Tim,
You can go to a 15" rim that features a 5 on 4.5" Ford bolt pattern, consistant with your 14" rims, but Tony's' advice is sound about space. Unless your axles' swing arms can be adjusted, you may be reducing your space between the top of the tire and the underside of the fenders enough to create illicit contact.
You also need to watch the space between your fenders' inner coverings (Fender pads) and the sidewalls of the bigger tires.
We spent about 13 months trying to marry up all of our suspension and frame components, to accomodate 17" tires and rims. That will make your head spin.
Once we found a couple of 17" rims, with the correct offset, that feature the same lug stud pattern, in quantities sufficient for product demands; we had to narrow our frame and axles 2", find a wider fender, and raise the boat on the trailer about 1.5", in order to use the 17s and stay within the USDOT maxium width requirements of 102".
The USDOT max of 102" caused the 20" low profile tire and rim combos offered by our west coast competitor, to pushed the boats up 4 - 6" higher on the trailer, making them virtually useless on most any ramp that did not feature a fair degree of steepness. The tow boat market was a good niche for this style, but only if you stayed away from the north central markets that typically feature shallow ramps.
Goober says, "Hey!" Andy!! WoooHooo!!