know what you're thinking, having gone to a 32 footer after a life time of those 'tiny' boats. What am I getting myself into! I asked myself, over and over again, but I'm really glad I did. They are great boats!
Here's what to do:

1. Find a good surveyor.

2. Use him!

3. Then do an extensive sea trial WITH the surveyor.

Now, on the aluminum hull... Properly protected (galvanically isolated, properly zinced, with the hull painted properly) a Marinette will last a loooong time--even in salt water. The trick is how was this boat maintained throughout the years, and where? Again, your best friend is a good surveyor, but you can do some intelligent looking on your own: Open every hatch and climb into the bilge with a good flashlight. Look for white, powdery areas--especially at welds--that might indicate a problem.

It's a gamble, but a better one than a fiberglass boat of that age. Fiberglass blisters, once started, are a dog and a half to repair--if then can be repaired at all. And massive bulkhead delaminations are death for a boat!

Good luck, and keep us informed,

Fastjeff

I am having corrosion problemss in 67 32' Express which has 1.2 kw ONAN. These corrosion meter goes from 700 to 900 when shore power is disconnected.

More particularly, disconnecting the DC ground from generator to its starting battery raises the corrosion meter from 700 to 900.

The generator is isolated from ship ground: white neutral is separated from green earth on boat wiring. Generator shell is isolated from boat hull. The problem appears to be in generator with some connection between its AC neutral and the DC battery ground.

Any similar experiences out there???

LogJim

You may need a galvanic isolator which is mounted in the green line coming in from 110v. Nemar makes them. I recently had a surveyor out and he was checking my generator, 110v and 12 volt systems and found that I needed to install one. My 12V and 110V systems were allowing about a 5 volt crossover.
Before installing one my meter never read over.7 and now it always reads .85 to .9 He stated this was fine for fresh water. I am going to go through my corrosion meter system this spring before launch to see if there are any other problems. Hope this helps some.

DEBAL

>>More particularly, disconnecting the DC ground from generator to its starting battery raises the corrosion meter from 700 to 900.>>

that's rather interesting. the manual for the Capac meter with my Marinette says .9 is the usual lower limit reading with up to about 1.2 being the max. According to that instruction then, your reading of "700" sounds too low. Unless, that is you have a different brand or different instructions. Making certain your isolator is connected properly, as other posters are pointing out, is always good advice.

I had a situation this summer where I got readings out of range and blamed it on either a transient boat nearby or a problem with some newly installed A/C wiring done by the marina. I did not notice any corrosion when I hauled for the winter... as a matter of fact the zincs are good for another season (which could also be a sign that the boat and not the zincs is being attacked).

JRAlbert

thanks for your answer:
I am going to try to isolate the green ground at shore connection to see it that makes any difference. (temporarily of course)

the meter we used (my boat's meter is inaccurate) said 700-900 was safe range for aluminum
.
seems to me the question is where does the reduced voltage come from when the generator case DC ground is attached to boat DC ground.

Logjim

See Test an Isolator