earth in a steel yacht

[bigbear69]

Hi there,
Put a new engine in my steel yacht. It's a marinised nissan Td27.
everything is earthed through the block which is then taken back to the negative on the battery Just like a car. This also means the earth connects to the hull through the driveshaft. I'm just wondering if this is ok or shall I put some sort of insulator between the engine and shaft to break this connection.
Was just thinking of putting a thin bit of plastic between the drive plate at the back of the gearbox and the cv shaft. I'd also put plastic washers/ sleeves on the bolts.
Or shall I just leave it as it is. any help much appreciated

[gonzo]

You need a good connection between the negative and the hull or grounding plate. That is, a thick wire.

[bigbear69]

Originally Posted by gonzo

You need a good connection between the negative and the hull or grounding plate. That is, a thick wire.

HI cheers for reply. Could you explain why I need the hull connected to the batt neg? None of the electrics negative are connected to the hull. they all go direct back to the batt or through the engine block back to the batt. it is a steel hull so no grounding block. just wondering why I would want to connect the hull to anything electrical. could set up galvanic corrosion in marinas etc.
Is it to do with shore power ( AC) and safety? what if i never run off shore power?
[/i]

[gonzo]

The sacrificial anodes need a good connection to work. Also, you will have a voltage difference between adjacent metal parts submerged in water which will increase electrolisis.

[MikeJohns]

There are differing views on bonding.

One scenario is tht the hull is unbonded to the negative and a fitting or wire shorts the positive to the hull:
No fuse blows but the rubber mounted engine unless it has isolated starter and alternator is now massively negative relative to the hull as the prop and shaft can be as well.

The resulting corrosion would be woefull.
The anodes become low resistance immersed cathodes and will be very happily protected by all the other metals driven negative that are in contact with the water.

If the hull was bonded to the negative then no electrical fault will drive any part negative wrt the hull and anodes but the fuse will blow or the battery will flatten.

AC shore power is a different consideration and there are regulations that cover the requirements here, you can use an isolating transformer or a galvanic isolator to protect yourself in a 'hot' marina.