Jacking up boat trailer

Where would I place the jack to jack up the boat trailer? All I want to do is lift it just enough to turn the tires about a half a turn. Mind you the boat is on the trailer.

 

If it is a tandem axle, you may be able to raise the tongue jack high enough to lift the front tires off the ground, or lower it enough to lift the rear tires off the ground. I can do both of my tandems that way, but they're not boat trailers, and I don't recall ever trying it with a load on them. Otherwise, I would put it under the axle, directly under or as close to the spring mount as possible.

 

If you can get a jack under the axle that will allow you to get the tire off the ground the easiest but you need a jack that won't slip off as many axles are round. You can jack on the frame of the trailer but you have to lift higher as you take the travel out of the springs before the tires after leave the ground.

It's impossible to give specific advice without knowing anything about your trailer or jack but keep safety in mind. Don't get under the jacked trailer in case the jack slips. Think where the trailer and boat above will go if something shifts or moves. You don't want to be the squishy bit underneath.

 

How big is the boat? What's the make & model? I ask because your car jack may not be rated to lift a large boat.