Is Your Engine Earthed?

Dougal

Dougal

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Almost all my sailing in the last 15 years has been in France, so I can't really comment on British built boats.
I'm totally amazed at number of 12v installation's I'm seeing here, where the engine is earthed. I was taught never ever to do this, yet it seems to have been common practice here. Can't comment on more recent builds. All the stuff I'm seeing is 1980s to maybe mid 1990s.
Is this simply a cost cutting thing? Maybe I'm just out of sync with the times? I know many skin fit tings these days aren't susceptible to electrolysis, but props and shafts etc most definitely are.
Have I missed something new?
 
P bracket and rudder connected to the external anode as is the negative side of the 12v circuits. Mains earth connected to the negative side of the 12v circuit and therefore to the anode. Prop protected with separate anode. All seacocks are isolated.

Think this is normal for UK boats isn't it? Mericans like theirs all wired up but what is the point?

Pete
 
Not sure what you mean by "earthed" when you then go onto describe a bonding circuit. You can't "earth" a boat electrics except as described by pete when a shorepower circuit is connected to earth through the marina supply.

What I think you are talking about is bonding certain metal parts to an anode to prevent corrosion through galvanic action. so, yes you are out of date. It is no longer considered necessary to connect items such as seacocks to an anode, so you will not find it done on boats built in the last 30 years or so, whether British or European.

You may, however see the stern gear connected to an anode to protect the yellow metal prop from the stainless shaft. This connection commonly uses the engine/gearbox as a pathway to connect the shaft electrically to the anode mounted on the hull. You are mistakenly thinking this is "earthing".

However you are right in thinking bonding circuits were common in the past before the subject was fully understood. The process now is to look at each individual metal fitting underwater and determine if the conditions for galvanic action exist and then take appropriate action to provide protection. These conditions do not normally exist in seacocks or skin fittings so they do not need anodes. However they do exist in various ways in things like props and shafts, saildrive units, folding and feathering props, bow thrusters and sometimes rudder assemblies so you will find anodes used on these.
 
sorry, maybe didn't explain clearly. I'm ONLY referring to 12vDC and having a HD battery negative to engine. I do not have this. Nor would ever install anything like it. All my senses etc have an isolated return.
 
Surely having an engine connected to -ve battery provides some protection against corrosion? My schoolboy chemistry was obviously years ago, but my father who was a Chemistry Teacher told me that cars used to be "+ve earthed" (car body connected to +ve battery terminal) but that got changed to -ve in "newer vehicles" to reduce corrosion.
 
Surely it's not that unusual to connect the battery -ve to the engine block to avoid needing two thick cable runs to the starter motor?

Earthing to shore power earth is debatable but if you have a GI it's sorted anyway. :)

Richard
 
Dougal said:
sorry, maybe didn't explain clearly. I'm ONLY referring to 12vDC and having a HD battery negative to engine. I do not have this. Nor would ever install anything like it. All my senses etc have an isolated return.
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A good few popular small yacht engines use the engine block as the common negative return. Probably the majority. There are also "hybrid" systems requiring the block to be connected to DC negative by an earthing relay during preheating, stopping and/or starting.

By and large you are stuck with whatever system the engine manufacturer chooses unless you are prepared to order a special version or go to the lengths of changing sensors, alternator , glow plugs, stop solenoid and starter motor.
 
It is common to have the -ve battery cable connected to the engine block. This has been the case for decades.

Some engines (VP in particular) use isolated negative starters, alternators and sensors. However, they typically connect the -ve battery cable to the block, the separate wires from the starter, alternator and sensors also connect to the same point on the block. Seems a pointless excersise to me.

Some engines will have -ve cables going to the starter, alternator and sensors, with no connection to the block.

As far as bonding goes, it's common for the whole engine/gearbox to be connected to the hull anodes, although some (most ?) sail drive installations have the saildrive isolated.

Shore power on modern boats can be connected to the anode too and often the -ve, by virtue of the -ve being bonded to the anode. In such cases a galvanic isolator should be fitted. If no connection between the shore power earth and the boats -ve or anodes a galvanic isolator is not required, it will have no effect.
 
