Propeller Shaft Installation

Propeller Fit:

In order to properly mate the propeller to the shaft taper, the following is one of the most commonly used methods:

1. The taper should be coated with Prussian Blue.

2. The propeller should then be put on the shaft without the key and gently tapped snug onto the taper.

3. If the Prussian Blue is uniformly removed – the fit is good.

4. If the Prussian Blue is not uniformly removed, the fit is not good. An appropriate lapping compound should then be used

and the test repeated.

Since a 100% fit is not always possible, an effective seal such as waterproof grease should be used to exclude water. This seal will

also protect against dissimilar metal corrosion.

 Alignment:

Bearings, shaft and couplings shall be aligned to a tolerance of no more than 0.004 inches (0.10mm) 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.

Note: The shaft should be aligned after the boat is in the water.

Do not force the shaft to accommodate the engine bed, bearings or stuffing box.

Propeller Overhang:

The distance between the forward end of the propeller hub and the aft end of the last strut bearing shall be limited to one shaft diameter.

Propeller and Key Installation:

The propeller should always be installed on the shaft taper first without the key, and its position marked with a pencil. The propeller is

then removed, the key installed and the propeller installed so its position is at the pencil 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 Specification J-755.

Propeller Diameter vs. Shaft Diameter

The ratio of propeller diameter to shaft diameter should not exceed 15:1. Higher ratios may create excessive stresses and may shorten

shaft life. High performance boats, such as racing boats, should consider a maximum ratio of 12:1.

Shaft Seals and Shaft Logs:

Shaft seals shall be readily accessible. 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.

 Cathodic protection should be used to prevent localized corrosion in bearing areas or in other crevices (Refer to Galvanic

Series Chart). An impressed current system or Navy-grade zinc anodes should be used for salt water applications; boats which see

brackish and fresh water should consider Aluminum anodes as Zincs can become inactive in non salt water applications. If the boat

sits idle for extended periods of time, it is strongly recommended the shaft be rotated several turns at regular intervals. For more

detailed information, refer to ABYC E-2 "Cathodic Protection."

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.