I have a 1974 I 30 MK II. It has 5 feet of draft and I am a bit surprised that you need to reef or change jibs
as soon as you do. I have carried a 170 jib
to 18 knots before reefing. Yes, the boat is designed for larger jibs
, and that is where the power is. I almost always reef before changing jibs
. However, I need to amend my reefing statement a bit. That is if there are 4 of us on boar racing, with three on the rail I can use the 170 to 18 knots. With myself and my wife the 150 needs to be reefed at about 18 knots, of wind. We can sail at very close to 30 degrees of aparent wind on either tack. Also with either sail.
As a general rule, I figure the boat speed should be at least half the apparent wind, i.e. closed hauled at 6 knots appaent, I aim for, and usually get 3 knots of boat speed, 10 wind, 5 boat speed. On a reach the speed sholud be even closer to the apparent wind. Of course when hull speed it reached, that's it. Hope this helps.
John on Aquila
New Bern, NC