Now I might be imagining this, though freestylers have told me they notice sliding tricks go on forever in shallow water, but how much easier is it to gybe in the shallows? Is the ground effect significant? We occasionally get a gust over flat deep water in the lee of a local reef. Thinking about it I reckon you have to watch your trim a bit more carefully to come out with a respectable amount of speed in that deeper water. But there are a lot of variables. What do you think? Is it more than just the flatness of the water?

As an engineer ( geeze I love saying that , cos it stirs ) I would think , while planing , ( because water is so thick ) that at 15cm or less that it won't make much or any difference. Apart from fin digging catapult.
Of your skimming over water I would doubt the water pressure under the board would increase under 5cm , ..... I know ..., it's a call !

As a ship's pilot I know that ship's (i.e tankers, container ships etc) that if you are in shallow water the turning radius is basically doubled, as the water can't escape underneath the hull. If you where banking a board over so that say 20cm of board was immersed in 40cm of water in a gybe I would expect that the water pressure could be double that of deep water so you could bank the board over twice as hard to get the same turning circle as in deep water so I think you are on to something Ian. Never really thought about it when not at work though! Normally the definition of shallow water is less than double the draft of the vessel.

I remember gybing in the shore break as it washed up on the sand. The turns were nice and smooth and easy to stay on the plane because the water was so flat. I don't recall the turning radius being any different. However I do recall the pain of being catapulted onto the sand when the turn was too slow and the wave receded.

Sounds good to me when displacing water.
When sliding ( planing or skimming ) is a different question.
The question was gybing .
Need more info.
Was it planing gybe or bogging gybe ?
If I was to slowly stuff up a turn I could disperse ten bathtubs of water,
If I was head down ass up and going for it, mabee one.

I think there is something to the different effects of shallow water with less area for the water to compress, in terms of speed and control.
We used to sail in this large back bay in my area years ago, and all of us seemed to think that we were sailing faster in the shallow water during low tide, we're talking about 20" deep (or 50 cm) or less. Then we would slow down as the water became deeper, that's what we were thinking anyway, but it might have been a visual thing, when you can see the bottom whizzing by, it feels very fast. This was back before GPS and none of us had a speed gun to see for sure, but we agreed that shallower was faster, and cranking hard gybes seemed easier.
I'm pretty sure speed records have mostly been set in shallow water.
Anyway, shallow can be very fun, fast, and a bit scary.

For me, because the water surface is smoother in shallow water, I find it significantly easier to gybe.

I'm with John. My shallow (make that flat) water gybes are way better / easier / smoother than the rough deep stuff. Can relax and draw longer lines at greater speeds- helping plane cleanly around the whole arc. Deep and rough for me equals, sharper harder turns- in order to get around between chop / swells. Half of which I drop off the plane.

I know from kayaking in shin deep water (say 25-30 cm or so) there's a vacuum effect that draws the craft to the bottom. Not sure if planning speeds negate this drag though.

Mastbenders pic reminds me of Harrington.

Not sure what it adds to the discussion -- but, to be precise, water can't compress (well, not to any meaningful sense in this context) -- it only displaces.

I think that the difference between smooth and rough has 1000x more effect on how easy it is to gybe than depth -- so it would only be, as some have noted, that shallows are usually less rough. Just my two bobs worth.

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Mastbender said..

Anyway, shallow can be very fun, fast, and a bit scary.

Have hit and broken things. Multiple times. Can confirm; is scary for a reason.
Still alive.

Man, you think catapulting into water is bad? Shells and stuff add to the grittiness of the whole situation.

My comment and example was w/in the context of the surface of deep water, being as smooth as the shallow water.
Does anybody know of any speed records being set on smooth but deep water? Smooth deep water does exist for sailing.
I don't know the answer, but all the pics I've seen of speed records being set, are what looks like being set in pretty shallow water.
So I can buy the shallow ground effect (or bottom effect), less space for water displacement, beings that it can't compress, so at speed, the water may actually be harder thus faster.

