Thought that I would throw this out here to see if anyone has had a similar problem. This is not a problem that has started all of the sudden. It was doing the same thing last year with a different set of engines.
On my port engine, every gauge reads less than the coresponding starboard counterpart and each individual gauge "ticks" every 6 seconds (I know that that symptom is coming from the hourmeter). When I turn on the ignition for the port engine, the voltmeter will go to 12 and then very slowly drop.

I have checked all of the engine grounds (although the two engines are not grounded with a strap...should they be?) and all of the panel terminations and wiring under the dash panels are tight and the connections look good with no corrosion. The alternator on the port engine doesn't charge as well as it's starboard counterpart, but I'm assuming that could be a result of the symptoms as well.

Any ideas?????

Yes put a common strap between the motors for ground.....you dont want a long thin ground cable to be the power source for the opposite engine if/when you kill one and switch the battery selector switch over to the opposite battery to try and start the engine up! That will probably cure the symptoms but you need to find the problem.....i would first start by starting the engine and disconnecting that hour meter if you suspect it is the problem...and see what happens. Also take your alternator over to PepBoys or a local auto store and have it tested...you may have one with a faulty regulator or the brushes are starting to out....

Most definitly put a cable between engine blocks. This is needed for safety reasons if nothing else. If you lose your main ground connection between engine and battery and crank your engine your starter will use what ever ground it can find. Even a 10g that happens to be strung under your dash between your engines. It will instantly melt that wire and anything elese that is in contact with it. I think that it is a Coast Gaurd requirment too.

Thanks, I'll ground the engines together tonight. What gauge ground? I'm assuming that bellhousing would be the spot on both engines?

The problem isn't the hourmeter (I disconnected it and something is still draining). I've pretty much resigned to the fact that it will take time to figure it out. You guys know how it goes..... The problem will be the last place that I look!

I'd use #4 or something in that range....

If you know something is drawing juice then first thing i would do is get a digital volt meter and set it to amps.....disconnect the neg cable and read how many amps you are drawing...then start either pulling fuses or dissconnecting acc's till it reads 0amps....or very close to it if you have an efi engine...one you hit that, then you will know what the item is giving you the problems

Its always the last place, cuz you stop looking for it after you found it!

PS I hate electrical gremlins

OK, I replaced the alternator and installed a 2/0 ground between the bellhousings. Still there!!! When I turn on the key for the port engine, the voltmeter goes to 12 and then slowly drops a little. Also on the port, every gauge "jumps" every 6 seconds.

I'm thinking that it's either a bad gauge or a short somewhere in one of the "common" wires that goes to all gauges.

I did put a voltmeter on the batteries when the engines were running and the alternator was bad on the port engine. Replaced it and both batteries now run at 13.8 volts (although the port volt gauge reads 12.5 and the starboard reads correctly). Something is robbing power from the port gauges. I've double-checked the grounds at the batteries and on the engine blocks.

There are only two other symtoms and I'm not sure that they are related but I'll throw them out here to see if it rings a bell for anyone. On my main electric breaker panel, when I turn on the breaker for the blower...it lights up the light next to the breaker for the horn (even with the breaker off) and vice versa. The other thing is that the radio will cut off when I'm flushing the head.

Any ideas???????

ok....your going to need to take the panel out and test behind it when you flip the breakers and see whats going on back there....if its lighting up the breaker for the horn, but the horn itself is not going off then i'd say you have a short to ground somewhere real close along the horn/blower circut........not being able to see the stuff first hand is hard to guess at a solution.
The radio cutting off sounds like the main power wire is not big enough to supply enough juice.....or another short

Thanks Wally, I'll check it out. Ahhhhh, the joys of owning an old boat!!!!

.....but it sure is nice not having a boat payment!!!