Sail with New Freebooters

CS Forrester, Patrick O’Brian, Dudley Pope… These and other authors have all written about young officers in Nelson’s Navy squirming under the intimidating pressure of oral examinations. Their young heroes usually had a tough time with the mathematics of navigation, but delighted in proposing ways of coping with steering gear and rigs severely damaged by storm or combat, even when their examiners then posited further damage to their ingenious jury rigs.

Rigs have probably been the weakest link in seagoing vessels since the earliest days, and the first two days of Vendée Globe 2008 have been no exception. There’s a good reason for this: if you over-engineer the rig, the increased weight reduces stability, and hence the ability to carry sail. The Ocean 60 class rules take this into account by including a stability test, so the challenge is to produce a tall, strong rig for the minimum possible weight.

The stress analysis challenges are like those for civil engineering structures such as viaducts, only more complex. It’s easy to calculate the static loads, but what really matters in civil engineering structures is the resonances caused either by traffic or by high winds. Victorian engineering is littered with examples of failures caused by not understanding or being able to assess this. The damping effect of having a sail attached to it makes it a far less common concern for a mast, but there is a far more serious problem which viaduct designers don’t have to cope with.

Masts are attached to boats, and when the sea gets rough boats leap half out of the water and come crashing down - sometimes into a wave that is coming up to meet them, and possibly knocking them sideways at the same time. That’s what smashed masts in the Bay of Biscay this year, often after the wind had abated significantly.

Places like Southampton University’s Wolfson Unit are now highly skilled at using tank tests to assess the stresses on hulls, and modelling the way they react to those stresses. I don’t know of any group that has made similar progress in calculating the stresses on masts while attached to a freely moving boat. The number of degrees of freedom involved make for pretty computationally-intensive mathematical modelling, and the problem sounds too chaotic to model physically in a tank. How do you reproduce the kind of sea that builds up when strong winds shift through 90 degrees?

Racing in extreme classes like the Ocean 60 has helped collect the data that has led to class rules and  international standards, for hull construction and materials. I would expect it to contribute to similar standards for rigs at some future date. Certainly examination of the failures that have occurred in recent years will establish what probably caused them, but I don’t know of any boat carrying recording instruments that could give an idea of the actual stresses that occurred in specific incidents.

For instance, does any boat carry an accelerometer? Many years ago, offshore powerboats suffered serious damage without even hitting solid objects in the water.  In one case, a heat exchanger broke off and went through the bottom of the boat when it landed after jumping off a wave, and in another, one crewman finished a race with compression  fractures in both legs.

If there is anyone reading this who can point me in the direction of research being done on stresses on rigs in rough seas, I’ll be very grateful. It’s well beyond my own mathematical and computational skills, but I’d still like to read about it.