Comments for HydroComp Musings http://hydrocompmusings.wordpress.com [efficiency in motion] Wed, 15 Apr 2009 16:15:32 +0000 http://wordpress.com/ hourly 1 Comment on Time trial racing – deep water versus shallow water by HydroComp http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-18 HydroComp Wed, 15 Apr 2009 16:15:32 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-18 Is 10 cm or 50% beam reasonable? On purely hydrodynamic grounds, it does seem like a reasonable figure. The water depth to chine beam ratio (H/Bch) for the deeper draft condition in the Khattab figure (152 mm) was approximately 50%. On the other hand, is it reasonable in terms of safety or other aspects? I have no background in this type of racing to answer that. Natural waterways are not nicely flat and smooth. And, mechanical contact with a 9 cm rock has a really high drag coefficient... Is 10 cm or 50% beam reasonable? On purely hydrodynamic grounds, it does seem like a reasonable figure. The water depth to chine beam ratio (H/Bch) for the deeper draft condition in the Khattab figure (152 mm) was approximately 50%. On the other hand, is it reasonable in terms of safety or other aspects? I have no background in this type of racing to answer that. Natural waterways are not nicely flat and smooth. And, mechanical contact with a 9 cm rock has a really high drag coefficient…

]]> Comment on Time trial racing – deep water versus shallow water by Markus http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-17 Markus Wed, 15 Apr 2009 15:58:38 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-17 What I am refering to is the "skimming" effekt - you know this summer sport done with a wide and thin board, you throw it along the beach when the wave breaks and jump on it and slide on a film of water - quit fast until you come into deeper water and the skimboard begins to sink. It glides perfectly as long as the water film is thin (lets say a couple of cm), but stops working when it gets deep. Also, everybody of us knows aquaplaning on his car, I think this is the same effect ? What the WSSRC did is trying to investigate the ground effect - they want to eliminate super shallow water advantages as in skimming or, as you said, an icesailor, or a landsailor in the rain. Thats why they defined the minimum water depth to 10 cm or 50% of the beam, whatever is greater. On a kite-speedboard this is usually 12-15cm. Now the big question: is this reasonable ? What I am refering to is the “skimming” effekt – you know this summer sport done with a wide and thin board, you throw it along the beach when the wave breaks and jump on it and slide on a film of water – quit fast until you come into deeper water and the skimboard begins to sink. It glides perfectly as long as the water film is thin (lets say a couple of cm), but stops working when it gets deep. Also, everybody of us knows aquaplaning on his car, I think this is the same effect ?
What the WSSRC did is trying to investigate the ground effect – they want to eliminate super shallow water advantages as in skimming or, as you said, an icesailor, or a landsailor in the rain. Thats why they defined the minimum water depth to 10 cm or 50% of the beam, whatever is greater. On a kite-speedboard this is usually 12-15cm.
Now the big question: is this reasonable ?

]]> Comment on Time trial racing – deep water versus shallow water by HydroComp http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-15 HydroComp Wed, 15 Apr 2009 12:28:48 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-15 The sensitivity of water depth to changes in drag is modest at "super-critical" speeds. That was one of my original contentions in the article - once you have gone "super-critical" there is not much change in drag due to water depth. Critical speed is defined as speed [m/s] / Sqrt(G [m/s2] * water depth [m]). For a depth of one meter, for example, critical speed is about 6 knots. (Consider the second figure in the article. This was for a 1.1 m, 7 kg model of a planing hull. It's critical speed was only about 1.5 m/s.) Of course, there is a limit to what is "shallow" and what is just lubrication of the earth - which you have noted. (Isn't an ice boat really just running in extremely shallow water?) I would think, however, that it could be easy to establish some simple criteria for an acceptable range of water depth. To some extent you will always have some "third party" influence unless you are sailing in the middle of the ocean, as the shore and its geometry dictate the nature of breaking waves. The sensitivity of water depth to changes in drag is modest at “super-critical” speeds. That was one of my original contentions in the article – once you have gone “super-critical” there is not much change in drag due to water depth. Critical speed is defined as speed [m/s] / Sqrt(G [m/s2] * water depth [m]). For a depth of one meter, for example, critical speed is about 6 knots. (Consider the second figure in the article. This was for a 1.1 m, 7 kg model of a planing hull. It’s critical speed was only about 1.5 m/s.) Of course, there is a limit to what is “shallow” and what is just lubrication of the earth – which you have noted. (Isn’t an ice boat really just running in extremely shallow water?) I would think, however, that it could be easy to establish some simple criteria for an acceptable range of water depth. To some extent you will always have some “third party” influence unless you are sailing in the middle of the ocean, as the shore and its geometry dictate the nature of breaking waves.

]]> Comment on Time trial racing – deep water versus shallow water by Markus http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-13 Markus Wed, 15 Apr 2009 09:00:46 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-13 Hi, I think the main issue is the definition of sailing by ISAF, allowing only natural wind and water. The question was, if adding a third party (the ground) is defined as sailing, especially when the water gets very shallow (think of the skimboarders that can only plan on a thin film of water). So the main question remains - gives sailing in super shallow water an advantage, and if so, how big is the advantage and when does it begin (at which water depth in correlation to the beam). Thank you for all your explanations, but it seems we need to go a little bit further into detail on this :) Feel free to email me any time. Hi,

I think the main issue is the definition of sailing by ISAF, allowing only natural wind and water. The question was, if adding a third party (the ground) is defined as sailing, especially when the water gets very shallow (think of the skimboarders that can only plan on a thin film of water).

So the main question remains – gives sailing in super shallow water an advantage, and if so, how big is the advantage and when does it begin (at which water depth in correlation to the beam).

Thank you for all your explanations, but it seems we need to go a little bit further into detail on this :)

Feel free to email me any time.

]]> Comment on Time trial racing – deep water versus shallow water by HydroComp http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-14 HydroComp Wed, 15 Apr 2009 01:00:29 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-14 Modest changes in water temperature (i.e., a five degree swing) will have a very, very small effect on drag. The change in density and viscosity due to temperature is much less than going from salt to fresh water, for instance. Please take a look at a past <em>Marine Performance Technology Exchange</em> newsletter for an article about "Performance differences in fresh and salt water". This will give you a sense of the magnitude of changes to drag. It also has another article about shallow water effects. You can grab it from our <a href="http://www.hydrocompinc.com/knowledge/library.htm" rel="nofollow">Knowledge Library</a>. Scroll down to the October 2005 issue. Modest changes in water temperature (i.e., a five degree swing) will have a very, very small effect on drag. The change in density and viscosity due to temperature is much less than going from salt to fresh water, for instance. Please take a look at a past Marine Performance Technology Exchange newsletter for an article about “Performance differences in fresh and salt water”. This will give you a sense of the magnitude of changes to drag. It also has another article about shallow water effects. You can grab it from our Knowledge Library. Scroll down to the October 2005 issue.

]]> Comment on Time trial racing – deep water versus shallow water by Bruce McPherson http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-12 Bruce McPherson Tue, 14 Apr 2009 22:22:53 +0000 http://hydrocompmusings.wordpress.com/2009/03/20/time-trial-racing-deep-water-versus-shallow-water/#comment-12 I've seen nothing regarding water temperature. Less drag? Old timers sailing in the Port Huron to Mack. Island race find that certain conditions make sailing the much longer route north along the Canadian shore where the water is shallower and WARMER to be successful.... I’ve seen nothing regarding water temperature. Less drag? Old timers sailing in the Port Huron to Mack. Island race find that certain conditions make sailing the much longer route north along the Canadian shore where the water is shallower and WARMER to be successful….

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