I was just reading a thread on GPSS about water depth which has come about from Tillmans amazing kiteboard runs in next to no water.
See text below:
3) Water Depth

This is the one aspect I really cannot accept unless it is objectively proved otherwise. To me the 'ground effect' of sailing in shallower water MUST result in higher speeds. 2 examples:

a) Skim Disc (probably the ultimate low-tech 'ground effect' planing hull). In 5mm water depth it feels like there is no friction and will glide for very considerable distances using the water as a lubricant but the ground effect to provide sufficient lift so that the small planing surface can support the rider. As the water depth increases, 10mm, 15mm, 20mm the skim disc very quickly loses lift with a massive resultant increase in drag, coming to a stop very quickly - Having read Hilman's reports I believe it is this effect that is what he describes as 'liquid ice'.

b) Traditional rowing boat (displacement hull) - I have done a lot of racing in these boats at a fairly serious level. We often rowed in tidal estuaries where the water depth undulates over sandbars etc. The effect of crossing a sandbar where the water depth decreases (from many meters down to perhaps 250 - 500mm) would be a very noticeable and quite dramatic reduction in drag (much easier to row) and a corresponding increase in speed.

My view is that water depth can have a BIG effect on drag and this must therefore have an equal impact on speed. This effect is perhaps significantly greater for kiteboards (because of their small fins) than for boards or boats.

Out of curiosity, what is the depth of water at Sandy Point? I know obviously the water is super flat but I'm curious to also know the kind of depth of water you are running in while sailing along the shore. Could this aspect possibly have much to do with the speeds that come out of there? Is it possible this is also why the Mandurah mob have such success too due to the shallow conditions in which they sail?
Don't get me wrong, I'm not saying anyone out there is not worthy of the speeds thay have done, far from it, I'm just curious as to maybe how much an effect the shallower water makes. I know that certainly our small speed strip in Botany Bay is actually still fairly deep (at least 6 foot deep), albeit flat so I presume we would be getting next to no help from the ground effect.
Thoughts anyone???

I think the ground effect doesnt really affect board lift for sailboards- since most of this is aerodynamic at speed, and the board surface is at least 25cm from the bottom. But it would make the fin more efficient by constraining the tip vortex. Basically you get more lift for the same fin size. The fin feels a bit longer than it is. On a kiteboard- the water can be 5cm deep or less making it a bigger effect by increasing both vertical and lateral lift for the same drag.

At SP the water is deep at low tide, because the edge drops away very suddenly- I'd estimate the water depth is ~50cm when we are close to the bank. At high tide its a different story- the water is over the bank, where it has a much flatter slope. You cant get anywhere near the shore line but the water is very shallow. Last time I sailed there at high tide I was hitting the bottom with a 20cm fin at speed- quite a way off the water line... was taken at medium tide. About an hour after it was possible to drive in. Notice we are not sailing close to the edge... I hit a few times.

Ian, after reading that about 10 times I think I almost understand you. Does the rate of compression (water being thrown down) of the water increase as speed increases or does it almost decrease due to less wetted surface area?

Chris I guess the main reason I asked this was because of what the guy I quoted mentioned about the shallow sandbank effecting his rowboat speed. I imagine the draught of a rowing boat would be more than 25cm and yet he says he still felt a distinct difference when in the shallower water. I'm curious as to what depth the ground effect actually starts to make a difference.

I think he got it wrong- a rowboat is displacing. Lift is generated predominantly by buoyancy. Normally in shallow water the drag increases for displacing hulls.

I'm glad that's been clarified. Now if you'll excuse me, I'm just off to find a two km long, low sandbar angled at 125 degrees to the prevailing 35 knot wind, and where the water is also the ideal depth, and is free of weed. Is there anything else I should be looking for? Absence of kiters?

Kiting in 5 cm of water end plates the board. High aspect surfaces are more efficient at large AoA. This is the case whether planning on the surface or submerged foils. A kite board is a angled planning surface where the vertical component provides lift equal to the riders weight minus whatever lift the kite provides and the horizonal component provides the lateral resistance. But the Kite board is a very LOW aspect foil (AR = ~ 0.25) which WOULD have a huge induced drag component as the high pressure water escapes under the bottom edge to the low pressure side setting up a large vortex which is the source of induced drag. By endplateing on the sand the AR effectively becomes infinite and the lift/drag ratio of the board would be improved by a huge amount.
Additionally, the wing-in-ground-effect will make a huge reduction of the drag. Wieselsberger has (theoretically) found this in the 1920's by applying Prandtls lifting line theory . From this theory it follows that induced drag reduces to approximately 50% at a ground clearance of 10% of the wingspan.

From this it can be seen that for a planning windsurf board even in shallow water there is little benefit from drag reduction. A pin tailed board may have an average span of 12-15cm on the planning surface at speed and looking at the top right of the graph the drag reduction is negligible at H (depth)/B (span) = 1.
You would need to go down to about 1/10 of the span (ie 1.5cm) to get a 50% reduction in drag. Impossible for a windsurfer but doable at Westerheven where Tilman was sailing in only centremetres of water at 50 knots.
Anyway that is the drag aspect of WIG effect which is related to span (width of board). The other aspect is lift which is related to chord or in our case the waterline length at speed. This results in increased lift from the increase pressure as the water can't escape (ram air effect but in water). So with reduced drag and increased lift there is a large increase in the efficiency of the board when at skimming depths.

Already asked. No go. The place was used as a bombing range during the 2nd world war and is full of unexploded ordinance.

The place to do it Pt Gregory, near the Beta Carotene farm there.

Much windier than Perth (OK,it's a bit of a drive) but there is a huge expanse of dry salt bed that is basically worthless. If the shire was prepared to do a little earthworks, they would a have a new specialty tourist attraction on their hands.

I'll just go and arrange for the "no Kites" sign to be made up now.

yoyo said...

Already asked. No go. The place was used as a bombing range during the 2nd world war and is full of unexploded ordinance.

Damn!!!

AUS1111 said...

The place to do it Pt Gregory, near the Beta Carotene farm there.

Much windier than Perth (OK,it's a bit of a drive) but there is a huge expanse of dry salt bed that is basically worthless. If the shire was prepared to do a little earthworks, they would a have a new specialty tourist attraction on their hands.

I'll just go and arrange for the "no Kites" sign to be made up now.

hmmmm, a quick trip back to Corro's if the swell comes up

sounds pretty good to me

And after we've done that experiment Nebs we'll mount a board on a shopping trolly, fitted with sand tyres, and run the fin in water on an outrigger to provide lateral resistance. I bet it will go faster than your device Nebs hence proving once and for all that hull resistance is greater than fin resistance. I doubt either would qualify as sailing though.

I recall once seeing "sailing" technically defined as the exploitation of the velocity discontinuity between two fluids. We use the discontinuity between water and air but it doesn't have to be different fluids. Albatross extract energy from the velocity profile above the ocean - they gain momentum in the faster air higher up in the profile, swoop down into the lower slower air, do a 180 degree turn and rise to their original height. They do it for days at a time with barely a wing flap. That's sailing. Kite surfers also fully satisfy that definition if they're in deep water. But if one of the fluids is shallow and solid earth becomes involved the fluid component of definition is not satisfied. If we deliberately chased shallow water for the advantage we're moving towards the physics of the solid riding ice yachts and blow carts, OK faster but not strictly sailing.