A1Sailor said:
Surely having an engine connected to -ve battery provides some protection against corrosion? My schoolboy chemistry was obviously years ago, but my father who was a Chemistry Teacher told me that cars used to be "+ve earthed" (car body connected to +ve battery terminal) but that got changed to -ve in "newer vehicles" to reduce corrosion.
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That is not relevant to boats as they do not use the "body" so there is no corrosion. The only time there is a possibility of corrosion is if there are mixed metals in the seawater cooling system. If this is the case then an anode is is used in the system. Most engines though if heat exchanger cooled avoid this by not using dissimilar metals in the seawater circuit.
 
PaulRainbow said:
It is common to have the -ve battery cable connected to the engine block. This has been the case for decades.

Some engines (VP in particular) use isolated negative starters, alternators and sensors. However, they typically connect the -ve battery cable to the block, the separate wires from the starter, alternator and sensors also connect to the same point on the block. Seems a pointless excersise to me.

Some engines will have -ve cables going to the starter, alternator and sensors, with no connection to the block.

As far as bonding goes, it's common for the whole engine/gearbox to be connected to the hull anodes, although some (most ?) sail drive installations have the saildrive isolated.

Shore power on modern boats can be connected to the anode too and often the -ve, by virtue of the -ve being bonded to the anode. In such cases a galvanic isolator should be fitted. If no connection between the shore power earth and the boats -ve or anodes a galvanic isolator is not required, it will have no effect.
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My MD22 is not earthed to the block, the common negative is on the starter motor. IIRC the MD2030 in the previous boat was the same.
 
Tranona said:
That is not relevant to boats as they do not use the "body" so there is no corrosion. The only time there is a possibility of corrosion is if there are mixed metals in the seawater cooling system. If this is the case then an anode is is used in the system. Most engines though if heat exchanger cooled avoid this by not using dissimilar metals in the seawater circuit.
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My MD22 has an aluminium heat exchanger body with a yellow and red metal tube bundle plus a yellow metal seawater pump.
 
My Prima M50 is mainly not earthed. But there is an earthing relay that is activated when the glow plugs are on and when the stop button is pressed. Both the plugs and stop solenoid -ve returns are via the engine block.
 
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skipper_stu said:
My MD22 has an aluminium heat exchanger body with a yellow and red metal tube bundle plus a yellow metal seawater pump.
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But the seawater only comes in contact with the yellow metal and red metal parts, not with the aluminium HE body I'd guess ..... or you'd be able to power your underwater lights from it. :) ...... at least until the aluminium all corroded away.
 
Hoolie said:
My Prima M50 is mainly not earthed. But there is an earthing relay that is activated when the glow plugs are on and when the stop button is pressed. Both the plugs and stop solenoid -ve returns are via the engine block.
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The Prima 50 is exactly the same as the MD22, VP bought the Perkins and painted it green. You are correct, the earthing relay works to make the block earthed for the few seconds that the heater and shut down solenoid are working BUT the main negative battery wire goes to the starter motor from the batteries and is insulated from the block.
 
VicS said:
But the seawater only comes in contact with the yellow metal and red metal parts, not with the aluminium HE body I'd guess ..... or you'd be able to power your underwater lights from it. :) ...... at least until the aluminium all corroded away.
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Blue!
 
A1Sailor said:
Surely having an engine connected to -ve battery provides some protection against corrosion? My schoolboy chemistry was obviously years ago, but my father who was a Chemistry Teacher told me that cars used to be "+ve earthed" (car body connected to +ve battery terminal) but that got changed to -ve in "newer vehicles" to reduce corrosion.
Click to expand...

Cars were changed from +ve earth to -ve earth when cars started to be fitted with alternators to replace the dynamo dur the the change from commutator to rectifiers
 
Rogershaw said:
Cars were changed from +ve earth to -ve earth when cars started to be fitted with alternators to replace the dynamo dur the the change from commutator to rectifiers
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Thanks. My father told me, when I asked him, why grass was green - he "helpfully" told me it was because it reflected green light.
What I should have asked, but didn't aged 6, was why it reflected green light!
 
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