# Water stiffness with speed increase

I am sure that you guys have discussed this before, but I do not know what keywords to use to find the answer in the search engine. So I apologize…

Why is it that it is more difficult (hurt more) to penetrate the surface of water as your speed increases?

Also, why is it that surfboards float higher on the surface of the water when they are moving faster?

And, are these two observations based on the same physical phenomenon? What is the name of this phenomenon?

Ok, so I think the phenomenon that I am asking about is “surface tension”.

Why does surface tension seemingly change as speed increases?

Something about the inertia being mostly in a horizontal direction (momentum) as opposed to the vertical (gravity)

Air also enters the equation, broken water mixed with air…

I think one of the things is also that water cannot be compressed, so when you get in the water or put your board in, you have to displace a certain amount of water, but the water has mass (as well as surface tension) to it, so it resists displacement. The faster you go, the stronger the forces invlolved.

Something like that… been a while since any physics courses.

Ah- good questions. And they might be related…lets think about it.

First off, a board that’s moving along a wave isn’t floating, it’s planing. That is, it is going fast enough that the board is generating lift against the surface of the water. The faster you go, the more lift…I suspect it varies not just with the speed but with the square of the speed.

Okay, so you’re going whipping along the wave and if you’re like me, sooner or later ya screw up and ‘whoops’. If you’re not pretty much doing a dive, hitting like you were trying for a 10 in a diving competition, you kinda skip like a stone. Though if you’re going slower it’s not as bad. Why is that, anyways?

Surface tension? Not really, that’s a property of the water molecules that doesn’t change with speed. But remember, planing and lift?

Think of skipping a flat-sided stone across the water. Throw it too slow and it goes ‘plop’ right in. Or throw that stone so the flat side isn’t on the bottom. But that flat side generates lift, the faster the stone goes the more lift and the more it’ll skip.

The reason you spin the stone is to keep the flat side down, gyroscopic kind of thing. It doesn’t otherwise affect whether or not the stone skips. Anyways… you go off the board at fairly high speed and what happens?

Unless you manage to do a perfect swan dive, so you hit the water like a spear point and go right in, you hit on a kinda flat surface of your body. Or at least rounded, not sharp, so you don’t dig in and zip under the water, instead, you generate lift, enough lift so that you skip over the top of the water until you bleed off enough speed so you’re not planing any more. You’re not as dense as a rock…least we hope not…so you can do that planing thing pretty well with less speed a than a rock needs, or a relatively non-flat part of your body. Think about those barefoot water skier guys and you’ll see that it doesn’t take a really good planing surface, it takes speed.

And that’s how it works. I think…

Hope that’s of some use

doc…

I kind of hope (wish?) I am as dense as a rock.

Well, it kind of would beat being as dense as, well, butter or something.

Ok, so I’m dreaming.

Doc!

What are you doing up at this hour? Don’t you have a boat to prep in the morning?

BTW - (I actually have something to contribute to this thread besides sarcasm) I had an interesting experience last week. I went on vacation up to the mountains and there was a lake. A big lake. And a friend had a boat (with a motor).

I took along my kiteboard to see if it worked like a wakeboard.

Yes & no.

With low rocker, going both ways, and a design for the pull coming from up in the air (like a kite), it seemed awfully critical that I lean like 90% on the back foot while being pulled horizontally by a competition-style ski boat. My back foot got tired fast (good thing I nailed my first strapped switch-foot, ever!) because I had to jam it deep in the water to keep myself from pearling.

And yes, that happened too. And I skipped like a stone (but not like butter).

I had my buddy going 16-19 kts, much like a kiteboard, but the pull direction was so different, that it was hard to maintain control. And yes - when I fell, the water was harder to penetrate than water at low speeds (like surfing - sorry, I ain’t Roy).

But really, it was interesting to think about how you would design differently when the speed is external (a boat) rather than when you have to generate speed from finding the pocket in a wave. Much more for control (my board felt really skatey except when I was playing with the outside “ramp” edges of the wake) and much less for speed. No need to worry about eliminating drag (you are the drag) because it’s overcome by 75, 90, 150 HP or more.

So back to the skipping…I think the planing is all about resistance. As was said before, water is incompressible. So the harder (faster) you press on it, the faster you’re going to encounter that incompressibility, and the more you’re going to skip across it like a stick of butter on a kiteboard.

Say you’ve got a jet of fresh water, from a hose say, and you’ve directed it at a wall (or a friend). Say the water leaving the nozzle is traveling at 2 m/s with a cross-sectional area of say .2 sq m (pretty strange hose); how much force will be imparted to the wall (or friend) that its directed at when it finally hits it?

Lets see a train leaving the station at noon travels…

Ah!

For an approximation, use what engineers call stagnation pressure, defined as the density of the fluid times the cross-sectional area time the square of the water velocity all divided by two. Here, this equals, 400 N. Big deal? (Check my Math please.) Say you weigh 145 lb, that’s 644 N.

