Boat­yard

Pro­pel­ler shafts and their ac­cou­trements are pretty tough, but poor main­te­nance can shorten their life spans. By Mike Smith

Power & Motor Yacht - - IN THIS ISSUE -

Ne­glect­ing your driv­e­train could spell big trou­ble af­ter a few years, but sim­ple main­te­nance can pre­vent catas­tro­phe.

One of the eas­i­est chores on a boat is main­tain­ing the driv­e­train, ar­guably the most ba­sic, most re­li­able on­board sys­tem. A typ­i­cal shaft-and-pro­pel­ler driv­e­train is com­prised of sim­ple com­po­nents that last for decades and de­mand just ba­sic TLC—un­less you hit some­thing and knock things out of whack, or you want to fix some­thing that ain’t broke. I’ll bet that, some­where, there’s a cen­tury-old stuff­ing box that’s still work­ing fine, need­ing only repack­ing ev­ery few years. If you’re a “hands-off ” owner, driv­e­train main­te­nance is just the thing.

But that doesn’t mean you can com­pletely ig­nore your driv­e­train. If you do, af­ter a num­ber of years you might find wa­ter com­ing in around the pro­pel­ler shaft, thanks to worn-out pack­ing or a rot­ted shaft-seal bel­lows, a sit­u­a­tion whose con­se­quences can range from an­noy­ing to cat­a­strophic. While a cas­cade of un­wanted sea­wa­ter is most dra­matic, there are other, more in­sid­i­ous, con­se­quences of driv­e­train ne­glect, in­clud­ing worn en­gine mounts and ex­ces­sive vi­bra­tion. For­tu­nately, it’s easy to pre­vent all of the above.

A Driv­e­train by Any Other Name

Many ex­perts con­sider the driv­e­train to be the string of com­po­nents be­tween the gear­box and pro­pel­ler, start­ing at the shaft cou­pling. I dis­agree. I say the driv­e­train starts with the en­gine mounts. In most boats, they trans­fer the thrust of the pro­pel­ler to the hull. The prop pushes against the shaft, the shaft against the gear­box (there’s a thrust bear­ing in­side), the gear­box is bolted to the en­gine and the en­gine’s at­tached to the boat via its mounts. A solid mount is best at trans­fer­ring prop thrust, but also trans­fers vi­bra­tion to the hull; a su­per-soft mount makes for a nice ride, but lets the en­gine shift too much. It’s a com­pro­mise, but one with a good, al­beit costly, work­around; get out your check­book and keep read­ing.

Pro­pel­ler thrust both pushes the en­gine for­ward, against the

mounts, and im­parts a twist­ing force due to the torque of the spin­ning blades. Th­ese forces grad­u­ally wear out the flex­i­ble com­po­nents in the mounts. Since the en­gine is more of­ten in for­ward gear than re­verse, the twist­ing causes the mounts on one side to wear out sooner than the other. We’re talk­ing years for this to hap­pen, of course—given the light us­age most yachts get, maybe a decade or more. You can’t see the wear, but if your boat starts vi­brat­ing un­der­way, and a feeler gauge shows the shaft is prop­erly aligned at rest, maybe one or more en­gine mounts has got­ten soft and is let­ting the en­gine shift a bit too far un­der load. Time to call in an ex­pert. If you’re buy­ing a used boat, es­pe­cially one that’s been heav­ily fished, or used for char­ter, it’s some­thing to check, and to bud­get for.

Check the align­ment be­tween the gear­box and shaft cou­pling ev­ery once in a while—how of­ten ranges from never, if ev­ery­thing is run­ning smoothly, to of­ten, if you’ve picked up vi­bra­tion un­der­way, or your en­gine’s jump­ing around like a jack-in-the-box, or you’re wear­ing out Cut­less bear­ings too of­ten, or you’ve hit some­thing hard enough to da­m­age the run­ning gear. But be­fore pulling things apart, take a swim and check that there’s not some­thing wrapped around the shaft.

In wooden-boat days, me­chan­ics would break the cou­pling be­fore the boat was hauled, so any flex­ing of the hull when out of the wa­ter— com­mon with wooden boats—wouldn’t throw things out of whack. Once the boat was back over­board, the me­chanic would check the align­ment when re­con­nect­ing the cou­pling flanges. Modern fiber­glass boats don’t flex much, so that pre­cau­tion is sel­dom ob­served to­day. Al­though it’s still not a bad idea.

