Heavy Metal (Part 2)

In the second of a two-part series, we look at the tools and techniques needed to pull the shaft, replace the cutless bearing, repack the stuffing box and put things back together

August 7, 2002

Attention to detail pays off for those working on the hardware that connects the reverse gear to the propeller. This drive train endures considerable wear and tear. Small things like a few too many thousandths of an inch misalignment can cause real trouble. Good engineering practice counts. Here’s how to make sure all the bases have been covered.

The coupling’s clearance, or alignment, is an often discussed subject, just as often misunderstood or mismeasured. The general rule of thumb is that one-thousandth of an inch of clearance (measured at the outer edge of the coupling) is permitted for each inch of coupling diameter. A coupling five inches in diameter, a common diesel auxiliary size, can tolerate five-thousandths of an inch of misalignment. While this is considered acceptable, a much more desirable figure would be below three-thousandths of an inch.

High-quality hose clamps for the stuffing box and a straight shaft are linch-pins of an effective drive system. A good machine shop can check the trueness of a shaft, cut a taper for the prop and fit and face couplings.


The procedure for measuring this clearance requires some patience and, depending on the installation, some contortions. First, the coupling bolts must be removed. These should be stowed in a safe location, preferably where they can’t be knocked into the bilge. Inspect these bolts carefully for excessive rust or signs of stretching and fatigue. Now separate the coupling from the output flange. The shaft should spin freely, but with some effort. If the prop is installed, this adds quite a bit of mass to the shaft. If it is a folding model, its flopping blades can make spinning even more difficult. Loosening the packing nut on the stuffing box may also ease resistance. The surface of the flanges should be clean and free of rust. They should be sprayed with a silicone-type lubricant and buffed lightly with a scuff pad, then wiped dry and sprayed again. It must be possible to push the shaft coupling tightly against the output coupling to carry out the measurements. Once the two halves have been pushed together, attempt to fit the largest feeler gauge possible into the joint where they meet. Once a feeler gauge has been found that will slide into this space, again, be sure that it is the largest one that will fit with a slight amount of resistance. The gauge should penetrate only a portion of the joint. If it fits in through 360 degrees, the two halves were not pushed together tightly enough. If the largest feeler gauge that fits into any portion of the joint is three-thousandths of an inch, for example, and the coupling diameter is five inches, then no further alignment is necessary. However, if the clearance is nine-thousandths of an inch some engine movement will be required. If this clearance is at the 9 o’clock position on the flange (viewed from aft looking forward) then the front of the engine would have to be moved to port. If the clearance is at the 4 o’clock position, then the front of the engine would have to be moved to starboard and the back of the engine raised, or the front of the engine moved to starboard and lowered.

This procedure is time-consuming, even for seasoned professionals using hydraulic expansion tools. Perseverance and patience are essential. The entire procedure will be made much easier if the adjustable motor mounts are in good condition and still adjustable. Most motor mounts use a threaded rod with a nut above and below a flange that is bolted to the engine or transmission. Raising or lowering the nut will raise or lower the engine. This is difficult even when the mounts are new. Unfortunately they are usually constructed from mild steel and they seem to rust almost immediately. When I install a new engine or begin work on a new boat, I always spray these threads with corrosion inhibitor so that I, or the owner, can be saved some grief down the road. If these threads are rusted beyond repair, then alignment will be virtually impossible. It will be necessary to replace these mounts before beginning. This might not be a bad idea anyway, as the rubber in these mounts wears out from the vibration. This will cause excessive movement of the engine, resulting in further misalignment.

Moving the engine to port or starboard is usually a bit easier. The motor mounts should have oval-shaped holes either at their bases, where they are bolted to the stringer, or in the flange where the threaded rod portion of the mount passed through it. Loosening the nuts and using a bar or two-by-four will allow lateral movement. If the mounts are lagged into a stringer and these lags are made of mild steel, remove a few entirely to inspect the threads. If these lags have lived in a wet stringer, then the depth of the threads has probably been diminished. The “wet” stringer should be repaired, and the lags replaced with stainless steel or bronze hanger bolts — fastenings that are half lag bolt, half threaded rod. Remember, while under power, the thrust of the prop is connected to the vessel entirely by the motor mounts and their associated hardware. As a result, it is vitally important that they be kept in excellent condition.


Double nutting a prop is done with the smaller nut closer to the propeller. Check the cotter key for wear and corrosion. A prop strut should be firmly bonded or bolted to the hull and the cutless bearing replaced when the shaft shows excessive side play. A split coupling makes shaft removal much easier. Conventional couplings often must be cut to free them from a shaft.

