A steel lug would be great. No matter what material you use for ground plane you can't 'set it and forget it'. You should be inspecting it frequently, keep a good coating of corrosion block or similar and it will work well. A steel boat is the ultimate ground plane, consider yourself lucky! As for antenna length, this is far less important than it used to be. Modern tuners will literally tune anything. A short wire antenna is by far the easiest to set up on a boat. Good practice is to use 1/2 wavelength of the frequency you most use. i.e. for 14.3MHz use 34.4 feet, but longer is better.
Keep in mind there is no ideal or perfect antenna install for a sailboat. Anyone that tells you otherwise is full of themselves. No matter what you do, esp on a metal boat every bit of
rigging, and all the wires going up the mast will work as parasitic elements. Your antenna will be directional to more or less of an extent. There is no way to calculate this, but use trial and error to get to know it, even use it to your advantage.