Onboard Tool Kit and Spare Parts: What to Carry on a Day Boat or a Cruiser

Every boat, even a small one, should carry (BoatUS and Discover Boating lists):

• A good multi-tool, if you carry only one tool
• Screwdrivers: flat and Phillips in a couple of sizes, plus stubby ones for tight spots
• Pliers: regular, needle-nose and locking pliers (needed for cotter pins on the prop nut)
• Adjustable wrench and the wrenches that came with your engine, plus the sizes you use most
• Small socket set in the system your engine uses (metric or standard)
• Nut drivers in common sizes
• Small hammer and a hacksaw
• Telescoping magnet and mirror for parts dropped in the bilge
• Knife (sailors often carry a rigging knife)
• Headlamp, so your hands stay free

Electrical problems are some of the most common on-water failures:

• Multimeter (to test the battery, fuses and live circuits)
• Wire cutter/stripper with a crimping tool and a few marine crimp connectors (heat-shrink type)
• Electrical tape and self-amalgamating (self-fusing) tape
• Spare fuses of every type and size on your boat: blade fuses for the panel and any special fuses your engine uses (check the manual)
• Spare bulbs for navigation lights if they aren't LED
• Jumper cables (Discover Boating notes they let you start from a borrowed battery)

BoatUS warns: never replace a fuse with a higher-rated one. A fuse that keeps blowing means a fault to find at the dock.

For each engine, typical spares include:

• Fuel filter / water separator element (at least one; Discover Boating suggests one or two per engine). Ethanol fuel makes clogged filters common.
• Drive belt for engines that use one (inboards, sterndrives, many diesels, some outboards).
• Spark plugs (a full set) for gas engines.
• Engine oil to top up, plus a funnel and absorbent pads.
• Raw-water pump impeller and gasket for inboards, sterndrives with an accessible pump, generators and diesels. Discover Boating notes an outboard's water pump is in the lower unit, so it isn't a realistic underway repair.
• Primer bulb and fuel line clamp for portable-tank outboards.
• Spare kill-switch lanyard (the engine won't start without one).

A damaged prop is one of the most common ways a single-engine trip ends. Bring:

• A spare prop (Discover Boating suggests a used prop of the right size, or an affordable aluminum backup for a stainless prop)
• Spare prop nut, thrust washer, tab washer or cotter pin and spacer
• A hub kit if your prop uses a replaceable hub
• Prop wrench or socket and a block of wood to stop the prop turning
• A lanyard to tie the wrench to your wrist when working near the water

Counter-rotating engines use different props, so twin-engine boats need the right hand for each.

Mercury's spare-prop advice: on lakes and inshore, the spare only has to be good enough to get you home; far offshore, carry one rated for your engine and close in size to your main prop so the boat can still plane. If your prop has a sacrificial hub system, keep a matching hub kit aboard too, since after a hard hit the prop itself may be fine. A floating prop wrench is handy if you ever have to change it in deep water. In calm, safe water the easiest swap is to back into shallows near shore and stand; in cold or rough water, call for a tow instead.

• Soft wooden or foam tapered plugs in sizes that fit every through-hull. Many owners tie the right-size plug to each seacock.
• Stainless hose clamps in several sizes (BoatUS notes a large clamp around a spin-on filter even gives you extra grip).
• Duct tape and rescue (self-fusing) tape for temporary hose and leak fixes.
• Underwater-curing epoxy putty and a tube of removable marine sealant.
• A short piece of each common hose size on bigger boats.

• Cable ties in several sizes (a very temporary backup for a failed clamp)
• Assorted stainless nuts, bolts, washers and screws
• Thread locker
• Spare line and a couple of shackles
• Penetrating and water-displacing spray
• Rags, nitrile gloves and absorbent pads
• A marker and a notepad
• Mask and snorkel (Discover Boating suggests these for prop and intake checks; only when it's safe to get in the water)

Boats that run offshore, cruise overnight or have more systems should add:

• A second fuel filter per engine and the tools to bleed a diesel fuel system
• Spare belts, impellers and pump parts for each engine and the generator
• Spare complete bilge pump and float switch, and a spare washdown or livewell pump
• Head repair parts (joker valve, seals) if you have a marine toilet
• Cordless drill with bits (Practical Sailor's pick for longer trips)
• Oil filter and filter wrench if you're far from service
• Spare prop for each engine; a slightly lower-pitch spare can help a twin get home on one engine
• Handheld VHF, a handheld GPS and a PLB as backups to the main electronics

Practical Sailor suggests splitting tools into smaller, focused kits (general, electrical, rigging, plumbing) so you can find things fast. Label each bag or box. Keep the plugs, a flashlight and the multi-tool where you can grab them without digging.

Use a waterproof box (BoatUS) or a canvas roll (Practical Sailor prefers canvas because tools can breathe and dry). Rinse off salt, dry the tools and wipe them with a lightly oiled rag or a corrosion-block spray. Cheap tools rust fastest.

At commissioning, check that spares still match the engine (a new prop or fuel filter type means a new spare), batteries in the headlamp and multimeter work, fuses are all there and sealants haven't cured in the tube. Replace anything you used during the season.

At the dock, practice changing a fuse, a fuel filter, the prop and (on inboards) the impeller. Time yourself. Knowing the steps makes it much calmer when it happens offshore.

Mercury's prop quick tip has two habits worth practicing: lay the washers and spacers out in the order they came off (or snap a phone photo before you start) so they go back the same way, and wipe the shaft and look for fishing line wrapped behind the prop, which can cut into the prop shaft seal. Hand-start the prop nut, then tighten it to the spec in your owner's manual and spin the prop by hand to check it turns freely.

BoatUS's 2025 list of New Year's resolutions makes the same point about why the kit matters: without basic tools and a few spares aboard, an engine problem underway leaves you with almost no options except a tow. It doesn't have to be elaborate: a socket set, screwdrivers, a few pliers, any tool specific to your engine, and spares like plugs, belts, hoses and fuel filters.