My situation is a bit tricky, since I tow with a car. I don't have a lot of tongue height. I have explored all options to raise the height of the hitch ball, and short of a new vehicle, there is really nothing out there. I plan on getting a truck, but a car is what I have to work with for now.
Luckily, the boat ramp is 3.0 miles and 4 turns away, top speed 45MPH. If I go out every weekend for a year, that's only 300 miles. I will not be towing the boat to any other ramp until after I get a truck.
The tongue and main support of the trailer are 2"x3" square tubing. At the axle, from the bottom of the tube to the ground is 16 inches. At the hitch, when I hook it up to my car, measuring from the the bottom of the tube, it is only 12 inches. There is a 4 inch drop from axle to hitch that makes me uncomfortable. That is one of the main reasons I want to lower the back half of the trailer, and not just how the boat sits on the trailer.
The trailer seems too short to me, so I eventually will bolt on a longer tongue to move everything forward. It definitely came with 8-inch wheels when this boat/motor/trailer package was new. It looks like the fenders were removed to make way for the larger tires. I would like fenders to avoid spraying my boat with sand on the way home. Also, fenders with a step would allow my 5 foot tall wife to reach into the boat to help load and unload, instead of just watching me do it!
If I were to flip the axle to the top of the springs, the larger tires would be really close to the bottom of the boat. It is a really wide boat, and there isn't much clearance.
That is why I think 8-inch wheels are ideal in my situation. For now, it seems wisest to improve the tongue angle
and lower the boat a few inches. 8-inch tires do that with with 10 bolts, 10 minutes, and $100.
My trailer does have rollers. They work really well, although they also add height. The saltwater is eating them fast, so they'll need to be replaced in the next year. I will likely be able to lower the boat by about two inches when I switch to bunks, if necessary. The boat sits parallel to the main trailer support.
With smaller tires, I might even be able to flip the axle. That would make the trailer level, and put the transom about 21 or 22 inches above the ground. That's why I am wondering if there is an acceptable range, or a minimum clearance. If everyone's boat sits 18 inches above the ground, and mine is 27 inches, I want to know!