May 6, 2012

White water perils

ONE OF MY FIRST LESSONS in seamanship came in a rather unexpected way. I was swimming in the surf off an African beach bordering the sub-tropical Indian Ocean when a large breaking swell picked me up, tumbled me over, and whumped me into the swirling, foamy depths deep below.

I fought to regain the surface, but found I couldn't get my head above the white water to breathe. I could feel panic rising until, fortuitously, my feet found bottom, dragged along a bit, and slowed my progress toward the shore. The breaker moved on, and I found myself in solid water once more, able to rise to the surface and breathe.

Some time afterward I realized that things don't float properly in water that is full of air bubbles. And by things, I mean people and boats. I know now why body surfers dive underneath the white water of oncoming breakers and emerge in the solid water behind them. 

The simple fact is that aerated water is not as dense as normal water and will not offer the same support to bodies floating in it. Just as a boat will float lower in fresh water than in salt water, which is denser, that same boat will float lower in aerated water, perhaps even dangerously lower, depending on how much freeboard she has.

But my lesson in seamanship came when it also occurred to me that a boat's rudder is less effective in water that's less dense, which explains why a sailboat's helm goes dead when a wave breaks under the stern and throws her forward.  It's a nasty feeling when she's about to broach and you can't get her to respond to the helm.

The answer, as I found out, is to slow her down with drogues or warps streamed aft, so that she is not carried along for any distance with her rudder in the breaking, foamy water. Let the breakers overtake you as quickly as possible, so that you get your steering control back before she can make mischief by turning broadside-on.

The same rule applies to boats operating in any white water; that is, water filled with bubbles of air, especially if you're coming in to a beach through lines of surf in a dinghy.  If you don't have the engine power to stay safely on the back of a breaker all the way in, you must slow down and let each line of breakers overtake you as quickly as possible.  In a boat powered by oars, it's often best to turn and face out to sea, backing in slowly and then pulling forward just before the next breaking swell arrives.  Sometimes, if you have enough line,  it's possible to drop a light anchor and let the line run, snubbing it as new breaker arrives.

There may be unexpected danger, too, in areas where strong tidal currents clash or run over rippled bottoms, causing the sea surface to bobble and dance.  This can aerate the water so that a boat entering the area floats lower, and loses grip on its rudder and propeller.

So watch out for white water. It's not something we think about often, and it sure looks innocuous, but those pretty little bubbles can spell trouble if you don't know the science behind them.

Today's Thought
 "Wouldst thou,"—so the helmsman answered,
"Learn the secret of the sea?
"Only those who brave its dangers
"Comprehend its mystery!"
— Longfellow, The Secret of the Sea

 What do you call a fish with no eyes?
A fsh.

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