Yes that is how to use the halyard as a vang. I’ve not seen anyone else run it around the gooseneck twice , but it should be fine!
And a cleat for the vang is illegal. You can put one halyard cleat on the mast. Also not sure what you mean about halyard swapping.
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My understanding is you untie top halyard so you can lower top spar to depower rig. If you’re using halyard for vang would you have to untie whole thing or just use bottom halyard for vang?
You can either use the bottom halyard for the vang, or use a mast halyard cleat for the top halyard. If you do the latter, you can still use the top halyard as the vang if you want.
But I’m with Wavedancer. The two halyard system usually comes into play only when it’s blowing above 15, so it’s a lot for a new Sunfisher to deal with for conditions you don’t see that often.
How did the first races go Sunday?
I would sail with a regular set up first. You need to get the feel of the boat and what makes it fast. Do you have a hiking strap?
Some top sailors are not adjusting the outhaul even downwind as they want max sail size. Cunningham on in heavier air and chop by feel. The main thing is keeping the boat flat so a lot of sheeting and smooth tiller work.
If you are on the lighter side sail with a small jens all the time.
I race with a very fast group and some have regular setups, some have two halyards and some jens all the time and they are all fast. They get great starts, hit every shift correctly, boats are flat and boat handling is perfect. On a Sunday they get at least 12 sunfish out and get five races in.
Even in 15 mph if you adjust from full rig to jens, you still will be hiking, steering and sheeting a lot.