Check­ing and ad­just­ing align­ment takes a skilled me­chanic about half an hour. It in­volves loos­en­ing the cou­pling, then us­ing a very thin feeler gauge to check that the two flanges of the cou­pling line up al­most per­fectly, side to side and top to bot­tom. The rule of thumb is no more than .001 inches of mis­align­ment per inch of flange di­am­e­ter. The me­chanic will check the align­ment, then ro­tate the shaft 90 de­grees and check it again; changes in align­ment dur­ing ro­ta­tion can in­di­cate a bent shaft. Align­ing an en­gine in­volves mak­ing mi­nor ad­just­ments at the en­gine mounts, but it’s a bit like cut­ting all the legs of a wob­bly ta­ble to the same length—be­fore long the ta­ble’s about knee-high and the top’s still not level. It’s best left to a pro­fes­sional.

Flex Your Cou­pling

Many boats to­day have a flex­i­ble cou­pling be­tween the gear­box and prop shaft; if yours doesn’t, you might want to add one. A flex­i­ble cou­pling—Drivesaver from Global Ma­rine Prod­ucts ( gc­s­ma­rine.com) is one, but there are sev­eral other brands—can com­pen­sate for slight shaft mis­align­ment while re­duc­ing vi­bra­tion from the pro­pel­ler. It’s ba­si­cally a thick, hard plas­tic washer that fits be­tween the halves of the shaft cou­pling; it’s not very flex­i­ble, re­ally, but more flex­i­ble than a steel-on-steel con­nec­tion.

A flex­i­ble cou­pling also in­su­lates the prop shaft from the gear­box, pro­tect­ing against da­m­age from stray cur­rents. (It’s easy to make an elec­tri­cal con­nec­tion if de­sired, us­ing a piece of cop­per strap; some cou­plings use a pro­pri­etary con­nec­tor in­stead.) But the big­gest ad­van­tage of a flex­i­ble cou­pling is its weak­ness: Should the pro­pel­ler strike an im­mov­able ob­ject, the cou­pling is de­signed to fail, to break apart and pre­vent da­m­age to the gear­box and en­gine. (Okay, the prop and maybe shaft and strut are his­tory, but at least the other ma­chin­ery is still in­tact.)

In­stalling a flex­i­ble cou­pling is easy, at least on pa­per. Break the shaft cou­pling, slide the shaft back just enough, but not so far you knock the pro­pel­ler against the rud­der. Sup­port the un­cou­pled shaft; don’t just let it hang. Then bolt the flex cou­pling to the gear flange. Slide the shaft into place and bolt the shaft flange to the cou­pling. (Two sets of bolts are used to at­tach the cou­pling in­de­pen­dently to the two flanges.)

Sounds easy, right? In most cases it is, un­less the dis­tance the cou­pling shifts the prop shaft aft makes a dif­fer­ence. Most flex cou­plings are from 1- to 1.5-inches thick, de­pend­ing on the cou­pling di­am­e­ter. In some cases, the cou­pling moves the prop far enough aft of the strut to leave too long a span of un­sup­ported shaft—what Naval Ar­chi­tect Dave Gerr calls “pro­pel­ler over­hang.” It’s the dis­tance be­tween the af­ter side of the strut and the for­ward side of the pro­pel­ler hub; some over­hang is nec­es­sary to al­low free flow of wa­ter through the Cut­less bear­ing, but too much can cause vi­bra­tion.

Gerr rec­om­mends an ideal over­hang of 1x shaft di­am­e­ter; 1.5x is ac­cept­able, 2x the max­i­mum—and only if it’s un­avoid­able. For ex­am­ple, if a 2-inch shaft has a 2 inch over­hang, adding the 1-inch-thick flex cou­pling brings the over­hang to 3 inches, or 1.5x shaft di­am­e­ter. Okay, but if the over­hang is al­ready 3 inches, the cou­pling makes it 4 inches, 2x shaft di­am­e­ter, and you’re up against the max­i­mum. To be safe, mea­sure first, or check with your yard man­ager for ad­vice. If it’s nec­es­sary to cut the shaft to com­pen­sate for the width of the flex cou­pling, that usu­ally means re-ma­chin­ing the key­way, and the shaft, too, if it’s ta­pered. A sim­ple job be­comes not so sim­ple and more ex­pen­sive.

