Changing sheets is an annoying affair on most boats...our aft cabin is big and beautiful, but having to climb onto the bed and tuck in sheets and secure them, so the don't come loose easily is a pain in the "you know what"! So instead of using fitted or other sheets I have long ago changed to using a thin cotton quilt enveloped in a duvet cover...the quilt is used like a thin mattress pad where you sleep on and not as a cover up...this way it lies flat on the bed and nothing needs to be tucked in anymore...the duvet covered quilt does not move around much when laying on it, and is easily pulled straight in the morning. When it is time to change bed linen I simply take off the duvet cover and wash it...easy peasy! This system has worked extremely well in our V-berth (front cabin) where we usually sleep :) 
Here is a photo of how I had set up the aft cabin for our visitors...notice that the duvet covered quilt was tucked in on all sides of the mattress...looks great, but is really difficult to maintain that way and even more troublesome when having to change the duvet!
Here is a photo of the new set up...although it is hard to notice, the duvet covered quilt is now only half the size...and instead of the sides being tucked under the mattress it is now shortened to fit the dimensions of the bed...no more tucking! I simply doubled the quilt over (yes it was that big)...out of the kingsize duvet cover I made two smaller duvets that fit the doubled over quilt :) So now I even have a spare duvet cover! My sewing skills come in very handy at times...I even altered the kingsize pillow covers that came with the initial duvet set...cut one to make it fit a normal pillow and extended the other to make it fit one of the big nautica body pillows.
When not in use for sleeping we can either roll up the duvet covered quilt and hide it behind the two blue body pillows underneath the portholes (windows) or simply leave it in place and cover with another small piece of quilt...either way looks great and is practical! Hah...I can see us snoozing in both front and aft cabin at random now, hahahaha. Mission accomplished :)
