Re: [Paddlewise] Rocker: too many sea kayaks

>I am perplexed by the variety of sea kayaks and the use of rocker.

Does get confusing especially since the sectional shape also influences how
a boat turns (or doesn't turn). For example, "U" shaped sections can have
less resistance to lateral motion than "V'd" sections. Attributing how a
boat handles just to rocker probably simplifies things too much. A boat with
"U" sections aft and a straight keel could have less controllability than a
boat with some rocker and "V'd" sections aft.


>you decide on total load and your level of skill (stability), it seems
>to me that the intended use dictates the rest of the design.  Yet every
>manufacturer seems to offer several boats with variation in rocker being
>one of the principal differences.  What am I missing?

People differ, paddling objectives differ, conditions differ and designers
differ so I suppose it only reasonable that boats differ.

>Assumptions:
>
>(1) The boat should be usable in rough conditions without its rudder.

So long as you have an ARPEE it won't matter much. :-)
I guess it would help to know what "useable" means and at what point "rough"
becomes "who cares how well it handles I just hope it blows over soon". :-)

>(2) Leaning the boat a little causes it to turn away from the lean
>because the low side is longer on the water than the high side, causing
>lift aft.  Thus lean can be used to steer in open water.

Not always or for all boats. Once a turn gets initiated lean can accelerate
the turn in either direction. I don't think that "lift" occurs aft (At least
not on the hull. Different matter with a rudder) . I think heel just reduces
the lateral resistance aft. If lift did get generated it does not appear
that it would act aft of the center of gravity given the forward motion of
the boat. Some boats also trim down by the bow when heeled and that reduces
lateral area aft. Some trim down by the stern and that increases lateral
area aft. ETC.

I have met a fair number of paddlers who don't want to lean their boat to
control it. In fact, some prefer that the boat have insensitivity to heel so
that a bit of a mistake in balance won't cause an undesired turn. All part
of the "different strokes for different folks".

(SNIP)


>
>(4) Increasing base rocker makes the boat more maneuverable with the
>paddle, but decreases tracking ability.

Perhaps, with all other things equal and assuming no other shape effects.

One could offset the effect of rocker by decreasing beam, decreasing the
block coefficient (ratio of the boat's volume to the volume of a cube having
the same length, beam, and draft), deploying the skeg or using a fixed skeg
or deploying the rudder. Adding rocker when one has a rudder increases the
rudder's effectiveness in turning the boat. In experiments with sprint boats
we found that a rudder improved performance over boats with straight keel
lines aft. I believe the primary advantage had to do with the reduced wetted
surface due to rocker offsetting the area of the rudder plus the rudder
seemed to give the paddlers better control for wake riding etc. without
upsetting their stroke cadence. Not everyone involved with the project
agreed with me, however.


>Based on the foregoing, it seems to me that a sea kayak should have
>little if any base rocker; otherwise it will be difficult to paddle in
>windy conditions without a rudder.  It should also be designed so that
>at high lean, it has abundant rocker and a shorter waterline.  This will
>permit maneuvering in tight quarters.  The rudder is only used to help a
>bit in windy conditions and when surfing.

Not sure how much "abundant" comes to nor am I convinced that rocker means
poor handling in windy conditions. Not even sure that one needs to design
around the absence of a rudder. I don't feel that all the evidence has had
proper evaluation yet. So many variables in this.

>If the foregoing is true, why would anyone choose say a Seawards Vision
>over a Seawards Navigator for sea kayaking?


Ask the people who bought them.

>Similarly, why do many people like the Looksha IV?
>Without its rudder, it is hard to paddle in strong wind.

They may have their reasons.

I used to believe that no boat should need a rudder but the more I paddle
and the  more people I meet the more it becomes apparent to me that "no boat
I paddle should need a rudder". I consider that difference significant.

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


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