Stuff It

No mat­ter what’s go­ing on at the gear­box, the pro­pel­ler shaft has to exit the hull, and that means a stuff­ing box or shaft seal to keep the wa­ter out. A stuff­ing box is ba­si­cally a hous­ing to hold wa­ter­tight pack­ing, and an ad­justable com­pres­sion sleeve to tamp it down around the shaft. (Stuff­ing box de­signs dif­fer, but the prin­ci­ple is the same.) The stuff­ing box can be bolted di­rectly to the shaft log—a “fixed” box—or mounted in a heavy hose that’s clamped to a shaft tube or a “flex­i­ble” box. A flex­i­ble box, more com­mon on modern boats, ab­sorbs mi­nor shaft mis­align­ment, but the hose and its clamps need pe­ri­odic at­ten­tion; if the hose fails, lots of wa­ter will fol­low. A fixed box is more se­cure, in my opin­ion, but mis­align­ment can wear

out its pack­ing quickly, and even loosen or break its fas­ten­ings, al­low­ing wa­ter to leak in around it. Reg­u­lar in­spec­tions should pre­vent both is­sues.

No mat­ter the style of stuff­ing box, it should be ad­justed so no wa­ter seeps in when the boat’s idle, but there’s an oc­ca­sional drip for lu­bri­ca­tion when the shaft’s turn­ing. What’s “oc­ca­sional?” De­pends on who you ask. Some ex­perts say two or three drops per minute, oth­ers one per minute; some say one ev­ery few min­utes. I’d err on the side of more, within rea­son; too lit­tle wa­ter, and the shaft will soon get as hot as a pis­tol, which can da­m­age it. As a young, know-it-all mate, I once repacked a stuff­ing box too tightly, and af­ter about five min­utes un­der­way the shaft was so hot the wa­ter that drib­bled onto it boiled away. (Good thing I checked.) Keep the area around the stuff­ing box/shaft seal dry and clean, so if ex­ces­sive wa­ter does start leak­ing in, it will be eas­ier to no­tice dur­ing your in­spec­tions.

If wa­ter is drip­ping in steadily, like a leaky bath­room faucet, it’s time to tighten the gland a bit, or maybe repack it. It’s an easy, do-it-your­self job with the boat out of the wa­ter. Modern pack­ing is made of graphite im­preg­nated with Te­flon; it looks like square braided rope. Pull out the old pack­ing (there’s a tool for it, ba­si­cally a corkscrew on a flex­i­ble stalk), then re­in­stall three or four sep­a­rate turns of the cor­rect size pack­ing, stag­ger­ing the joints. Push each one into place with the com­pres­sion sleeve be­fore adding the next. Fi­nally, tighten the sleeve handtight, plus a bit more, and set up the lock­ing nut. When the boat’s back in the wa­ter, check your work; if a lit­tle wa­ter shows up, that’s okay. Start the en­gine, mea­sure the drips and cor­rect ac­cord­ingly. There’s a stuff­ing box on your rud­der, too, so check it as well.

Seal It

Maybe it’s time to up­grade to a drip­less shaft seal. It does away with the stuff­ing box, and the drips, and re­quires very lit­tle main­te­nance. PYI, Inc. builds a pack­less shaft seal that’s easy to in­stall with the boat out of the wa­ter.

The PYI shaft seal con­sists of a rub­ber bel­lows that fits over the shaft and clamps to the stern tube; there’s a non-ro­tat­ing car­bon flange at­tached to the bel­lows that makes up one-half of the seal. The other half is a ro­tat­ing stain­lesssteel disk se­cured to the shaft. Push­ing the disk against the flange to com­press the bel­lows slightly, and lock­ing it in place with set screws pro­vides the seal. Sim­ple so far, but ves­sels faster than 12 knots re­quire a sep­a­rate sup­ply of cool­ing wa­ter in­jected via a hose barb on the car­bon flange. PYI rec­om­mends tap­ping off an ex­ist­ing sup­ply of raw wa­ter. That com­pli­cates the in­stal­la­tion some­what, but once it’s done, the shaft seal is a set-and-for­get sys­tem, at least for a few years. PYI rec­om­mends in­spect­ing it reg­u­larly, with a re­build, in­clud­ing new bel­lows, ev­ery six years.

Maybe I’m just get­ting old, but I don’t see any­thing wrong with a con­ven­tional, time-proven stuff­ing box, one that doesn’t need cool­ing wa­ter plumbed in. Okay, so they take a bit of main­te­nance, but re­new­ing the pack­ing is a mi­nor job, and one most skip­pers won’t have to do more than ev­ery cou­ple of sea­sons. Some­times old stuff is the best.

Proper rud­der in­stal­la­tion is just one com­po­nent of cre­at­ing an ef­fi­cient driv­e­train.

When was the last time you in­spected your shaft seals? Be hon­est.

Proper driv­e­train align­ment starts at the prop and con­tin­ues all the way to your en­gine mounts.

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