Boris, I think you just made the slip of the fingers I was guilty of: NMEA--Natinoal Marine Electronics association--vs NMMA--National Marine Manufactures Association.
C Dory was NOT NMMA certified or ABYC complient in 2006 when I purchased my Tom Cat. NMMA was pending--and did not occur until 2007. I am not sure of history prior to 2003--my 2003 C Dory 25 was not AYBC or NMMA complient. Just because a boat was ABYC complient at one time, does not mean it is at other times. Materials, management and workers change.
One must remember that ABYC is a set of standards (these are available to members cheaper than to non members). NMMA means that the boat complies to their standards--and these cost money--both for the organizations and the surveys. It is very possiable that C Dory (Fluid Marine) does not feel that it is worth the costs.
This does not excuse them from not building to the standards. i cannot comment if the current boats are built to the standards or not. Currently I could not find on the NMMA web site C Dory or Fluid Marine. I could not find mention of ABYC or NMMA on the C Dory web site (but I may have missed it--it does not seem to currently be in the PDF brochure).
Getting back to the questions. If there is no ventillation, then you should put in a couple of vents--no big deal to do this, and it would be a good idea for condensation alone.
The fuel tank is a more difficult question. My 25 has the plastic tank, which is not leaking--and I decided to not replace it when I replaced the cockpit deck (older 25's did not have a removiable floor). I am told that only one plastic tank leaked and that was replaced.
A properly built and installed aluminum tank should have the following characteristics. 1: be built of a aluminum alloy which is not salt water corrosive. 2 be properly prepared, and then painted with an appropiate two part epoxy primer (usually stronchium chromate or zinc chromate) and then the bottom of the tank should rest of plywood which is sealed with epoxy--so it will not soak up water (or the tank can rest on the inner hull). In either case, the proper way to install the tank is to 5200 some strips of plastic to the bottom, so that it will never sit in water, and then put 5200 under this plastic to bond it in place. I believe in a discussion in the past, that C Dory claimed that they were using a non carbon containing rubber compound to space the hull off the floor--and if this is the case, and there is no water, then it is OK.
However, the way the C Dory 25 sits, the bilge pump is just aft of the tank, and there is often water in the tank compartment when the boat is in the water. You need to be sure that the boat sits to that any water will drain out the aft drain plug when the boat is on the trailer (tongue high).
Yes, 20 years is about average for many fuel tanks (both steel and aluminum) but I recently heard of one boat where the tank was leaking (not C Dory) after 3 years due to corrosion. Worse is the tanks which are foamed in. The water sits in contact with the tank.
