Notice how the rudders are hung. If you have offset rudders and your combined thrust (inboard) is channeled more between the rudders and dead astern, it seems that you would have most effective bollard pull. combining wheelwash (inboard) just makes sense for minmizzing friction loss and drag. definitely not scientific. Net Gain may be calculated from difference in drag coefficient, given water flow outboard of the rudders is different with IB vs OB.