G'Day Matt and Nick and many thanks for the suggested explanations. 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 Nick responded: >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. Nick, Your explanation sounds plausible and I hadn't thought of it. I wasn't looking at the rear deck but as you say it is very low, sufficient that I'm able to lie right back on it, while I don't even come close to doing the same with the Pittarak. I'll get someone to photo what's happening with the deck the next time I'm in a Tahe Greenlander. Matt responded: >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 tendency to weathercock when paddling forward in >a side wind 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 centre of lateral resistance (due to the water) to the >rear and reduce the weatherhelm the wind/water balance was causing in the Pittarak. Matt there wasn't much wind but I noted your reply was before you saw the length of the Tahe was 5.45m (I got it right in the first email and forgot the decimal places in the second). Also as you say the Tahe is a narrower boat, with significant areas of vertical hull surface, tracks well and turns relatively easily but does have a well defined cessation of turning as soon as the paddle stops moving. With the Pittarak, dropping the skeg does reduces its tendency to keep turning after the sweep has stopped and makes the boat somewhat harder to turn. Likewise as you say side winds have a similar effect. Your subsequent analysis sounded consistent with what I'd observed, particularly that the Tahe does sit deeper in the water, although the chined hull changes to a rounded hull towards both the bow and stern. You're also right that the Pittarak has a great deal of buoyancy near the cockpit, and it reduces quite rapidly and symmetrically towards both the bow and stern. So on its side and when unloaded the rocker is considerable and this would not be true for the Tahe. Next time I'm in a Tahe I'll try and get some photos of it edged to see what exactly is happening at the bow, the stern and the deck. BTW the Pittarak's 'side' rocker when edged might account for its ability to respond relatively easily to the brace position to move from a broach back into a break out or surfing stance. Matt continued >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... When I read this it made me want to jump on the next plane and come over and try a Mariner! Something I'm still struggling to understand is why the Pittarak turns more easily upwind with a reverse sweep and more easily downwind with a forward sweep. The centre of rotation (if that's the right term) must be quite different between the forward and reverse sweep. I've started trying to develop an equation of motion treating the kayak as a single line with points of contact at butt, knees and feet. One problem is how to make enough realistic assumptions to reduce the variables. In particular how to partition the forces between each point of contact and also to assign realistic ranges of value to the relative angular velocities of the boat and paddle with respect to the water. A friend commented I wouldn't get an explanation with this approach as it was more likely to be related to wind wave action but I find that just as hard to understand. Nick and Matt. Thanks again and all the best, PeterO *************************************************************************** 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/ ***************************************************************************Received on Thu Jun 10 2010 - 06:07:48 PDT
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