Nick wrote; > Primary stability has almost nothing to do with the cross sectional > shape of a boat. Chines, no chines, makes no difference. It is the > shape of the water plane and the height of the center of gravity that > will determine initial stability. Hmmm. Out of curiosity I drew two boats with the same displacement, same CG and same waterline area and shape. One had a "V" bottom and one a flat bottom. The flat bottom boat had a righting arm at 5 degrees 38% greater than the "V" bottom boat. That seems like a lot of effect. Granted this is an extreme example. Maybe what Nick means is that, given two similarly shaped boats, having a chine or no chine does not produce a significant difference. Am I reading you wrong on this, Nick? Robert Livingston wrote; > The fact is that secondary stability does not have a universally agreed > upon > definition. Certainly not among paddlers - possibly because so much of the boat's stability depends upon the paddler and her skills. It seems to me that one's individual perception of stability has a lot to do with the shape of the static stability curve and the range of stability (the range of inclination through which the boat is statically stable with a fixed CG). Slow, creaky people like me seem to prefer a wide range of stability and a stability curve that has a slow rate of change throughout the range because we can't shift our weight so well anymore. Athletic types seem to get along fine with boats that have less static stability at all angles of heel, a smaller range and more rapid changes in stability with heel (as revealed by the slope of the stability curve) . > We have stability curves for hundreds of kayaks. I have seen many of them. > Secondary stability can not be identified as something on those curves or > at > least there is no agreement on what that something is. Once you have found a boat with a stability curve shape and magnitude that you like would not other boats with the same or similar curves "Feel" good to you? It seems to me that this suggests that the curve can tell you something. > So secondary stability is something mystical that cannot be "measured" > objectively by a stability curve or anything else for that matter. > > Basically is a subjective "impression" and thus cannot really be refuted. If you can measure the stability and relate it to your preferences have you not accomplished something useful and possibly important? In short, the quantities may have no menaing to some one else but they would have meaning to the paddler. I agree that to say a boat has "good" secondary stability is rather meaningless but to say I "like" (or don't like) the secondary stability of a boat has a lot of meaning. Cheers John Winters *************************************************************************** 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 May 12, 2005, at 7:22 AM, John Winters wrote: > Nick wrote; > > >> Primary stability has almost nothing to do with the cross sectional >> shape of a boat. Chines, no chines, makes no difference. It is the >> shape of the water plane and the height of the center of gravity that >> will determine initial stability. >> > > Hmmm. Out of curiosity I drew two boats with the same displacement, > same CG and same waterline area and shape. One had a "V" bottom and > one a flat bottom. The flat bottom boat had a righting arm at 5 > degrees 38% greater than the "V" bottom boat. That seems like a lot > of effect. Granted this is an extreme example. > > Maybe what Nick means is that, given two similarly shaped boats, > having a chine or no chine does not produce a significant > difference. Am I reading you wrong on this, Nick? I based my statement on several things. One being a similar analysis I did on 5 different cross sections each with the same displacement, waterplane shape and CG height: http://www.guillemot-kayaks.com/ Design/ConstWL.gif. It is also based on the standard naval architecture definition of Initial/primary stability which is the slope of the stability curve at zero degrees of heel. 5 degrees is much greater than zero My "V" bottom sample has plumb sides and a chine below the waterline. As soon as the chine on one side leaves the water (around 10 deg in my example), the stability starts to fall off rapidly. A pure "V" bottom with chines or eventually a sheer above the waterline and the same displacement, waterplane shape and CG height will have the same initial stability by this definition, however the slope won't be maintained for very long, so 5 degrees is enough for a big change. *************************************************************************** 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 07:22 AM 5/12/2005, John Winters wrote: >Nick wrote; > >> Primary stability has almost nothing to do with the cross sectional >>shape of a boat. Chines, no chines, makes no difference. It is the >>shape of the water plane and the height of the center of gravity that >>will determine initial stability. > >Hmmm. Out of curiosity I drew two boats with the same displacement, same >CG and same waterline area and shape. One had a "V" bottom and one a flat >bottom. The flat bottom boat had a righting arm at 5 degrees 38% greater >than the "V" bottom boat. That seems like a lot of effect. Granted this is >an extreme example. > >Maybe what Nick means is that, given two similarly shaped boats, having a >chine or no chine does not produce a significant difference. Am I reading >you wrong on this, Nick? Doesn't the "shape of the water plane" account for the differences you're seeing? Maybe I'm missing what "shape of the water plane" means. I also assume that "height of center gravity" is in relation to center of buoyancy (yes, I've read Nicks excellent stability article). Would the center of buoyancy change for differently shaped bottoms? Having paddled lots of different boats with different hull shapes I've noticed a different "feel" in the initial stability in different. Boats with very flat hull feel very resistant to tipping. Boats with a shallow V have a gentle rocking feel. Boats with a deeper V start to feel "twitchy", Rounded hull boats feel "slippery" (sort of like sitting on a basketball). That's not very scientific but that's just how different shaped hulls *feel* to me. *************************************************************************** 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/ ***************************************************************************
