Just about any length over about 23' will do. Longer lengths favor the lower bands; shorter lengths favor the higher bands. Somewhere around 40' is a good compromise for both ham and marine
Re: insulators, you don't need two in many situations. Sometimes, it makes good sense to put one insulator at the top, and feed the antenna
directly from the chainplate belowdecks, with antenna
tuner mounted closeby. This works very well, indeed. And, if you're worried about potential RF burns, you can cover the lower portion of the backstay with a nylon or PVC boot.