So estimate the surface area of your feet say A sq meters, figure the density of water is 1000 kg/ cubic meter, and jump off a bridge say 30 m high (acceleration due to gravity is 9.8 m/sq sec). If you did, you’d hit the water in about 2.5 sec traveling at 24 m/s (neglecting air resistance, which admittedly is pretty important here, but what the hell…) that means that the force of impact would be, A times 1000 times 24 times 24 divided by 2 newtons. Nasty. (Again check my Math.)

What’s this got to do with your question? Whether the water is moving towards you or your moving towards the water – its the same old, same old (at least sometimes it is.)

Also, dido what Johan1 said. If you want to move mass, you’ll need a force. The quicker you want to move the mass (in the same amount of time), the more force you’ll need.

kc

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First off, a board that’s moving along a wave isn’t floating, it’s planing.

doc…

All planing craft are ‘floating’ (i.e producing lift via displacement of water) to some degree, less as the craft goes faster. but there is always some water displacement, planing craft cannot produce lift without displacing water.

.

Oh contrare’ Tomas, can a displacement craft plane without displacement, or vice versa…I beg to differ. Planing can only displace when it is floating and therefore, hence , in conclusion only floating when it’s displacing, or subsequently planing while displacing. It’s a paradim shift in accordance to the Kelvin principle, only 212 degrees and your floating! Remember Newtons 23 law…Thou shall float, eat, then displace.

Water cannot be compressed, Right. But water can expand, and it can evaporate, right. So what happens when you take that same surfboard and ride a frozen,compressed gas wave?..exactly you plane. I think I proved my point.

I got what Doc was saying, of course planing craft are floating. If they weren’t they’d be submarines. Why split hairs. In insuferable as always…keep up the good work.

-Jay

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I kind of hope (wish?) I am as dense as a rock.

I’ve been told I’m as dense as a rock… mostly by ex-girlfriends… oh… you mean my body… that’s more like butter.

As far as stiffness goes, isn’t this really a viscosity/velocity relationship?

famous power boat quote from Albert Hickman

"the drag of water at 40 knots is equivalent to a

piece of flat bar pulled through hard cheese at 4 knots"

hope that doesn’t muddy it up too much, I’d hate to

wipe out hard on cheese.

Pete

Quote:

As far as stiffness goes, isn’t this really a viscosity/velocity relationship?

famous power boat quote from Albert Hickman

"the drag of water at 40 knots is equivalent to a

piece of flat bar pulled through hard cheese at 4 knots"

hope that doesn’t muddy it up too much, I’d hate to

wipe out hard on cheese.

Pete

Isn’t that exactly what I just said? “Flat bar pulled through hard cheese” Great minds think a like. You are my new brother.

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I got what Doc was saying, of course planing craft are floating. If they weren’t they’d be submarines. Why split hairs. In insuferable as always…

-Jay

No, Doc was saying that planing craft are not ‘floating’, however as you can see all craft are ‘floating’ to some degree as they displace water, nothing contrary about it, just being exact, if you think that’s insufferable then posssibly it’s the polyester resin factor. . . plays havoc with the brain you know.

.

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As far as stiffness goes, isn’t this really a viscosity/velocity relationship?

Water does not exhibit significant changes in viscostiy based on velocity. The phenomenon you are thinking of is shear thickening, and is tyipically associated with fluids with lots of solids suspended in them. At low speeds the solids move past each other relatively easily. At high shear rates (high velocity) the solids tangle, interfere, etc and the viscosity increases. Waves and surfing would be wildly different in such a fluid.

You can make examples in the kitchen using corn starch and water:

Quote Bloke: You and I both know that the Duke probably wouldn’t mind me putting my ride alongside his, and that’s what counts. . …I’m just buzzing on that way of surfing, it’s a blast !

**Interesting, your a mind reader and a self confirmed Doper all in the same sentence. At least I wear a mask when I work with toxins.

xxoo,

Jay

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Quote:

As far as stiffness goes, isn’t this really a viscosity/velocity relationship?

Water does not exhibit significant changes in viscostiy based on velocity. The phenomenon you are thinking of is shear thickening, and is tyipically associated with fluids with lots of solids suspended in them. At low speeds the solids move past each other relatively easily. At high shear rates (high velocity) the solids tangle, interfere, etc and the viscosity increases. Waves and surfing would be wildly different in such a fluid.

You can make examples in the kitchen using corn starch and water:

http://www.coldwater.k12.mi.us/…i-dept/6th/goop.html

That reminds me of one of my favorite “Myth Busters” episodes! http://video.google.com/videoplay?docid=6903509899806041831&q=mythbusters+cornstarch&total=1&start=0&num=10&so=0&type=search&plindex=0

-Carl