Sorry to delay an answer. I've been busy with other projects.
First time poster here. I also am planning to build the Lissa/Otter and was thrilled to find such clear pictures of your pretty boat. I hope I can ask you a few questions. I have purchased the plans for the Lissa and have redrawn them in 3D CAD to adjust the sheer and bow shape a little.
Let me be clear. Are you using the Otter or Dobler Lissa plans? I had both sets and used parts of each. Since I wanted to use 4MM on the side panels, most of the lines for my boat came from the Lissa plans. I believe the bottom panel for the Lissa was a little wider than the OT. I followed the interior framing of the Otter which was much simpler than the Lissa. I built the boat in a "basket mold".
1. I want to use a balanced lug rig and am curious if the gunter as designed at 60 sf area is too much, too little or about right for sail area. I'd like to go bigger, like 72 sf unless you think that might be over-canvassed.
Because of the errors made by the sailmaker, my actual sail is less than 60 SF. When I was considering the build, I was impressed with the reported performance of the lug rig on the Goat Island Skiff, a boat about the same size. I did some low grade graphics comparing the gunter and the lug rig with the same 60 SFA. It seemed that the sail centers of effort were very close. The mast, as I remembered, was longer than the 10 foot spars for the gunter. Another boat of interest is the lug rigged SCAMP. This boat has been pretty highly developed in kits and hardware. I would track down how the SCAMP rig was rigged and in particular how the reefing rigging was finally worked out. The Lissa is over a foot narrower than the GIS or the SCAMP and the shape is conducive to rowing not sailing...tippy! Going forward in the boat to reef the lug rig is chancy. I decided to add the side seats so I could shift position more easily and balance the boat. I have only sailed the boat once since completing the seat mod. I definitely would build the boat with the seat modification.
In short, I think the lug rig would be a good choice. The sail can be dropped by just lowering the halyard. Given a SCAMP like reefing system, the sail should be manageable. Although one wouldn't choose the LISSA as a performance sailer, the reports of the lug rig on the GIS indicate the lug rig is no slouch.
As to sail area, I think 60 SF is fine. The hull is very easily driven in light air. In even moderate air, the boat is scary fast downwind. Having even a little sail area on the windward side could help.
2. Did you build the sail and daggerboard as designed? Does she exhibit much weather or lee helm?
I followed most of Dobler's plan for spars as soon as I figured them out. The daggerboard is also Dobler's design. Joe claimed the board might kick up in shallows. Jacques was skeptical. I haven't sailed the boat enough to comment on weather or lee helm. Mostly, it has a different feel. The length of the hull/beam seems to have some effect here. The bottom has enough rocker that the boat responds easily to the helm. My daggerboard board is rather small but carefully shaped so the boat has to be moving to avoid side slip. The board is about the same as I used in the 7-3 version of the Lissa. In that boat, the board could have used more area...about 12 in chord x 24 inch draft when compared with the race-able Sabots [an 8 foot pram like the D5].
If I were starting from scratch, I probably would have built an off-center centerboard along side the side seats. Dobler has a plan for a pin-less centerboard that might have been ideal... and simple.
3. I am planning 4mm sides and 6 mm bottom, would you still recommend this? I will probably go with 6 oz cloth on the outside and 4 oz on the inside for strength and paint longevity. I also want it to be light enough to pick up and move around.
I followed Jacques' fiberglass schedule, 12 oz biax, one layer inside and one out. He was striving for a very tough boat. My hull weighed about 200 pounds before the seat modification. The 6 oz/4oz was what I used in the Pepita, the 7'3" version of the Lissa. That little boat was very stiff. Particularly with the seat modification, I think the Lissa would be just fine with that layup and you would save considerable time and materials in fill and finish.
4. How many sheets of each thickness ply did you need?
I used 2 sheets 4 mm, 2 sheets of 6mm plus a part sheet of 9 mm and some scraps of 6 mm. For the side seats, I used the better part of a 6mm and part of the 9 mm sheet. The nesting of the Dobler Lissa got four side panels out of two 4 mm sheets. I wonder how long Joe spent fitting that on the drawing. He was working with graphical methods... before CAD.
Let me know if you have further questions. I think your suggested ideas are on the right track.
