Propellers & Anodes

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Propellers and Anodes

Please note that this article plots the early experiences of propellor/saildrive corrosion. Much progress has been made and the issue seems to be better understood, although a full explanation and solution is still elusive.

Those of us with folding bronze propellers should take a close look at the anodes and condition of the propeller body 'bronze alloy'. Any pinkish streaks mean that your anodes are not doing their job and that the Zinc is leaching out of the alloy.

This picture clearly shows the pink coppery areas that have corroded when anodes erode too quickly.

In my experience both two bladed and three bladed propellers suffer as the anodes erode very quickly, the anodes on my two and three bladed propellers only lasted 3 months! I can tell you, from experience, that an unprotected propeller will last four years before the teeth on the folding blades disintegrate. If your anodes are not eroding there may also be a problem.

Older folding two-bladed propellers use the anode on the bottom of the sail-drive leg for protection, these propellers should be electrically connected to the sail-drive leg via the propeller shaft. The propeller body is electrically connected to the shaft by a washer under the fixing nut.

Newer folding propellers have the protection anodes fixed to the propeller, these propellers are electrically isolated from the propeller shaft and leg-anode.

You can test electrical connectivity (conductivity) with a cheap digital multimeter, if you don't have one buy one from Maplin or similar, it will be very useful.

Testing electrical conductivity (resistance).

Set your multi-meter to measure resistance, choose the lowest resistance range. i.e. Less than 100 Ohms.

First practice to confirm that you can measure LOW resistance ( less than 10 ohms and typically 1 ohm) between the two points on the same piece of metal (anode or propeller), if you're not sure how the meter works connect the two probes together to see how the meter-display changes when a circuit is completed. You will find that most meters will display OL. (overload) when there is no connection or high resistance. When you are confident in your ability to make the measurement continue.

Warning: On no account scratch the body of the sail-drive leg, also do not hit the leg with a hammer to try to loosen the oil drain plug (all including Volvo agents take note!)

Now measure the resistance between propeller and leg anode. To do this, place one multi-meter probe on the propeller, you may need to make a fine scratch to find clean metal. Place the other probe on to the sail-drive leg ring-anode, you will need to scratch this harder to get an electrical connection.

If the test shows high resistance, change the multi-meter measurement range to 1M ohms (or highest range) and repeat the test. If OL. is displayed change to the highest range and repeat. Note the resistance measured.

Results of the conductivity test:

Note these results assume that the propeller and leg are dry as water has LOW resistance.

Propellers without propeller anodes should have LOW resistance (Less than 100 ohms, typically 1 ohms) resistance between propeller and leg-anode.

Propellers with propeller anodes should have HIGH resistance (greater than 1M ohm between the propeller and leg-anode.

Action

If the results seem odd consult an expert. If you have real premature anode erosion problems or a propeller that has been destroyed by anode problems or conductivity problems complain to Volvo.

Latest news

The initial 3 bladed propellers have been changed because of a problem with corrosion. Now the propeller is cast from a new alloy and the hub is now electrically connected to the leg & leg anode. This means that if the hub anodes corrode away the leg anode will take over. If the propeller then causes the leg anode to erode away then the leg is unprotected and could be very expensive to replace. You may be better off keeping the original propeller or changing to another manufacturer.

An alternative

Really fed up with your existing propeller, read about an alternative corrosion resistant propellor.

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