42' sounds like ballpark, but it's very hard to say without modeling.
Might I suggest that you rig a temporary antenna made of whatever material you have readily available and try different lengths? You won't be able to duplicate what the shroud will be unless you remove the existing shroud and use comparable material (probably totally impractical), but you might get a close approximation by rigging
the temporary antenna just forward, aft, or outside the shrouds, using PVC standoffs as necessary.
Certainly worth a try, I'd think.
BTW, I assume you're just gonna add ONE insulator at the top, and feed the shroud from the chainplate bolts belowdecks, right?
PS...nothing, but nothing will equal a 20m vertical dipole, so I hope you build and try one early on. If you have one, then you won't worry so much about 20m with the shroud -- which in any case ain't gonna be great -- and could optimize it for lower bands :-)
Be sure to check out this thread: http://www.cruisersforum.com/forums/...led-18130.html