Engines and onboard systems: fuel, cooling, electrics and bilge
Most trouble at sea is a small thing that was not checked: a blocked filter, a closed valve, a flat battery, water in the bilge. Learn the main systems of a boat, what each part does, and the checks that keep them working.
Know your own boat
Every boat is a little different. This lesson shows the typical layout of a small cruising boat with an inboard diesel. Find each part on your own boat, read the engine manual, and ask the previous owner or a mechanic to show you.
Check before you need it
Engines and pumps fail at the worst moment, in a narrow harbour or a rising wind. Daily checks of oil, coolant, fuel, belts, filters and the bilge catch most problems early. Carry spares for the parts that wear: filters, belts, an impeller, fuses.
Safety first
Diesel fuel burns, engine parts get hot, batteries can burn or give off gas, and cooking gas explodes. Engine exhaust contains carbon monoxide, which you cannot smell and which can kill. Keep the bilge and engine space ventilated, and fit a carbon monoxide alarm.
Before you start the engine
Tap the checks in the order you would do them. Each right tap locks in; a wrong tap gives you a hint.
From fuel tank to propeller
The sea water cooling circuit
Batteries, charging and fuses
Bilge, fresh water and the gas locker
Study sheet
All the words in one place, by level.
Level 1: Fuel and drive
Fuel tank
Holds the diesel. Know how much you have and how far it takes you, and keep a reserve. A full tank has less air in it, so less water condenses inside. Never fill it with the engine running.
Fuel filter
Catches dirt and water before they reach the engine. Many boats have a water separator with a clear bowl you can look at. Change the filter on the maker's schedule and carry spares, because a blocked filter is the most common reason for a stopped diesel.
Engine
Most cruising boats have an inboard diesel. It turns a shaft that turns the propeller. Check the oil, coolant and belt before every start, and read the manual for your own engine.
Gearbox
Lets you choose ahead, neutral or astern while the engine keeps turning one way. Start the engine in neutral, and pause in neutral when you change between ahead and astern.
Propeller
Turns the engine power into thrust. Keep ropes, nets and weed away from it, and never go near it when the engine could start. A fouled propeller can stop the engine and leave the boat helpless.
Level 2: Cooling and exhaust
Seacock
A valve on a hole through the hull. Opened, it lets sea water in for engine cooling. Closed, it stops the water if a hose breaks. Open it before starting the engine, close the ones you do not use, and move them now and then so they do not seize. Keep tapered wooden plugs nearby in case a hose fails.
Raw water strainer
A filter, often with a clear lid, that catches weed, plastic and sand before they reach the pump. Look at it often and clean it. A blocked strainer starves the engine of cooling water.
Impeller
A rubber wheel with vanes inside the sea water pump. It wears out, and runs dry in seconds if no water arrives. If no water leaves the exhaust, stop the engine and check the strainer and impeller. Carry a spare and a pair of pliers.
Heat exchanger
Works like a car radiator. The engine is cooled by fresh coolant, and the coolant is cooled by sea water passing through the exchanger. That keeps salt water out of the engine itself. Check the coolant level when the engine is cold.
Wet exhaust
After cooling, the sea water is mixed into the exhaust gas and leaves at the stern. Water spitting out is the sign that cooling works. The exhaust gas contains carbon monoxide, so keep it out of the cabin and the cockpit.
Level 3: Electrics
Start battery
A battery kept just for starting the engine. Because the lights and the fridge never touch it, it still has power when everything else is flat. Keep it charged.
House battery
Powers the lights, fridge, instruments and pumps. Do not drain it too low, because that shortens its life. Check how much power you use and how you charge it: engine, solar, wind or shore power.
Alternator
A generator driven by a belt on the engine. It charges the batteries while the engine runs. If the belt is loose or broken, nothing charges, so check it with the oil and coolant.
Battery switch
Connects or cuts off the batteries. It lets you use one bank, the other, or both in an emergency. Do not switch it off while the engine is running, because it can damage the alternator. Switch it off to isolate the boat when you leave it.
Fuse panel
Each circuit has a fuse or breaker. If a wire overheats, the fuse breaks first so the wire does not start a fire. Never replace a fuse with a bigger one, and carry spare fuses of each size.
Level 4: Bilge, water and gas
Bilge
The lowest part inside the hull, where water collects. Look at it every day. More water than usual means a leak, and oil or fuel in it means a problem to find. Keep it clean so the pump does not clog.
Bilge pump
Pumps water from the bilge overboard. Most boats have an electric one and a manual one that works without power. A pump only buys time: find and stop the leak.
Float switch
A float that rises with the water and starts the electric pump by itself. Test it often by lifting the float. If it fails, you only find out when the bilge is full.
Fresh water tank
Holds your drinking and washing water. Fill it from a source you trust and keep the tank clean. Know how much is left, and plan the water for a trip: a person needs a few litres a day.
Gas locker
Cooking gas (propane or butane) is heavier than air and sinks into the bilge, where one spark can explode it. It lives in a sealed locker that drains overboard. Close the valve on the bottle after cooking, and check hoses with soapy water.