These boats all have traveller alternatives. Note that they all have blocks or a bridle
line off center. This allows you to pull the boom to the centerline, which for most boats is as far as you want to go.
The first just has a windward block that allows the boom to be centered.
The middle boat picture is set up like a high performance dinghy
like a 505. It's called vang sheeting. The bridle
allows you to pull the boom to the centerline without much down force and the vang is used to control leech twist. For this to work
well you need a powerful vang. The 505's have something like a 24:1 purchase
. It doesn't look like the boat in the picture has enough purchase
The third has an old system. I've seen this traveller on Cal
20s, old Lightnings, and a Morgan
OI 41. It's not a great traveller.
As you sheet in, the boom will stop approximately over the leeward block. If you want the boom sheeted in to nearly the centerline of the boat, you grab the boom with your hand and pull it to the centerline and sheet in some more. The perimeter of the triangle formed by the sheets
with the boom over the leeward block is bigger than the perimeter of the triangle with the boom between the blocks in the centerline, so once you have sheeted in more with the boom over the centerline, it can't go back out to leeward. (Very much anyway.)
On the Morgan
I never tried pushing the boom to the centerline. The jib
sheets so far outboard
you don't want the boom farther inboard.
All the designs require a powerful vang for reaches.