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Below is a list of frequently asked questions we often receive. We have also provided helpful tips on the following subjects:

Saw Cutting
Cathodic Protection
Stray Current
Buildup Of Worn Areas On Boat Shafting
Shaft Seals And Shaft Logs
Weld Overlay Fusion Process
Additional Suggestions For Installation Of Boat Shafts

Frequently Asked Questions

Should a sportfish consider a safety factor higher than 5?
Yes. A sportfish, although not categorized as a commercial workboat (with a recommended safety factor of 10), sportfish yachts endure more rigorous maneuvers and overall operating hours than the typical pleasure craft (with a recommended safety factor of 5). Therefore, a safety factor of 7.5 should be considered.

What are the typical applications for each Aqualoy grade?
Aqualoy 17 is designed with the commercial workboat in mind. In the larger diameter shafts, Aqualoy 17, unlike the other grades, maintains its strength throughout all diameters. In the smaller diameters, it is not uncommon to use 17 for fresh water environments. Aqualoy 19 provides better corrosion resistance than 17 and is used on fishing boats including shrimpers, trawlers and the like. It is also used on pleasure craft.

Aqualoy 22 Alloy provides the very best combination of strength and corrosion resistance for 2 inches diameter and under. For 2-1/4 inches through 6 inches diameter, Aqualoy 22 High Strength offers the same mechanical properties as Aqualoy 17 while providing the same corrosion resistance as the 22. From smaller pleasure craft to the larger yachts, Aqualoy 22 and Aqualoy 22 High Strength prove to be the boat shafting alloys of choice.

What is the number one cause of pre-mature shaft failure?
Based on our experiences, improper propeller fit is the primary cause of pre-mature shaft failures. A properly mated propeller will reduce fatigue and avoid propeller wobble, two very key issues in ensuring smooth operation of the vessel.

Can Western Branch Metals provide tighter tolerances in both
diameter and straightness?

Yes. In most cases, we can provide bars requiring abnormal or tighter than normal diameter tolerances in a much shorter lead time than the usual mill delivery at a competitive price. With our on-site straightening presses, we are able to provide straightness equal to or better than ABYC tolerances. For more information, please inquire.

Are there special care instructions for stainless steel half oval?

Yes. It is important to remember, the material is stain"less", not stain"never". To maintain the finish, you should clean the material regularly with soap and water and/or with a glass cleaner. Rust spots should be removed with a brass, silver, or chrome cleaner immediately as it may develop pitting if left unattended for any period of time. For added beauty and protection, a good car wax can be used. Never use coarse abrasives, a cleaner with mineral acids or bleaches, or leave the half oval in contact with iron, steel or other metals which may cause contamination. Lastly, when selecting screws, it is important to choose an alloy equal to or better than that of the half oval alloy.


Useful Tips

Protect the precision straightness and fine surface finish of each shaft by careful handling during machining and installation. Don't use wire slings or tools that can damage the surface of the shaft. When lifting shafts, it is strongly suggested that slings with spreaders be used to distribute the weight more evenly, thereby avoiding straightness problems.

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Saw Cutting
Only cold saw cutting is recommended for cutting boat shafting. Abrasive saw cutting, wet or dry, can create excessive heat and stresses which may induce cracking at the saw-cut face. Shafting shipped as random lengths, or multiple lengths, when cut to exact lengths may go out of straightness due to relief of stresses.

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As with any shafting material, keyways and tapers must be properly machined to insure maximum service life. Machined surfaces of the tapers and keyways should have a surface finish of 32 RMS maximum. Improper machining can result in propeller wobble, crevices, and sharp corners which can cause added bending stresses, potential crevice corrosion problems, or stress raisers, all of which will result in premature failure. Proper size, shape and alignment of keyways are essential for maximum shaft life. Likewise, keyways with sharp corners and vertical ends produce stress concentrations that can significantly reduce shaft life, especially when service puts high stresses on the shaft. Such stress concentrations can be prevented easily by rounding top corners and filleting interior corners on all shafting. Sled-runner type ends to keyways are suggested. It is recommended that the top corners of keyways be rounded to one-half the radius that is recommended for interior corners. For specific information, consult SAE Specification J755, section on Marine Propellers - Shaft Ends and Hubs.

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The propeller should always be properly mated to the shaft taper. To insure the best fit, the taper should be coated with Prussian blue. The propeller should then be put on without the key and turned slowly by hand. If inspection of the shaft taper indicates the Prussian blue is uniformly removed then the propeller fit is good. If not, use an appropriate lapping compound and repeat the test. Since 100% fits are not always possible, an effective seal should be used to exclude water such as waterproof grease. This seal will also provide corrosion protection between dissimilar metals. Avoid key pressing against forward position of sled-runner keyway. Do not use graphite packing in the stuffing box. Instead, use a packing material that causes the least abrasion after the lubricant wears away. Be sure to adjust the stuffing box with the clutch disengaged to avoid excessive tightness.