Once the alignment procedure has been carried out ashore it is recommended that it be carried out again afloat. This practice is a necessity for wood vessels due to their flexibility. As for FRP vessels, the movement varies with hull stiffness. Because these measurements are made to the thousandths of an inch, it is not unrealistic to expect that amount of movement or settling, especially if the shaft is long (over five feet). Recheck alignment after the vessel has been allowed to rest afloat (four to six hours for FRP construction and 24 hours or longer for wood). If the boat is built of steel or aluminum, then it is safe to assume that very little resting time will be required.

Before you begin, determine what tools will be needed. Specialized marine service tools can’t be rented at the local tool rental shop. Some tasks may have to be left up to yard personnel who possess the necessary tools and are experienced with them.


In order to remove and replace a full complement of running gear, you’ll need a prop puller. There are many different varieties; expect to spend about $200 for a good one. This may only pull conventional props, not feathering or folding models. Alternatively, if you are able to cut and drill 3/8-inch plate steel, then you can construct a puller from this plate and a few lengths of 1/2-inch threaded rod. This type of puller works well, and with some ingenuity it can be adapted to pull most any type of prop. Using an automotive-type gear puller usually does not work because the leading edge of the prop hub is too small and rounded, causing the puller jaws to slip. Worse yet is what I call a “bang nut” puller, usually with an adjective preceding it. This is simply a large piece of bronze round stock that is threaded on one end to fit the shaft. The user removes the prop nuts, screws this device into place, then proceeds to hammer the living daylights out of it without mercy, tormenting the output bearings in the transmission. Theoretically, the vibration and shock action cause the prop to surrender its grip on the shaft. It could take in excess of 25 blows for this to occur. I do not recommend the use of this “tool.”

The coupling also may need to be coaxed off. Dedicated coupling removal tools are manufactured, but their cost is usually prohibitive for boat owners. Some of these tools use a slide hammer arrangement that tugs the shaft out of the coupling with a few sharp jerks. They require an adapter to fit each size shaft, so they are usually sold as a kit. Again, a simpler arrangement can be constructed using some plate steel and threaded rod. With these materials, you can make a “pusher” that attaches to the coupling. Using a short section of shaft material and the compression action of the threaded rods, the shaft is pushed out of the coupling. This pushing method is a slower but also gentler way to remove the shaft without placing any load whatsoever on the transmission.

The cutless bearing is the only other component that should require some type of special tool for service. Professional repair yards use a tool that will allow removal of a cutless bearing from a strut (but not from a stern tube) without removing the shaft. This is accomplished by using dedicated split drifts and hydraulic rams. It goes without saying that this is not an option for the do-it-yourselfer. A reputable yard may charge two hours to carry out this procedure. If shaft removal is not in the plan, then this may be the way to go assuming the yard possesses this tool.


Removing the shaft first renders bearing removal a bit easier for the novice. Once the shaft has been removed, the inside of the bearing will be accessible. After removing the set screw(s) and using a hacksaw blade, cut a groove into the brass or fiberglass shell of the bearing blade, until it is close to the strut. A low-profile blade-holding tool can be purchased at most hardware stores, or constructed from a section of wood trim. Milwaukee Tool also makes a handle that will hold a Sawzall blade. This works very well, because a more aggressive blade can be fitted for faster cutting. An electric Sawzall can be used, but it is very easy to overcut and damage the strut material. Once this cut has been made, drive a punch into the set screw hole to collapse the bearing. If the bearing proves stubborn, it may be necessary to fabricate some wedges from brass flat stock, which you can drive between the cutless bearing and the strut at the hacksaw cut. The wedge will cause the bearing to give way, separating the interference fit.

Installing a new cutless bearing is relatively straightforward. The inside of the strut should be clean of all debris. The new bearing should be placed in the freezer for several hours before attempting to fit it. This will cause it to shrink, which makes for easier installation. Some bearings may require custom fitting by sanding their exterior lightly. The fit should be tight, so don’t overdo this.

A simple bearing installation tool can be fabricated out of two blocks of wood or aluminum and the versatile length of threaded rod. The blocks should be drilled to accept the rod with washers and nuts. The rod is passed through the strut, a block placed at one end, the bearing and another block placed at the other end. As the nuts are tightened the bearing is pulled into the strut. A word of warning: Some struts are tapered, or are machined with a lip in one end. The tapering and lip are most often found at the trailing end. If so, the bearing can be installed only from the leading end of the strut. Once the bearing has been installed, the set screws should then be replaced using a thread-locking agent such as red Loctite.

If shaft removal is contemplated, a logical course of action would be to remove and recondition the stuffing box. This can only be accomplished with the shaft separated, if not removed altogether, from the coupling. The weak links in the stuffing box are the flexible rubber hose, the hose clamps and the packing. The hose, as mentioned earlier, is not ordinary rubber hose. It should be especially heavy walled. If this hose shows wear, or is over five years old, replace it. Be sure that it fits properly on the stern tube as well as the stuffing box. The clamps should be replaced at the same time.

