I have not paddled any of those boats but that does not stop me from having some opinions!
I have been actively racing for 3+ years and am also about to start building another race boat. I currently have a Wahoo (18.5’ x 20.5”) that I race when there is a dedicated kayak or FSK division. The Wahoo is quite competitive in those races, being every bit as fast as similar boats like the Epic 18x and Stellar S18S or short skis like the V8. Where the Wahoo is not competitive is in those races where all types of kayak/surfski are lumped together. For those races and for those with big waves, I recently bought an intermediate surfski (Stellar SEI, 20’ x 18.1”). As much as I hate paddling a plastic (actually it is a 25# composite) boat, it is very much faster than the Wahoo (30+ seconds per mile). As soon as I switched boats, I went from being way in the back to being well into the front half of the fleet in those unlimited races.
I want to build a boat that will be competitive with an intermediate surfski on flat water. Going through the small list of designs out there that are viable for racing (Mystery, Yukon and Razor Billed Auk by Nick Schade, Pax 18 & 20 by CLC, Sea Racer, Spray, Spindrift, Panthera and Jehu by Bjorn Thomasson, and the Wahoo by Winters), my short list is the same as yours, Mystery, Sea Racer and Pax 20. In my opinion, those are the only three that have a chance of being as fast as my SEI.
There is very little info available on the Sea Racer besides that posted on Bjorn’s website. Given the dimensions, I am sure that this is a fast boat, but I’d have to redesign the cockpit area. As drawn, this boat has a low deck and small cockpit which interfere with leg pumping. You can’t be competitive without good rotation and leg drive. I’d also add facets forward of the cockpit to make the boat narrower at the catch.
I suspect that the Pax 20 is slightly slower than the SR due to width but still fast. If you dig around the web you will find a few reports that say this boat is not very good in waves because of low maneuverability. I suspect this is directly related to the hard chines inherent in S&G construction. The cockpit on the Pax 20 is larger than the SR but its key hole shape still interferes with leg drive a bit. Rounding out the front of the cockpit should eliminate that problem (I paddled a Pax 18 which has same cockpit). If I build a Pax 20, I will make it hybrid construction and add facets forward for a narrower catch.
My leading candidate is the Mystery. It is slightly longer than the other two and the drag data indicates that it is slightly faster than the SR (I don’t have data on Pax 20). I suspect that it handles a bit better than the Pax 20 because of the rounded strip hull but it is probably not as stable. Another advantage for the Mystery is that it flares above the waterline to a beam of 20”. Here on the east coast, many races use the 20” beam measurement as the break between “kayak” and “surfski” divisions. The narrower SR and Pax 20 would be competing against all the rock stars paddling elite surfskis, where the Mystery would be competing against shorter/wider boats.
My plan is to equip the new boat the same as I have the Wahoo. I will install a surfski style footbrace from Stellar and overstern rudder. I use a small rudder blade for flat water with few turns but go to a larger blade for waves and/or lots of turns. For seating, I have a raised rotating seat from Nelo that I use for short races (<10 miles) and flat water. For waves or longer races, I have a very comfortable foam seat. Compared to the foam seat, the raised rotating seat is 0.1-0.2 mph faster but much less stable in waves and a real PITA after about 90 minutes.
Now, to translate my long dissertation into some recommendations for you.
When I envision racing in SoCal against surfskis and OCs, I see coastal racing which means paddling in waves. Depending on the size and spacing of the waves, long kayaks with zero rocker like the SR, Pax 20 and Mystery can be a handful. It is possible for the boat to “bridge” two waves which basically leaves you sitting on a balance beam. A skilled kayaker can keep upright but certainly can’t paddle with any power in those conditions. Lastly, surf/breaking waves can be problematic for a kayak, especially one with a large racing cockpit. Beach starts/launches are especially problematic because it is hard to get the skirt on before taking a bunch of water. All of that being said, I do believe that all three of the boats would be good for the milder coastal conditions.
Assuming you are comfortable with its stability, I would submit that the Mystery would be the best of the three for you. It is likely the fastest and the only one that is designed with a large cockpit and facets. Do give the stability issue some real thought. You said you paddled the Mystery and “liked it a lot.” Does that mean that you were comfortable with the stability? Did you paddle it in conditions like those that you will race in? Will you be stable enough in it that you can paddle hard even when you are tired? One of the biggest errors that racers make is to overestimate their stability when choosing a boat. They test paddle an elite ski in calm conditions and decide to buy it. Then come race day, the conditions are a little worse or part way through the race they get boat fatigue and go for a swim. Hardly a race goes by when I don’t pass people swimming next to their very fast but very unstable boats. Stability is only a little slower, but it allows you to produce much more power.
If you are looking for some rougher water racing capability, I would look at one of Bjorn’s two sufskis, the Spray and Spindrift. There really is no substitute for some rocker and a self-bailing cockpit when paddling in waves. Both boats will be slower than the Mystery in flat water. If you want a decked boat, look at the Panthera which is a kayak (decked) version of the Spray. A faster option would be to design your own deck for the Spindrift hull.
The recommendations above are based upon the standard 6-12 mile races that we have on this coast. If the primary use of this boat will be for longer (15+ mile) races, you might want to look at the RBA or Wahoo. On the longer races, comfort, stability and the amount of gear (water) you can carry become more important than top speed capability. I find that at 20 miles I am just as fast in the Wahoo as the SEI, but much more comfortable.
As part of the selection process, do some research the races that you will be competing in. What are the conditions generally like? Do the division classifications favor one design over the other? If there is a +20” class, the Mystery should dominate if well paddled. Lastly, look at which boats the other paddlers are using. If the beginner skis like the V8 or S18S dominate, then rough conditions predominate, and you will want a similar boat like the Spray/Panthera. If unstable flatwater boats like the V14 are common, then a Mystery is probably a good choice.
One last suggestion regardless of which design you choose. Get a good beginner ski and wing paddle now so that you can start developing your race skills while you are building a faster boat. Even after the fast boat is complete, having a stable ski in your arsenal is good because it gives you a more competitive option when you show up for a race and the conditions are tough. Many racers (myself included) take several boats to each race and choose the best given the conditions that day.
I hope that this helps
First picture from the USCA Touring Class Nationals Mar 2017. I won my age division. Second picture is my first race in the ski at FCPA Silver Springs Race last Dec. Placed fifth out of thirty some boats and won my age division. Things are not always so good. Last weekend a guy in my age group beat me by 6 minutes on an 11 mile course.
