Thanks much for all the feedback so quick all ! I really am listening to the rudder ventilation talk as that is exactly what we have been feeling and seeing. To us the cavitation felt like a reaction to the amount of push force needed to "break" the hard lee turning issue and that we were basically pushing so hard on the stick / rudder that we simply pushed the rudder out of its normal flow to the point of ventilation (and yes as SRM said, see and hear it !) - especially when only 1 rudder was in the water. The odd thing is you can feel the same thing, but not as extreme with light air and both hulls down and running pretty flat. If a wave comes (normal boat wake wave on a lake) especially from the rear and the nose goes down (rudders up) it will pull to lee and a slight push on the rudder will break it right away and all is good again.
Rudders are original fiberglass we restored. Outer gel coat was pretty thin and you could see and feel fibers in many spots but straight and no damage so we re-gel coated them. Had the boat out in 25 MPH winds on the big lake (only folks on the lake that day:-) last summer in 3' – 4’ max seas (some over cross bar at times) with no jib / jib, mast raked back about 10" from straight up and gave everything a pretty good stress test with no issues at all. Had a blast, except still same issue of constant fighting the lee turn under load.
We definitely have normal weather helm until the lee pull comes suddenly with some type of "disturbance to good flow" and under power we have no tiller pressure most of the time.
Rather than re drill the rudders we have been using stainless flat washers and long screws to test various rudder rake attempts and was planning on drilling and filling once we found the best set up. Attached is a photo of current rudder set up and rake. We had a few more washers last summer with about 1.5" at 12" down from Gudgeon, but nothing changed in handling and it caused the pin to bind so we went to current to save pins and bushings.
According to SabresfortheCup we should be about the 2" mark, rudder is about 9" wide at the 12" down point so 2 1/8" would be 1/4 blade. We have been from 0 to about 1.5" 12" down pin center line to front edge of blade with no change. toed in and out over 1/2" with no change. The original rudder holes were not too bad but bushings were gone and tons of slop and play everywhere which we thought would fix the issue. We drilled everything out to 3/8" (drill press) just to make sure the rudder is held tight as possible installed the washers for rake to suggested specs, set toe 0" - 1/8" in and had no change at all.
The part that is hard to take is where the original factory rudder rake was almost right at the edge of the rudder. Our previous 18 had no rudder rake, loose pins and bushings, toe who knows where and sailed perfectly with mast rake all over the place and floppy dagger boards.
Maybe next attempt should be polishing the rudders smooth and getting more rudder rake again to the 2" ? Mast rake was right on the mark checking the suggested way and other ways,but made no change. Problem we have is changing mast rake is a bit of a problem now because the new Hobie side stays were about 3/4" shorter than the originals and the front stays were longer so getting close to straight up or a slight rake is tough because we are in the top hole for side stays and have to remove the forestay adjuster and go direct to the furler tube to get correct slight rake adjustment. Straight up required using the old shorter forestays. We have a teflon washer under the mast base we could take out which might help.
Thanks again for your thoughts!