[Chris Burkard]

The picture posted above looks more like cavitation, but it may also be electrolysis.

Cavitation:

If the issue is cavitation, then you likely have the wrong props on the boat.

1) Call a good prop shop and have a discussion with them about your boat. They are going to need; Make, Model, Hull design, Length, Beam, Engine, Drive and Weight. From there they can probably tell you what props you need. Aluminum props are cheap, but they have a much shorter life span and they are subject to much greater damage when grounded. Stainless props are what I suggest.

2) Make sure you go over your zincs, bonding system and electrical system so you do not damage your brand new props.

3) Once you have them installed, make sure your bottom is clean, engine is properly tuned and run the boat. Your props should allow your engine turn within its is rated WOT range. I believe the 7.4L is designed to turn between 4000 - 4400 RPM (please check as I am not 100% on this). Ideally, your engine should turn in just over the minimum in the range when fully loaded (fuel, water, gear, stores & people). When your vessel is light, it should turn towards the upper range, but not over. Engine damage can occur if your engine is unable to turn to the minimum WOT range. This is known as Over-Proping. Under-Proping can also be harmful if you exceed the maximum Rated RPM of the engine.

I hope this helps.