Re: Is a single axle enough for a 21' Bowrider?
Adding a second axle would be a way to go in my opinion. I know Rwidman won't agree(of course he never does) but how can adding a second axle to an existing one axle trailer make it any worse? Its still the same load! Its not like you are putting on a bigger boat on than what you have. You are just adding some safety margin.
You can add a second axle or even a third if you wish. Like a chain, a trailer is only as strong as the weakest link. If the frame is rated for 3500 lb, an additional axle or two will not increase the capacity. You would have to contact the manufacturer or hire an engineer to determine the capacity of the frame.
You cannot just add an axle, you must move the existing axle forward or back so each is equidistant from the original location. While you may feel that one set of brakes is sufficient, many states require brakes on all axles, even unecessary ones. Of course, you will need wheels and tires and you will need new fenders. You will have to readjust the tongue weight once you are finished. A multi axle trailer must be towed level so you may have to purchase a new ball mount for the tow vehicle.
There may be a safety factor in case of a blowout, but thousands of trailer haulers are using single axle trailers and there doesn't seem to be a rash of blowout related incidents. A multi axle trailer is harder to manuver in tight spaces, and of course, the tire and brake maintenance is double with two axles.
If the boat is properly supported and legal on a single axle trailer, I would suggest leaving it on one. If not, I would suggest purchasing a ready made trailer with the capacity for the boat.