[AdamLein]

Originally Posted by smackdaddy

However, my smarty-pants-physicist-nephew corrected my thinking that waves in water are actually "moving forward" as they appear to be... he explained that the standard non-breaking wave is simply an up/down movement of the water - not the forward motion that it appears to be.

Individual water molecules move in a circle: in the direction of wave motion at the crest, then downward as your nephew points out, then in the direction opposite wave motion in the trough, then back up to the crest.

If a breaking wave poops you, the water is not only moving forward, it's moving forward faster than your rudder is.

And that forward "surfing" motion of the boat is due to its sliding down the face of the wave - it's not really being "pushed" because this type of wave has no velocity, just amplitude. (I think that's all right).

The wave has two velocities: the velocity at which the geometric shape that you perceive propagates along the sea's surface (called the group velocity) and the velocity of individual particles. It's the group velocity that determines whether or not you're surfing... surfing is sailing on the leading edge of a wave at approximately the wave's group velocity.

He went on to say that breaking waves were a completely different set of equations that got very complicated very quick. That water IS moving and has a lot of "forward force".

He's right that the math is a lot more complicated for breaking waves, but there's still a straightforward lay description. The main difference is that normally when a water molecule arrives at the crest of the wave (moving forward), there's some water in front of it and below it to support its weight and to resist its further forward motion. In a breaking wave there's just air there. So the water has some kinetic energy from its forward motion like it always does, but it also has its potential energy from towering high above your boat. When a swell passes under you you don't experience that energy. When the waves breaks on your transom, all of that energy is converted into heat, sound, and deformation. It's the deformation you're concerned about.

But, as someone said earlier in this debate, as long as the rudder stays in that lower "strata" of water, it's fine.

The water continues to be affected by surface waves quite deep -- to a depth equal to twice the wavelength... or maybe it was half the wavelength, I don't remember. Either way the water just below the surface (where your rudder is) is still moving about quite a bit, and in a 15' breaking wave you can expect that that water will be tossed about quite violently when the wave crest breaks on top of it.

Imagine pouring some red water from one container into another which already contains clear water. The red water won't just stay at the surface, it'll plunge down below the surface of the clear water and mix things up and create lots of turbulence... and you're pouring a small amount of water from a few inches away. Now imagine pouring tons of water from ten or twenty feet above you... you will not find shelter just below the surface.