Marinettes were designed before many ABYC standards were developed, and grounding to the hull was common practice in those days.
There are some drawbacks:
=> Any connection between copper and aluminum is unreliable, and a hull return relies on two of them.
=> If you use the hull for return, then you have no real ground for the frames of your equipment.
=> The return path through the hull is unpredictable, so there is likely a "loop" that may affect the compass.
=> There is a possibility of electrolysis when current flows through the hull, admittedly less with a Marinette because of the all-welded construction but still there.
=> Resistance to lightning strikes is greatly reduced.
The best way to wire stuff is to use a 2-conductor cable, red and yellow, and run it back to the switch panel. The red goes on the switch, of course, and the yellow goes to the ground bus in the panel. The cable should be supported every 18". It is best to run a separate cable for each blower, and join them back at the panel, for redundancy. This is often easier than troubleshooting old stuff.
Skycraft has 12 AWG 2-conductor cable for $0.63/ft.
http://skycraftsurplus.com/marine.aspx
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
