I'm trying to understand why rocker makes a boat turn easier. My current idea is that if a boat drafts 6 inches all the way along, a sweep stroke or other force such as a wave has 6 inches to work against at the end of the boat. However, if there is rocker, the power of the sweep stroke is pushing against only a couple of inches and it takes less force to move the bow on a pivot point back near the paddler. Is this correct? Also, when a kayak is leaned with the right knee, does the boat turn because the water on the right side has a longer path and thus the left side is able to move quicker through the water so that the boat ends up turning to the right? Scot Buy the perfect holiday gifts at Yahoo! Shopping. http://shopping.yahoo.com *************************************************************************** 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/ ***************************************************************************
Scott wrote: >However, if there is rocker, the power of the sweep >stroke is pushing against only a couple of inches and >it takes less force to move the bow on a pivot point >back near the paddler. Is this correct? G'Day Scott, Sounds as if your explanation of rocker is pretty much correct. I didn't quite follow the carving (edging) explanation. Looking at the hull of my boat, the bottom is long, straight, flat and pointed at each end. The sides are highly curved so the shape of the boat makes it look a vertical sheet near the bow and stern. When the boat is upright its whole length is in contact with the water and the bow and stern are acting like a fixed rudder. Thats one factor making the boat go straight when its upright. Also imagine the boat broken up into sections. Each section contributes to the effort needed to turn the boat, by its depth in the water multiplied by its distance from somewhere near the paddler. This sounds like your explanation also. But! When you lean my boat on its side it is no longer floating on a flat surface. The boat is now supported mostly near the middle rather than the bow and stern ie the bow and stern are somewhat lifted out of the water and don't contribute so much to drag at each end when trying to turn. Also the rudder effect of bow and stern goes away because they act like a sheet lying flat on the water. I think this is also why it takes less effort to turn and steer boats in choppy water, because it becomes possible to take advantage of turning strokes when the stern and bow are out of the water. Of course not all kayaks are designed like this, horses for courses! Mine is optimised for paddling at sea without a rudder. 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/ ***************************************************************************
Thanks PeterO, My reasoning on why a boat turns better on edge is that if the boat is 24 inches wide and you put it on edge, it could end up with an effective 12 inch rocker plus an asymetrical shape with a longer water path around what used to be the bottom of the boat. I have a hard time thinking through why if I lift my right knee, my touring boat turns to the outside of the lift (right) but my shorter ww boat usually turns to the inside (left). If I just build up a little speed and coast straight then lift a knee on calm water, I no longer have a steady state and my boats always want to turn. I don't think I know how to keep it straight without correcting paddle strokes. Am I on the right track? Scot Buy the perfect holiday gifts at Yahoo! Shopping. http://shopping.yahoo.com *************************************************************************** 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/ ***************************************************************************
Carving a kayak works just like a ski does, you are following the arc that is defined by the turn of the bilge. The more rocker, the more extreme (shorter radius) the arc is and quicker the turn. The sharper the chine at the bilge the more defined the arc is and easier the turn will be. John Blackburn *************************************************************************** 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/ ***************************************************************************
At 3:03 PM -0800 12/2/01, Scot Hume wrote: >I'm trying to understand why rocker makes a boat turn >easier. My current idea is that if a boat drafts 6 >inches all the way along, a sweep stroke or other >force such as a wave has 6 inches to work against at >the end of the boat. However, if there is rocker, the >power of the sweep stroke is pushing against only a >couple of inches and it takes less force to move the >bow on a pivot point back near the paddler. Is this >correct? > >Also, when a kayak is leaned with the right knee, does >the boat turn because the water on the right side has >a longer path and thus the left side is able to move >quicker through the water so that the boat ends up >turning to the right? > Anything sticking into the water is going to resist moving through it. When you try to turn the boat you need to overcome the resistance of the water to allow you to turn. Because the ends are farther away from the center of rotation they create a larger moment arm to create more resistance. Rocker reduces the area at the ends of the boat, thus decreasing the resistance. Leaning the boat can have many different effects depending on the design of the boat, but one effect is almost universal regardless of the kayak. Because kayaks get narrow towards the ends, when you lean the boat you give it more rocker and more rocker makes it easier to turn (see above). Nick -- Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 (860) 659-8847 *************************************************************************** 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/ ***************************************************************************