Lets take a really weird example: A box shaped hull with two fully submerged pontoons well down and out to the side. --[____________]---- < waterline / \ O O Stability occurs because the CB moves as a boat tips. This is due to the lifing side of the boat losing buoyancy, and the sinking side gaining. The effect of the pontoons on the stability curve will not be evident until one of them comes out of the water because until that point the pontoons have little relative effect on the CB. The primary factor changing the buoyancy will be the box at the surface of the water. At some large angle a pontoon will lift free of the surface. At that point its effect will be very noticeable. The buoyancy contributions of the pontoons puts the center of buoyancy of the pontoons halfway between the two pontoons. Now imagine moving the pontoons somehow: --[____________]---- < waterline \ / O O Notice that the buoyancy of the boat does not change. The CB of the pontoons does not change so their contribution to the overall CB does not change. The only time you will notice any change is when one of the pontoons comes out of the water, at some large angle of heel. Moving the pontoons in or out will only effect the angle of heel at which they emerge from the water. Placing the deeper will have more effect: --[____________]---- < waterline / \ / \ / \ O O Lowering the pontoons lowers the overall CB of the boat. This can make the CG move out beyond the CB more quickly, but at small angles, the difference will be small. Nick On May 12, 2005, at 3:07 PM, John Fereira wrote: > At 07:22 AM 5/12/2005, John Winters wrote: > >> Nick wrote; >> >> >>> Primary stability has almost nothing to do with the cross sectional >>> shape of a boat. Chines, no chines, makes no difference. It is the >>> shape of the water plane and the height of the center of gravity >>> that >>> will determine initial stability. >>> >> >> Hmmm. Out of curiosity I drew two boats with the same >> displacement, same CG and same waterline area and shape. One had a >> "V" bottom and one a flat bottom. The flat bottom boat had a >> righting arm at 5 degrees 38% greater than the "V" bottom boat. >> That seems like a lot of effect. Granted this is an extreme example. >> >> Maybe what Nick means is that, given two similarly shaped boats, >> having a chine or no chine does not produce a significant >> difference. Am I reading you wrong on this, Nick? >> > > Doesn't the "shape of the water plane" account for the differences > you're seeing? Maybe I'm missing what "shape of the water plane" > means. > > I also assume that "height of center gravity" is in relation to > center of buoyancy (yes, I've read Nicks excellent stability > article). Would the center of buoyancy change for differently > shaped bottoms? Having paddled lots of different boats with > different hull shapes I've noticed a different "feel" in the > initial stability in different. Boats with very flat hull feel > very resistant to tipping. Boats with a shallow V have a gentle > rocking feel. Boats with a deeper V start to feel "twitchy", > Rounded hull boats feel "slippery" (sort of like sitting on a > basketball). That's not very scientific but that's just how > different shaped hulls *feel* to me. *************************************************************************** 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/ ***************************************************************************
> Having paddled > lots of different boats with different hull shapes I've noticed a different > "feel" in the initial stability in different. Boats with very flat hull > feel very resistant to tipping. Boats with a shallow V have a gentle > rocking feel. Boats with a deeper V start to feel "twitchy", Rounded hull > boats feel "slippery" (sort of like sitting on a basketball). That's not > very scientific but that's just how different shaped hulls *feel* to me. I think that you are correct in your impression. But this is not "primary" stability as defined by Schade (basically the definition that I would accept) It is my contention that kayaks are VERY different from normal craft because the user shifts his/her weight constantly and this represents a substantial part of the weight of the craft. Stability curves are based on the concept of the paddler sitting rigidly in the boat as it heels which does not happen in real life. Imagine this "thought" experiment. Sit in some narrow 18" kayak. Extending out is a board 6" wide and 1" thick and it extends out 4 feet in each direction. It is fiberglassed to the bottom of the kayak. That kayak will feel "rock stable" as you sit in calm water. You would have little trouble taking out your camera and taking pictures. Its "primary stability" as measured on a classic stability curve is little different than the the craft that has no such board fiberglassed to the bottom. But it feels ENTIRELY different. The dynamic resistance to tipping created by the resistance of the board moving through the water gives the kayaker ample time to adjust his/her weight as the boat starts to tip one direction or another. That adjustment is practically subconscious. You could probably stand in that boat. For that matter, fiberglass a 4 foot dagger board to extend down from the middle of the hull. Make it out of a slightly buoyant material like wood and it will reduce the primary stability as measured on a stability curve. But it will feel very steady at 0 degrees of heel. I have paddled square logs that are about 16" wide and I can stay upright on them which I cannot do in a 18" kayak. My contention is that the square cross-section kayak feels initially more stable than the round kayak because there is much more dynamic resistance to the boat rotating around its long axis. It is NOT because of its stability curve which at 0-2 degrees might be identical to a boat with a rounded hull. It is like a built in paddle brace. Rock a flat bottom boat back and forth. You will kick up a lot of waves because of the resistance to rotation of those flat surfaces against the water. The rounded hull will rock back and forth with little wave action created. In the world of heavy shipping this is not much of an issue because the cargo weight cannot be shifted quickly and an unstable boat will slowly turn turtle whether it is square bottomed or rounded. That said, on passenger ships they will sometimes deploy stabilizers that extends out and resist the rolling back and forth around the long axis of the boat to increase the comfort of the passengers. *************************************************************************** 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/ ***************************************************************************
> In short, the > quantities may have no menaing to some one else but they would have meaning > to the paddler. Sure they might have meaning to me. But words are generally to communicate with others. Perhaps "secondary stability" is like poetry, the words evoke something. But for it to be a scientific concept it needs some kind of definition that people can agree on. Secondary Stability = slope of the curve of area * righting arm^2 at 30 degrees of tilt. Whatever. *************************************************************************** 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/ ***************************************************************************
>> We have stability curves for hundreds of kayaks. I have seen many of them. >> Secondary stability can not be identified as something on those curves or >> at >> least there is no agreement on what that something is. > > Once you have found a boat with a stability curve shape and magnitude that > you like would not other boats with the same or similar curves "Feel" good > to you? It seems to me that this suggests that the curve can tell you > something. In answer to this, I would say quite possibly NO. I think that two boats tweaked so as to have essentially identical stability curves could be perceived as being very different in their "stability"characteristics because of the dynamic resistance of the viscosity of water on their cross-section shapes. *************************************************************************** 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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