How to make an intelligent mast

Surely every voyager, beset by heavy weather, has gazed apprehensively at his or her mast and wondered: “Is the mast being stressed to the breaking point as we pound into these seas?” “Is my rig about to come crashing down?”

When caught in this situation most voyagers probably fall back on a determined faith in the strength of their masts based more on familiarity than hard data. But what if there was a way to check on the stress that your mast was experiencing? What if you had advance warning that your mast tube was nearing its breaking load and needed an immediate sail reduction before it collapsed into the drink?

Providing that type of advance warning could be one use of a new “intelligent mast” system developed at Aston University in Birmingham, U.K., and now offered by a company called Smart Fibres Ltd., in Hamble, U.K. This stress-sensing technology uses optical fibers, just like the thin, light-carrying fibers used to carry phone calls, to gauge the stresses imposed on a mast as it works in a seaway. And, for increasingly common composite masts, the fiber strain gauges can be built right into the structure during the layup process.

Smart Fibres has built a few of their Smart Fibre System masts in association with Carbospars, also in Hamble. These boats include: Jacquelina, a 115-foot sloop that uses a carbon-fiber AeroRig setup; Velsheda, a 126-foot stayed mast J-class sloop; and a spar for the 105-foot yacht Never Say Never. This optical-fiber approach has also recently been built into an AeroRig mast for Smart, a Beneteau 42-s7 that Smart Fibres is using as a technology demonstrator.

This strain-sensing technology works by sending light pulses up the optical fibers built into the mast. However, while most optical fibers are built to be clear and unobstructed, so as to lose as little of the light passing through them as possible, the fibers used in this mast system are built with small sections that contain a series of tiny bumps called a Bragg grating. The bump pattern of a Bragg grating is tuned to interfere with a specific wavelength of light. Light that is not the right wavelength will pass by the Bragg grating without a second glance. But when the wavelength of light matches the period of the grating, some of the light is reflected back down the fiber. Thus, by building Bragg gratings with different bump patterns into a length of fiber and pulsing light of appropriate wavelengths down the fiber, you can get a reflection from each grating.

When the mast is put under strain, the mast material flexes, and this causes the optical fibers to flex as well. Flexing causes the distance between the bumps in the grating to increase or decrease. This flexing of the bump pattern will slightly change the wavelength of light the grating will reflect. Changes in reflectivity are directly related to the amount of flexing of the fiber and, therefore, to the strain experienced by the mast. Since most masts have multiple sensors, the photodiode array and control computer that runs the system needs a way of recognizing which Bragg grating is reflecting at any given time. This is done by constructing each Bragg grating to reflect a specific frequency range or color of light. Each grating has a notch of frequency it will respond to. So if the control unit sees a reflection in that range it knows which grating is responding to each broadband light pulse.

This system provides a sophisticated method for determining the actual loads on a mast, rather than the merely theoretical loads derived from computer modeling programs. “Our guys have been really excited at getting their hands on real data,” said Dr. Lorna Everall, opto-electronics engineer for Smart Fibres. “They’ve never had actual load data to compare to the multiframe program [they use].” This information is, obviously, extremely helpful for use in mast design. Another approach that some superyacht building programs have taken is to build the system into the mast for the purpose of speeding the tuning of a newly installed mast. “We’ve seen it used as a setup tool for tensioning a mast,” said Everall. “They just use it once and perhaps never use it again.” At least not unless the mast needs to be unstepped and then restepped and tuned.

Strain gauges are nothing new, of course. There are mechanical and electronic strain gauges available that have been used to monitor mast stress. These devices, however, are bulkier and less reliable than the opto-electronic method of embedded fiber optics, Bragg gratings, and a computer controller. While it makes more sense to build a new mast with this system, it should also be possible to retrofit the fiber sensors to an existing mast. Smart Fibres is selling this technology for other applications as well. These optical-fiber sensor systems can be used in aircraft and automobiles, and even for measuring the health of structures like bridges and buildings. The British Millennium Dome at Greenwich has a Smart Fibres installation for measuring the tension in the dome material.

Of course, this mast sensor setup isn’t something that most voyaging sailors will readily order for their boats. The experimental work has been done and the system is commercially available, but the question is who, other than superyacht owners, will actually want such a system on their boats. While some sparmakers may see the British Smart Fibre System as a valuable tool for mariners, two of the leading carbon-fiber mast builders here in the U.S.neither of which is offering the option of installing such a systemdon’t predict much of a market for intelligent masts. “I don’t see the need for this type of equipment unless it’s for a test mast,” said Eric Hall of Hall Spars, one of the premier carbon fiber mast manufacturers. “I don’t think [knowing] stresses and strains are going to help you much. If you need to know when the mast is about to fail, then the mast isn’t strong enough.” And Ben Sprauge, marketing director for Goetz Marine Technology in Bristol, R.I., a high-tech mast and yacht builder, said that GMT doesn’t have any plans to offer such a mast system.

According to Hall, such a fiber-optic strain-sensing system would be valuable for testing high-end masts to be used in rigorous conditions. “For very ambitious masts for America’s Cup, or Whitbread, or Admiral’s Cup boats, we like to know, on a test basis, all the stresses on a mast,” said Hall. But for voyaging sailors Hall felt dependability was more important than embedded strain gauges. “I’d rather develop a rig that gets a cruiser where they’re going in a reliable way.”

Still, when the wind is really cranking and you’re flying little more than a handkerchief, wouldn’t it be nice to have a system offering reassurance that your mast is not about to snap in two like a popsicle stick?

By Ocean Navigator