Re: [Paddlewise] Comparing sweeps chine and round hull - 'inertial' vs 'damped' responses

 PeterO wrote:
>
> On Saturday I was comparing two sea kayaks: a 5m round hulled Pittarak
> (similar to an Icefloe - http://www.pittarak.com.au/pittaraksingle.html)
vs.
> a 5m Tahe Greenland style kayak with a chined hull that extended either
side
> of the midsection along about half the length of the boat
> http://www.tahemarine.com/?module=Product&id=49. Neither boat has much
> rocker, they both rely on edging to make a sharp sweep turn. The links are
> given to show what they look like, I don't have any commercial interest in
> either.
>
> There were two differences in turning performance that puzzled me:
>
> 1. In conditions of low wind the Pittarak continues to turn for quite a
> while after a single sweep, provided I hold the edge, whereas the Tahe
> responded very precisely to the forward sweep stroke, only turning as the
> paddle moved almost as if it were damped, perhaps by turbulence or some
> other hydraulic resistance at the chined section of the hull?
>
> 2. The Tahe has a much shallower deck (and less knee room) with lower wind
> resistance so its ability to turn using a forward sweep on one side or the
> other was symmetrical i.e.. independent of orientation to the wind and
there
> was no need to consider reverse sweeps. On the other hand the fastest sweep
> (forward or reverse) to turn a Pittarak depends on its orientation to the
> wind and the direction in which the boat is being turned. I don't really
> understand why this should be but am told it's commonplace with boats that
> have high deck wind resistance.
>
> I'm not suggesting that one hull is 'better' than the other, but I'm
curious
> to understand the reasons for these different characteristics, their pro's
> and cons, and whether the first inertial vs. damped characteristic, can be
> generalised to most rounded hull vs. chined kayaks.


Nick responded:


>>>>A couple difference I would note on the two boats. Pittarak has a high
back deck and Tahe Greenland has a low back deck. When leaned to turn the high
deck will tend to lift the boat slightly and the low deck will allow the boat
to sink down. I notice with many Greenland style boats that the low back deck
will become awash with a relatively small amount of lean. When turning this
allows water to pile up on the back deck which will tend to slow down any
turn.<<<<<<<<<





Peter,

With just the title to go on earlier, I was expecting a question about how the
chines effected the relative tipping motions of the kayaks but that wasn't the
case. I agree with Nick that a back deck that gets water on it when leaned can
snag and stop the turn (and even cause a capsize, as happened to me the only
time I capsized a kayak during my lean to turn testing of over 1000 kayaks)
but I don't think that is what is happening here. I also don't think it is due
to the greater overall depth of the Pittarak kayak.  I think the difference
you noticed has nothing much to do with the difference in the chines but
rather the differences in the wind/water couple of the kayaks. The Pittarak
appears to have a tendence to weathercock when paddling forward in a sidewind
with no skeg (and the Tahe apparently doesn't). I'll bet if you drop the skeg
on the Pittarak you will find that it handles a lot more like the Tahe because
that will move the center of lateral resistance (due to the water) to the rear
and reduce the weatherhelm the wind/water balance was causing in the Pittarak.



The Pittarack would keep turning once the turn started (if not in a side wind)
because it was less directionally stable than the Tahe. The Tahe could be
stiffer tracking because it sits deeper in the water, is trimmed lower at the
stern, has more stern keel, has more vertical surfaces on the stern, is more
fishformed, or is narrower than the Pittarak (so doesn't have as much curve in
the stern quarter causing "lift" to the side at the stern to drive the
turn--this is also why a fish-form hull is more stable directionally--if all
other things are equal). It appears to me that the Tahe is achieving its
neutral balance when paddling forward in a sidewind by somehow increasing the
tracking stiffness. That tracking stiffness increase prevents the lean driven
turn from continuing once the turning force (from the paddle) is removed.



I wrote the above before looking at the links you provided. Now that I said
the above I looked up the relative dimensions of the two kayaks. It appears
rather than both being 5 meters as Peter wrote the Tahe Greenland is over a
foot longer (30.5cm) than the Pittarak and is about 4" (10cm) narrower as
well. From Sea Kayaker magazine's review of the Tahe Greenland I see it is
also slightly fish-form. I don't know anything about the center of buoyancy of
the Pittarak but given how wide it is I'll bet it has a lot more side curve in
the stern quarter that gives it more side "lift" when leaned to help keep it
turning on its own once a leaned turn has been started. The hard chine on the
Tahe also likely contributes to a more vertical side in the stern half that
helps prevent the stern from moving sideways as much. Being narrower the Tahe
probably sits a lot lower in the water and therefore has more keel in the
water at the stern. Also being narrower the keel won't be lifted nearly as
much from the water when the kayak is leaned as the with the wider Pittarak.
With more curve at the sides than the bottom, when you push the side down to
lean the kayak is lifted and has more rocker so it turns easier. The Pittarak
would probably do this lean turn with more precision were it hard chined in
the stern quarter to provide a curved keel when leaned that acts sort of like
a rudder. The more you lean it the tighter the turn should be with the
Pittarak.



I personally like a kayak that continues to turn when you lean it and I hate
fighting a constant tendency to weatherhelm (and also hate using rudders--and
to a lesser extent adjustable skegs to control weatherhelm). Getting both in a
kayak without a rudder or an adjustable skeg was a major design criteria for
us so your question was right up my alley, so to speak.
***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************