Keep your prop on
Other than keeping the barnacles off it each year at hauling time, many boat owners give little thought to the installation or maintenance of the propeller. While propeller installation is generally a routine operation, a useful and occasionally critical step is often overlooked. In most instances, the propeller is simply slid onto the prop shaft, with the key in place and secured with the prop nuts and cotter pin. However, there are occasions when doing the job in this manner can lead to later unhappiness, including shaft breakage and loss of the prop.
A second problem arises when the prop is not removed from the shaft for a long period of time, perhaps years. This type of neglect can lead to failure of the locking key, resulting in the prop free-wheeling on the shaft. This situation is especially likely to occur if a bushing must be used to fit an oversize prop bore to the prop shaft. A couple of simple steps can prevent shaft breakage and reduce the chance that the locking key will fail.
There are occasional reports of propeller shafts mysteriously breaking, usually just aft of the prop shaft bracket. Although many of these reports involve high-powered planing-hull boats, trawlers and sailboats are occasionally involved. This type of shaft breakage usually results in loss of the prop. Upon investigation it is usually found that the prop shaft was the correct diameter for the torque being transmitted, that it was true and properly supported and was made of well-finished, quality material. Yet, it failed. Numerous theories are advanced for the cause of the failure. The shaft and prop are replaced and in many instances the replacement functions without problem for many years. The cause of the failure remains obscure. One major cause of this type of prop-shaft failure isn't a mystery. It results from the manner in which the prop is mounted on the prop shaft.
The section of the propeller shaft onto which the propeller is fixed is tapered, with a matching taper bored into the hub of the propeller. It is generally assumed that the taper of the shaft matches that of the prop bore. Unfortunately, the match between these tapered surfaces is often not as precise as we assume it to be. The prop slides onto the shaft easily, and when the securing nut is tightened it appears to be securely seated on the shaft. In reality, the contact area between the shaft and the prop bore may be less than total, with a minute amount of space between the shaft and prop over sections of the mating surfaces. On many boats, especially those powered with modest-size engines, the fit of the prop to the shaft is sufficiently close and allows trouble-free transfer of energy from the shaft to the prop. In more highly powered boats the slight looseness of the prop on the shaft allows a small amount of rocking movement between the prop and the shaft. This movement imparts a cyclic stress on the shaft that adds to the cyclic loading always present as a consequence of the presence of underwater hardware ahead of the rotating prop.
The cyclic loading created by what we may call prop rocking accumulates rapidly, at the rate of one cycle per revolution of the shaft. At typical shaft speeds of 1,200 rpm, 72,000 cycles occur in one hour of operation, more than seven million in 100 hours. While the prop shaft is designed to deal with the normal-magnitude cyclic loading, it may be unable to withstand the combined effect of the normal and prop fit-induced loads. The shaft fails, usually just aft of the shaft support bracket, with loss of the propeller. In many instances, the prop cannot be recovered, and it is assumed that the failure was caused by impact with an underwater object. Prop shaft failure can be prevented by grinding or lapping the bore in the prop so that it precisely matches the taper of the prop shaft.
Most propellers are made of a bronze alloy that is significantly softer than the prop shaft, which is often a stainless steel alloy. The procedure is simple. With no key inserted in the shaft keyway, the prop shaft is coated with a small amount of abrasive valve-grinding compound. The prop is slipped onto the shaft and rotated by hand against the stationary prop shaft as it is forced forward onto the taper of the shaft by slowly tightening the prop nut. The valve-grinding compound, aided by the usually sharp edges of the keyway slot in the shaft, will quickly lap the interior bore of the prop so that it precisely matches the taper of the shaft. The prop is periodically removed to check progress and to apply additional grinding compound. In most cases, the lapping process takes less than 15 minutes to accomplish. The task is complete when the residue of grinding compound on the shaft and in the prop bore appears smooth and uniform. Clean all of the compound from the surfaces and mount the prop, remembering to check the key for a tight fit. Plan on removing the prop from the shaft every year or two for inspection and possible replacement of the bronze key.
There is yet another problem that is unique to propellers installed on shafts with a diameter smaller than the prop-bore diameter. In such cases, a prop shaft bushing is used to mate the prop to the shaft. This problem is more common with sailboats, where relatively small-diameter prop shafts are frequently used. The prop shaft bushing kit consists of a longitudinally slit, tapered plastic sleeve and a stepped-width brass key. The width of one edge of the key fits into the slot in the prop shaft; the wider dimension fits into the slot in the propeller. When the bushing is installed on the prop shaft it is usually necessary to trim the edges so that they lie snugly against the sides of the key. The prop is then installed in the usual manner. Although Îse of the bushing makes it impossible to lap the prop to the shaft, the relatively low power involved usually precludes the cyclic-loading failure noted above. However, a different failure mode can occur.
The key that secures the prop to the shaft must carry almost all of the torque load transferred from the engine. In a normal installation, in which the prop bore matches the prop shaft, the force imposed on the key is a shear load. When a bushing is used, the load can consist of both a shear and a bending force. Over time the combination of the shear and bending forces may degrade the key, to the point where it fails, allowing the prop to freewheel on the shaft. When using a bushing it is particularly important to tighten the prop onto the shaft very securely. There is no metal-to-metal contact along the length of the prop bore, only the intervening plastic sleeve. In addition, it is advisable to remove a prop installed with a plastic bushing annually, replacing the bushing and the key with new parts when reinstalling the prop. With a bit of care, the prop's reliability in propelling the boat can be assured.