Align the shaft after the boat is in the water. Don't force the shaft to accommodate the engine bed, bearings, or stuffing box. Stuffing boxes should be permitted to leak slightly, thereby avoiding a stagnant water condition in the packing gland area and to avoid overheating during operation. Generally, seven drops per minute is adequate for small diameter shafts. Increased amounts are advisable for larger diameters. Installed as recommended, an Aqualoy propeller shafts can be expected to give more satisfactory service than any other shafting material under the same operating conditions.

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Cathodic Protection
To prevent localized corrosion in bearing areas and other crevices, proper cathodic protection should be used. This should consist of Navy Grade zinc anodes – or even better – an impressed current cathodic protection system. In addition, it is recommended that the shaft be rotated at regular intervals if the boat sits idle for long time periods of time. Several turns will be sufficient.

When zinc anodes cannot be placed effectively on shafts (e.g. vessels with shaft logs or stern tubes) and are, therefore, placed at several hull locations, proper electrical grounding is necessary. A shaft brush grounding assembly is the only effective means to accomplish this since lubricating oils in engines and reduction gears will almost completely electrically isolate propeller and shaft from the common ground.

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Stray Current
Stray battery current is the most dangerous cause of corrosion. It can be suspected in cases of rapid and deep pitting, loss of metal, bright discoloration, extensive formation of corrosion products, and weakening of batteries. In older boats without digital electronics the testing procedure was quite simple. A multimeter with a current range from 10 amperes to 10 milliamperes can be used to check for possible current leaking while the boat is in the water.

The testing procedure is to first turn off all electrical equipment and fixtures aboard, then disconnect the positive connector from the boat's battery terminal and connect the positive lead of the multimeter to the battery post and the negative lead to the connector. Starting at the 10 ampere (or highest current) range of the meter, check to see that there is no current flow, then switch the meter to low current ranges. With perfect boat wiring and equipment, no current should flow at any setting. A normally "clean" electrical system will have a leakage current of less than 1 milliampere (0.001 amp). Leakage of a few milliamperes indicates a small amount of corrosion - hardly dangerous -but something to locate and correct if possible. However, if the meter shows a sizable fraction of an ampere, separate circuits should be switched off one at a time to find which is at fault, and the wire or equipment repaired or replaced promptly. One problem with this type of test is modern electronics will typically consume electricity even when switched off. In this case some current would always be measured as long as the battery is connected.

A better way to do this test (probably not something your typical owner could do) is to use a reference cell and measure hull potential with batteries disconnected and then measure again connected. Then measure while starting loads one at a time and note the effect on the hull potential. This test requires some specialized equipment and the knowledge of how to use it. There are several ABYC approved corrosion surveyors with the equipment and expertise to conduct a corrosion survey of your vessel and recommend remediation should stray currents be found.

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Buildup of Worn Areas on Boat Shafting
When bearing areas become worn, a buildup of this area can be accomplished with reasonable success providing some precautions are taken. Types of buildup processes are:

  1. Metalizing by either oxyacetylene arc-spray or plasma flame methods, and
  2. Weld overlay fusion process.

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  1. Low work temperature of 200-400° F (93-204° C) minimizes distortion.
  2. The bond to the pre-roughened area is mostly mechanical.
  3. The plasma flame process gives the densest coating with a minimum of oxide formation.
  4. Type 304 stainless steel is the most readily available coating to be considered for metalizing boat shafts.
  5. The effect on the base metal shaft is regarded as little or no effect.
  6. One problem is that of questionable reliability of the shaft-coating bond.

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Shaft Seals and Shaft Logs

  1. Shaft seals shall be readily accessible.
  2. The shaft log, shaft seal, and specifically the rubber hose must be inspected yearly for signs of deterioration or excessive wear. The rubber hose should be replaced every other year. Every shaft seal shall be constructed in such a manner that if the sealing element locks or freezes to the shaft, the resulting rotation of the seal will not create a condition wherein more than two gallons of water per minute can enter the hull with the shaft continuing to operate at low speed.
  3. A shaft seal utilizing a rubber hose for the mechanical connection between the shaft seal and the shaft log may be used provided the design meets the requirements called for in the preceding paragraph.
  4. A shaft seal utilizing replaceable packing material must be installed in the boat with sufficient clearance along the shaft line to permit replacement of the packing without uncoupling the shaft or moving the engine. Face-seal and lip-seal types are not considered replaceable within the meaning of this paragraph.
  5. The use of graphite-impregnated packing is not recommended because of the possibility of galvanic corrosion of the shaft material. FEP fluorocarbon (Teflon***) - impregnated asbestos braid, the fibers of which are impregnated with the plastic prior to braiding, is considered to be excellent for shaft seals. Wax impregnated flax packing may also be used.
  6. A shaft seal of the face-seal, lip-seal, "O" ring or frictionless types must be designed in such a manner that it can be operated at low speeds after failure of the seal without permitting more than two gallons of water per minute to enter the hull.
  7. Every shaft log that is not part of the hull structure, such as a cast metal log, should be secured with a minimum of four through-bolts. Additional fastening in the flange may be screws.
  8. A shaft seal or stuffing box utilizing plastic or rubber as a component must be designed so that over tightening of the packing gland or misalignment will not raise the temperature of the nonmetallic material above its normal working-temperature limit.
  9. A boat having a long stern tube or any other type of construction that tends to trap water next to the shaft should be provided with a positive means of water circulation. This recommendation is to avoid the corrosive effects of stagnant water. Ideally, the water circulation should be through the shaft seal, which will also provide a positive pressure to cool the packing.

