I apologise for not following this thread more carefully so I may have things wrong here. Correct me if I do. Some one wrote (I think) that they could determine a boat's displacement from the wave system. Could they explain the mechanism for this. Some one else said that a boat could "climb its bow wave". I have heard this so often that I am beginning to believe it must be true. I don't understand the physical mechanism for this either. Could some one explain this. Cheers, John Winters Web site address http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
John Winters <735769_at_ican.net> wrote: I apologise for not following this thread more carefully so I may have things wrong here. Correct me if I do. Some one wrote (I think) that they could determine a boat's displacement from the wave system. Could they explain the mechanism for this. Some one else said that a boat could "climb its bow wave". I have heard this so often that I am beginning to believe it must be true. I don't understand the physical mechanism for this either. Could some one explain this. I (Matt Broze) reviewed the thread and added my comments (see below). This is probably what you were remembering (or mis-remembering). If all the other factors were equal (boat shape, water density and temperature etc.) the higher the waves in the "wave system" the greater the displacement. In regards to the "climb its bow wave" comments you made I can only say that that's not what I said. I have used that terminology in the past and we've been around this particular wording once before on paddlewise. This time I was careful to use "climb the furthest out of the hole it made in the water" which I believe is the wording for this that we agreed at that time was more accurate than saying "climbing its bow wave" which you had valid technical objections to. >>>>Robert Lawson asks; > > Tell me more about what you could tell from observing their wake. How > does it look when the bow wake is over run? > Peter A. Chopelas <pac_at_premier1.net> replied: >>Actually quite a lot of information could be gained from the wake. If you could accurately measure the angle of the wake from a photo you could determine the boat's speed. The size of the wake combined with the LWL would also tell you something about the boat's displacement (though I am afraid I am not smart enough to figure that one out, though it is possible I'm sure).<< Actually in deep water the angle of the wake stays the same at all speeds, 38 degrees 56 minutes (at least until the wave barrier is broken and you start to plane--and you don't do that in a human powered kayak). Therefore knowing the angle won't tell you much about the speed of the kayak, but a photo's measured angle might be able tell you about the depth of the water (at least if the angle is greater than 38 deg 56' and you know some of the other parameters in the picture). The amplitude of the waves in the wake of a non-planing hull will be determined by the boat's displacement and boat speed. Moving more water aside more quickly stacks that moved water up higher and results in higher waves. Energy equals mass times velocity squared. More energy imparted to the water makes bigger waves. At the racing speeds the faster kayak will be the one that has managed to climb the furthest out of the hole it made in the water. If all the kayaks were the same and the paddlers and kayaks all weighed the same measuring the amount of the bow that hung out over the water before touching it would probably be the best predictor (for an observe from the side) of the fastest moving kayak at these sub-planing speeds. Measurements of the wave height could also tell which of these identical boats was moving fastest since mass would be controlled and speed would be the variable that created the differing wave heights. The length of the transverse waves (the long ones directly behind the kayak) in the photo will also tell you the speed of the kayak. A wave moves (in knots) at 1.34 times the square root of the wave-length (in feet). [Note: the next sentence has been edited to include a correction I made a bit later with a second post] Wavelength (in feet) = .558 times velocity (in knots) squared. If you know the length of the kayak in the photo and with Olympic sprint kayaks we do, I believe 5.2 meters (about 17' 1") is the rule. You should be able to use the kayaks known length to measure the wavelength of the long following waves and therefore deduce that kayaks speed. Matt Broze http://www.marinerkayaks.com Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Matt wrote: > > I (Matt Broze) reviewed the thread and added my comments (see below). This > is probably what you were remembering (or mis-remembering). (SNIPS) Thanks Matt.. I could not remember what and who said what. I have been doing a No-No in reading my e-mail from several different computers and locations and if I don't have time to answer them right away I lose track of things. Hence the question. Cheers, John Winters Web site address http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Matt's response brings up antoher question. Matt Broze wrote: >>Actually in deep water the angle of the wake stays the same at all speeds, 38 degrees 56 minutes (at least until the wave barrier is broken and you start to plane--and you don't do that in a human powered kayak). Therefore knowing the angle won't tell you much about the speed of the kayak, but a photo's measured angle might be able tell you about the depth of the water (at least if the angle is greater than 38 deg 56' and you know some of the other parameters in the picture). << I imagine this angle is related to viscosity or how much molecules of a particular substance are attracted to each other (I can't remember the word for this, cohesion?). If I were paddling in mercurey would the angle be more obtuse and in gasoline more acute? Robert *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Robert wrote: > > I imagine this angle is related to viscosity or how much molecules of a > particular substance are attracted to each other (I can't remember