Trailer Axles (sorry, long)
Ford and GM recommend 10-15% tongue weight for trailers, it does not vary based upon the number of axles. If you add a second axle, then you will probably need to move the existing axle to keep the right balance.If the boat, stuff and trailer is over 3000 lbs, you will need brakes to be legal in most states, some require them at even lower weights. Many states will require them on both axles. You will need to be legal in the states that you tow in, being legal in just your home state will not stop a ticket when you are out-of-state. I would think that 4 inches of suspension travel would be adequate, if you are cutting it close, then a rubber snubber is a good idea. These can be pirated at a junkyard off of a small truck or Jeep with leaf spring suspension.For a really low slung trailer, if you are willing to spend, torsion axles are a possibility. The spring is a rubber element inside the axle. The axle just bolts to the frame and does not need any travel.Springs are underslung to axles all the time. You will need bigger U bolts - because now they are in tension and they also get significant tensile load from being torqued down.You are adding weight to your trailer. To be legal, depending on how the trailer is rated, and where you live, you may have the problem of exceeding the trailer's TAGGED GVWR. GVWR is a legal rating of how much the trailer can weigh with it's load. If your added axle puts the trailer over its current tagged GVWR on the VIN plate, then you will not be legal, regardless of your potential safety improvement. There are several choices, they depend upon the laws of your state. When I restored a trailer that I had lost the tags for in Colorado, I did it with parts from a Harbor Frieght trailer. Because the trailer was a mix of parts from multipls sources, they let me tag it as homebuilt. Colorado does not put a GVWR on homebuilt trailers, so theoretically, my little traier can carry whatever I am dumb enough to put on it. By adding weight to the trailer, you may need to work with your state's DMV to retag it if the GVWR thing is an issue. Maybe you can retag as homebuilt, with a GVWR of your choice, or one that is the sum of the new axle and tire ratings. This has nothing to do with the safety of your mods - just the way the law looks at them, right or wrong.Try these sites for trailer parts and advice:http://www.championtrailers.com/http://www.etrailerpart.com/Best o' Luck,OC