I have a 7m plate boat and have recenly cut all my rollers off the trailer and replaced them with a series of V's, each V is stainless steel with 20mm hi density polyethelene strips screwed to them,and are the width of the trailer, the boat is supported along the keel and bottom/sides. its hard for me to explain but ill put some photos up, when driving the boat up as soon as the bow touches the V on the end of the trailer regardless if im way off centre and the wind or current is moving me away i then jus give it some power and it automatically self aligns and goes straight up the centre like a arrow, i launchn and retrieve my 7m boat single handed much easier than 2 blokes with a 5m, each V is set at the same angle of the hull in its corresponding point for ex the back V is 15d while the front is 45d,also the boat sits so snug on the trailer as the Vs act like a cradle and it is supported along the keel and both sides, so my point is have a look at this kind of system ill never own another trailer like it