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I am currently restoring a 66 moppie. The painter is finishing up here in the next few weeks. Transom has been replaced. Stringers were inspected, but looked good and dry. The hull has been painted with an alexseal silver metallic finish. The inside is matterhorn white. The bottom is Vc performance epoxy. I have a "new" (certified pre owned) 4.3 MPI. I want to move the batteries to the bow. My question is the run is about 20' do i really need 4/0 wire? The merc chart says this, but it seems rather excessive.

Michael,

Cable sizes are particularly important for low voltage battery load cables.

Voltage drops through incorrectly sized cables are one of the most common reasons for low voltage (12v, 24v or 48v) system faults. If the cable is far too small, it can be very dangerous as the cable will heat up and potentially cause a fire. Undersized cables also waste energy.

For a 20' run, I wouldn't use anything less than 4/0 tinned marine battery cable. Better safe than sorry...

I wouldn't move the batteries to the bow. They are gonna get pounded to crap up there. Seen multiple internal connection failures due to that type of abuse.
If your still of the mind to do it a 20' run by the numbers which is current flow vs resistance(length of run) to eliminate any voltage drop which equates to current increase, 4/0 is the go to.

In reality 2/0 if the ends are tinned and soldered on and the battery voltage is kept up should work just fine for that engine.

Keep the batteries inthe stern, if you need to trim, use something else for weight like: water bottle;sand bags; lead pigs; etc...Keep the batteries near the motor, have short cables and, as Bruce says, the softest ride in the boat. I am assuming trim is why you want the batteries forward, am I missing something? Also, the 20's operate very nicely a little stern heavy, the chines on my baron are 4" underwater at rest and it's running attitude is perfect.

I speak from experience on both issues. I had a battery fail that was bow mounted for a windless. I caught it before there was a fire only due to the rotten egg smell it gave off when it let go.

On the cable size as everyone has been saying go bigger. Even one or two sizes bigger than factory recommends. Remember they're only giving you their chart for starting, not including radios, fish finder, radar, lights etc... I just did a job for a guy that swore he had a battery or alt prob, tested them all and no issues after a little more questing and test determined it was the wire size. His comment "that's what they installed from the factory it should be good enough" you should of seen his face when he got the bill for the wire!!! Tinned marine wire not cheap!

Mike,
Don't put the batteries in the bow area, they will end up braking loose and long runs on cables is asking for a fire due to heat build-up, plus it's just not the correct place for them. Keep them in the stern area, short run with heavy cable will never give you a problem. As a surveyor I do not like anything up in the bow except anchors, line and chain and maybe life jackets in that boat. I'm a former owner of a 1968 Moppie and I have owned 1961 31 express for 35 years. Batteries in the bow like Bruce said is asking for trouble, don't do it.
Pete Fallon

Thanks for all the responses. I was wanting to put the batteries forward to remove some weight from the stern.
I was not considering it for trim, and I plan on adding volvo qls. I like the idea of the battery switch forward and the easy access at the helm for the starting and house battery. I dont plan on running it offshore unless it is "nice."
If you guys still believe it is not a good idea, I will take your advice. Again thanks for your responses, long time lurker Michael.

I have same 4.3 (albeit Volvo) going into my B20 Bahia Mar.

The batteries will be just forward of the engine.

I purchased 1/0, and these things are huge compared with what was in the boat and what I have typically seen installed on repowers with batteries in same position.

Perhaps I undersized, but for the run that I will have I think I have plenty of copper in there.

Ian.

I also struggle with the desire to have the battery switch near the helm.

I am looking are remote battery switch.

http://bluesea.com/category/78/79/productline/385

Ian.

Ian: Other than at installation and during battery changes you will never regret going oversized on the cables!! You know you wouldn't be happy otherwise!!

Been there done it To save a lot of cash go to your local welding supply get 4/0 cable it is u/v bullet proof solder ends and cover with shrink shield Only use solid copper terminals BR goober,