Bonding the Prop Shaft

[Charlie]

While on my cruise in the PNW I had a nasty feeling that I should change or at least check my zincs. There was a diver on the dock so I asked him to do it. He said that the Max prop zinc was fine. The rudder zinc was OK. There were no zincs on my prop shaft. I bought two zincs and the diver installed them for me.

The boat is currently kept at a private dock with no other boats within 300' I keep it plugged in all the time and do not have a galvanic transformer -- yet. the prop shaft has one of those hard plastic flexible couplings. The guy who installed the engine didn't know whether or not I needed to put a bonding strap on or not. He said it depended on the rest of the boats bonding. Does anyone know what I should look for and whether or not I should just go ahead and bond the shaft and the coupling. Or most likely is there more information required.

[cooper]

If the prop shaft has its own protection (zinc annode) then you dont need to bond it. Treat it as a "circuit " in its own right.

[delmarrey]

If you have a bronze prop and a SS shaft your going to get a little electrolysis even if it is isolated. It's better to spend the extra $10 or what ever, then to neglect.

When I had a bronze prop, in which the shaft is isolated by a soft coupler, the zink would last about two months. It could be just natural errosion to the zink or it might be the current, donno. But either way I know it's covered.

[GordMay]

1. First, you NEED either a Galvanic Isolator, or an Isolation Transformer.

2. The test method recommended by EngNate, at http://www.cruisersforum.com/forums/...rina-8445.html
(and the ABYC) for testing (& sizing) sacrificial anodes is to place the reference electrode (silver/silver chloride) in the water outside the hull near the item that is to be protected, and measure the relative (zinced & un-zinced) voltages.

This simple test will determine if your shaft zinc is actually working as intended.

Measure the relative voltages using a digital multimeter:
1. Connect the negative terminal to the reference electrode.
2. Connect the positive terminal to the metal part or bonded collection of parts that are to be protected, assuring that there is good contact.
3. Note the DMM reading.
4. Connect a sacrificial zinc anode or anodes of the proposed size to the metal part or bonded collection of parts that are to be protected.
5.Note the DMM reading.
6. Compare the two voltages.

“Protection is adequate when the voltage measured is 200 millivolts more negative than the reading noted without the sacrificial zinc anodes.”

[Charlie]

Del:

I did have the diver put two zincs on. Curious that your zincs only lasted a couple of months. BTW. I love the PNW. Can't wait to go back up in September.

Gord:

Thanks for the simplified version of testing the Zincs. YOu have a real knack for clairifying the important details.

I still do not have a good idea whether I should bond my shaft to the coupling. Calder says to do it but does not say why. It isn't a matter of money but of whether it should be done or not. Any comments on bonding?

[GordMay]

The propeller shaft should be bonded when:
1. The voltage test indicates inadequate protection.
2. The shaft zinc forms a significant part of the grounding electrode (ground plate).
3.The transmission and/or shaft-saver type coupling isolates (electrically) the shaft from the engine.

Seldom does a small-craft shaft require more than a single zinc (you mentioned zincs).

The common reason for installing a shaft brush is to improve the protection from zincs attached to the propeller shaft and other underwater parts of the boat. However, owners of some inboard engines have reported that pencil zincs in the cooling system last up to twice as long with a shaft brush installed. Simply stated, if all underwater metal hardware is bonded [*1], and electrically attached to a zinc it is better protected from galvanic corrosion. A shaft brush (wiper) provides a better connection between the shaft and the bonding system, than the packing, coupling, and transmission [*2]would offer.

*1 A bonded system is when you connect all your immersed metals in the boat to each other, and then to a sacrificial anode. The sacrificial metal will erode protecting the boat metal that is bonded to it.
If a boat is not bonded, and you took readings on all the underwater metals with a DC milli-Amp meter, the readings will be slightly different from each other.
Bonding, in effect, makes all the immersed metals into one piece of metal. The voltage throughout the bonding system will be the same, and will average out depending on the size and voltage of each metal.
One piece of zinc, of the appropriate size, attached to the bonding system as the sacrificial anode, will protect all the bonded metals.

*2 The lubricant in the gearbox works as an insulator preventing good electrical connection between the shaft and the engine.

Not all boats need a shaft brush, and there are cases where one may do more harm than good. Before spending a lot of money on new hardware, you should perform (or have performed) the voltage test.

The stuffing box, stern tube and strut should all be attached to the bonding system with at least #6 AWG (Green) wire or copper bonding strap. Obviously the shaft should be provided with a zinc and so should the strut, if provisions are made for it.

BTW: Shaft zinc's are generally expended more quickly than stationary zincs, because of the velocity of the shaft and the abrasion of the water.

[Charlie]

Thanks Gord:

Clear and concise as usual. I think you should write a book. I would buy it.

[delmarrey]

Originally Posted by Charlie

Del:

I did have the diver put two zincs on. Curious that your zincs only lasted a couple of months.

As Gord stated above:

BTW: Shaft zinc's are generally expended more quickly than stationary zincs, because of the velocity of the shaft and the abrasion of the water.

My boat doesn't sit in the water long but only when I'm out on a weekend or a vacation cruise. The rest of the time it's on the hard. And even on the hard the zink actually errodes slowy away once it's been in the salt.

I don't bond mine due to the fact the only raw metal I have under the water is the prop & shaft. The strut & keel are sealed in epoxy with exception of the cutlass bearing which I change out every other year.