Depending on where you use your boat, whether it is up and down the Intracoastal waterway (ICW), coastal cruising, or ocean passaging, an appropriate number of spare parts should always be carried in the event of equipment failure. If you are on the ICW, or even coast-hopping you will probably be able to get spares as needed for nearly anything from the nearest marine supplier or even delivered on the internet. Ocean cruising is very different and I have sometimes thought I needed another boat full of spares towed astern.
Stowing larger items is not so much of a problem on Britannia with its 14-foot beam because there are lots of lockers and drawers to accommodate everything from a spare alternator, a large engine starter, and all the other things a large cruising boat might need as a 
replacement.
I kept a multitude of fasteners, nuts and bolts and other small items in multi-compartment plastic boxes available from most hardware stores and made in all sorts of shapes and sizes, most with adjustable dividers to accommodate nearly any item. I had five of these boxes for small parts and fasteners in both imperial and metric up to 6-inch-long rigging screws, along with all the other things that I never threw away and which tend to accumulate. The heaviest of these weighed 15Llbs and I kept them all in the same place, behind a saloon seat piled one on top of the other and where it seemed that the one I wanted was always on the bottom. The combined weight of all the boxes was a whopping 5Llbs!
A pull-out drawer system like those used in garage workshops would be much better, but they are mainly designed for tools, wrenches, screwdrivers, etc., and much bigger than my available space. Eventually, I found an ideal small parts organizer system from Husky.
Each of these units has two 18-inch wide drawers that can be added to by locking units on top of each other to make a stack of any number of drawers. The inside dividers can also be adjusted to form different-sized compartments. I bought two of these from the Home Depot for $45.00 each.
The drawers are on roller-bearing sliders that are easy to pull out even when completely loaded with steel fasteners. Better still the drawers can be locked shut so they won't slide open when the boat rocks. The outside dimensions of each two-drawer unit is 7⅜-inches high x 21¾-inches long x 12¼-inches deep and there are 16 interior compartments in each drawer, 4¼-inches long x 2¼" Wide x 2" Deep.
I decided to install them behind the seats where the original loose boxes had been kept but it meant modifying the opening a little. I then screwed a wooden batten along the inside for the front of the drawers to sit on and supported the back of the units with a block of wood. The front was then fastened with self-tapping screws through the wooden front panel into the plastic frame of the drawers. I didn't lose any real storage space by fitting them in the same space either because I never stored anything on top of the boxes, except the other boxes and the parts are now much easier to access.
I removed the locking levers on one side of the drawers to make it easier to fit the units and the other side still locks them securely. I modified the original plywood panel so it would fit over the front of the drawers and when the seat back is clipped in place the drawers cannot be seen.
After the screw holes had been filled and all exposed wood painted white the installation was finished. There was now only one other thing to do – transfer the hundreds of small parts into the new drawers.
All this took as long as installing the drawers and the bottom drawer carries all 15Llbs. of the heavier fasteners but still opens effortlessly on its roller gliders.
The finished project is a vast improvement on all the heavy boxes I had to manhandle. Now, if I need anything in the drawers I just remove the seat back pull the drawer out and pick whatever I need then lock them until next time. Anyone sitting on the seat would never know that hundreds of fasteners and boating bits are stored directly behind where they are sitting.
Stowing spare parts all over the boat is one thing, but remembering where they all are is another. Many times it had taken me longer to find something than to fit it. So I devised a method of quickly locating anything and everything, that has saved me lots of time and frustration. If you have similar problems finding things, go to "Get organized," it will be worth it.