WSSR Newsletter No 156 Water depth rule 05/07/08
WSSR Newsletter No 156
The WSSRC's new rule on minimum water depth for speed record attempts has seemingly caused a great deal of excitement among the kite sailing community with strong opinions being expressed both for and against. Some people have sent several pages of requests for technical explanations of the most detailed kind while others have demanded that the new rule be cancelled or delayed. We really cannot be expected to give a detailed reply to hundreds of questions so the following is an attempt to give a general explanation of the new rule and what led to it.
Every individual record attempt or observed speed event generates a report from the supervising WSSRC Observer. Last year it became clear from these that kite sailors were sometimes running in very shallow water - for instance in the lee of a mud-bank or sand-bar. Their objective was to find water as smooth as possible but it also rang alarm bells with members of WSSRC who wondered if there was some other benefit of running in shallow water which could give an unfair advantage to a particular type of craft.
The alarm bells began to ring more loudly last year when German newspapers reported that a kite sailor had exceeded 50 knots. It turned out that this 'claim' was neither observed nor measured in any acceptable way and was therefore rejected by virtually all reasonable sailors but certainly caused a good deal of confusion at the time. It was reported that this run had been made over mud-flats covered by a thin film of water and raised the questions:
- is this fair?
- is it sailing?
- does it have wider implications for future record attempts?
As the body responsible for ratifying records, WSSRC felt it must investigate.
One WSSRC member pointed to the game of 'skim-boarding' in which a flat board is thrown down onto very shallow water followed by a rider who runs and jumps onto it. If you do it right, the board skims along for many metres with apparently very small resistance. This proves the existence of some kind of 'ground effect' in which the water acts as lubricant rather than support. Most of us have scared ourselves at some time by driving into water on the highway when the tyres unexpectedly 'aquaplane' and lose all contact with the road. This also shows how a very thin film of water can act as lubricant.
The questions that arise are:
- how significant is the effect?
- how shallow must the water be to create this effect?
The Council therefore decided to consult the Wolfson Unit of Southampton University to see if they could come up with some hard evidence. They did - and the results are really surprising. When investigating the performance of seaplanes, a full programme of tank tests had been done years ago by NACA in the United States and the Wolfson team extracted and analysed the relevant data, also adding their own knowledge gained from tests on fast ferries and the science of sailing in general. To greatly simplify these results, they show that a planing surface experiences progressively lower resistance in water depths less than about 50cms. When the water is shallower than the width (beam) of the planing surface, the reduction of drag can be dramatic and in the extreme case when the water depth is less than half the beam of the board, the drag reduction can be as much as 50%. This clearly explains the performance of the wide, flat, skim-board running in a few centimetres of water.
Going back to the earlier questions -
- Is this fair? WSSRC thinks not because it is an advantage only open to a very specific craft; one that is effectively flat-bottomed and can be sailed at very deep wind angles without needing a skeg, board, hydrofoil or other device that counters leeway.
- Is it sailing? We do not believe that the public could possibly accept running in 10cm of water as a World Sailing Speed Record and neither would it accept that only kite-boards could hold the record in future.
- Are there wider implications for the future? Yes. Once it is generally understood that there is a big advantage to be had from running in shallow water, there will surely be efforts to create a perfect artificial course (just as the windsurfers created the 'trench' course in the past). This could be a flat piece of ground such as a car-park on which is laid plastic sheeting to contain the necessary few centimetres of water. And what is meant by 'water'? Must we analyse it to ensure that polymers such as soap solution have not been added to lower resistance further? The safety of competitors is not the responsibility of WSSRC but even so it might not be advisable to recognize a 'sport' in which there is real risk of falling onto concrete at 50 knots. So far, the ISAF has not played any part in these discussions but if they found out that WSSRC was intending to ratify a World Record under such circumstances, they would surely be horrified.
Three more points need to be made.
It is not possible for an Observer to carry out a complete and accurate survey of the area in which record attempts will be run. In practice he will use all means at his disposal to check that in general the water is at least 50cm deep. There could easily be stones, bumps or sand ripples standing higher and these will surely not affect the speed.
This new rule is nothing to do with measuring speed by hand-held GPS, which is a completely different discussion.
The intention of the new rule is to ensure that a wide variety of craft have a fair chance of establishing records. With the 50 knot record currently under attack by multihulls, hydrofoils, planing craft, windsurfers and kiteboards, it is particularly important that all should have the same opportunity of success. Nothing prevents kite-surfers from finding or creating a course where the water is 50cms in depth very close to the shore.
Secretary to the WSSR Council