Battery question
 

Any problems with using 2 deep cycle batterys in a boat???
instead of cranking marine batterys???

Why would you want to?

Deep cycle batteries are made to discharge small to medium amounts of current over a period of time and can be drawn down. Cranking batteries are made to discharge large amounts of current over brief periods of time and are sensitive to significant discharge (it ruins them)

A DC may not crank your engine- especially if you've run them down a bit and your engines are hot/run alot of timing.

Tell us what you want to accomplish and maybe we can suggest something.

I have then in the boat now,and both of my alternators failed,they burned the studs on the back off,I thought the deep cycle battery might be the cause
also when I start/crank the port motor the radio goes off and turns back on about 5 seconds after engine starts,I dont know if thats normal or not

You have wiring problems. Quite possibly with your grounds.

The alternators don't know what batteries you have in there- they just ouput current. DC's don't take more than a regular battery to charge, but they don't live as long if you blast them instead of trickle charging them, but that's a different issue.

I thought the same,did a voltage drop test on the ground side,with the engines running ,from the battery ground to the back of the bellhoushings stud I have .045mv
I also did a voltage drop on the positive side,with the engines
running from the alternator stud to the battery positive I have .160mv all seems fine there.
Now the other thing I also had was only 1 Perco battery shut off switch and always ran the boat with the switch on the ALL
position,I was told thats a No No,
I installed a second switch now so I could keep the alternators and batterys separate,
I thought maybe the alternators were fighting each other to charge the batterys

The alternators can't tell that each other are in the system. They sense voltage and switch to charge mode when necessary.

Meter tests are often inaccurate or inconclusive- you have to duplicate the load to see what's really going on. I've seen connections that had low impedance with a meter that were 90% corroded. It doesn't take much current to run an ohmmeter. One single strand of wire will show very low impedance, but you can't run a 70 amp alternator through it. I'd pull all the grounds and reseal them, replacing the star washers. I'd also pull the alternators and clean everthing up really good.

I have ran two "dual purpose" batteries for the last two seasons and they have worked well. And I run my stereo a lot while sitting.

I know what you mean about the single strand,
the readings above where from a voltage drop test.
Tonight I ran a 8 gauge ground wire from the back of the block to a bus bar behind the dash panel and ran the grounds to it,that took care of my gauge issues,lights and gauge readings dont bounce when I trim up or turn the nav lights on,also tachs now seem to work fine.
Now my radio still goes out when I crank the port motor and when I use the trim tabs,I checked the voltage at the radio circuit breaker,it drops down to 9.5 volts during cranking or trim use.
I was thinking of doing the same with a power wire ,running a 8 gauge wire to a bus bar behind the dash,
I believe that the correct way of doing it,
where would you get the power supply from??and would you tap into the ignition switch??

Your systems legs should come right off the battery switch and have fusible links in them. One goes to the tabs/drives supply, one for dash/engine circuits and one for cabin circuits. Obviously this is for a basic performance boat. If you have electric fuel pumps that draw big current, another circuit is waranted. Big stereo? Another one. BIG stereo- separate battery.

Make sure your bilge pump circuit comes off of the feed side (input) of your battery cutoffs.

Your boat sounds just like cbgnrl's '97 290 PQ. I tease him all the time it was wired by monkeys. PQ makes a nice boat but their wiring left a lot to be desired. He is constantly chasing electrical gremlins.