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Many cruisers are fitted with an auxiliary marine engine that, in addition to a motoring capability, charges the boat's electrical system. As well as using them for motoring or motor-sailing, many sailors use boat motors in preference to sails when entering harbour and berthing.
This approach means that some sailors fail to become adept at handling a boat in close quarters under sail and have trouble when the marine engine fails. Sailors that rely on boat motors for handling and for operating an intricate electrical system should understand how the marine engine is installed so that scheduled maintenance can be carried out.
Two types of boat motors are the inboard and outboard motor. Improvements in recent years have made them smaller and lighter for a given power output and more reliable. Originally, inboard marine engines were fuelled by gasoline or paraffin (kerosene), but during the past years there has been a steady, inevitable move to marine diesel engines.
The cost of the fuel is a strong reason for favouring marine diesel engines but the main influences for choosing diesel have to do with safety and reliability. The opposition to diesel engines in the past has been that they were big, heavy, noisy converted truck diesel engines.
This is now a thing of the past as many cars and vans have compact, lightweight diesel engines, and are able to be converted into marine diesel engines giving safe, economical and reliable motors. Companies such as Sabre, Westerbeke and Volvo Penta completely transform a stock diesel engine to turn it into a marine auxiliary.
Diesel engines ignite the fuel by the heat of air compressed in the cylinder. An injector pump and fuel injectors deliver at high pressure measured amounts of diesel to the combustion chamber. This is done at exactly the right moment in the [ compression cycle ].
High compression ratios are instrumental in achieving the temperature needed to ignite the fuel, so a marine diesel engine is more robust and heavier than a gasoline marine engine. Marine diesel engines tend to be more costly to buy than gasoline units, but have lower running costs.
When a marine diesel engine refuses to start, check the marine diesel fuel is clean where it reaches the injectors and the fuel system contains no air. If air is found, bleed the injectors to remove trapped air from the fuel supply and try to source the point where air is being introduced into the system usually through some seal in the fuel system. Check the battery is sufficiently charged and can turn the starter motor at a speed to fire the marine diesel engine.
Well maintained, marine diesel engines are reliable, but it is essential that clean fuel is used. Two filters in the marine diesel fuel line should be installed, one that separates off water
and the other, to filter out fine particles which may clog the fuel jets. Water accumulation in marine diesel fuel tanks, caused by condensation, promotes the growth of diesel bacteria which ruins the fuel pump and injectors.
Keeping the tank topped up reduces condensation and cleaning and changing the filters regularly, controls the situation. The use of antibacterial and water-absorbing additives also reduces the incidence of [ water and bacteria ] in the fuel.
Similar to car engines, gasoline marine engines are widespread on high-speed power boats and require an electrical circuit to provide a spark to ignite the fuel. Gasoline marine engines have lower compression ratios than a diesel engine and therefore lighter.
A drawback to gasoline marine engines has always been the unreliability of spark-plug ignition systems in the damp marine environment. Electronic ignition is superior in this case to the magnetos and [ distributors ] of old, but there still is the unknown effect of dampness and corrosion caused by sea air on these systems.
Gasoline vapour sinks downward being heavier than air. This is not a problem for vehicles, as gasoline evaporates safely but in boats the vapour collects in the lowest part of the hull. It then presents a deadly danger of explosion or asphyxiation.
To make a gasoline marine engine boats safe:
- The fuel system should be designed to avoid the effects of abrasion and vibration which cause fuel leaks to appear
- An extractor fan to extract fumes should be ducted to the base of the marine engine compartment. This fan is run for five minutes prior to the engine being started and while the engine is running.
- The motor is spark proofed.
- An action plan to combat any spilt gasoline while refuelling, should be implemented.
- Install suitable fire extinguishers to fight oil or electrical fires handy to the marine engine bay.
- The fuel tank is fitted with a shut-off valve that is closed when the marine engine is not in use.
- Include checking the gearbox oil level in your pre-start marine engine checks.
- Check the engine oil level every 3 - 4 hours while the engine is running.
- If the marine engine is not used regularly change the oil more frequently than recommended.
- If possible, install an oversized oil filter which helps to prolong the life of diesel engines. Diesel engines do not respond well to low load, short periods of use then followed by long periods of being idle.
- Be aware of any signs of water contaminating the oil on the dipstick. This is a sign that the cooling system has broken a seal and coolant water is mixing with the oil.
Look for a marine engine where all the main maintenance points, such as oil dipstick, sump pump, filters and belts are mounted on the [ front of the engine ] where they are accessible without dismantling the marine engine bay compartment. A small yacht’s inboard marine engine is often located under the companionway steps with the steps being removable allowing access to the engine.
