Tanka. I wrestled with this same topic and settled on a prop that works very well for me, but is not the popular choice.
Grady recommends a 15.75 x 15 for the 258, which is the same hull hull as your 248 except with 2' more running bottom, extending to the transom. When we bought our boat the dealer let us try the standard prop and gave us a 15.5 x 17 to try. We have run the larger prop now for 5 years.
The benefit is great fuel economy. I regularly see 3.3 mpg at 26 mph and have seen 3.5 mpg in flat clam and lightly loaded.
Drawback is It tops out at 5400 rpm. Some will suggest the larger prop may cause premature engine failure. That was a concern. So I've taken two Yamaha techs out on my boat and both have concluded that the engine seems very happy on plane and that the set up is fine. Both said that to allay any fears, I should limit time spent running over 4,000 rpm. Not a problem since 4,000 rpm gets me 31 mph.
I do a lot of long distance boating, so the fuel economy is attractive to me.
Good luck.
Rob
