If you're MacGyver, all you need to keep your boat running is a Swiss Army Knife, some wire and duct tape, but lacking that resourcefulness, it is best to keep a good set of spare parts aboard the boat. Like so many things in boating, how a boat will be used drives many of the decisions made. The list of spares kept aboard could be very different for a day boater in confined water versus a boat intending to be days at sea in the open ocean. But those differences notwithstanding, there are common items all boaters should have on board. Of course a basic tool kit should be maintained, enabling you to change the defective part with the spare. Like spares, the contents of the tool kit will vary with the boat and type of cruising. One of the most important items to consider in utilizing any spare part is the boat's anchor. It is much safer to perform a repair while securely anchored than while drifting with the wind or current and yes many boaters carry a spare anchor as well. Even if you're not very mechanically inclined, having the right spare part can still save the day. Quite often a mechanic is easy to find, but without the spare part on hand to repair the problem, the mechanic is of no help.
Every in-board marine engine I'm aware of, whether it's the main engine or a generator uses at least one belt. The belts typically drive alternators and water cooling pumps, and they are critical to the operation of the boat. Most belts are also easy to replace.
Whether on an overnight run or just returning to the marina late from a long day on the water, running a boat at night can be an exciting experience. However running a boat at night without proper lighting is extremely dangerous. The corrosive bumpy environment we boat in is tough on light bulbs. Having the ability to replace a burned-out bulb with a spare will get you home safely.
They keep the music playing on the boat's stereo and without one the VHF radio and GPS will not work. Fuses are located in many devices aboard the boat. Locate all of them and make an inventory of each type and size. Be careful when re-applying power to a device with a blown fuse, the fuse failure may be an indication of a larger problem. Fuses are safety devices, to protect the device they are in and for the boat. Never bypass a blown fuse with direct wiring.
From problems created with ethanol gasoline or infrequently used diesel fuel, we ask a lot of our boat's fuel filters. A clogged fuel filter is one of the most common reasons boaters call an on-water tow service. They are inexpensive and usually easy to replace. Depending on the length of your cruise, you may want to consider keeping more than one spare aboard.
Carrying a spare and changing a prop is fairly easy if your boat has an out-board or an in-board/out-board engine. The props are relatively small and easy to access. A larger in-board con figuration could be a different story. However, I've seen some pretty ingenious ways boaters have managed to carry these larger props aboard. With larger props, just having spares specific to your boat could make a big difference. I know of an instance where a boater doing the Great Loop had the spare prop overnighted from their home in Virginia to their location on the Mississippi River and were underway again the next day, without that spare they could have waited a week or more getting the exact size and pitch they needed.
Different sizes and types of boats can use a wide variety of fluids, from engine oil and coolant, to power steering fluid, or gear and hydraulic oil. Each could be critical to the boat's operation. Loss of fluids can come from an engine overheating to a leaky fitting or hose failure. Repairing the problem or leak is only half of the solution, you'll also need to replace the lost fluid in order to get underway again. Be sure to keep plenty of oil absorbent pads aboard. Keep all the fluids clean and in their original container and make sure to keep lost fluid from being pumped overboard.
Even with the proper spares, repairing many items aboard can still require some creativity and ingenuity. Having a variety of items like silicone sealant tape, an epoxy mesh kit and wire ties can help you play MacGyver if you needed.
These items are only the beginning of what could be hundreds of spares kept aboard. Consult with the manufacturers of your boat's engines and more complex components, many of them have spare kits already assembled or at least a suggested list of commonly used spare parts.
Remember to keep an organized inventory of what spares you have on board and where they are located. Digging under life jackets, fishing poles and beach towels is no fun when looking for that needed spare.
Spreadsheet apps on smart phones and tablets make it easy to keep updated information on spare parts, they also enable the information to be shared across devices and backed up. Having a spare but not being able to find it is just as bad as not having it.