How Close to Rated Capacity?
 

I ordered a 2011 FX4 4x4 with a 157" wheelbase (5.0 liter engine and 3:73 gears). The papers say the rated weight of the vehicle is 7350 pounds. I'm considering also getting a boat. The papers on the boat put the dry weight at 5,600 pounds. For all intents and purposes, I'm assuming the actual weight of the boat is no less than 6,000 lbs, which is roughly 82% of the vehicles rated towing capacity. While legally, I should be fine, realistically, what is the maximum towing capacity of a truck with the configuration I ordered? Does a weight-distributing hitch increase this limit or is the engine and transmission (something else) the limiting factor?

I understand that I'm within the legal limits of the vehicle, but I don't like the idea of towing at maximum capacity --especially considering I'll be carrying at least one passenger and whatever other goodies we might bring along with us.

You have your numbers confused. You are looking at the GVWR as if that is what your tow rating is. All that is is the max weight of your vehicle with everything loaded in it including people, cargo, fuel, and tongue weight if towing. You will have other numbers to consider. GAWR, which is the max weight on each axle (they will be different and if added up be higher than your GVWR, so if heavier on the rear, you can't be near the max in front. Then there is the max tow rating, which will probably be around 10k or so. This number needs to really be ignored, since if you tow that much you will be over just about every other rating. The final rating is the GCWR, the combined vehicle rating of the truck and whatever is being towed. That and the GVWR are the main rating to be concerned about.
Also if the weight of the boat is 5600lbs dry, then between boat trailer and everything in it, you are going to be near max. My guess is the fuel for a boat that size is going to be at least 100 gallons, which is by itself another 500lbs. Trailer will be close to a 1000lbs. With that weight, you will also need a weight distribution hitch as your hitch is rated only to 500/5000 lbs(tongue/trailer) without a WD hitch. If the weight you listed is including the trailer, then you will be better off, but still should have the WD. Tongue weight needs to be between 8 and 15% of towed weight to tow a trailer safely.
Put up the other numbers and we can give you better info. Those would be your GVWR (on the door sticker), GCWR (in your owners manual), boat dry weight and whether that includes trailer, boat fuel capacity, etc.

I appreciate the info. The truck is still a few more weeks ago (ordered it April 9th, but it won't be delivered until June 20th), so I have some time to think.

If you want to take an early look at documentation like the owners manual, you can download that from www.motorcraftservice.com in the owners manual section.

Congrats on getting a new F150! I like to go by the 75% rule: If you frequently tow at or above 75% of your truck's rated towing capacity you probably should get a larger truck. A larger truck can cope with the heavier loads better and will last longer and be safer towing than a maxed out light duty truck. Just my 2 cents worth, your mileage may vary. :D

As stated earlier, you need to look at your tow rating, which is 9100 lbs if you have a SCREW and 9300 lbs if you have a SCAB. (see http://www.ford.com/trucks/f150/specifications/towing/)

My '09 5.4 is rated for 9700 lbs and tows my 32' travel trailer well. My total rig on the road is 14,180 lbs, my GCWR is 15,700.

I am not sure why your 5.0 is rated lower. Some say it is to sell the 3.5; perhaps the 5.0 doesn't handle long periods of full (or near full) throttle as well as the 5.4.

In general, the big issue is how close to your GVWR you will be when towing a heavy trailer. Unlike travel trailers, boats typically have only 5% - 7% tongue weight, so you shouldn't have a GVWR issue.

Bottom line - you shouldn't have any problem towing that boat. Personally, I wouldn't be concerned with a 8500lb boat, which leaves some room for the weight of your cargo and passengers.