April 26, 2015

Beware the dreaded hinge effect

HAVE YOU EVER FELT the deck of a fiberglass boat “give” with your weight as you walk over it? Has it ever given you concern, because you were thinking of buying that boat? And did you come to the conclusion that if the springy deck can carry the load of your walking about on it, all must be well?

Ah, but it’s not, apparently. According to one of Britain’s foremost naval architects and surveyors, Ian Nicholson, no boat can be deemed seaworthy if it has flexible panels that encourage the dreaded “hinge effect.”

This is a condition in which the edges of the panel flex back and forth, giving rise to a series of approximately parallel cracks along the edge of a panel. The repeated bending eventually cracks the gel coat.

In his book, Surveying Small Craft, Nicholson points out that you’ll often see this trouble along each side of the bottom of a cockpit well. It happens where there is no cockpit grating is fitted to distribute the crew’s weight evenly over a large area, or where the grating is too flimsy for the job.

It is widely found where the cockpit sole is weakly supported or where the bottom beams are not carried to the full width of the well,” says Nicholson. “It is sometimes seen when these beams are spaced too far apart, or end too sharply.”

And here’s the big warning from the expert: “If the hinge effect is allowed to develop, eventually the whole panel will drop out.”

Nicholson says this is admittedly rare but is occasionally found in the bottoms of high-speed powerboats where the whole hull pants in and out at each impact with the sea’s surface. “The reversing strain, occurring thousands of times each day, soon cracks through the gel coat and then sets about fracturing the glass fibers.”

The hinge effect is particularly troublesome with sandwich construction, which is how many sailboat decks are built, and even some hulls. “If the outer skin does not continue to grip the core, the trouble may develop very quickly,” Nicholson adds. “The first indications of the hinge effect are, of course. the roughly parallel lines of cracks along the ‘hinge line.’”

So now you have been warned. Large areas of fiberglass, especially flattish areas, should be absolutely stiff under load. In the old days, we learned the hard way about metal fatigue when the flexing wings of Comet airplanes broke off in flight.  Fiberglass fatigue may not be as dramatic, but it’s not something you want to experience on your boat.

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Today’s Thought
Whenever your preparations for sea are poor, the sea worms its way in and finds the problems.
— Francis Stokes

Tailpiece “Waiter! Get your thumb off that steak.”
“Very well, sir, but if it falls on the floor again it’s your fault.”

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

4 comments:

Alden Smith said...

For most yachts that spend their live mooring in marinas the hinge effect doesn't matter. But it matters in serious cruising yachts.
I have heard of cleats being ripped out of the deck because of this problem (often NO wooden backing plates planned into the design).
The worst problem is having the main anchor bollard simply bolted to a hinge effect deck instead of a proper anchor capstan / bollard post that runs from the deck to the the lower stem of the boat - being through bolted or glassed in at its base.

Boon Chiew Cheng said...

The Comet jetliners crashed, not because the wings broke off, but because of "catastrophic metal fatigue in the airframe". They had large square windows and cracks developed at the corners due to metal flexing caused by pressuring and depressuring the fuselage. That's why all aircraft now have round or oval windows or large radii in corners of square/rectangular windows.

Boon Chiew Cheng said...

Mr Vigo
I posted a comment earlier about the Comet, that it was not the wings breaking off that caused the crash. If I sounded a bit abrupt like a know-it-all, I apologise. That was not my intention at all. I have every respect for your knowledge and experience about boats, the sea and sailing. In fact, I look forward to reading your posts almost every day and I have learnt a great deal from them. I wish you fair winds always and many many more days of sailing.

John Vigor said...

Hi Boon:

You have nothing to apologize for. Thank you for putting me straight about the Comet. It was a lovely-looking airplane. So sad that its square windows eventually led to tragic loss of life.

John V.