Steve: I think you'd want to keep about a 3 to 1 ratio for the overall length of the slider to the span between the bearing points. In my case, I am using a 9 foot tube and my supports are 40 inches apart. Some tube is lost to the coupler mounting and the end stops, but I still will get about 5 feet of extension. My parker 25 used a full 10 feet of tubing and had the supports 3 1/2 feet apart so it extended just over 6 feet. The key is how heavy a tube you'd like to used. I am using schedule 80 stainless pipe. The most significant load comes when you are making tight turns with the trailer. If you have lots of room to turn, the tubing can be lighter.
One of the things I've done that limits the length of the tube is to mount the slider in the center of the channels. The Parker had it mounted under the trailer, so the tube could be longer without potential interference with the bow of the boat. The installation on the Parker was done 8 years ago and the oilite is holding up just fine. The boat is in commercial service and is being launched almost daily.
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