Every boat owner keeps a stock of spares, be he a novice or an experienced sailor, all require a certain amount of spares and boating parts to keep the boat afloat. Some of the major differences are how close and attainable acquiring new spares are, you may have a chandlery just around the corner or you may be cruising grounds where no chandley can be found for hundreds of miles. Another consideration is what systems one has onboard, as an example, performance sailboats would keep a large amount of spare sails and deck gear, motorboats would have a good stock of engine parts and some boats are more dependent on electronics nowadays that if a single component fails may result in a malfunction of several systems. Depending on the nature of your cruising and area of navigation should determine what spares should be carried onboard.
I shall cover the basics but add some small, and sometimes regarded as insignificant, parts that might prove helpful.
Engine and boat parts -
For the engine and generators the basics are service kits, quantity depending on area of navigation and length of voyage and this would include engine oil and, depending on the model, an additional spare impeller pump seal kit. For some, cruising long distances and away from shops, more is required, such as: spare alternator and starter, spare impeller pump in addition to a rebuild kit and a fuel feed pump, not to forget the associated gaskets and seals. Sometimes, a small but crucial part can lead to a faulty system such as the secondary fuel filter bleed screw gasket, if lost or damaged the engine may not run smoothly or may not run at all, in any case, this is a small and cheap spare to keep that might prove vital.
Electrical components and boat navigation lights -
Some boats nowadays are dependent upon electrical and electronic components more than in the past, it is not an easy task to decide which spares are most important and nothing is better than experience. Nevertheless, the fundamentals are fuses and relays, light and compact to hold. The ANL or MEGA fuses at the batteries are easily attainable and then the automotive blade fuses can be bought as a set and kept for the moment needed, some manufacturers prefer the use of resettable fuses which eliminate the need for spares. Other components many systems rely on are relays or solenoids, some are rated for small currents, such as the 15 seconds timer relay for the shower pump and others are larger, such as the windlass contactor, a large and expensive spare to carry on board but they do fail. Also, some engines may have several relays, such as the glow plugs, fuel feed pump (if electrical), starter and engine stop switch.
In addition to spare navigation LED and light bulbs, carrying emergency boat navigation lights could prove useful one day.
For boats sailing in warmer climates with air conditioning on board, should consider having spares such as a cooling pump , but also capacitors, which are very cheap and are vital for the system, are good to have.
Water systems -
Pumps in general do fail and keeping spares such as a fresh water pump , or a rebuild kit , on board so you never get stuck without water pressure might prove to be a lifesaver one day, especially if you have guests on board. Some pumps, such as the electric bilge pumps are mandatory to keep as spares, but a manual hand operated bilge pump membrane service kit would suffice and not necessarily a whole new unit.
Sailing Deck Gear -
It is not an easy task to list what deck equipment spares are necessary for a sailboat, the variations of type of boat and type of sailor make it impossible to account, each to his own as they say. But allow me to list a few basic parts, such as the winch service kit but more specifically pawls and springs, some spare blocks; mostly for heavy load and use such as the main sheet and traveler system.
Having shackles and snaps are the usual spares found, what is important for a sailboat that normally wouldn't be found are the clevis pins and associated split or ring pins, these are small and inexpensive parts that give a lot of peace of mind when needed to be replaced.
It takes time to know what you need for your boat and the type of use you make of her and It is easy to overdo spares, with all the many places to fit parts onboard you may find, several years down the line, parts that are corroding away and some might not even be recognizable. So an important aspect of running your spares is good stock keeping; keeping a log, and once in a while removing the spares for a good clean and inspection, best is to keep as many spares together as possible instead of having boat parts scattered all over.
A well-kept stock of spares will increase sailing time and less running-for-spares time.