* From Safety Standards for Small Craft, Project P-6. American Boat & Yacht Council, Inc.
** These opinions are not included in Safety Standards of American Boat & Yacht Council, Inc.
*** Reg. U.S. Pat. Off. for duPont de Nemours & Co. (Inc.)

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Weld Overlay Fusion Process
A metallurgically sound bond is attainable when matching fillers are used in conjunction with highly skilled craftsmanship for weld overlay and full welded joints.

  1. A sound metallurgical bond is obtained. No preheating is necessary for welding these alloys.
  2. To minimize distortion, a semi-automated process (gas metal-arc) is preferred by clamping a welding torch to a lathe tool holder for indexing as the shaft rotates in the lathe under the weld head. If this cannot be accomplished, longitudinal overlay weld beads may be deposited alternately 180 degrees opposite the previous overlay.
  3. For Aqualoy shafts, matching weld overlay fillers are preferred. However, where availability may be a problem, alternate fillers may be used, i.e., Types 308L, 309 and 316L stainless steels. These alternate fillers will have lower a hardness and possible lower strength and corrosion resistance than the base metal shaft.

A note of caution is offered concerning the weld overlay fusion process for Aqualoy 17. Welding such an alloy will create a small area of untempered martensite in the shaft, which is somewhat lower in ductility; however, such an area appears satisfactory for normal seawater service. When the matching filler is used on Aqualoy 17, the overlay will also contain untempered martensite. While the use of this process (as-overlayed Aqualoy 17) is generally not sanctioned, it is frequently done with satisfactory results. A postweld heat treatment at 1150° F (621° C) for four hours will restore the weld area properties to nearly those of the shaft. Straightening after such a heat treatment is probably necessary.
Aqualoy 19 and 22 shafting in their as-welded conditions have mechanical properties which closely match the base metal in diameters over 3 inches (76 mm). Under 3 inch (76mm) diameters, a loss in such properties will probably occur.
Aqualoy 22 retains basically the same corrosion resistance properties when overlaid using the matching filler.

Shaft Fillers Matching Filler Alternate
#17 W 17-4 PH (AWS E630 & ER630) Types 308L. 309, 316L
#19 Type 308L (AWS E308L & ER308L) Types 309. 316L
#22 NITRONIC 50W (AWS E209 8 ER209) Types 308L. 309, 316L, 309LMo

"E" designates coated electrodes
"ER" designated bare filler wire

Please note that Nitronic 50 weld filler metal is hard to find and expensive. A common material which is very close in composition and recommended is 309LMo. This material should be used if possible when welding Aqualoy 22 and can be used for both 19 and 17.

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Additional Suggestions for Installation of Boat Shafts
The following recommendations apply to the installation of boat shafts of any kind, as well as the selection and installation of shaft seals and logs.


  1. Alignment. Bearings, shaft and couplings shall be aligned to a tolerance of no more than .004 inch (. 102 mm) measured between the parallel flange of the coupling with the coupling bolts loose. Smaller shaft sizes should turn freely by hand with the clutch disengaged when the coupling bolts are drawn tight.
  2. Propeller Overhang. The distance between the forward end of the propeller hub and the aft end of the last strut bearing should be limited to one shaft diameter. This does not apply where the last bearing is installed aft of the propeller. (High performance craft, such as racing boats, should restrict this distance as much as practical without obstructing water flow through the bearing.**)
  3. Propeller & Key Installation. The propeller should always be installed on the shaft taper first without the key, and its position marked with a nongraphite-bearing marker. The propeller is then removed, the key installed and the propeller installed so its position is at the mark. This precaution will indicate if the propeller is riding the key up the keyway end radius, forcing the propeller off center. Every propeller installation should be provided with a system consisting of a straight key, propeller nut, jam nut and cotter key. Other positive-locking systems may be used if they protect against accidental loss of the propeller. The length of the key shall not be greater than dimension "X" shown in SAE Standard J 755.
  4. Propeller Diameter vs. Shaft Diameter. It is suggested that some consideration be given to the ratio of propeller diameter to shaft diameter. It is felt that a ratio of approximately 15:1 max should be considered. Higher ratios may create excessive stresses and early failure. High performance boats, such as racing boats, should consider a ratio of 12:1.**
  5. Boats with engines on floating mounts should provide sufficient distance between the engine coupling and the shaft seal to prevent the shaft from taking a permanent set with sudden bursts of power. Flexible shaft seals may help to prevent such shaft deformation from taking place. Please consult your marine engineer.**

* From Safety Standards for Small Craft, Project P-6. American Boat & Yacht Council, Inc.
** These opinions are not included in Safety Standards of American Boat & Yacht Council, Inc.
*** Reg. U.S. Pat. Off. for duPont de Nemours & Co. (Inc.)

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