Your best spare......
......is yourself. At one time or another repairs will be needed for everything aboard. Eventually, everything that can fail, will. At the worst possible time. If you are 8 miles off a lee shore and 50 from the nearest port, a call to vessal assist won't get it. Redundancy, that is, having 2 of everything is the best solution, but is a bit ridiculous. Carry tools-lots of 'em. You can't have too much line or ground tackle aboard. Carry spares for: filters, belts, impeller, pumps(including head), bulbs, fuses, everything that you might wear out, clog or blow. A good supply of electrical connectors, screws, pins, shackles, needles, thread, sail repair tape and various raw materials including a good sized 2x4, other wood/plastic pieces and some WIRE. Electrical as well as rigging. Cable clamps. Oil. Antifreeze. Hose/hose clamps. All of this takes up less room than you might imagine if you get a few of those "organizer" carry cases at Home Depot. Build your inventory over time. Sort and consolidate once in a while so that you can find what you need when you need it. Having all of this aboard gives a good chance of fabricating some sort of fix to get you home. Making you the hero! Hey, did you check to see if your manual bilge pump actually works lately? How about your emergency tiller? Plugs at every thru-hull? It truly is a consuming activity, this sailing safely, isn't it? Happy Sailing! Ron Hughes S/V "Best Revenge" H 40.5