I'm learning navigation using Burches book "Kayak Navigation" (thanks for the suggestions some months ago). In it, he mentions that speed efficiency is dramatically reduced in shallow water (1-2 feet) as opposed to deeper water (generally over 3 feet) due to the affects of the hull tubulance and so forth running into resistance from the bottom. Okay, I'll buy that, because I've noticed it myself as I travel across the Poquoson Flats near the Plum Tree Island Wildlife Refuge at the lower end of the Cheseapeake Bay. Seems like I always slow down when I cross them. Brings up a thought, though, that he didn't (or hasn't yet) discussed. Water temperature is a result of the speed of the atoms/ neurons/molecules or whatever (I'm not all that scientific) - the faster they move, the higher the temperature. So, if you are passing through a body of water with the atoms (or whatever it is - humor me) themselves moving at varying speeds, will it affect your speed? Seems like the colder water (therefore slower moving atoms, and therefore less resistence / counteraction to other motion) would allow for faster boat speed. But, it also seems like it might be measureable in highly controlled experiments, but to the average paddler would be unnoticable. I'm also starting to wonder about water salinity, but we can save that one for later.... So, what do you think? Any thoughts? Rick - Poquoson,VA *************************************************************************** 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'll tackle both of the questions below. First, you can ignore any thermodynamic affects caused by the vibration of molecules. They are all vibrating in random directions and have a net motion of zero. So you can imagine them hitting your hull from all possible directions and cancelling themselves out. That leaves us with the other thing that temperature does to water, change the density. In my limited naval architecture experience, I have found that warmer water is faster. The water is less dense and thus the volume displaced is easier to move out of the way. This also assumes that the change in water density doesn't adversely affect your wetted surface or wave making shape of the hull. Your kayak may sink deeper into the less dense water. My experience is in submarines where those features never change. You can then extrapolate to salinity. The more saline, the denser the water. So you can say that fresher water will be faster. This does not take into effect that your paddling will be more efficient in the more dense (cold, salty) water. You will be moving more mass per paddle stroke. You would have to determine whether the gain from the paddle in dense water is greater than the loss to hull drag. I believe the advantage from less dense water (reduced hull drag) is the more effective. I remember from my rowing days that any records set when the water temperature was above a certain point were in question. Also these changes are quite small. If I remember correctly that our submarine hull speed was around 8.25 knots in 60F water and 8.45 knots in 80F. This is an advantage only seen at the elite athlete level or when scaled up to full ship size. Daniel On Wed, 16 May 2001, Rick Sylvia wrote: SNIP > Brings up a thought, though, that he didn't (or hasn't yet) discussed. > Water temperature is a result of the speed of the atoms/ neurons/molecules > or whatever (I'm not all that scientific) - the faster they move, the higher > the temperature. So, if you are passing through a body of water with the > atoms (or whatever it is - humor me) themselves moving at varying speeds, > will it affect your speed? Seems like the colder water (therefore slower > moving atoms, and therefore less resistence / counteraction to other motion) > would allow for faster boat speed. But, it also seems like it might be > measureable in highly controlled experiments, but to the average paddler > would be unnoticable. > > I'm also starting to wonder about water salinity, but we can save that one > for later.... *************************************************************************** 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/ ***************************************************************************
Rick Sylvia wrote: > Brings up a thought, though, that he didn't (or hasn't yet) discussed. > Water temperature is a result of the speed of the atoms/ neurons/molecules > or whatever (I'm not all that scientific) - the faster they move, the higher > the temperature. So, if you are passing through a body of water with the > atoms (or whatever it is - humor me) themselves moving at varying speeds, > will it affect your speed? Seems like the colder water (therefore slower > moving atoms, and therefore less resistence / counteraction to other motion) > would allow for faster boat speed. [snip] No. Actually, the viscosity of water is mainly what affects the kind of drag you are describing: viscous drag. And, as the water's temperature is increased, the viscosity drops, so as the temp rises, the viscous drag decreases. I guess I disagree with Daniel Key that the change in density is more significant, inasmuch as the increase in density (have to move more mass aside) is offset by less hull in the