Explanations for the carving given here are OK, but they donīt give a full account of why when you lean, the boat turns. If lower resistance to the water would be the answer then the shorter side would move faster and the larger curvature side would move slower because it is longer and deeper. That would mean turning in the opposite direction to the one observed. I have wondered about this subject also, and have come to a conclusion, that may be added to other reasons. In order to visualize the effect better I put my kayak on the floor tilted, and looked at it from front and back. Placing the kayak sideways one can see that in the inner turn side the stern is tilted from vertical but also moves sideways from the center axis and is lifted. The outer side water sees the stern as moving away and more water has to come in to fill the gap as the boat is moving. The inner side stern deflects the water coming on its side. In other words the outer stern is pulled due to the void of water generated as the displacing big mass on that side moves forward. On the inside the water coming straight along the boat is bounced by the "tilted with respect to vertical" stern. This creates a pushing force on the in side and a pulling force on the out side. The small difference, among other things account for the boat turning. In similar fashion, once the boat starts to turn, just a little bit, then incoming water faces the outside bow, pushing it, (as if it wanted to bow to broach), but the inside bow as it moves forward and slightly turning, leaves a void that must be filled with water. Therefore there is a pulling force on the inside bow and a pushing force on the outside bow. These two small forces together make the boat turn. The effect is there in the stern all the time, but in the bow it takes a little bit of turning before the effect occurs. That is why sometimes leaning is not enough to get started turning and a little push is needed. Just a small sweep stroke is enough to get going. The effect is small but effective, and that is why paddling on one side, even tilted counteracts the turning force with ease. It seems to me that the longer the bow and stern the more noticeable the effect is. In white water kayaks it happens also but it has to do more with the hull turning effect that one does with the legs. Of course a small sweep stroke makes a quick turn because there is no mass far away that has to move sideways against the water. The more rocker and shorter it is, the faster it turns with a sweep stroke, but the better defined deep stern and bow will give a better turning by leaning. Best Regards Rafael. At 10:23 a.m. 03/12/01 -0500, you wrote: >At 3:03 PM -0800 12/2/01, Scot Hume wrote: >>I'm trying to understand why rocker makes a boat turn >>easier. My current idea is that if a boat drafts 6 >>inches all the way along, a sweep stroke or other >>force such as a wave has 6 inches to work against at >>the end of the boat. However, if there is rocker, the >>power of the sweep stroke is pushing against only a >>couple of inches and it takes less force to move the >>bow on a pivot point back near the paddler. Is this >>correct? >> >>Also, when a kayak is leaned with the right knee, does >>the boat turn because the water on the right side has >>a longer path and thus the left side is able to move >>quicker through the water so that the boat ends up >>turning to the right? > >Anything sticking into the water is going to resist moving through it. >When you try to turn the boat you need to overcome the resistance of the >water to allow you to turn. Because the ends are farther away from the >center of rotation they create a larger moment arm to create more >resistance. Rocker reduces the area at the ends of the boat, thus >decreasing the resistance. > >Leaning the boat can have many different effects depending on the design >of the boat, but one effect is almost universal regardless of the kayak. >Because kayaks get narrow towards the ends, when you lean the boat you >give it more rocker and more rocker makes it easier to turn (see above). > >Nick >-- >Nick Schade >Guillemot Kayaks >824 Thompson St >Glastonbury, CT 06033 >(860) 659-8847 >*************************************************************************** >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/ >*************************************************************************** *************************************************************************** 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/ ***************************************************************************
On Sun 02 Dec 2001, Scot Hume wrote: > I'm trying to understand why rocker makes a boat turn > easier. My current idea is that if a boat drafts 6 > inches all the way along, a sweep stroke or other > force such as a wave has 6 inches to work against at > the end of the boat. However, if there is rocker, the > power of the sweep stroke is pushing against only a > couple of inches and it takes less force to move the > bow on a pivot point back near the paddler. Is this > correct? You are correct. Visualize using a 3ft pry bar and then you try prying with a 6ft pry bar. The resistance created by the water has similar leverage out from the center of rotation. Force of rotation is called a moment of rotation and is defined as F x d where F is the force (in this case the force of resistance provided by the water) and d is the distance out from the center of rotation. Hey, I gots no books here, so I may have just butchered that, but I think it gives some perspective on how force works, and where the water has more leverage. And another thing to consider is the shape that the water has to flow across during the rotation. Is the bottom squarish like some boats or more smooth and eliptical like others? This design factor could also help to allow the water to flow easily across the hull during sharp turns. -- Mike McNally mmcnally3_at_prodigy.net *************************************************************************** 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/ ***************************************************************************
