Define "relatively calm"?
Do you plane on your long board?
What size rig are you using (your beat up sail)?
At 150 lbs, you could go with something like a Rio M and have the best possible compromise
between a long board, and a short board.
With the retractable centerboard down, you have a longboard (well, not so long as what you have
now, but certainly not a "shortboard".
Retract the centerboard and you have a large shortboard that can test your shortboard skills, and which
actually planes fairly early.
Depending on your "relatively calm" lake's windspeeds, you will most likely want a larger rig with a >75%carbon content mast.
For the Rio M in around 10-12 knots, I would suggest at least an 8.5 m2 rig.
Hope this helps,