water. I believe the decrease in viscosity will be more significant. > I'm also starting to wonder about water salinity, but we can save that one > for later.... Salinity has a very small effect on viscosity. Nothing like temperature. Probably not measurable under the usual test conditions, except that there will be less wetted surface area in salt water because it is more buoyant, hence less boat contacting the water. Note, however, per the comments of Daniel Key, that this is offset by the greater mass of the fluid displaced as you push you craft through water. Just my 2 cents ... although seems like others have half-dollars invested in the "paddle efficiency" debate. Read a lot of bad physics over the last week or so. -- Dave Kruger Astoria, OR *************************************************************************** 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/ ***************************************************************************
Daniel Key <dtheman_at_u.washington.edu> wrote: >>>>>.I'll tackle both of the questions below. First, you can ignore any thermodynamic affects caused by the vibration of molecules. They are all vibrating in random directions and have a net motion of zero. So you can imagine them hitting your hull from all possible directions and cancelling themselves out. That leaves us with the other thing that temperature does to water, change the density. In my limited naval architecture experience, I have found that warmer water is faster. The water is less dense and thus the volume displaced is easier to move out of the way. This also assumes that the change in water density doesn't adversely affect your wetted surface or wave making shape of the hull. Your kayak may sink deeper into the less dense water. My experience is in submarines where those features never change. You can then extrapolate to salinity. The more saline, the denser the water. So you can say that fresher water will be faster.<<<<<<< You are ALL wet (I suspect because you were in a submarine surrounded by water). In that situation your conclusions are correct but mostly for the wrong reasons. Denser is slower and down to at 40 deg. F fresh water gets denser as it gets colder (then it becomes less dense until it turns to ice). The difference is that on the surface the denser water floats the kayak higher, as well as being harder to push aside, so while it may be a little harder to move to the side the denser water cuts the wetted surface and that removes most of the difference even at higher speeds so that salt water is only a little bit slower than fresh water of the same temperature when racing. Thinking about it now it could well be that the decrease in wetted surface makes salt water easier to paddle in than fresh water at cruising speeds and below where wave making (shoving the water aside) is a small percentage of the drag. However, the property of water that changes with temperature that you missed is by far the most important one. Colder water is more viscous (stickier in layman's terms). I test a lot of kayaks and do timed turn and sprint tests with them when I can. I observed that I was slower with the same kayak in winter than in summer on my fixed course (on a freshwater lake) by several percent. I decided I needed to find out what was going on and how big the effect was (and write down the month and year of the tests so I could estimate or research the water temperatures then if I wanted to make accurate comparisons between boats tested at different times). If I didn't I would always have to compare the kayak tested to a known kayak--like I did each time when I originally started comparing kayaks for top speed in the late 70's). Almost any good Naval Architecture text will provide conversion tables to convert your water temperatures drag to the standard used in N.A.--which is 59 deg. F. If I remember correctly the rule of thumb I took from the tables was that a 10 deg. F. change in temperature resulted in about a 2.5% change in drag when both viscous and density changes were taken into account. >>>>>Also these changes are quite small. If I remember correctly that our submarine hull speed was around 8.25 knots in 60F water and 8.45 knots in 80F. This is an advantage only seen at the elite athlete level or when scaled up to full ship size.<<<<< The difference you site is about 2.5%. In my opinion, nothing to sneeze at even for a recreational paddler. I'd be willing to bet that due to viscosity the drag differences would be about twice that for the 20 degree F. difference you sight. Now your differences in "hull speed" might be correct if you were in a real short submarine of about 40 feet long that was operating on the waters surface. A submarine under water does not have a "hull speed" (in the way that term is traditionally used) since it does not make waves to get stuck behind. I'm assuming that you were using the term to mean the speed the sub went with the same amount of power because if 8.45 knots was as fast as the sub could go underwater you must have been in a slow sub indeed. 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/ ***************************************************************************
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