I was getting the bottom washed once or twice a year on my 22ft crownline and then I was told about slimy grimy. It is a dry mix that you put in a garden sprayer with warm water, spray the bottom of your boat and in a few minutes, all of the grime and yellowing is gone. It's not like on/off that harms your trailer, I do this over grass and the only thing it does is clean the boat bottom. I know i sound like an ad for it, but its the best boat product I have found that does exactly what it says it will.
A 1lb tub gets me 3 washings and is so easy to do, no scrubbing. So much cheaper than marina washing.
Parafunalia in Gilford carries it and you can get it online but it is hard to find at times.
Here is the link to the video. It really works.
https://www.youtube.com/watch?v=BqK1x6BSB1c