With our old rig double trouble.,
The previous owner towed her with a landcruiser ute which had a lift kit so he moved the axle set as far backwards as possible so he didnt blow the rear tyres out on the trailer.
When we bought her we towed her forabout 2 years with a ford territory and a nissan patrol which were both alot lower in the rear than a cruiser ute, and was constantly going through tyres, every 200kms of travel we would have to change one of the front tyres due to blow outs, tyre damage etc.
About 12months ago we spent 2 weekends on the trailer getting the setup right so we would stop getting blowouts etc.
We never had 2 change a tyre on the trailer in 12 months, and they were in top condition when we sold her.
They were 9 inch wheels inflated to 38PSI each.
I was told by a VERY experienced four wheel driver that the right setup for your trailer is when the hitch has 200lb's of weight on it heres the method
step 1--take a set of bathroom scales and a car stand, place the carstand on the scales and drop the hitch of the trailer on it and see how much it weighs.
step2--if the weight exceeds 200lb's you will have to move the wheel carrige forwards, if it is under 200lb's move the carrige backwards. untill you get the correct weight.
We were lucky with the old boat that the mudgaurds, axles, wheels etc were all connected via a large metal plate so no weldding or griding was needed.
cheers james