the > word for this, cohesion?). If I were paddling in mercurey would the > angle be more obtuse and in gasoline more acute? > I searched could find nothing about the effect of viscosity. Perhaps because when Lord Kelvin proposed the theory of wave patterns he did not think about bodies moving through anything but water. I don't know that since I have not read his paper but, like Matt, I could not find anything that says otherwise.. Anyway I did find some interesting stuff. For example the angle 19 degrees 28 minutes (half angle not full angle. For some reason naval architecture books seem to prefer half angle over full angle) does not seem absolute. For example, Commander Hovgaard presented a paper to the Society of Naval Architects in 1909 reporting that observations of full size ships showed angles between 16 degrees and 19 degrees. On one Danish torpedo boat he observed an angle of 11 degrees. His observations in the David Taylor model basin showed even smaller values particularly at high speeds. (non-planing craft and no reference to what "high" meant. Admiral Taylor attributes the difference to the forces being distributed over the entire boat rather than a single pressure point. Nevertheless he also says that the Kelvin wave patterns agree "reasonably well" with practical results. Gilmer (professor at the U.S. Naval Naval Academy) writes that the 19 degree 28 minute angle applies to a thin body or pressure point moving in a straight line through the water. So the hull shape seems to affect the angle. This ideal condition does not seem to take into consideration the stern wave that boats develop. Apparently Kelvin's paper contains mountains of complex mathematics to arrive at his conclusions. That discourages me from trying to obtain the paper and report on its contents. :-) Sorry to bore everyone but I found it interesting. %0 Cheers, John Winters Web site address http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Robert wrote: >>>>Matt Broze wrote: >>Actually in deep water the angle of the wake stays the same at all speeds, 38 degrees 56 minutes (at least until the wave barrier is broken and you start to plane--and you don't do that in a human powered kayak). Therefore knowing the angle won't tell you much about the speed of the kayak, but a photo's measured angle might be able tell you about the depth of the water (at least if the angle is greater than 38 deg 56' and you know some of the other parameters in the picture). << I imagine this angle is related to viscosity or how much molecules of a particular substance are attracted to each other (I can't remember the word for this, cohesion?). If I were paddling in mercurey would the angle be more obtuse and in gasoline more acute? Robert<<<< I believe this angle is independent of density and viscosity of the fluid (but have not yet found this stated in a textbook so I hedged a bit here by using "believe"). This wake angle is the result of one wave train (transverse or following waves) keeping station with the speed of the hull (therefore getting longer the faster the boat moves) interacting with the bow (or divergent) waves radiating away from the disturbance created by the boat's bow (much like when a stone is dropped into water making a circle of waves). The most visible "wake" is at the point where these two different wave trains intersect (here the wave crests reinforce each other making the very noticeable steep waves that we call the wake). Since both sets of waves are traveling in the same medium their relationship to each other should remain the same irrespective of the density or viscosity of the fluid. If density or viscosity made a difference the texts I have read would probably have hedged and said the angles were slightly different in salt (denser) and fresh water or in cold (more viscous) and warm water. They don't hedge here so I am pretty confident I'm right about this even though I have never seen it stated anywhere in a reliable source (or even in an unreliable source for that matter). Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On a totally non-technical note: yesterday morning in an incredibly clear and glassy mountain lake with the sun low, my shadow and wake's refraction of sunlight on the bottom were mesmerizing. I zigged around for an hour just digging it. The wake looked like those wind tunnel pictures of sound barrier-breaking. A bulbous shadow amidships and dollops when the paddle entered water. Wonder if I could make glass-ended tube to affix to my vidcam to film subsurface. Hmm. *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
From: "E. Sullivan" <sullivaned_at_pop.mts.kpnw.org> > Wonder if I could make glass-ended tube to affix to my > vidcam to film subsurface. Hmm. An open-ended tube would work just fine. You just have to have it sealed from light at the camera end. You'd want to have a cone shape to match the angle of the lens. Channel surf til you find Flipper re-runs (are they on?) I remember that Flipper's friends used to use such a thing to see underwater from their boat. Mike *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Various questions about the Kelvin wave angle: I looked in a book _Fluid Dynamics for Physicists_, by T E Faber. As John Winters suggested, it's geometric. It depends on the "dispersion relation" for gravity waves, i.e. wavelength being proportional to the square of the frequency. This is true when wave amplitude is small, water is deep, and wave length is large enough (much greater than an inch or so ( a few cm.)) that viscosity can be neglected. Also, as John said, it's based on a point source: the object creating the wave is very small. In practise, the waves created by each point on the hull add together. My guess is that finite wave amplitude is the major reason for departure from the ideal angle. Hope this helps, Bruce Bruce Winterbon bwinterb_at_magma.ca http://magma.ca:80/~bwinterb Capitalism means the greatest goods for the greediest. *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Bruce Winterbon wrote: > > Various questions about the Kelvin wave