hobie 21 downhaul cheekblock

The sheave is not replaceable on most cheek blocks. It is pretty simple to drill out a few rivets and rivet a new one though. I just finished removing, replacing and rivetting a pile of them on my mast. 3/16" drill bit will make short work of it.

My Prindle 18 was missing sheaves on the 2 block riveted to the mast when I got it. I used it that way for 3 seasons. I just bought and installed two harken micro cheek blocks . They did not match the exact same holes so I put one rivet in the unused hole to fill it. I then used the other hole and drilled a new one to fit. I then bought a small downhaul with a quickpin and two micro blocks on sale at Murrays.

I really wasn't to hard drill out old rivets probably 3/16. Just drill of the head then use a punch to knock the rivet into the mast. This helps to not make the hole any larger or you will have to go to a larger rivet. I bought a long handled rivet tool at harbor freight for $20 and some stainless steel 3/16 rivets from West Marine.

If I remember I'll take some pictures. Seal all the spots you rivet with silicone to keep the mast water tight. I probably oversealed mine I am not an artist with silicone so my mast is not pretty.

You don't need to seal the mast at the bottom, you really want the water to run out at that end. The top of the mast is another story...

If your turtle long enough the water is still going to get in at the bottom. If your sealed no water gets in. you might could rig a drain like some have at the bottom but I think the more sealed the better.

I would seal the entire mast. Drill a small hole at the bottom & put a screw with rubber washer in. Remove if you think you need to drain.
The mast sits on top the DS, above the tramp. If you turtle, the tramp will be in the water, meaning the entire mast is under water. If there are any openings water will quickly run in, & fall to the mast tip. One lousy gallon, 10 lbs, at the end of a 30' lever will make righting much more difficult. If you have much of a delay, getting things sorted out, passengers calmed down etc you just might find the mast is full of water. Righting will now be the biggest beeyitch you have encountered.
For a demonstration, toss your unsealed mast in the lake, (only try this in shallow water) let it fill, then try to carry it back out.

Ah, sometimes it pays to sail in an 8 ft deep lake.

Ya, the fact that the bay my cottage in on is only 6-15' saved me once. I went over in close to 30 mph wind, & the drift drove the mast under til it hit the mud. I got the boat up, then felt smag dripping on me. Looked up to see a huge ball of mud & weeds at the mast head, so I flipped the boat near the dock to wash it all clean.
Once I clear the point the water goes to 80+ feet, so I sealed everything.