Don't ! The alternatives are not better. The internal downhaul should be a 12:1 double cascading setup. We tried several different setups and this was the best by a significant amount. You are not going to win anything by replacing it.

You may want to check whether it is indeed a 12:1 (measure line travell) and whether it is double cascading (measure the height of the internal bolt/plate that holds the blocks, you can do this on the outside of the mast). Going to 16:1 will not lower the feel of line pull but will add alot of line to your tramp. Look at it this way 400/12 = 33 kg pull, 400/16 = 25 kg pull. Do you think those 8 kg are going to matter much on a total of about 30 kg ?

I do agree with you that playing the downhaul isn't comfortable on the F16's with the alu masts. The response is both to slow and too small. When singlehanding you are also likely to not have sufficient leech tension to make pulling downhaul on further practical. Basically, you are trying to bend the mast more by pulling very hard on basically a 3 inch leverage. This is never going to work. The F16's with an alu mast require alot of downhaul tension even in moderate conditions as the downhaul is taking out the initial mast bend and preventing the top from springing back and making the top very full when the mainsheet is released.

When the mainsheet is tight (and I mean tight !) you'll notice the difference in pull on the downhaul line, then there is almost nothing there. If you release the mainsheet the pull is back again. The very stiff pentex mainsails make this behaviour even more pronounced. Most 1-up crews tend to slacken their mainsheet before trying to pull the downhaul tighter when you actually want to do the opposite.

Experiences so far is to set the downhaul at a good average for the upwind legs and leave it there while you play the mainsheet.

With respect to water in the mast and turtling. Most water in the F16's masts get inside through the T-terminal endings of the diamond wires. Just rivetting end plates on the mast is a much better and cheaper construction but ... If you have these then seal them with kit and take tape them over. If there are no leaks in the top 3 mtr of the mast then your boat will never turtle. There is simply to much volume in the top to press the mast sufficiently deep under water to go turtle. As such the water surface will never got close to the openings of your downhaul system. Several of us also have sealing blocks just above the downhaul setup to make us feel better, but I don't think I would ever install one again.

Wouter