"Switching from MAg to ALm will be a great weight penalty... 150lbs IMO"
immaengineer,
The weight difference between a sand cast aluminum or magnesium gearbox housing is usually not much. There are several reasons for this.
First, while magnesium is less dense than aluminum (.064 vs .098 lb/cu.in.), the minimum wall thickness most sand foundries will pour is .16 inch for mag and .12 inch for aluminum. Lightweight aircraft gearbox housings tend to have lots of surface area (especially with cast lube system pipes) with minimum thickness. So that extra .04 inch of mag can make a difference with a large housing.
Second, maintaining accurate gear geometry under load is very important. Housing deflections can cause problems with gear and bearing life, so having a stiff housing structure is critical. Aluminum has a higher modulus than mag (10.3 vs. 6.5 x 10^6 lb/sq.in.), so it gives better stiffness for a similar structural configuration.
Third, if the gearbox is in an area that must meet a fire rating, large thin wall areas in a mag housing must sometimes be thickened to meet the burn through requirement.
As for using cast mag in a marine environment, this is something that the Navy (NAVAIR) usually frowns on, mostly due to corrosion/maintenance issues. There are some newer sand cast mag alloys (WE43) that have decent corrosion resistance, but they are much harder to cast than the more common mag alloys such as ZE41.
riff_raff