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What determines the center of gravity on a boat trailer?? The height of the bottom of the boat off the ground?? The height of the frame rails?? The height of the cross members where the weight of the boat sits on them???

Pretty much every boat trailer I see with spring suspension has the axle mounted on top of the leaf spring to get the frame rails closer to the ground. But in return the rollers or bunks make the boat sit around 10 inches off the cross members of the trailer so the boat will have an inch or two of clearance above the fenders. So if you were to mount the axle under the spring, you could bring the fender lower on the frame & in turn bring the boat closer to the cross members ( to a certain point ). Too close & the boat hits the cross members when launching & loading.

Would the center of gravity be lower with the boat closer to the trailer or the trailer closer to the ground?? Or does it not matter????

It's not so much the CG. were worried about as that will as high as the top of the tires or his a little higher. Is what we worrie about is being able to launch and recover the boat without turning the truck into a submarine. Of course you have to have the bunks high enough that the keel does not hit the cross members when loading/unloading due to the boat string level and the trsler dropping down the ramp, provided you took the astern straps off.

http://physics.bu.edu/~duffy/py105/Rotationalkin.html

Hope this helps.