skipjack27
Petty Officer 3rd Class
- Joined
- Oct 6, 2009
- Messages
- 79
My Volvo Duoprop 290 (1987 vintage) appeared to be dissolving away over a period of months. When great chunks finally started to fall off the blades, I called in an electrolysis expert. His measurements of the voltage in the water next to the props showed perfectly normal results, and he was initially dumbfounded - had no idea why the props were being so viciously and rapidly eroded. The shaft zincs appeared to be doing their job very well. I rarely connect to shore power, there was no suggestion of stray currents from other boats in the marina, and my onboard electrical systems checked out A-OK.
Later consultation with a leading expert in the area revealed what was going on. The props were effectively "floating free" in electrical terms. i.e., they were not properly bonded to the shaft, and thus to the shaft anode. The reason for this, he claims, is that the screw used to hold the prop onto the shaft was altered by Volvo in recent years from metal to carbon fibre, which apparently does not provide an adequate electrical bond to the shaft.
His proffered solution is to drill 3 holes equally spaced around the hub and insert metal screws that connect to the hub (without prevent the props from breaking free from the hub in the case of hitting a reef or whatever). This should ensure adequate electrical contact.
The mechanic in charge of the repair agrees with the principle, but has some reservations about this particular arrangement and is currently looking for alternative solutions that might be more robust.
Has anyone faced a similar problem and come up with an effective solution? As the above account probably reveals, I have only a vague idea of how the props connect to the shaft, and would also appreciate any light that anyone can throw on this - so that I don't look like a complete idiot when the mechanic tries to explain things to me.
Jeff
Later consultation with a leading expert in the area revealed what was going on. The props were effectively "floating free" in electrical terms. i.e., they were not properly bonded to the shaft, and thus to the shaft anode. The reason for this, he claims, is that the screw used to hold the prop onto the shaft was altered by Volvo in recent years from metal to carbon fibre, which apparently does not provide an adequate electrical bond to the shaft.
His proffered solution is to drill 3 holes equally spaced around the hub and insert metal screws that connect to the hub (without prevent the props from breaking free from the hub in the case of hitting a reef or whatever). This should ensure adequate electrical contact.
The mechanic in charge of the repair agrees with the principle, but has some reservations about this particular arrangement and is currently looking for alternative solutions that might be more robust.
Has anyone faced a similar problem and come up with an effective solution? As the above account probably reveals, I have only a vague idea of how the props connect to the shaft, and would also appreciate any light that anyone can throw on this - so that I don't look like a complete idiot when the mechanic tries to explain things to me.
Jeff
