Hello John,

One thing I would consider with the lexan is "flutter" at higher speeds. This is something I deal with a lot in designing paddle board and surfboard fins. They have to be very stiff or they start to vibrate at speed and sometimes you can even hear them hum. Have you thought about trying nylon sheet? It is friction reducing, very durable, and pretty darned stiff. The friction reduction may help solve your issue with rocks sticking in the slot. http://www.interstateplastics.com/plastic-sheets-details.php?sku=NYLNE&thickness=0.125&dim2=12&dim3=12&utm_source=froogle&utm_campaign=froogle&utm_medium=na&utm_content=sheet-NYLNE&gclid=COWQ9tbMsrECFSQDQAod_yAAvw

Here is the latest fin I designed for a displacement hull paddleboard that is basically a "stand-up kayak" more than a SUP. The hull has a vee all the way down the keel so I don't need too much fin. The idea came from a tuna's fins that are narrow and swept back. The leading edge is at 30 degrees. I often paddle over shallow reefs and want a fin that will tend to deflect up rather than snag. Also it makes the fin weedless. We have this type of seaweed that floats on the surface in large groups and it tends to hang on fins. I am probably going to cut it off flat at the bottom, reducing that point at the end by about 1.5".