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). 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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/ ***************************************************************************
> From: rebyl_kayak_at_energysustained.com > 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. You didn't say if this is when the Pittarak is starting from a standing position or when it is moving forward. If when moving forward, try it again when you are paddling backwards. I suspect it may now work the opposite way. Also try this without any wind. Next try to separate the effects of the wind and the waves. Maybe use boat wakes for no-wind waves and an offsore wind that hasn't had enough fetch to build waves yet. You also didn't say if you are leaning the kayak during these turns (if moving forward). If so are you leaning them exactly the same? Or perhaps you are leaning inside with the reverse sweep and to the outside with the forward sweep? That will change things. I think you can quit worrying about your points of contact with the kayak and concentrate on the kayaks pivot point (which changes with speed due to the resulting water pressure differential on the hull from front to back) and where in the water the paddle blade is acting. You will also have to take into account how your weight shift (to put the paddle blade where it is acting) may be affecting your trim and what change that trim change will make to your center of rotation. Leaning forward will move the center of rotation of the hull forward and visa-versa. If you give me a more detailed description of the conditions when you experience this (and just how you make these turns) that addresses my confusion I'll try to figure out what might be happening. *************************************************************************** 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/ ***************************************************************************
Thanks Matt, Will run those variations and send you the results, I can say for sure is that I lean forward and edge away from the turn when using a sweep and edge into the turn when using a reverse sweep. I think I lean towards the stern for a reverse sweep and that the effect is similar whether turning on the spot or moving forward, usually I'm turning on the spot when going upwind, but its worth checking again. I'll be down the south coast this weekend with 25 to 30 knot winds and 3m swell forecast so will try to find a more sheltered spot otherwise might have to be next weekend to try your suggestions. 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/ ***************************************************************************
PeterO asked: >>>>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. <<<<<< And when I asked, Peter clarified the way he leans for me: >>>>>>....I can say for sure is that I lean forward and edge away from the turn when using a sweep and edge into the turn when using a reverse sweep. I think I lean towards the stern for a reverse sweep and that the effect is similar whether turning on the spot or moving forward, usually I'm turning on the spot when going upwind, but its worth checking again....<<<<<<< I'll bet, like most folks, when turning into the wind you are more comfortable leaning/bracing more into the wind doing a reverse sweep than leaning downwind to the outside doing a forward sweep/high brace. As I said earlier, the more you lean the kayak the quicker it will turn. I suspect turning into the wind you are willing to lean further to the inside (and into the wind) while low bracing on a reverse sweep than you are willing do to the outside (leaning downwind) doing a forward stroke. Turning downwind you are in the opposite situation where you get to lean into the wind with the outside lean and must lean downwind doing a reverse sweep. Since you naturally lean into a wind some to compensate for the wind strength (to maintain balance) you aren't so far off balance when leaning into the wind than when leaning at an equal angle downwind. With forward speed an outside lean with a forward stroke is most effective in turning a wider kayak (and most kayaks) into a wind because maintaining speed means you are being helped by the weathercocking tendency and for an equal lean angle the stern keel will shed water better when the kayak is tilted to the outside. You also gain the advantage of the side "lift" I talked about earlier helping drive the turn. Everybody should practice fast outside lean turns while maintaining good bracing. The reason is that when the wind is strong you might still be able to turn into it if you are willing to tilt the kayak away from the wind but might not be able turn into the wind at all if you are afraid to tilt the kayak that way. Practice this in strong winds when in a safe place. Being able to turn into a stronger wind can be a real safety advantage. At speed, most sea kayaks can turn nearly twice as fast with a good outside lean as they can with the kayak level. I'm not making this up. When testing kayaks I time turns both leaned and kept level, I do this comparison both when moving forward at cruising speed and when spinning the kayak in place (in place the difference is not as great but you can't normally turn a kayak into a strong wind as easily in place as you can when getting some speed up accross the wind first and just using forward sweep strokes). I have collected this data from most of the 1000+ kayaks I tested on some spreadsheets (that include dimension data on some 4000 one-person kayaks I know about--not including WW kayaks, surf skis, wave skis, or downriver and flatwater racing kayaks). In a strong wind with steep wind waves what happens is you make a turn into the wind in the trough and then lose ground (water? angle?) when the bow rises over the crest and is caught by the wind. The more angle you can gain in the trough the more you can lose at the crest and stillhave made some progress by the next trough. Turning speed is an important variable in this situation. The length of the bow from the paddler (lever arm) and the windage at the bow are some other variables that matter then as well. Weight in the bow (or the whole kayak) is a big help and the weight and strength of the paddler are also big helps when the wind is strong. *************************************************************************** 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/ ***************************************************************************
