Marine vs Auto alternator types?

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grahama

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I am wondering if someone can explain the fundemental reasons why these two type differ? By this I mean my marine alternator (Ample Power Company) has both a heavy duty +ve and -ve terminal and has a separate 3 stage regulator.
Most auto alternators only have the heavy duty +ve and most often built in reg.

What is the main reason for marine alternators using a fully isolated electrical changing system and does my 'hot' standby spare alternator configuration below sound sensible or have I forgotten something fundemental?

The reason for the query is I am looking at setting up a 'hot' spare alternator for if the main high capacity one fails. I was thinking of using an auto one with built in regulator as a cheap keep me going spare. Clearly I would need to disconnect the existing sepatate regulator and connect the heavy duty -ve cable to the engine earth. The engine earth already has the main battery -ve connected to the starter motor mounting bolt so I could use location this as a temporary fix.

The boat engine is a marinised Ford FSD 2.5DI.

Thanks in advance,

Graham
 
Im not sure what the main reason is but as CreakyDecks has said sometimes the engine is isolated as with my own VP2001. I think this is due to the saildrive which is attached. The whole system is isolated the only exception to this is when starting.

A negative solenoid is used as i guess an isolated starter motor was big money and its really needed just for the sake of a few seconds.
 
Many marine engines, particularly those connected to a saildrive, are isolated from the battery 'earth' (negative) most of the time, presumably to avoid any possibility of electrolysis or galvanic corrosion due to stray currents.

To allow isolation, the negative return for the alternator and the starter are wired, rather than use the engine block. However, some items, like glow plugs and stop solenoids need an earth return through the block. To allow this a separate earthing relay is included that connects the block to negative when required.

Your set up sounds like a mix of isolated (e.g. the alternator) and grounded (e.g. the starter). If the block is grounded all the time at the starter motor then using a grounded alternator should be OK.
 
Oh interesting...............now I know it's for saildrives & galvanic corrision issues.

Thanks to the_barnflake and playtime for clearing this up.

Do saildrive units suffer more corrosion issues than say my set up with a s/s propshaft and bronze 3 bladed prop?
 
grahama said:
Do saildrive units suffer more corrosion issues than say my set up with a s/s propshaft and bronze 3 bladed prop?
Click to expand...

Only if they are not properly isolated and stray currents are present.

The saildrive housing is generally aluminium which is one of the more anodic metals and therefore has more potential ( ;) ) for erosion.
 
Playtime said:
Many marine engines, particularly those connected to a saildrive, are isolated from the battery 'earth' (negative) most of the time, presumably to avoid any possibility of electrolysis or galvanic corrosion due to stray currents.
Click to expand...

Also important on aluminium boats.

Most new so-called marine engines are fitted with standard automotive alternators and they're much easier to obtain and cheaper.
 
I have the same engine and replaced the original 55A alternator with a 95 amp from a auto supplier. The original only had the internal reg, and I keep it as a spare. The replacement was upgraded to take an Adverc reg.

On stray currents, I have a flex coupling that electrically isolates the engine from the prop shaft, so no conductive route there.
 
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