Have you ever wondered the water velocity in waves?

[tuomas]

Hi,

Using e.g. Airy's equations one can calculate that the water particle speed on the crest may be many m/s and equally in the trough it may be many m/s in the opposite direction. Also water accelerations are large.

To my knowledge, the effect of horizontal velocity of water is hardly ever explained in textbooks or in sailing theory of sailing in waves. Is it because it is negligible or because it is not understood?

[tkeithlu]

Perhaps because there is little effect on a sailboat. Water, as a wave passes, indeed moves with and then against the direction of the wave, such that a molecule in the wave traces an upright oval pattern. The boat going against waves does indeed get hit by the water moving against it, it's called hitting a wave, and our boats are designed to minimize that effect by being narrow and sharp. You definitely feel this motion in the rudder. Combining the effect of a series of waves becomes almost neutral, with each "hit" followed by a "push".

[tkeithlu]

By the way, tuomas, that was a very interesting question. Thank you for posting it.

[tuomas]

Let me give you some calculations based on linear theory: in the Baltic sea (size of Great Lakes) you might get 2m/s flow on a crest and -2 m/s in a trough. if your speed is 5 kn (2.5 m/s) close hauled your keel would see alternating flow of +4 m/s aoa +35 deg and 0.5 m/s aoa -35 deg. with the phase speed of 8 m/s you would only spend a second or so in this situation, so the weight of the boat most likely filters this effectively. when running you get longer periods and them you feel it. In close hauled the flow stabilizes the boat as it turns you correctly upwind when climbing and downwind when falling but on running it works the other way around and tries to destroy your course. In fact this also explains why a boat seems to stop on a crest and then fall to the trough if you fo not turn it. Well, academic thinking but very interesting explanation for phenomena in the waves.

[Sailing Ohm]

Very thought provoking...you got me sitting thinking deeply in a new way about the waves I experience daily.
I would think boat builders think of this when designing hulls. I know there is an optimal waterline flow pattern (not sure I’m saying the right terms here). Basically the wave pattern at the waterline does have an optimal pattern and hull design can help or hinder that flow which correlates to the boats efficiency thus speed.
I don’t know if they use math to figure it out or just the human eye and water flow tests against a model.
Now all of that info has come from a guy I know who designs TP boats and catamarans. So it’s not my info and I could have misunderstood.

[tuomas]

And also note that in the half depth of the keel, say 1 m, the velocities are 90% of the surface velocities so the whole keel feels this.

[tkeithlu]

"And also note that in the half depth of the keel, say 1 m, the velocities are 90% of the surface velocities so the whole keel feels this."

Doesn't that depend on the height of the wave?

[Cadence]

Originally Posted by tuomas

Hi,

Using e.g. Airy's equations one can calculate that the water particle speed on the crest may be many m/s and equally in the trough it may be many m/s in the opposite direction. Also water accelerations are large.

To my knowledge, the effect of horizontal velocity of water is hardly ever explained in textbooks or in sailing theory of sailing in waves. Is it because it is negligible or because it is not understood?

You can calculate it by the no. of brown stains in your skivies when surfing down the face of one.

[Dave_S]

I tried to find good information on this not long ago. Not just horizontal flow but vertical speeds and at what depths. I couldn't find any worthwhile information. I even asked on here but had no luck.

I used best guestimates based on wave speeds of 15kn at various heights and steepness of wave faces. It becomes very complex (and for my maths probably a high degree of inaccuracy) when you add in the curvature leading into and out of the wells and crests. That's without surface wind/water speeds, currents, underwater topography......

[Cadence]

Originally Posted by Dave_S

I tried to find good information on this not long ago. Not just horizontal flow but vertical speeds and at what depths. I couldn't find any worthwhile information. I even asked on here but had no luck.

I used best guestimates based on wave speeds of 15kn at various heights and steepness of wave faces. It becomes very complex (and for my maths probably a high degree of inaccuracy) when you add in the curvature leading into and out of the wells and crests. That's without surface wind/water speeds, currents, underwater topography......

I think you nailed it. The variables are infinite.