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Bearings and Shafts on Your Boat

Bearings and Shafts

These simple technologies keep your boat from sinking or shaking herself to pieces.

Many years ago while “vacationing” in Thailand courtesy of the U.S. Army, I ended up in a harbor. Southeast Asia is full of oddities, marine and otherwise, and they’re so ubiquitous that you tend to become inured to them. But one I saw there really impressed me. It’s called the ruea hang yao or long-tail boat, and it is a marvel of ingenuity and resourcefulness. It consists of an automotive engine mounted on the back of a native canoe—actually on top where the gunwales come together in the canoe stern. Directly connected to it—there is no transmission—is a long shaft that terminates in a propeller. The engine is swivel-mounted so it can be turned to steer the boat or depressed so that the propeller comes out of the water, allowing for shallow-water operation and, at the extreme, stopping forward motion. There being no marine gear, there is not only no neutral but no reverse, a deficiency that is handily overcome by the owners’ extraordinary piloting skills.

Occidentals routinely mock and lampoon the long-tail, but the design has many advantages, not the least of which is that it is easily and cheaply constructed using locally available components. Another one that was often pointed out to me by locals is that it does not require a hole in the boat below the waterline to accommodate a propeller shaft. Nor, for that matter, does it require any underwater gear at all. Indeed, I recall one local Thai gentleman expressing wonderment that any sane individual would put a hole in a perfectly watertight hull.

Thai fishing boat

The ways in which we crafty westerners have figured out how to do precisely that is the subject of this month’s column. Unless your boat is powered by an outboard, you have at least one hole in your boat’s bottom through which a propeller shaft passes. How this is accomplished without sinking your boat is a story worth telling.

The most basic mechanism involved here is the stuffing box, which as the name implies compresses material around the shaft so that sea water cannot pass yet allows the shaft to turn freely. (Also termed a gland, the stuffing box is not restricted to boats; most conventional faucets in your home use a variation that allows you to turn the tap without incurring leakage.) The basic components of a boat’s stuffing box are a cylindrical enclosure slightly larger than the propshaft that is typically constructed of brass or bronze and is threaded at one end to accept a compression nut and a lock nut; packing, which is usually braided fiber treated with wax or a similar substance to make it waterproof and lower its frictional resistance; and a rubber hose that attaches the stuffing box to the shaft log, the part which actually passes though the hull.

Operation is straightforward. Once you uniformly stuff the packing between the propshaft and the enclosure, you turn the compression nut until you feel considerable resistance (yes, this is a matter of trial and error), after which you turn down the lock nut to prevent the compression nut from backing off. The resulting seal is not meant to be watertight; there should be occasional dripping—exactly how much is a matter of personal opinion. Most service people I’ve talked to like to see dripping even when the shaft is not turning, and more frequent dripping—say, a drip every 30 seconds—while the shaft is turning. Others say there should be no drips when the shaft is idle. For what it’s worth, I subscribe to the former opinion. 

Because the packing further compresses and wears over time, you must periodically tighten the compression nut (after loosening the lock nut); eventually you will need to replace the packing material altogether. Failure of this seal has catastrophic consequences, so the prudent mariner checks it every time he or she leaves the dock. Simply shining a flashlight on the lock nut to check for the telltale drip is all it takes. Note that overtightening will overheat the seal and so is as much to be avoided as not enough compression.

Most newer boats have what are termed dripless shaft seals, which differ mainly in that they have synthetic packing material, typically (various composites and highly burnished stainless-steel surfaces) and require little maintenance compared to traditional packing glands. These are basically closed systems—hence the term dripless—and typically require infrequent tightening or none at all. (For a thorough discussion of maintenance for dripless shaft seals, check out “Maintaining Dripless Shaft Seals”.)

Another component that long-shaft boats don’t need to mess with is a support bearing for the propshaft. The result is some disconcerting (to westerners at least) flexing that doesn’t seem to affect the long-tail’s performance. When conventional inboard boats of the sort you and I are used to reach a certain length, their propshafts also start to flex, which can produce jaw-grinding vibration and extra drivetrain wear. To avoid this, we have the stave bearing, encased in a bronze strut roughly midway between propeller and shaft log. 

The nomenclature stems from the use of water-lubricated wooden (typically lignum vitae) slats or staves as bearing material in early powerboats. Today a more common term is “cutless bearing,” which is actually a trademark of Duramax Marine’s rubber bearing material. Over the years “cutless” devolved into “cutlass,” which despite what your pals at the pub say, has nothing to do with pirates. Boatyard columnist Mike Smith offers a very interesting explanation for how all this happened, by the way, in “It’s Easier On The Hard” in our October 2014 issue. 

In any case, modern stave bearings employ synthetics such as Duramax’s in place of wood. Still water-lubricated, they are as close to maintenance-free as possible, although some bronze struts have a small hole to admit water. Of course, it’s important to keep this hole free of marine growth and paint. So yes, ruea hang yao operators may have some navigational challenges on their hands due to a lack of gears, but hey, at least they don’t have to worry about stuffing boxes, shaft seals, and bearings.

This article originally appeared in the October 2014 issue of Power & Motoryacht magazine.