I'm trying to conceptualize a design for a hydro generator that basically operates by moving an airfoil up an down along a straight path. Most of it is all worked out, except for the issue of water sealing. If you can make sense of my not-so-great drawings, the airfoil is attached to the "block", which has a linear bearing that allows it to move up and down along the guide rod. The "block" is also attached to a belt that will drive pulleys (and then gears and eventually to a generator, not drawn.) The issue is allowing the airfoil to be exposed to the water flow while sealing the guide rod, pulley, belt, etc. One idea I had was to use a bellows/accordion type system (shown in the bottom half of 'sketch.pdf'.) The main concern with this idea is that the two bellows might act as springs and restrict the motion of the airfoil too much. Another idea (that I have less faith in) is to use some type of linear seal (I'm picturing something like the weather sealing used around door frames, but something a little more suitable.) This idea is shown in 'linear seal.pdf'. I think for this to have any chance of working, the part that moves through the seal needs to be long and sharp to minimize any water leaking through. I have a feeling that even if this is possible to make work, the friction due to the seal would be very large. This is where I'm at now. I would think that something like this HAS to have been done in the past, but I haven't been able to think of any applications. (Anyone with a lot of familiarity with either industrial underwater processes or satellites know of anything similar?) Let me know if anything here is unclear!