Some of the comments Matt made: >SNIP I suspect turning into the wind you are willing to lean further to the >inside (and into the wind) while low bracing on a reverse sweep than you are >willing do to the outside (leaning downwind) doing a forward stroke. Turning >downwind you are in the opposite situation where you get to lean into the wind >with the outside lean and must lean downwind doing a reverse sweep. Since you >naturally lean into a wind some to compensate for the wind strength (to >maintain balance) you aren't so far off balance when leaning into the wind >than when leaning at an equal angle downwind. >SNIP With forward speed an outside lean with a forward stroke is most effective in >turning a wider kayak (and most kayaks) into a wind because maintaining speed >means you are being helped by the weathercocking tendency and for an equal >lean angle the stern keel will shed water better when the kayak is tilted to >the outside. You also gain the advantage of the side "lift" I talked about >earlier helping drive the turn. >SNIP Everybody should practice fast outside lean turns while maintaining good >bracing. The reason is that when the wind is strong you might still be able to >turn into it if you are willing to tilt the kayak away from the wind but might >not be able turn into the wind at all if you are afraid to tilt the kayak that >way. Practice this in strong winds when in a safe place. Being able to turn >into a stronger wind can be a real safety advantage. At speed, most sea kayaks >can turn nearly twice as fast with a good outside lean as they can with the >kayak level. G'Day, Thanks Matt for the interesting post. Some of your points caught me by surprise! Your point about the tendency to lean into or against the wind sounds very likely though I have yet to check it. On Saturday the winds were fluky, about 20-25knots blowing off shore with short period wind waves and rebound. We had to stay near the cliffs so as not to be blown out to sea. So very little wind sheltering behind cliffs and about 18knots surface wind crossing bays near the beach and rounding points. I found it hard to experiment and had to trust to reflex reactions. I'll try your suggestions when it's about 15 knot surface wind with sub 1m waves, and/or long period swell. I tested as much as I could in a sheltered spot. You're right about the comfort factor comparing forward and reverse sweeps although it worked in reverse for me and I was more hesitant with my port reverse sweep and very comfortable with the forward sweeps. This is due to paddling with people who are stronger and in faster, ruddered boats when I can't afford the drag associated with reverse sweeps and practise forward sweeps often. Also I try to focus on edging rather than leaning to get the hull further over (and after years of being trashed in the surf). When the Pittarak has very little load apart from my 70kg weight it goes beyond its stable point with the cockpit rim edged to the water line and the deck at about 45 degrees to the water surface. When using a forward sweep I edge the cockpit rim about 1 cm below the water with some lift in the sweep stroke. With a reverse sweep I edge the cockpit rim about 1cm above the water line, i.e. a bit more hesitant! Larry Gray can take the Pittarak almost to 90 degrees and seems to turn without any trouble at all in conditions I'd find very challenging. He must be using a forward sweep with a lot of lift, is that called a sculling sweep? I usually follow a forward sweep with a slightly reduced edge, skimming the blade back across the water in a low brace position trying not to touch the water but prepared to brace if necessary. Likewise I return the reverse sweep by skimming the blade forward above the water in a high brace position and with a slightly reduced edge. In light winds and sea the Pittarak will turn on edge without any need for a sweep stroke as per your weather cocking comment! All the sweep does is make the turn faster. Taking advantage of this makes the boat a bit more controllable in a following sea despite the weather cocking tendency, otherwise in bigger following seas resorting to the skeg helps. Regarding your prediction on turning while moving. Its certainly less work and a bit more stable than turning on the spot, as though the forward movement is translating into a turning movement and contributing lift to the forward sweep/scull stroke. So your comments about "weather cocking tendency" and "side lift" seem to be right on the button. The comments on safety were also useful, intrinsically and because sometimes I've found paddlers not used to wind or following sea to be intimidated when turning back. I'll remember your point when helping out, it will be a trade against leaving them exposed for longer periods to beam seas and ease of turning in a wide arc compared to on the spot. Thanks again - I'll provide some more pertinent feedback when I've tried your suggestions and observations in wind. 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/ ***************************************************************************
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