My old girlfriend broke her arm being catapulted in water about 12" deep at the same spot I was referring to, yikes, it wasn't pretty.

I thing what you said earlier about the visual effect of shallow water is probably right. The"hardness" is the same, deep or shallow. It feels harder the faster you hit it!

Interesting thought, those skim boards used on a few centimeters of water have almost no resistance at all. You see kids dragging each other while running flat out. And there was that video from the other day of the guy skurfing behind a drone, I don't reckon he would have got started if he hadn't run and jumped on in water that was a couple of centimeters deep. If someone was crazy enough to put a sail on a skim board in a tidal pool they would probably set a speed record... or die.

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Agrid said..
If someone was crazy enough to put a sail on a skim board in a tidal pool they would probably set a speed record... or die.

You'll always need a bit of fin in water. Quad delta fins? All across the tail? Or maybe two amidships on a double-stepped hull?

I wouldn't be surprised if there was some kind of effect related to turbulence. I could imagine that the turbulence at the tip of the fin would be effected. I did a quick google of skim board physics which revealed both some nonsense and something called the navier-stokes equation which was beyond my high school level physics knowledge.

Drone surfing...

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Mastbender said..




I think there is something to the different effects of shallow water with less area for the water to compress, in terms of speed and control.
We used to sail in this large back bay in my area years ago, and all of us seemed to think that we were sailing faster in the shallow water during low tide, we're talking about 20" deep (or 50 cm) or less. Then we would slow down as the water became deeper, that's what we were thinking anyway, but it might have been a visual thing, when you can see the bottom whizzing by, it feels very fast. This was back before GPS and none of us had a speed gun to see for sure, but we agreed that shallower was faster, and cranking hard gybes seemed easier.
I'm pretty sure speed records have mostly been set in shallow water.
Anyway, shallow can be very fun, fast, and a bi

More balls than me

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PhilSWR said..
I'm with John. My shallow (make that flat) water gybes are way better / easier / smoother than the rough deep stuff. Can relax and draw longer lines at greater speeds- helping plane cleanly around the whole arc. Deep and rough for me equals, sharper harder turns- in order to get around between chop / swells. Half of which I drop off the plane.

I know from kayaking in shin deep water (say 25-30 cm or so) there's a vacuum effect that draws the craft to the bottom. Not sure if planning speeds negate this drag though.

Mastbenders pic reminds me of Harrington.

Phil when your kayak sinks lower in shallow water that is known as squat as you are basically creating a current under the kayak which because of the increased velocity of water under the kayak it creates a suction effect. In ship's it normally drags you down about 10%, but is a function of speed and how "blockey" the water craft is. I think planning would be easier in shallow water as it is harder for the water to escape making the board ride higher and faster. So what we need to do is set up a speed strip in the Dead Sea in shallow water in a windy position if there such a thing. When sailing in Lake Budgewoi the water is often smooth as of the weed but you can definitely feel the board accelerating as you get into the shallow water. I have done a fair bit of testing and found the bottom of the lake to be soft but stinky!

Here's another theory ...the water is usually warmer closer to the beach ..
Is Warmer water easier to gybe in ,and go faster in .? Maybe it could be .eg..
can you move a spoon faster through boiling water or cold .
Is it to do with the oxygen levels in the water.

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sboardcrazy said..

More balls than me

What? Eye-balls?

"...they notice sliding tricks go on forever in shallow water."

That I believe is due to the fact that being close to the beach, you have a frame of reference. This being the beach plus the sea floor. While you are on deeper water, you do not have that. A close analogy is the moon seems to be ridiculously large when first appearing on the horizon.

I doubt very much that shallow water does exert a reactive pressure back onto the board. One may be able to gybe better in the shallow because the water is calmer. Perhaps there is a bit of psychological effect in it too. You are not as afraid to fall off. And if you do, you can just do a beach start anyway. A bit risky if you keep too close to the beach though.