The packing, a material that has probably received more than its share of expletives, is easily serviced. If the shaft has been removed from the boat, then the stuffing box can be reconditioned and repacked on the workbench. The hose should be removed as well as the packing nut or ram, and the old packing material. All metal parts should be thoroughly cleaned with a brass or stainless wire wheel. If a mild-steel wire wheel is used, it will embed tiny particles of steel in the bronze. This steel will then rust. While this condition may not be debilitating in any way, it is unsightly and will cause the bilge area beneath the stuffing box to turn brown. In a worst-case scenario, it may cause jamming of the packing nut threads. Once all of the parts have been cleaned, coat the threads with marine-grade grease. Then cut lengths of packing. Most boxes will accommodate three wraps, ever so slightly shorter than the circumference of the shaft, leaving a clearance of 1/8 to 1/16 of an inch. Each wrap should be pushed carefully into the packing nut with some grease. The butts of the wraps should be staggered approximately 120 degrees for a three-layered pack. Using more than three layers is probably not a good idea because it will not allow enough water to pass through, causing the forward wraps to overheat. Once they have overheated, the wax will melt out of them, rendering them porous. The stuffing box can then be slipped over a greased shaft and tightened down. Finally, it can be loosened, removed and installed on the stern tube. The packing will retain its shape until the shaft is installed through it. When installing the shaft, chamfer the forward end with a file so it will not catch on the packing as it passes through the stuffing box.

Products like drip-free packing are gaining approval in the marine industry. This stuffing box material is made of shredded Teflon and hydrocarbon lubricants and is used in conjunction with conventional flax packing (for more information call 412-843-2051). Another popular item is the non-packing-type stuffing boxes that use smooth, mated carbon/stainless steel surfaces to keep out water; P.Y.I. sells a good product called P.S.S. (Packless Sealing System). For more information call (206) 670-8915.

Once the vessel has been launched, the clamps on the stuffing box hose should be checked and the packing should be adjusted. It may take several hours of running time to establish an equilibrium for packing compression. It should be inspected frequently during the first five hours of running time and adjusted if necessary. Ensure that the locking nut or cotter pin is securely in place after each adjustment.

Some stuffing boxes are injected with excess seawater from the cooling system. This can be a mixed blessing. It will allow more cooling and lubricating water to reach the stuffing, thereby increasing its longevity. However, the nipple that enters the stuffing box to supply this water is usually brass, and brass should not be used below the waterline. I have broken several of these nipples off with just the slightest amount of pressure, all but one while the vessels were on the hard. The fracture usually reveals some pink, corroded brass and the threads are left in the stuffing box body. Many Asian-built vessels will utilize an even more dubious system of brazed or soldered copper tube. These are notorious for failing. If it were not for the vigilant eye of a launch driver at a yacht club where I was once employed, a 40-foot trawler would have been lost because of these failed soldered joints. If a suspect system is employed on your boat, consider eliminating it, or retrofitting with a more reliable material such as bronze (not as easily obtained in smaller-size nipples), or glass-reinforced polycarbonate (Marelon is the trade name).

According to the American Boat And Yacht Council, all underwater hardware should be bonded. This includes shaft, struts and stuffing boxes. Bonding performs several functions — most importantly, electrical shock prevention for shore-power equipped vessels. Next comes lightning protection/dissipation and third is corrosion protection. Simply stated, if all underwater hardware is bonded and electrically attached to a zinc it is better protected from galvanic corrosion. A shaft brush can be a worthwhile addition to any bonding system. This device provides a better connection between the shaft and the bonding system than the transmission would offer. The stuffing box, stern tube and strut should all be attached to the bonding system with at least #8 AWG wire or copper bonding strap. Obviously the shaft should be provided with at least one zinc and so should the strut, if provisions are made for it. Be sure that any underwater gear is free of all anti-fouling paint before installing a zinc.

Several innovations are becoming popular in the running gear field, most notably, dripless or packless stuffing boxes, folding and feathering props, line and weed cutters and flexible drive couplings shaft locks. All have some merit when used in certain applications. If long-term cruising is the plan, assume that all of your ordinary gear will fail at some point, hopefully not simultaneously, and all of your extraordinary gear will fail with greater regularity. Simply put, keep things simple and/or prepare with spares, especially for items that will not be available when off the beaten track.

The present running gear configuration has been around for a long time. Though not perfected, all its shortcomings and pitfalls are well known. As a result, the cruising sailor can be forewarned and thereby forearmed.

Steve D’Antonio is the director of operations at Harbourside Yachts in Wilmington, North Carolina.


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