Propeller Fitting Instructions

How to fit your propeller onto a tapered shaft correctly.

1. Check propeller shaft

After removing the old propeller check that the shaft taper, key and thread are undamaged. Try the propeller shaft nut on the shaft thread. The shaft taper should be clean and dry.

Five minutes with a dial indicator while the shaft is still installed can save a lot of agony and expense later. Even a slightly bent shaft can ruin your boating pleasure. If you don't have a dial indicator, you can use the following method. Rest a pointed stick on the rudder, aligning the pointed end with the machined centre of the shaft, than rotate the shaft. Any deviation will be apparent indicating the tapered end of the shaft may be bent.

2. Bearing check

Before installing your new prop ensure that the shaft bearing is not worn. A worn bearing or shaft will not be suitable for any propeller, so if it is worn, replace it. If there is too much play of the shaft in the  bearing, the bearing must be replaced or vibration and damage to the shaft can occur. Please note that certain types of bearings require some clearance. If not sure contact the manufacturer and ask them what the maximum allowable clearance is.

3. Check key and keyway

Check that the key fits the keyway. Ensure the key slides through the new prop’s keyway without jamming at any point or with no apparent slop. It will be helpful to mark the direction of the key in the keyway.

4. Propeller fit

Dry fit the propeller to the shaft, without the key in place first. Check that the propeller does not rock on the taper. Mark the shaft at the forward end of the propeller hub. This is most is important - to first fit the new prop onto the shaft without the key in place and to mark the shaft at the forward edge of the prop hub. Remove the prop and place the key into the shaft keyway. Slide the prop back onto the shaft and check that the forward edge of the hub comes to your shaft mark. If not then it is likely that the key is too large, and the propeller is not seated to the shaft taper correctly. Remove the prop and file the top of the key down until the prop will slide on to the shaft and reach the mark. This will ensure that the prop is now correctly seated on the shaft taper.

5. Fit propeller to shaft

It is good practice to "lap" the propeller to the shaft. It only takes a few minutes and will improve the fit. Purchase some coarse valve grinding compound from an automotive supply store. Liberally coat the tapered end of the shaft and the bore of the propeller with the grinding paste. Slide the propeller onto the shaft. Apply gentle pressure and rotate the propeller on the shaft 90° to the right , then 90° to the left and repeat this several times. Occasionally remove the propeller from the shaft and wipe out the valve grinding compound and visually inspect the bore. Continue this until a minimum fit of 75% is achieved. Most valve grinding compounds are water soluble and wash off easily with soap and water.

6. Check propeller position

Carefully clean the propeller and the shaft and check the "dry fit" once more. You will probably notice that the propeller goes to a different position on the shaft than before. Mark this new position.

7. Install propeller

Install the propeller with the key fitted to the shaft. Some people prefer to use a lubricant on the shaft, we do not recommend this. Check that the propeller goes up to the mark on the propeller shaft. If it doesn’t, the propeller is sitting on the key and you must reduce the height of the key to overcome this problem. Draw the propeller up the taper using the propeller locking nut, then lock this nut with the second nut. Don't forget to fit a new cotter pin.

8. Painting propellers

Painting your propeller will degrade the performance.  Barnacles, on the other hand  will degrade the performance more than properly applied paint. If you use the boat often painting is not necessary. If you have the bottom regularly cleaned then painting is also not required. On the other hand, if you are like most of us and use the boat not as often as you would like, then painting may be helpful. A good alternative is the specialized silicon propeller coatings e.g.PROPSPEED which works because they are slick;- any marine growth slides off the metal surface when moving through the water.

Here is one procedure for painting propellers:

A. The propellers will be clean when you receive them apart from a light coat of oil. Remove this oil film using alcohol or acetone.

B. Choose a good quality Zinc Chromate primer and lightly coat the propellers.

C. The anti-fouling paint to use on the propellers is sold under various trade names as ‘Outdrive Anti-fouling Paint’ in spray cans. Spray 2-3 light even coats of paint on the propellers taking care not to get any paint into the bore of the hub.

D. Allow at least 48 hours drying time before putting the propellers into service.

E. It is best not to apply standard anti foul paint with a brush as it tends to "spin off" the propellers quickly and cannot be applied as evenly as spray paint.

9. Alignment check

After the boat has been in the water for 24 hrs, the engine alignment should be checked.

10. Shaft zinc anode

Shaft anodes should be fitted as far forward on the shaft as possible, or as near to the cutlass bearing as practical so it does not disturb the water flow to the propeller. 

11. Vessel performance

Record your vessels performance after the hull is cleaned and while  the propeller is in good condition. Note of the top RPM and speeds achieved. This data will be very useful when fine tuning your propellers etc.