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Ian K said..
Now I might be imagining this, though freestylers have told me they notice sliding tricks go on forever in shallow water, but how much easier is it to gybe in the shallows? Is the ground effect significant? We occasionally get a gust over flat deep water in the lee of a local reef. Thinking about it I reckon you have to watch your trim a bit more carefully to come out with a respectable amount of speed in that deeper water. But there are a lot of variables. What do you think? Is it more than just the flatness of the water?

There definitely is a ground effect, but as somebody has already said, it doesn't really have much effect until depth is less than 1/2 hull width. So gybing in water that shallow is quite likely to result in a grounded fin.
And then if you slow down too much you're into the negative aspect of ground effect.
While planing, ground effect gives you more lift but subplaning creates more drag.

I think flat water is by far the greatest aid to gybing.

I experimented with a ground effect board for a while, but it was just an ancient heavy polyester board without much volume, a long way from ideal. After several modifications with different fin configurations I gave up. I decided I didn't really want to go fast in very shallow water anyway!

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Al Planet said...
I wouldn't be surprised if there was some kind of effect related to turbulence. I could imagine that the turbulence at the tip of the fin would be effected. I did a quick google of skim board physics which revealed both some nonsense and something called the navier-stokes equation which was beyond my high school level physics knowledge.

Might be an experiment for Mr slowboat maybe?

From what I've heard he can feel the fin tickling the bottom on some of those fast runs he does.

As an engineer ( geeze I love saying that , cos it stirs )

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decrepit said..

There definitely is a ground effect, but as somebody has already said, it doesn't really have much effect until depth is less than 1/2 hull width.

It was probably the WSSR that said that

"There is much anecdotal evidence about the positive effects on speed by the shallow water effect and certain published study including the effect on fast ferries when entering shallow water and using the effect to "unstick" seaplanes. However in order to obtain some hard data, the Council commissioned the Wolfson Unit of Southampton to prepare a paper on the specific subject. Their conclusion of this detailed study was that the drag of a planing board is reduced when the water depth is less than the beam of the board, with a possible reduction of 50% in very shallow water of less than half the beam of the board. A water depth of 50cm would be deep enough to avoid shallow water effects."

But it's all pretty flaky. I can imagine that the effect drops off sharply as water depth increases. But from a flaky 50% drag reduction at a flaky half board width to a magical zero at full board width is a big call. If it's just a regulatory line in the sand where ground effect is minimal enough for records then it should be called as such. Like the minimum tail wind allowable for the 100 metre dash record.

I was just hoping someone had done a lot of sailing at a location where they could make a confident subjective assessment of the flat deep water vs. flat shallow water differences.


How about trim angle?


Does anyone notice board trim going flatter as they enter shallow water?

That looks like a wing in air Ian, not sure that you can actually "stall" a planing hull?
There's no flow over the top surface to detach, there's only bottom pressure.

Getting a transition from flat deep to flat shallow is problematic, Fangy's had it a year ago, when there was heavy weed in waist deep water, going into knee deep. But how much effect does weed have? It must also impede the flow of water to a certain extent. And changes in weed density probably effect speed more than the ground effect.

There's also a speed bank, that gets shallower as you approach the bank, but the wind is a bit better away from the bank, and water not as flat. So I can't help you with that one.

Funny you should mention weed. I'm astounded by how much just a few strands of ribbon weed on the fin slows us down.

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decrepit said..
That looks like a wing in air Ian, not sure that you can actually "stall" a planing hull?
There's no flow over the top surface to detach, there's only bottom pressure.

No but what it does show, or at least it doesn't contradict the theory, that a wing or a planing surface affects the fluid/air BEFORE it comes to that point.

With the wing outside the ground effect, it stalls at a greater angle of attack because the air in front of the wing is being pulled down before it reaches the foil. This effect is reduced when the wing is in ground affect, as the trailing wing vortices are inhibited and the down wash angle is reduced.

I am not suggesting this is exactly what happens to a board or vessel is shallow water, but it does suggest there is more going on than just at the interface of the surface and the fluid.