding drag to the bow end that will increase the
broaching). So think about what you could do to a kayak's shape to make it
more neutral when it is facing the direction (to the wave motion) you want to
have it move. Now you are doing the kind of thinking I was doing when I was
designing a sea kayak.

A perfectly round hull will not be easier to turn if you lean it. A lean helps
partly because you are pushing the wider more curved part of the hull into the
water which raises the ends of the kayak more out of the water. The ends of
the kayak are also more rockered on the sides than at the keels when in that
tilted orientation. Leaning either way will help when the kayaks waterline
width is wider than twice its draft (is not round). Even Olympic Flatwater
kayaks are not round but are about 3 to 1 (WL width to draft). However, just
because you can lift the ends equally leaning to either direction that doesn't
mean they will work the same (or be equally effective) in use. They might be
in a perfectly symmetrical kayak that is sitting in one place but an
asymmetrical kayak that is moving one way or the other is usually what we are
trying to deal with.

To further complicate things, the paddler wants to accomplish some goals and
uses strokes with a paddle to do that. One of the main things needing to be
accomplished is to keep the kayak from capsizing while we are tilting it. Most
likely we also want to keep the kayak moving at speed rather than putting on
the brakes (although when you want to put on the brakes and turn at the same
time the inside lean and a constant brace for capsize prevention are often
employed). A forward stroke and a high brace are easily combined. A single
reverse stroke and a low brace are easily combined. A braking high brace (or
sculling brace) can work especially if you have some momentum to play with but
you wont be able to use any forward stroke effectively enough at that time to
maintain your momentum. Therefore, if you want to keep going forward and turn
your kayak quicker you will lean it to the outside of the turn.

With most kayaks there are several other advantages to leaning to the outside
of the turn as well. When water is flowing past the hull and you make one side
more curved than the other side (by leaning it) you create a wing shape in the
water (the "lift generated by this moving wing will be to one side). Add to
that, the fact that the stern of a forward moving kayak is a lot freer to move
sideways (than the bow) because there is less water pressure at the stern.
When moving forward, water is being pushed to the side by the bow half of the
kayak. It has momentum to the side (and up) and is having to reverse direction
again to fill in the hole the boat left as its stern withdraws. Therefore,
there is a lot more water pressure over the area of bow (which is doing the
pushing aside) than over the rest of the kayak. So far what I'm saying all
works even on well rounded symmetrical hulls (as longer as they are more
curved on the sides than at the bottom). For example, take an old fashioned
rounded slalom type kayak. Anyone who has paddled such a kayak knows how
difficult it is to keep it going straight at first. What I wrote above is the
reason for the river kayaks tendency for the stern to skid out to one side or
the other. Complicate the shape more (as is the case with most sea kayaks) and
you can counteract or enhance these tendencies depending on how you modify the
shape. One way to get the kayak to go straighter might be to add a fin, drop
skeg or more keel to the "loose" stern end to prevent it from going sideways
so easily. Once you have done that you will find that it is a whole lot easier
to turn the kayak by leaning it to the outside so that water sheds off the fin
or keel easier in the stern half (rather than to turn the fin into more of a
hook snagging the water by tilting the kayak to the inside of the turn). You
have to lean the kayak far more to the inside to get the fin or keel enough
out of the water not to snag than you have to lean to the outside. Your brace
will be even more important then and even it you use a high brace you will be
constantly braking with it rather than doing forward paddling.

Now, if you are a budding kayak designer, think about how you might be able to
change a kayaks shape to enhance the performance in the desired direction
without hurting it too much in other respects. In boat design everything is a
compromise, but your job as a boat designer is to learn to be good at
negotiating with Mother Nature. You want to be a lawyer looking for loopholes
in the laws of physics you can squeeze your kayak through.
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