Corrosion protection is always a concern for any boat used in salt water. Placing two different metals in an electrically conductive solution creates a voltaic cell - great for a flashlight, but not so beneficial when one of the metals is the bronze propeller and the other is a more noble metal, such as a stainless steel propeller shaft. The flow of current created by the cell will erode the less noble (but still costly) propeller. The time-tested answer to this problem is to offer the corrosion gods a more attractive sacrifice than the boat's prop. As a glance at the position of metals in the galvanic series will disclose, gold and platinum are the most noble of metals and from the standpoint of corrosion resistance would be the best material choice for all underwater gear. At the other extreme, magnesium, large quantities of which are naturally dissolved in seawater, is the least noble, and therefore props made of it might have an operating life of days. The third least noble metal, zinc, is an excellent choice for a sacrificial anode. It is relatively cheap; easy to cast into the required shape; sufficiently strong, at least when new and provided the purity is maintained (particularly with regard to iron content); and does a good job of protecting the remainder of the boat's underwater metal.
It is common practice to protect the prop by attaching a zinc collar to the prop shaft. As with any electrical circuit, resistance must be kept to a minimum. This is especially true when dealing with the very low voltages involved in the zinc/prop shaft/prop circuit. For this reason, it is important to thoroughly clean the prop shaft where the zinc will be fastened. Many quality shaft zincs are made with a small copper ball (a bb) embedded in the inner surface of each half. The small contact area offered by the ball is intended to penetrate any oxide or other coating on the prop shaft as the fastening screws are tightened, ensuring good electrical contact.
All of the foregoing is fine, provided that the prop itself is electrically connected to the prop shaft. Normally, the electrical connection between the prop and the shaft is ensured by intimate contact between the inner surface of the prop bore and the shaft, plus the contact between the shaft, fixing key, and prop, and the surface of the nut bearing on the face of the prop hub. Cleaning these surfaces before mounting the prop is always good practice. It is especially important when a prop is mounted using a plastic prop bushing since the bushing will eliminate a substantial part of the available contact area.
No discussion about galvanic corrosion would be complete without commenting on the everlasting dispute about the need to electrically bond the various bronze, underwater through-hull fittings typically used in a fiberglass boat. Bonding all such fittings together will allow all to be protected by a single sacrificial anode. However, fittings that are not bonded or in any other way electrically connected to the vessel's electrical or ground system cannot participate in a galvanic corrosion circuit and may therefore be immune from galvanic corrosion.
An adequate external metallic ground is a required part of the vessel's lightning-protection system. However, once that requirement is met, the decision to connect all other pieces of underwater metal together appears to be optional. In fact, if the prop were somehow mounted on a non-conducting shaft there would be no need for the protective zinc. It is interesting to note that on many boats the metal prop shaft support bracket, the P bracket, is not connected to the grounding system and is insulated from the prop shaft by the rubber lining of the cutlass bearing. The P bracket is therefore not protected by the shaft zinc. However, even though it is isolated from the grounding system, it doesn't corrode. It is possible that the prop shaft of your boat may be electrically isolated from the vessel's ground system.
It turns out that, unless a grounding brush is installed on the prop shaft, the only electrical connection with the engine will be via the transmission. In many cases the oil in the transmission acts as an insulator for the low voltages involved in the galvanic corrosion process. This need not create a corrosion problem for the prop since the dissimilar metal couple we are dealing with consists of the shaft and prop. A zinc on the shaft will take care of that problem. The prop shaft will be effectively grounded from the standpoint of both radio frequency currents and possible lightning-induced voltages. In the case of RF currents from a HF/SSB radio, the oil film will act as the insulator in a capacitor. The voltages involved in a lightning-induced surge will be sufficient to penetrate the oil film. Usually the prop shaft will be reasonably well grounded whenever the engine is in gear.
Silver test circuit
You can check the adequacy of the anodic protection on your boat with a circuit that is simple to assemble. One test lead from a sensitive voltmeter, capable of reading one volt full scale (either an analog or a digital meter will do), is connected to a small piece of pure silver so that only the silver is exposed. The other test lead is connected to the prop shaft. The lead with the silver is dropped into the water alongside the boat. The electrical circuit consisting of the immersed silver in salt water, connected through the meter to the prop shaft metal creates a voltage whose magnitude is determined by the electrical potential difference between the silver reference and the immersed metal of the prop shaft, prop, and zinc on the shaft. A voltage between 500 and 700 millivolts (mv) indicates that a suitable amount of zinc is present. Less than 500 mv indicates that the bronze prop is underprotected. More than 700 mv is an indication of an excess of zinc area exposed to sea water.
If the test reveals a lack of protection an additional piece of zinc can be connected to a wire fastened to the prop shaft and lowered over the side. The meter will indicate the changing degree of protection as the additional zinc is slowly lowered into the water.