I have to back part way in, get the keel aligned in the keel tray with the boat only partially on the trailer.
That is why a well designed keel guide, or at least a hull guide, is a godsend.
A boat can be a bit of a pain to get centered, depending on the slope of the ramp. Especially in windy conditions.
We had uprights at the back of our Catalina 22 trailer, the same as is common on power boat trailers.
Fairly cheap to build or install good ones.
Apparently the previous owner was so cheap that the ones on our boat would flop all over due to the horrible way he/she mounted them. They would have worked much better if they were nice and rigid.
A better bolt on option is a bunk guide. I had planned to make one for our Catalina 22. Here is a Hunter 18.5. on a trailer with them:
FWIW: The trailer for our fixed keel, 30ft, 5 foot draft boat, has welded steel guides that will line up the keel when the boat is floating 3 feet above the trailer. It has a keel stop at the front of the guides so that the boat stops at the exact spot fore and aft.
We can have the boat out in minutes. That $1000 trailer modification was worth every penny.
(After seeing the keel damage on a fixed keel Catalina 25, that was hauled out this fall, onto a badly designed trailer, I am even happier with the expenditure)