> I'm trying to understand why rocker makes a boat turn > easier. How about this... sea kayaks carve turns for the exact opposite reason that skis carve turns. A ski has sidecut and camber. I submit that a sea kayak has reverse sidecut and reverse camber (aka rocker). This causes the sea kayak to have the reverse turn of a ski when edged. Physically, here's what I think is happening.... Upon edging the kayak, water pressure builds up on the bow, pressing against the lower edge. This causes a turning momentum away from the edge that is dropped-down into the water. At this point, the stern keel/skeg/rudder resists the turn that is initiated above. However, if there is little resistance, then the turn will build momentum. If you have ever tried to paddle a whitewater kayak in a straight line, you know what this feels like. Whitewater boats have NO resistance to turning in the rear (eg. skeg/rudder/keel-like object). Some sea kayaks are similar... e.g. the P&H Cappella and the CD Gulfstream both have rounded sterns and a skeg. With the skeg retracted, edged turns happen very fast. With the skeg, edged turns don't happen without a lot coaxing. Incidently, weathercocking is also very bad with skeg up, and nonexistant with it down. Now, if your kayak has just enough keel so that the boat tracks well and doesn't weathercock when held flat, AND if that keel virtually disappears when the boat is edged, well then you have a boat that initiates and carves turns all by itself. You probably also have a Mariner kayak, since so few other designs behave thusly... AND you definitely have a kayak which absolutely does not need a prone-to-break-or-jam, cut-you-up-in-the-surf, spongy-foot-bracing, destroy-the-beautiful-lines, mechanical-contraption called a RUDDER! ;-) ;-) Kevin Whilden *************************************************************************** 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/ ***************************************************************************
Nick wrote: <SNIP>>>Leaning the boat can have many different effects depending on the design of the boat, but one effect is almost universal regardless of the kayak. Because kayaks get narrow towards the ends, when you lean the boat you give it more rocker and more rocker makes it easier to turn (see above).<<<<<< In the paddle flutter discussion I am in near total agreement with Nick. Here though, I think there is a slight flaw in his explanation. Imagine a perfectly cylindrical "torpedo" floating on the surface with a draft of 1/2 its diameter (and also having narrow ends) would not change its shape in the water or any handling characteristics by being tilted even though the ends might be narrow. Therefore the easier turning isn't just due to pointed ends. The reason a kayak turns easier when leaned (to either side--into or away from a turn) is that kayaks are usually wider from the midline than they are deep below the water so there is more curvature on the sides of the kayak than on the bottom. Leaning this shape puts the wider middle deeper into the water and the now more curved underwater shape decreases the depth of the ends (and often shortens the waterline as well especially if keels that are now shifted to the side are involved). Just what effect this tilt will have on a moving kayak will depend on a number of things such as the amount of lean, the relative depth, length, fineness and "V" angle of the keels at the bow and stern, and the shape of the kayak including chines and the longitudinal distribution of buoyancy. I'm a dealer so like other dealers I hear a lot of problems people have with their equipment. Some is due to faulty equipment but most likely it is due to some lack of understanding on the paddlers part about how to use the equipment or a false belief about what the equipment should be doing for them. While it might be more profitable to sell the paddler on a material solution (product) I think it is more profitable in the long run to help the paddler solve the problem most efficiently and with the least expenditure. The retailer will most likely be rewarded with repeat business and good word of mouth advertising from that customer. Matt Broze http://www.marinerkayaks.com *************************************************************************** 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/ ***************************************************************************
>Nick wrote: ><SNIP>>>Leaning the boat can have many different effects depending on the >design of the boat, but one effect is almost universal regardless of >the kayak. Because kayaks get narrow towards the ends, when you lean >the boat you give it more rocker and more rocker makes it easier to >turn (see above).<<<<<< > >In the paddle flutter discussion I am in near total agreement with Nick. >Here though, I think there is a slight flaw in his explanation. >Imagine a perfectly cylindrical "torpedo" floating on the surface with a >draft of 1/2 its diameter (and also having narrow ends) would not change its >shape in the water or any handling characteristics by being tilted even >though the ends might be narrow. Therefore the easier turning isn't just due >to pointed ends. The reason a kayak turns easier when leaned (to either >side--into or away from a turn) is that kayaks are usually wider from the >midline than they are deep below the water so there is more curvature on the >sides of the kayak than on the bottom. Leaning this shape puts the wider >middle deeper into the water and the now more curved underwater shape >decreases the depth of the ends (and often shortens the waterline as well >especially if keels that are now shifted to the side are involved). I actually think we are say the same thing. I just tried to be brief and thus was incomplete. A typical kayak floating level has a couple (2 to 4) inches of rocker (at best), and a kayak on its side has several (8 to 12) inches of "rocker". Even if some kayaks have fairly straight keels, almost all are narrow at the ends and wide in the middle and the relative difference is greater than the height towards the ends vs the height at the middle. So a result of leaning the kayak is effectively increasing the rocker as well as shortening the waterline. I think you are essentially saying the same thing, just more completely. I am not trying to enter the fray about how a shape can make a design actually initiate a turn. I don't have a coherent theory about that. Although I have paddled boats that turn better when leaned one way or another, I haven't consciously paddled a boat that on flat water shows a distinct tendency to turn due entirely to leaning with no other input. The number of boats I have tested is far out numbered by those that I haven't, so I don't feel qualified to comment. I do think that leaning to the outside of the turn will give any "skeginess" in the stern a more streamlined shape so that it offers less resistance to turning. Because it is angled away from the resisting water. Since the stern travels farther through the water than bow during a turn, this makes a relatively more significant difference. Nick -- Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 (860) 659-8847 *************************************************************************** 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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