From my experience with a 22', 4400# dry, 5.0GXi, heavily loaded sportdeck I can't help but think that a F5/D5 for light-load cruising or a F4/D4 heavy-load/watersports use might be best. If I recall correctly from my research, the VP DuoProps move more that one pitch on the scale for each size change so the rule of thumb regarding RPM change might not apply.. F3/D3 will likely result in too many RPM's. Even though currently more expensive, you may want to consider the "I" aluminum series as there are no speed limitations associated with them, but as the "D" series is discontinued you may can get a deal you can't refuse. (If your budget was unlimited, the Mercury Thunderbolt props would be worth considering. They are pitched sort of in between the VP's (I call them 4.5's, 5.5's, etc.) and could be a viable solution for your unique situation.)