Miscellaneous Musings from the Technical Director
I received an interesting email yesterday, and I thought I would share my reply with our blog readers. It comes from a speed record holder “kite surfer”.
In the world of speed sailing, kite surfers can sail in very shallow water (down to less than 10 cm), which gives us very flat water without chop. Last year the sailing world changed the rules to force us into deeper water saying that using shallow water was using ground effect and reducing drag which is cheating, but reading your report it seems the opposite is true for displacement hulls. Do the same rules apply to planing hulls that are pushing 50 knots in water depths less than half of the beam of the board?
This new rule is pushing us into choppier water and slowing us down and not letting us show our true potential to sail as fast as possible. We are trying to put a case together (if there is one) to challenge this rule that – as far as a bunch of surfer types can make out – is a bit unfair to us.
So the question I’m asking is “do we get an advantage in terms of speed from sailing in very shallow water with a planing hull in terms of ground effect or is it a disadvantage like your report states for displacement hulls”?
(Edited for space.)
First, my hat is off to anyone going 50 knots on a small board. I don’t have the same “need for speed”, but I’m impressed none-the-less…
The Rules Committee is making a correct technical statement about shallow water drag, but there are other aspects of the physics of operation in shallow water that may be on your side. Let’s first consider the general hydrodynamics of shallow water drag.
The attached plot [Hofman 2000] illustrates the basic principles of added drag due to shallow water. A non-dimensional depth-based speed parameter is on the X axis (usually noted as FNH, but shown here as FL), and a ratio of drag in shallow water to that in deep water is on the Y axis. You can see how shallow water drag increases rapidly as it approaches the “critical speed”, after which it just as rapidly drops off and produces negative added drag. Displacement hulls generally stay in the “sub-critical region” as they typically don’t have the necessary thrust-making “oomph” to get past the critical region drag hump. This is the mode that was described in the paper you read from our Knowledge Library. Planing hulls, however, typically can get past the critical hump, at which point shallow water becomes beneficial.
How beneficial? Let’s refer to another plot specifically for a planing hull [Khattab 1999].
You should be able to identify the various zones – sub-critical (0 to 0.9 m/s), critical (0.9 to 1.6 m/s), and super-critical (1.6+ m/s). You can also see that the super-critical (planing) drag benefit of shallow versus deep water is indeed significant.
However, in the context of making a case for racing in shallow water, I would acknowledge the drag benefit and ask “so what”? It would seem to me that one critical aspect of any type of time trial racing is uniformity in the venue. For example, you can also see from the plot that the planing mode benefit is affected to a much smaller degree by water depth or speed – the curve shapes in shallow water follow the deep water drag curve, and the drag offset is relatively uniform regardless of the “shallowness” of the water.
I would also have to question if there were any other performance benefits by running at an inland location (where you would find shallow water), rather than offshore. For example, does this give you more consistent wind velocity and direction? You also mentioned surface chop, which I would suggest has an even bigger effect on planing drag than shallow water, especially at 50 knots.
So, while I think the Rules Committee has a legitimate point to want to have uniformity in water depth (and the venue in general), it seems that the better solution is to do exactly the opposite of what they are suggesting. By requiring racing in shallow water, rather than in deep water offshore, it seems that you gain uniformity not only in the hydrodynamic aspects of added drag, but also in wind (your propulsion) and other potential variables (chop). Good luck.
References
Hofman, M. and Kozarski, V., “Shallow Water Resistance Charts for Preliminary Vessel Design”, International Shipbuilding Progress, Vol 47, No 449, April 2000
Khattab, O., “Investigation of the Behaviour of Planing Hull Forms in Shallow Waters”, International Conference Hydrodynamics of High Speed Craft, 1999
Filed under: Hydrodynamics
Uniformity suggests that there be records for different conditions and types of boats: ie displacement, planning, trimarans, catamarans, conventional board, kites, etc and shallow water, deep water, flat water conditions, wavy conditions, and an open classification where anything goes, except sailing on hard water (ice).
I would agree that this information is important for any empirical trial. However, in a racing setting, to compare “apples to apples” you would have to develop some correction for differing conditions. It would seem to me to be better to make the conditions as uniform as possible and eliminate any such correction.
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….
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.
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.
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.
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 ?
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…