This makes perfect sense
The Aqua Bound Sting Ray is designed as a touring paddle with 91 square inch blades. Sustaining 4.9 mph over 6 miles in a 14 ft kayak is a very good pace.
Counting your strokes per minute is easy if you have a deck mounted GPS, or watch that you can strap to the bungee cord on your deck. Your paddle swing should becone automatic over time. Since there is nothing else to do while paddling, the easiest, most accurate method is to simply count the strokes over 15 seconds (12 to 13 strokes), 30 seconds (25 strokes) or a full minute (50 strokes).
I use a Garmin deck mounted GPS, for real time readout to keep track of average speed, and a GeoTrack app on my android phone in a Pelican waterproof case hooked to my seat strap, which shows a graph for the entire trip.
I’m neither a racer or a kayaker, but I did some time paddling with racers years ago. Compared to the high-tec suggestions made so far this may seem simplistic and a bit Luddite - but folks I knew used to set racing cadence for practices using a simple battery powered music metronome (about the size of a cigarette pack, fits in pocket, and most could take ear buds.) Set the beats/minute and paddle to the rhythm.
I found in both bicycling and kayaking that a smooth cadence comes with practice. Efficiency requires practice, and as you master the mechanical process, cadence will improve. You must first master the mechanics of the stroke. Setting an artificially high cadence will most likely result in an inefficient stroke that pulls the paddle through the water rather than propel the kayak forward.
A few years ago, I believe @PaddleDog52 mentioned that the paddle must remain stationary (check the effectiveness by noticing how the paddle remains near a leaf or other floatsom where your paddle engages the catch), while the kayak moves forward. The paddle is always the lightest part with the smallest square inch area, so it will obviously move in the water with greater ease than the heavier boat that has greater weight and frontal area (for every action, there is an equal but opposite reaction). Therefore, the goal is to minimize that slippage (imagine your car engine spinning at 5,000 rpm while the car sits stationary with the tires smoking).
It might be worth pointing out that a heavy paddle is not necessary a negative, because the weight technically keeps the paddle in place and reduces the tendency to oscillate. So the question is how much those few additional ounces improves propulsion, compared to your ability to swing it accuracy between the exit and catch, then instantly transition to a smooth power stroke. Figure that out by testing and comparing paddles. It’s my “opinion” that a heavier paddle improves the feel of poor technique while a lighter paddle improves speed of the stroke but shows up inefficient technique. Stick with a cheaper but heavier paddle until you find features about the paddle that you want to fix.
Cadence changes, depending on the speed of the boat. You can vary the cadence under varying conditions of wind and current by varying the pitch of the blade or the depth of the stroke, with the goal being remaining aerobic. As soon as you go anerobic, you are tapping into a limited source of energy that can be depleted within 30 minutes (+/-).
The length and square inch area of the paddle and blade depends on how you transfer the power. Paddle technique, whether high angle or low angle, determines not only power transfer but the duration of the power stroke and the dead transition zone (how long does it take to transition between exit and catch - that transition period is longer for a high angle paddle, where the end of the power phase puts the blade up to 7 ft above the water; the interval between the exit and catch is lost time in the power cycle). Consequently, high angle paddling technique can never match the “efficiency” of the low angle stroke, and it will never achieve the same low angle cadence potential.
Rather than trying to match a cadence, it might be more beneficial to focus on stroke efficiency, and you might find that your cadence improves as you improve your paddle stroke. When I check my cadence by counting the swing as the seconds tick off on the GPS, I find that the cadence is typically within one or two stroke. If I increase my stroke rate slightly by about up to 10 spm, within about a minute I will go into the anerobic zone. Although my speed may go up by .5 mph, it then drops off until I manage to become aerobic again. You can’t fool your body into greater output. If your GPS speed graph shows a spike range of (+/-) 2 mph, it could mean you’re going in and out of the aerobic/anerobic zone.
Don’t confuse race techniques with a weekend outing. Racers not only train extensively, but they also stick with racing because they have the fitness and genetic ability to manage intense intervals. When they’re finished, their physical output has been depleted. Conditioning enables them to recover quickly.
I hope I haven’t misunderstood the post.
Coming late to this thread, but my Cheap-o Smart Watch, has a multitude of sport modes and sports it can track you at.
It even had a mode for canoeing and kayaking.
In these modes it will track your course and speed via GPS, your heart rate and SPO2. in addition it will also track your stroke rate.
at the end you can save all your data to your phone, and digest at your leisure. so if you look at your gps chart and were paying attention to conditions and how you felt you can go back through all the history and find your body’s optimal stroke rate. mine is about 58-60. faster than that I start burning muscle glycogen, and will burn out quickly. slower than that and I’m goofing off. but it’s handy to track your speed and strokes over time and if you are doing things right your stroke rate will be nearly a flat line.
Now this is all after the fact data analysis, during to keep my pace count I sing marching jodies, they are at 120 beats a minute so if I hit 1 stroke per beat I’m near enough optimal for government work.
Just don’t sing them out loud, most of them are quite raunchy. 
*Edit - math is hard. I said 1 stroke per beat, which would be 120 SPM, I should have said 1 stroke per two beats which is 60 SPM (and now I go hang my head in shame, who has the dunce cap.)
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Brilliant. Same thing with a march when you hike. That’s one reason the military does it. My Stroke Rate has become so automatic, I start counting when the second hand hits the the 12, and don’t look at the clock again until I reach 60 strokes. I look at the second hand sweep and stop counting once it hit 12 again. That way, you’re less apt to influence the cadence by chasing a specific number.
Cadence by itself is not some magic number or goal. It’s just a way to measure your technique and progress in managing form.
In bicycling, most riders will manage to maintain a cadence of 40 to 60 rpm (one full revolution of the chain ring, which is different than how I measure strokes per minute, or one stroke for each catch and exit). Until spinning (press down, pull rearward, then lift) becomes natural, its hard to hit a cadence of 70 to 80 rpm. Then it becomes easier to reach and sustain a 90 cadence. Until that motion becomes automatic, execution becomes choppy and awkward. The goal is building up to it by developing muscle memory. The same is true of paddling. Improving cadence is best accomplished by focusing on form. If you watch most kayakers in the distance, the paddles look like a cook on a short order grill chopping meat for a steak sub. I cantcomment on high angle, since I no longer use it due to physical limitations. In low angle, the more fluid you manage your form, the greater your potential cadence. Proper technique allows you to ignore the exit and focus on a proper catch.