Trapper, let me preface by saying I don't know. Then I'll get in over my head anyway. The prop is a SWS II 15.25 x 17. I do recall that when we switched out from the original F250 to the newer F300, the 4.2 engines had a different gear reduction. I don't know how much that contributed to my dealer's recommendation, if at all. The recommended range is 5,000 - 6,000 rpm. However, I believe you are correct, most techs will say closer to 6,000 is better.
The F300 gets considerably better fuel economy that our previous F250 and with significantly better low to mid-range torque.
Grady tested the 258 with both the 15 and 17 props. My dealer gave me their test data to consider. According to my dealer, the 15" prop fits a more wide variety of situations, including folks who frequently run heavy or run fast or who pull tubes or skiers. So that became Grady's default prop. But we don't fit any of those uses and he knew that.
I just Googled it and you can still find Yamaha's add highlighting Grady's test data where they got 3.34 mpg on a 257 Advance. I'm sure they got that using the 17" prop. I'll paste the relevant portion of that add and the link. It is on page five of the following link:
http://www.yamaha-motor.com/outboar...s/V-6 Offshore Hot Sheet FINAL FINAL (v2).pdf
BEST-IN-CLASS FUEL EFFICIENCY Yamaha’s 300hp V6 Offshore outboard has up to 17% better long-range fuel economy at 3500 rpm than other 300hp 4 stroke outboards. All air entering the engine block of each Yamaha V6 Offshore outboard is routed through a single 75mm electronically-controlled throttle valve. Working in concert with the Precision Multi-Point Fuel Injection System, together they help ensure the precise amount of air and fuel necessary for optimum power and fuel effi ciency. The reduction in friction from the Micro-Textured Cylinder Walls further adds to this outboard’s amazing fuel economy. FUEL EFFICIENCY* COMPARISON (3500 RPM) Yamaha F300 Mercury® Verado® 300 Suzuki® DF300 17% Better *Per Yamaha On-Water Testing (Grady White® 257 Advance) 3.34 MPG 2.84 MPG 2.32 MPG
The Yamaha tech who ran the boat with me said if this engine is getting better than 3 mpg with full fuel, it isn't lugging and is very happy.
As for the truck and trailer, they belong to the local guy who hauls my boat for these trips. The boat stays in an in/out boat shed. Since I usually only do one trip per year, it costs a lot less to pay his hauling costs than to own my own truck and trailer. Since I've been doing this for a while with him, he tries to give me a pretty good price.