Negative connection to starter

[pcatterall]

Sorry to post again on starters but your responses may be of generall ( file in the brain) interest to other forumites.

After several issues with my 4108 starter I am trying to find a less complex pre engaged starter. My original starter had an internal solonoid and both pos and neg terminals. I notice, however that the negative terminal is not insulated from the body of the starter. Is this terminal necessarry then, or could the neg connection come from the engine ( provided that the engine block has its own negative strap connection?
I understand that 'proper' marine starters are insulated from the engine, can someone refresh my mind as to why this is?
Thanks.

 

[oldharry]

Automotive wiring convention uses the metal chassis of the vehicle for the negative return. This saves a great deal of cabling in the vehicle.

Boats can have serious problems of electrolysis from the metal parts, and to help counter this, the power circuits are often insulated completely from the boats structure, particularly in metal builds. Also, in smaller boat installations, by providing an earth return directly from the starter casing there is a better chance of making a better return path for the very heavy currents involved.

 

[PCUK]

Agree with everything Old Harry says, but in practical terms if you can get a starter with the earth through the body it will give no problems as long as you have a heavy earth lead to the engine block.
Also my 25 year old steel boat has earth return starters and has shown no signs of corrosion problems in all that time.

 

[oldharry]

I have once had problems with a starter earthed through the block, due to corrosion between the starter body and the block - but it is rare.

Corrosion is not the only reason for isolating a steel hull: In certain circumstances a faulty shore power or earth connection can make the hull 'live' if it is part of the negative return. There have been a accounts on the forums some time back of people getting shocks from metal parts on their boats due to faulty earthing or bad wiring. Not worth the risk IMHO.

 

[davidbains]

On my Yanmar I think the negative connection on the starter is also the engine earth.
Either way you'd better wire it as supplied even if it means you have two engine earths.

 

[FlorealIII]

If this starter with internal solenoid is a Lucas CA 45 or M 45 or M 50 I advice you to connect the negative earth directly to the starter because they draw considerable current which can go up to 800 amp according to the Perkins manual . The engine can then be earthed from the starter .