The marine gearbox is only contains head, astern and neutral gears. The gearbox is often designed with a down-angle at the output end, enabling the marine engine to be installed on a horizontal level.
Volvo Penta Marine makes the [ Saildrive ] which combines the
advantages of both inboard and outboard motor installations. A right angle gearbox incorporates an outboard-type 'leg', projecting through the bottom of the boat with a folding propeller. The whole assembly is uncomplicated to install and the folding propeller eliminates drag when sailing.
Most inboard marine engines are cooled by water and the basic system has raw salt water circulating around the engine, cooling it and helping to silence the exhaust system. These marine engine cooling systems eventually corroded the engine's water channels.
The engine cooling system now used is a system whereby fresh water, containing corrosion inhibitors, circulates through the engine, and is cooled by a [ seawater heat exchanger ]. This method can incorporate a primary cooling circuit through another heat exchanger, producing hot water for use within the boat.
This system uses fresh water contained in a header tank and an impeller pump which circulates the water around the marine engine. A seawater heat exchanger inside the header tank uses salt water that is pumped through it, cooling the fresh water and is then expelled via the exhaust.
If a blockage occurs in the marine engine cooling system, the water flow reduces or stops causing the engine to overheat along with damage to the impeller pump. Ensure a good flow of water
goes through the marine engine cooling system by checking the exhaust and regularly cleaning the seacock's weed trap.
The secondary need of a motor is to generate electricity, resulting in marine engines having oversized alternators. Both alternator and starter need to be installed on the marine engine, clear of any bilge water. Boats usually have two sets of batteries; one deemed for general use and the other exclusively for engine starting and both are charged through a splitting diode stopping power trickling from one battery to the other.
As inboard boat motors have become smaller, outboard boat motors have become larger with stock outboard motors of 150 bhp from all the major manufacturers. These multi-cylinder two- stroke
marine engines have a formidable power/weight ratio but they are uneconomical considering the price of fuel. These big outboard motors have electric starting, as manual starting is impractical for boat motors above 25 bhp.
Four-stroke outboard motors are useful where economy is especially important. When it comes to noise level between the two, the four-stroke marine engines is less noisy.
With rigid inflatable boats (RIBS) used as tenders, outboard motors of 10 bhp or less are of interest to yachtsmen, doubling up either as auxiliaries for small yachts or as power units for tenders. Being ideal for the tender because of their light, self contained portability, they have the convenience of being lifted up when in shallow water.
Tiny outboard motors of 1.5 bhp that are easily pick up, are ideal boat motors for inflatable dinghies and usually have, no gearbox. Complete with a gas tank holding just a litre built into the motor, it avoids the expense and nuisance of a separate tank and fuel line.
Disadvantages of outboard motors are that they are relatively expensive to run being gasoline and are susceptible to theft. When buying an outboard motor, ensure that is the right size for
the tender. If it is too large it will be heavy and will consume more fuel and one too small will be unable to power the tender at a reasonable speed.
An outboard boat motor of 6-10 bhp range and fitted with a fine-pitch prop, has the power to drive a big yacht. A problem occurs when mounted on a stern bracket, where the propeller has a tendency be exposed in waves causing over revving.
The Hunter 707 sportsboat has a solution where the outboard motor is stowed in a well under the cockpit floor. When needed, a plug is removed from the hull skin and in a short time the motor is mounted in its place making it both easier and safer than a stern mounting.
Often clamped to a block on the pushpit when not used, outboard motors should be stored away from the elements when not sailing. Two-stroke outboard motors require an oil-and-gasoline mix and it is imperative that the right oil is used and is mixed in the correct ratio. Outboard motors are water-cooled so check that the cooling water flows freely, otherwise it will overheat the boat motor and may damaged it.
When laying up the boat ashore in the winter, take measures to winterize the marine engine from deterioration when unused.
- Run the marine engine for a few minutes heating and thinning the old oil making it easier to pump out.
- Change the oil filters.
- Change the oil
- Clean out the fuel line's water separation filter.
- Top up fuel tanks to reduce condensation forming in the tank.
- Remove the marine diesel engine injectors or spark plugs and inject oil in the bores while turning the engine over by hand.
- Thoroughly clean the outside surfaces of the engine.
- Grease moving parts lightly or spray with a water repellant.
- Drain the cooling system then flush and fill with an antifreeze mixture.
- Remove the impeller from the water pump and place it in a plastic bag.
- Disconnect the battery and grease the terminals.
- Remove and store the battery off the boat charging it occasionally.
- During the winter, occasionally turn the engine over by hand.