Stray voltages

[Dazedkipper]

Can any one tell me why I read 0.38v between my propshaft (+ve) and stern tube?
Reason for asking is that last season I consumed a new pear shaped anode, complete with steel bar and most of the studs too whereas I normally only use an anode every three seasons. Whilst trying to find out why I discovered a voltage existing between the anode studs (one fixed to the propshaft, system negative, and one fixed to the stren tube)
This has got me baffled and any assistance would be greatly welcomed.

 

[Bilgediver]

You need to tell us more as this would suggest the anode was not in place so was the boat in the water.

If there is a voltage difference between the two studs and the anode is in place then this suggests either one or both of the studs are NOT making electic contact with the anode.

 

[Dazedkipper]

Boat launched approx may last year and recovered Nov. On launching a new, 2.5kg anode had been fitted and resistances checked and found to be neglible. When the boat was recovered the anode, steel bar and most of the hull studs were missing, the remaining parts showing signs of electrolic corrosion.
With the boat ashore, checking around with a meter gives the stray voltage as above. The circuitry is as follows, 12v negatives from all supplies are fed through the engine casing to battery -ve. This is also connected to one side of the anode. The propshaft is rigidly mounted so this is also at -ve potential. The stern tube is bonded to the other side of the anode, the circuit being completed when the vessel is afloat by salt water, but there exists a voltage between the two, seemingly disconnected parts when ashore and dry.
My question is two fold, should there be a voltage between these parts, and if not could this have caused the rapid anode loss?

 

[lw395]

Is the voltage present with the battery isolated?
No Shorepower?
There should be no volts.
Yes it is probably causing wastage.
Is it possible to change to a shaft anode, so that the prop and shaft are directly protected?
If the prop is the only contact between the water and the electrics, there is no circuit. Unless you bring in an external ground, such as shorepower and connect it to the boat electrics....
Linking things together often seems to cause more problems than it solves. Stuff like thru hulls are better off with a good coat of epoxy and electrically isolated in my limited experience.
I'm not sure what is best for a stern tube- is the issue a metal stuffing box at the inboard end?
while you are measuring volts, also measure resistance, which should be less than 1 ohm between two 'bonded' grounds.
What other metalwork is in the water?
Do you moor somewhere odd? lots of scrap metal in the mud? (there are stories about this in Portsmouth...)