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/ ***************************************************************************Received on Thu May 17 2001 - 00:49:22 PDT
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