It is the right place to ask, but considering it's a highly specific question, not sure if you'll get a specific answer. I don't now have access to a reference where I could look it up. Normally, what I do when I have access to the actual implements/equipment, is do a run through where I measure distances needed. (It will help if you can make marks on the ground. I use paint.) If you can do the run-through and measuring on the actual site, it's better. Then add a small amount (10% to 15%) to the minimum measurements of reasonably proportioned turning areas as people don't always perform actions as precisely as they're supposed to. Then you might sketch up the proposed layout (at, or very close to, scale) and post it here in order to get feedback to see if you're overlooking something that someone here might find obvious. At the same time, it would be good to post a photo of the overall area as it may have indications that need to be accounted for but we can't have any clue about until we see. Maybe there is grade differential to be factored in ...?

Thank you Yardvaark. We are going to do as you suggest and measure the actual distances with the boat connected to the truck (and I hear you on adding the additional 10-15%)! We can't do it at the site because it is still rough, we are in the beginning stages of connecting the design to the property. We were out there yesterday with the builder and grader and they really know what they are doing as far as grading and siting, but asked us to give them the dimensions we're looking for for backing up the boat trailer. When things get farther along, I do plan to post a proposed layout and some photos, I have been very impressed with the advice here.

you'll need a huge space to change the angle of the boat after pulling up and trying to change the direction going backwards. one thing to consider would be to take measurements of the property to see where you need to get to. then take the boat and trailer to a large parking lot that is empty and try to recreate the path to see how much space it really needs. for a jon boat, another thing to consider is how often you'll be moving it. if it is twice a year, or not very often, you have the option of disconnecting the trailer, repositioning, and then attaching again. I use a tractor with a dump trailer, and have to do this all the time in tight spaces I can't turn around in.

Thank you film495. We won't have a dock so we will be moving the boat/trailer in and out of the property often. We can move the boat/trailer around pretty easily by hand when it is disconnected and it's on pavement, but I'm not certain we could on gravel. We're hopeful we can have the graded driveway area loop in a way which would allow backing under the covered parking at a fairly straight angle, that's what we're working on.