RE: what fin size for a 6.8m on a S type104
We seem to have some disagreements, some misunderstandings and some agreements that you seem to see as disagreements. I will try to group my response accordingly.
First of all you say that most forum users want ?micro level details?pro level details? I am not in that category. My objective is to make my ?good? level of wave sailing look the best that it can, given my advancing years, heavier weight and limited skills. I have until now found this forum an excellent medium for my objective and until recently Starboard had equipment that was perfect for me. It seems that Starboard?s gear is becoming more technical and advanced and specialized which I would have thought is away from the major market of guys like me. If the forum is also shifting to the ?pro? level then perhaps I should shift to the Hot Sails Maui Forum. It is your forum, but for now I will continue.
I agree that fin design, manufacturing, and selection is a complex area. I personally know that, as I had the assistance of some terrific experts to help me design and build my own line of Aero big wave fins. Within this complex field there are some basic, simple and straightforward scientific based principles. Let?s compare it to boards. It is a basic decision if I want a wave board, a formula board or a speed board. The same goes for the basic shape of a fin. I do not use my engineering degree to sort that out. Secondly I make a basic decision on what size of board I want. This used to be measured in cm of length and Starboard changed the industry by switching to litres of displacement. (The two are generally related but ?scientifically? this is more accurate.) This is perhaps a slightly less basic but still not a terribly hard to understand decision based on flotation and resistance. The same goes for fins. To prevent spin out for a given sailor weight, sail size and average anticipated speed, one needs a proportional fin area. Any more than that area gives unnecessary drag and, after a point, instability of the set up.
I have a simple guide to required area which is not as you state ?macro knowledge (29 vs 36 etc)?. It is NOT based on fin LENGTH, that is my whole point; it is based on fin AREA. And the answer is simple. Minimum required fin area (cm2) = 0.53 x sail size (m2) x sailor weight (kg.). It covers 95% of your market. The additional 5% ?pro? market could be covered by adjusting the 0.53 constant to suit the individual and speed if they wanted to. It is my experience that by far most fin issues are spin out caused by insufficient fin area, typically for heavier guy on bigger sails in lighter winds; and secondly by instability caused by oversized fins, typically for lighter guys on smaller sails in higher winds. I am convinced, based on significant personal experience that starboard would significantly enhance the sailing pleasure of a huge segment of their market by helping these people.
What do you mean ?(although a quick test of a Drake70NR with a similar area Drake40 Shallow will demonstrate that area alone does also not do it)? What does it not do? Are you being silly about my first point of selecting a beginner fin vs. a racing fin? Is it not obvious that first you pick the appropriate type of fin. Then you need the right area. If these two fins have the same area then they both meet that criterion. Am I missing something or are you being sarcastic?
You are more knowledgeable than I on the micro level details on the next levels of fin selection which I will not even attempt to go into. With the exception of some very poor design and manufacturing that I have found on too many fins on the market, it is beyond my interest, needs and wants. If you want to design a complex ?fin matrix? to aid in fin selection for the ?pro level? of the windsurfing market I wish you luck on such a daunting task.