Confused: A boat typically drifts beam on because...

Hi guys,

I've been pondering this question and gotten myself into a nice haze of confusion.

My question is, for your average vessel putting along under power, when you stop moving forward, your vessel typically will begin to drift beam on.

What causes this beam on drift?

My thoughts revolved around centres of lateral resistance?

Or perhaps the to do with the metacentres and moments of the waterplane?
That being KMl >> KMt so it would be harder to lift the bow, which slips down the wave and finds an easier path to "lift" / roll the vessel instead?

Or in my confusion have just stumbled past the really easy answer?


Thanks!

 

This varies a lot between boats, even raising the leg of an outboard or sterndrive can and does change the attitude. But I'd say you are right about centres of lateral resistance, and the areas and shapes of areas above and below waterline, wind strength etc.

 

For a start, the aerodynamics tend to make the boat rotate to 90 deg to the wind. When the boat is at a modest angle to the wind, in addition to the force parallel to the wind (drag) and the force perpendicular to the wind (lift), there's a yawing moment. The wind curves around the bow and around the windward quarter of the stern. There is a low pressure region associated with both of these curvatures in the flow. The low pressure regions are located at opposite ends of the boat and are pulling in opposite directions. That creates an unstable yawing moment that drives the boat to increase its angle to the wind.

Once the boat is near 90 deg to the wind, the forces from the wind curving around the bow and stern are nearly lined up and opposing each other. So there's no more yawing moment. If the boat rotates past this equilibrium point, the same yawing moment will tend to drive it back toward beam-on - it's a stable equilibrium.

I think there's a similar mechanism at work with the orbital velocity of the waves, too, that tends to make long things line up with the wave crests. If you consider a log floating in a regular seaway (waves all coming from the same direction), when it is at the point halfway between a trough and an advancing crest, the orbital velocity there is only vertical and has no horizontal component. But the end of the log that is pointed toward the trough sees an orbital current that is moving toward the crest. The end of the log that is pointed toward the crest sees an orbital current that is moving forward toward the trough. So the orbital motion is pushing both ends in opposite directions.

At the crest, if the log is at an angle to the wave and traveling with the velocity at the crest, then the advancing end is being pushed through slower moving water. The trailing end is also in slower moving water, so it's like the log is being driven through the water by the faster water near its middle. This would create an unstable yawing moment for much the same reasons as the unstable aerodynamic moment.

In the trough, you get the opposite effect. It's like the log is being driven backwards towards the next wave, and again, the moment would be unstable.

Halfway down the backside of the wave, the flow would be receding from the middle of the log toward each end, and the yawing moment would tend to line up at right angles to the wave.

So there would be different mechanisms at work on the log at different points in the wave cycle. But the unstable ones would outweigh the stable ones and the log would eventually drift to being beam on to the waves.

A boat would be affected by both the aerodynamic and hydrodynamic influences, and both are tending toward lying beam on to the wind and waves.

That said, a boat is also subject to influences that couple the aerodynamics and hydrodynamics together. As it sits beam to the wind, it will be drifting sideways through the water, making leeway. Now the distribution of the lateral area comes into consideration. If the center of lateral resistance is behind the aerodynamic center of pressure, then the boat will tend to turn downwind. And vice versa.

So a small riding sail at the stern will act like an air rudder and help the boat lie with its bow into the waves. Of course, a drogue is used for the same reason - it effectively shifts the hydrodynamic resistance to the end of the boat from which the drogue is deployed.

 

A boat is typically neither uniform in underwater profile, nor above water profile, so naturally it will weather-cock around according mainly to those variables. And can easily be changed by as little, as mentioned above, raising or lowering a drive leg, which will act as a drogue in the down position.

 

You can make the boat perfectly symmetrical in all axes and it will still lay beam on to the wind and waves. Kayakers have very personal experience with this tendency and make several attempts to control it, especially when going downwind. Skegs aft and body position help and an air rudder forward is often more effective than these. A paddle held straight forward and moved to create steering force works.

 

As Mr. Speer said. Since a picture is worth thousands words, you can find a link to an illustration and an animation of the wave motion in this post: http://www.boatdesign.net/forums/boat-design/designing-boat-doesnt-broach-46879.html#post625834.
Hopefully, they will make things clearer for you.