angle: > > I looked in a book _Fluid Dynamics for Physicists_, by T E Faber. > As John Winters suggested, it's geometric. It depends on the "dispersion > relation" for gravity waves, i.e. wavelength being proportional to the > square of the frequency. This is true when wave amplitude is small, water > is deep, and wave length is large enough (much greater than an inch or so ( > a few cm.)) that viscosity can be neglected. Also, as John said, it's based > on a point source: the object creating the wave is very small. In practise, > the waves created by each point on the hull add together. My guess is that > finite wave amplitude is the major reason for departure from the ideal angle. Don't have access to any fluid dynamics books so I'm stuck with my physical intuition ... which tells me that other properties of a fluid are involved here also -- not just viscosity, per se. What I'm thinking about is that it is the visco-elastic restoring forces which affect gravity wave propagation, and those will be a function of the medium. Tightly coupled molecules should respond to a wave disturbance differently than ones that are not tightly coupled, giving lesser amplitude for a given disturbance, but greater speed of gravity wave propagation ... and that might give a different "ideal angle" for the bow wave. Hmmmmm ... "more tightly coupled" may just translate into "more viscous." Maybe I'm all wet! If the oceans were made of alcohol, gravity waves would travel at a lesser speed, and could the bow wave angle be different than it is in water? (I assume the viscoelastic resotoring forces in sea water are pretty close to those in fresh water, though the density is some 3 % different.) This discussion may be more than most Paddlewisers want to know ... -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
This one has to be seen to be believed -- folks out kaying in giant pumpkins in Windsor, Nova Scotia. The CBC web coverage even has a video of the race. Looks like the winner used a wing blade. Check it out at http://cbc.ca/cgi-bin/view?/news/2000/10/15/pumpkins001015 Cheers, Richard Culpeper *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Richard Culpeper wrote: > This one has to be seen to be believed -- folks out kaying in giant pumpkins in > Windsor, Nova Scotia. Hmmmm..... think I'll add a skeg to mine next year. Any ideas on materials? I'm thinking along the lines of asparagus. ; ) PS: The major drawback to these (besides a severe lack of hydrodynamics) is that if you don't make land by midnight.........well, you know the story. Kerry Doubleday Halifax, N.S. Canada *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
From: "Richard Culpeper" <culpeper_at_tbaytel.net> > This one has to be seen to be believed -- folks out kaying in giant pumpkins in > Windsor, Nova Scotia. That's what they get for paddling after midnight! Mike PS - I heard recently that Cinderella's glass slippers come from a mis-translation of an old French word for fur that sounded like verre. Fur slippers. Hmmm... didn't our 'glass kayaks start out as seal skin? Babel, that's what it is! *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave Kruger wrote, in response to my earlier post: [I subscribe to thre digest, so it's easier for me to use cut and paste in replying. Thus Dave's words don't have any preceding ">", and I've indicated my replies with "BW>"] Don't have access to any fluid dynamics books so I'm stuck with my physical intuition ... which tells me that other properties of a fluid are involved here also -- not just viscosity, per se. What I'm thinking about is that it is the visco-elastic restoring forces which affect gravity wave propagation, and those BW> No. They're called gravity waves because the restoring force is gravity. At very short wave lengths(a few cm or less), the restoring force is surface tension, which is the only way in which inter-molecular forces can affect the wave motion. Remember, since we're talking about waves, we're talking about what is happening _away_ from the boat. Viscous drag is not a part of wavemaking. In fact you can see the capillary waves, those in which the restoring force is surface tension, in front of your bow wave, if the water is otherwise smooth enough. As we know, gravity waves are faster when the wavelength is longer; capillary waves are the opposite. So on the leading edge of the cusp on top of your bow wave, you can see very small waves running ahead, with the shorter-wavelength ones leading. Once you recognize them, you can see where the wind is putting capillary waves on top of gravity waves, which will eventually make the gravity waves bigger. Or you can see them around the beetles swimming on the (fresh-water) surface, who use them instead of vision. will be a function of the medium. Tightly coupled molecules should respond to a wave disturbance differently than ones that are not tightly coupled, giving lesser amplitude for a given disturbance, but greater speed of gravity wave propagation ... and that might give a different "ideal angle" for the bow wave. Hmmmmm ... "more tightly coupled" may just translate into "more viscous." BW> I suspect so. Maybe I'm all wet! If the oceans were made of alcohol, gravity waves would travel at a lesser speed, and could the bow wave angle be different than it is in water? (I assume the viscoelastic resotoring forces in sea water are pretty close to those in fresh water, though the density is some 3 % different.) BW> No. The critical thing is that the wavelength is proportional to the square of the frequency. The constant of proportionality, i.e., the strength of gravity, drops out. This discussion may be more than most Paddlewisers want to know ... BW> Maybe, but E. Sullivan very much enjoyed looking at her boat waves, and with any natural phenomenon, the more clearly you understand them, the more things you see in them to appreciate. Bruce Winterbon bwinterb_at_magma.ca http://magma.ca:80/~bwinterb A non-indexed pension is a fraud. *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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