Single Dagger Board vs Double Boards

[Speng]

Shuttleworth has a lot of good arguments for one board. Some of them are mine and some are his (since I've internalized his). this assumes a cruising boats that's not flying hull.
(1)boards and cases are expensive considering their size relative to the other aspects of boat building so you save disproportionate amounts of money and time.
(2) boards and cases take up space.
(3) your structural load cases for a board should dependent on a single board in operation anyway so there should be no structural difference. a board should be really strong. breaking a board due to just sailing loads (on a cruising boat) is a sign of underdesign. You should have to hit something to break one (IMHO). I think people often neglect shear loads espcially when they're doing cored boards...
(4) you might want to make the area a bit bigger. A lot of the time simple rule of thumb is something like 2-2.5% of sail area per board and really there's not much need to go to 4-5% if you only do one. Since you're draft limited you'd end up increasing the chord but chord may be limited by space in the boat (bulkhead spacing or something like that). For a cruising boat it isn't super critical. and yes your aspect ratio goes down so you more induced drag for the same side force but really it's a slow ass cruiser so it's not important.
(5) boards should be canted to either hull side to preserve interior space. For boats with 2 boards there are some good arguments regarding reducing pitching with outboard boards which I guess would apply to single boards as well. However inboard canted boards are easier as you can hoist with a line off the rig while outboard canters may need lines down in the case onto the board to lift them. Again that's more $$ and time to build. Of course on a cruising boat it also depends on how the coachroof is arranged. having a board coming through the coachroof can be a problem :-)