Re: Replacing trailer bunk boards
Here's how I did mine this spring. I used two 8 foot 2X4s, kiln dried douglas fir. I measured the bolt positions from the original bunks, predrilled the new bunk boards and countersunk for carriage bolt heads. I varnished the bunk boards and installed the bolts with some marine adhesive to fix them in place. I wrapped the bunks with bunk carpet and stapled them with SST staples. The old bunk bolts were completely rusted and I couldn't get a sawzall in, so I used an air chisel to knock the bolts off below the brackets.
I raised one side at a time with two floor jacks and a 2x4, and blocked them with jack stands against the trailer frame. Whatever you do, I would recommend using blocks to hold the boat up rather than relying on hydraulic jacks (I never trust my life or boat to jacks).
There are other ways that might be better, depending on your resources. If you have enough blocks and jacks, you can lift the whole boat from the ground with keel blocks. If your old bolts can be loosened, you could also just launch the boat and dock it, and replace the bunks in the parking lot. I'd considered that, but I had to shear the bolts and didn't want to drive the boat to the ramp without bunk bolts.
Here's a picture of the rig I used for lifting the boat. My trailer is galvanized and I wasn't too worried about scratching up the finish.
While the boat is on the trailer very carefully use a floor jack or a couple bottle jacks and lift the boat enough to take the pressure off the bunks. Don't try and lift too high, just enough to be able to remove one bunk at a time. That is one way but the safest way would be to remove the boat from the trailer then there is no saftey question.
Use the old bunk board as a template, match the boards up and reinstall the same way that they are on now.