: ...I have a problem with the weights swinging.

Instead of using a paint can for weight, use sections of pvc pipe. You can fill them with concrete and stuff in a loop of wire or an eyebolt to serve as an attachment point. Use 2 inch or 3 inch pipe, and cut the sections as long as you need for the weight you wish to work with. A cap on the bottom end will keep the concrete from running out before it sets, but you can remove the cap after the concrete is hard, if you don't glue it on.

Cut a 10 foot section of 4 inchdrain pipe (pvc or abs) in half and you'll have two 5-foot pieces which your weights can run in. A couple pieces of wood will stabilize those tubes, but you can try just leaning them against the wall. Caps at the bottom ends are optional, but I think I'd put on caps, and drop a big sponge or a chunk of foam rubber into the bottom of those guide tubes so the weights don't go crashing through the bottoms.

The pvc-over-concrete weights should slide nicely inside the larger tubes. A spray of silicone or wd-40 should lubricate them to avoid any squeeks.

For adjustable weights, cut your lengths of pvc pipe and get two caps for each pipe. Drill holes in the center of each cap. Put an eyebolt in the bottom cap, set so the eye will be inside the tube. Attach a wire cable to the screw eye, and then glue that cap onto the pipe. Thread the other end of your wire cable through the other cap. Allow about 10 inches of cable to stick out of the tube, and form the end of the cable into a loop. The cap should be able to slide freely along the short length of cable, but not come off of the cable because of the loop. Pour as much playground sand into the tube as you want, then put the cap over it to keep the sand from leaking out, and hook the loop in the wire cable to your pulley system. Put more rubber foam at the bottom of those tube guides so the threaded end of the weights doesn't act like a pile driver and dent your floor.

You may want to add two pulleys to your system and mount them on the floor, or, attach them to the bottom of those 4 inch guide tubes. Right now your paddling effort is pulling from above. You really want those ropes to come from below, to simulate resistance at a water line UNDER you, don't you?

A longer loom would place the pulling action in a better place, too.

If you want to use water resistance rather than weight, or in addition to weight, put about a 3 foot column of water in each of those 5 foot guide tubes. A disk of wood cut to about a 3 to 3 1/2 inch diameter and fasteneted to the weights will pull against the water in the tube on each stroke. the weight will pull it back under water on each recovery, but as it moves through the water the recovery will be a bit slower. It won't be sudden and accelerating, like a dropping rock. Cap the guide tube so you don't splash out a lot of water as you paddle. Or leave them open if you like the sound of the gurgling water.

Just some thoughts.

PGJ