Different rudder types provide improved performance for certain conditions while typically giving away something in another aspect of performance or handling.
Surf - typically larger, designed to provide more control on wave faces so one can turn better and at sharper angles without losing control (broaching). Don Kiesling (custom rudder maker) reports that going beyond some size limit starts to contribute to handling quirkiness and diminished control. 9“-10” is usually about as long as most surfski surfing rudders go.
Weedless - more swept back to allow weeds to slide off, larger ones can provide some surfing control while still being less likely to keep weeds
Weed Guards - either built-in to boat or small glue-ons that go directly in front of rudder to prevent catching of weeds, particularly in the gap between the rudder and boat
Postage Stamp - small rudders 4“ long or less, meant primarily for flat water.
Speed difference - somewhat guess work, local paddlers in Bellingham, WA estimate the speed difference between their 4” and surfing rudders (7“) may be roughly 0.1 mph on flat water (the small rudder gives them 1/10 mph more speed). Needless to say - in rough water that speed difference is more than reversed by the instability of a small rudder if you have to brace more and cannot confidently apply stroke power. You can use smaller rudders in rough water to work on improving balance.
Your Boat Manufacturer - Sometimes you can buy rudders for your boat from the manufacturers website and for some brands you have to buy from a distributor of your brand of surfski or OC.
Don Kiesling - in Seattle WA makes custom carbon fiber rudders for many different brands of surfski. Tiny to supersize. Hollow stainless steel shaft. Usually quite prompt to build and ship unless he's off paddling. Quality is professional grade. Cost is competitive with boat manufacturer's rudders.
Orka South Africa. We know their rudders are used with Fenn surfskis. They say that all they need is your rudder shaft measurement. We are not sure about whether the angle of the shaft to the rudder body works for all brands of surfskis?
Weed Guard Sources
Check major surfski brand and outlet websites - most offer weed guards