November 27, 2012

How fast will you sink?

THE TSUNAMI IN JAPAN washed out to sea a great deal of debris that is now headed for the Pacific Northwest coastline. Some of it has already arrived, of course, including a huge floating concrete dock that washed up on an Oregon beach — not the kind of thing you want to run into with your yacht in mid-ocean on a dark and stormy night.

But the fact that there’s lots more debris on the way got me wondering what I would do if I were in a boat whose hull was pierced by something good and solid. 

The depressing fact is that most bilge pumps cannot cope with the inflow of water from a reasonably sized hole in the hull. If it happens to you, your priority should be to try to stem the flow somehow, either by stuffing cushions or something similar in the hole from inside, or by covering the hole with a collision mat, or a sail, from the outside. That should slow down the flow enough to enable your pumps to cope while you make more permanent repairs.

But unless conditions are ideal, a surprisingly small hole will sink you in short order.

For the record, here’s the formula for rate of flooding from an underwater hole:

Incoming gallons per minute = D x square root of H x 20

(D = the diameter of the hole in inches and H = height in feet to which the water must rise to reach outside water level — in other words, the depth of the hole below water level.)

Note that a mere 2-inch-diameter hole 3 feet below the waterline will let in 69 gallons a minute, or more than 4,000 gallons an hour. A high-capacity power pump is rated at 3,000 gallons per hour. The pump is simply not going to win.

Today’s Thought
In smooth water God help me; in rough water I will help myself.
— George Herbert, Jacula Prudentum

“How was the concert?”
“Terrible. I had to change my seat four times.”
“Some man bother you?”
“Yeah — finally.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

1 comment:

Anonymous said...

Though I can't find it right now there was a blog post by someone who used expanding foam to seal a good sized hole quite a ways offshore.
It sealed perfectly and was actually difficult to remove when they hauled the boat out upon arrival.
Two qualifiers:
1. You should come to as complete a stop as possible to keep it from washing away and try to block as much as you can of the rest of the hole as you're spraying.
2. Religiously observe the expiration date on the can because it will eventually "set" inside the can and be unusable. I take it home a little bit early and use it wherever.