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Paducah said..Clever, like a mechanical Noer stick. Interesting to see how the screw jack (I'm assuming that's what's essentially inside) holds up to corrosion, sand and how much mechanical advantage is lost over time to both and the lack of lubrication. Agree with aeroegnr that smaller sails are simple and low tension enough not to need to spend the funds to complicate the process.
The only thing about carrying the driver/tool on the water is the opportunity for it to join about ten thousand pairs of sunglasses at the bottom.

Using my Duotone ratchet extension is getting a bit habit-forming. I can certainly see the appeal of alternatives to sitting on wet grass and pulling on the downhaul.
I agree with you on the ratchet extension; I've got the older North Sails version and it's great other than being particular about the kind of rope used. It's super convenient that you can easily adjust downhaul tension out on the water.
Like you, I assume this thing has a pretty fine thread jacking screw inside. That should be VERY easy to seal reasonably well with a carefully placed o-ring or piston-style seal to keep the water/sand/zebra mussels out and lubrication grease in.
However, adjustment out on the water means separating the sail from the board to get access to the screw point. It seems that a right angle drive with a worm gear would be a better choice for two reasons - easier adjustment (from the side, can be done while attached to board) and lower risk of the screw "unwinding" itself and releasing tension over time while in use.
Also, if you've got a high tension sail that only requires a bit of extension, you'll still be pulling on that downhaul rope pretty hard before you get to the crank mechanism, kinda defeating the main benefit here.