When land-based people want a new mattress they just go to a shop and buy one. For cruisers like us, that’s just the first step. Then comes the explanation where the thing should be delivered to (what, you have no address, what do you mean, just come to the parking space of the yacht club?), to be repeated several times a few days later on the telephone with the astonished (lost) delivery guy (of course all in broken French).
As our bed is creatively shaped, a bit like a butterfly lying in the aft cabin with his head facing forward, we need to buy two queen sized mattresses and then cut them into shape (not to be recommended on deck on a windy day like we did). As the new mattresses are a bit higher than the old ones, we also had to alter the covers, which turned into a 3 day sewing machine project (the fact that I have my left arm in a splint to immobilise my cyst-ridden wrist for 4 weeks doesn’t help).
Step one: undo the seams of the upper part of the old cover and all seams on the new cover.
Step two: Cut the new piece according to the old part leaving a margin to make it a bit higher, sew the new upper piece onto the old cover.
Step three: Curse the crooked thing thoroughly, undo all seams and begin all over again.
Step four: Measure the new cover directly on the mattress, adjust the thing several times with pins, sew all corners (none of them at right angles) and curves by hand and then transfer it again to the sewing machine for another seam.
Step five: Celebrate, have a wonderful night on one half of the new bed and prepare mentally to repeat the whole process (minus step 2 and 3) with the second half 
Good night and good luck!
