Question: I was wondering if you think the engine should be grounded to the hull or not [on an aluminum boat].
Answer: Definitely not, in our opinion, the engine chassis should be carefully insulated from the hull. Further, the propeller shaft should be insulated from the engine. (The easiest way to do the latter is with a DriveSaver).
What we are doing here is making sure that no metals that are more noble on the galvanic series are immersed in the same body of salt water and connected together.
One of the above conditions is OK, but not both. For example, Morgan’s Cloud has bronze seacocks, but even after 24 years there is no sign of any corrosion around them. The secret is that they are very carefully insulated from the hull using plastic through hulls, a spacer plate and insulating sleeves around the mounting bolts, so that there is no completed circuit between the bronze and the aluminum.
However, if you ground the engine to the hull, an electrical current can then run from the hull though the seawater to the bronze and steel in the engine’s salt water cooling system and back to the hull through the connection. You have constructed a battery and the least noble metal, in this case aluminum, will be eroded.
Of course if the boat is protected by hull zincs, as it should be, they will go first since they are even less noble than aluminum.
Isolating the shaft from the engine is a “belt and suspenders” approach if the engine is isolated, but well worth it since if the shaft becomes connected to the hull, the damage is likely to be inside the shaft log, usually consisting of a piece of thick walled aluminum pipe welded into the hull, a very difficult and expensive thing to repair.
We are pretty comfortable with our position on this, as detailed above, particularly since it seems to have worked well on Morgan’s Cloud for over 20 years. However, it is a complex subject, so if anyone has anything to add, please leave a comment.