Measuring Boat Engines: Power And Performance

Boat engines are measured in hours, as opposed to miles, to account for the different kinds of strain and environments they operate under compared to automobile engines. The number of hours a boat engine has been running is a key metric for maintenance and resale value. This is because it indicates how much use the engine has had, including idling time and slower speeds, which can impact the engine's performance and longevity. Most boats are fitted with a running hours meter, but older boats may require a computer to retrieve this information.

Horsepower

To market his new product to an audience that was still using horses for mechanical work, Watt had to find a way to compare the work of horses to the work of his steam engine. Through his experiments, he determined that a horse could do about 33,000 foot-pounds of work in one minute, which he defined as one horsepower. There were some flaws with this measurement, including the assumption that a horse could continue working at a consistent rate without tiring. However, the comparison showed that Watt's steam engine could do the work of five horses, and it became a key part of the Industrial Revolution.

Today, horsepower is calculated using the formula: Horsepower = Torque x RPM / 5,252. Torque refers to the turning force of an engine, and it is measured using a device called a dynamometer, which applies a load to the engine and measures the amount of power produced.

When it comes to boat engines, horsepower is an important factor that affects performance. The amount of horsepower needed for a boat depends on various factors, including the manufacturer's limits and recommendations, the boat's weight, its intended use, the number of people on board, and fuel efficiency.

It is important to note that overpowering a boat with more horsepower than recommended can be dangerous and may even be illegal in some cases. Federal laws and regulations are in place to ensure appropriate horsepower limits are listed on all boats, and exceeding these limits can result in fines and other consequences.

Engine maintenance

Boat engines are measured in hours of usage rather than miles travelled, as this gives a better indication of the wear and tear on the engine. This is because the hours will tell you how long the boat has been running, including idling time and slower speeds in no-wake zones.

Read the Owner's Manual

The marine engine owner's manual outlines basic maintenance procedures and has a schedule for regular maintenance. If your owner's manual is missing, a dealer can order you a new copy, or you may be able to order a paper or digital copy from the engine manufacturer's website.

Avoid Fuel Problems

When possible, fuel your boat with gasoline that does not contain ethanol. If only ethanol-blend fuel is available, make sure to use fuel with a maximum of 10% ethanol (E10). No marine engine is certified to operate on fuel with more than 10% ethanol. Try to buy fresh gas from a busy fuel dock or gas station. Use a fuel stabilizer additive if you don't plan on using most of the fuel within a week or two. Modern gasoline can begin to oxidize and form deposits in the fuel system within a few weeks, especially in older engines. Install a 10-micron water-separating fuel filter between the fuel tank and the engine to keep water and fine particles of debris out of the engine. Carry a spare filter on the boat.

Avoid Battery Problems

Always replace the engine cranking battery with a marine battery, which has thicker plates and a more robust design than an auto battery to withstand the vibration and pounding of a boat. Secure the marine battery with a good battery tray, which should be screwed or bolted to the boat and held with a rigid bracket or locking strap. Check the battery terminal connections frequently to ensure they are snug and free of corrosion. If you use the boat infrequently, use a maintenance-type battery charger to keep the battery charged between outings.

Annual Servicing

Follow the service schedule outlined in your marine engine owner's manual. Most outboard, sterndrive, and gasoline inboard engines require basic service every 100 hours or annually. This will include an oil and filter change, changing the gearcase lubricant, changing the fuel filter element, and perhaps installing fresh spark plugs.

Winterizing the Engine

If you will be storing the boat in a cold climate, winterize the engine. This procedure will be outlined in the owner's manual. A sterndrive or inboard engine will need to have its cooling system flushed with antifreeze to expel fresh water from the system, which could freeze and damage the engine. Even engines with a closed cooling system may need to be flushed to expel water from the exhaust and heat exchanger.

Check the Propshaft

Outboard and sterndrive owners should remove the propeller(s) several times a year to check the prop shaft for fishing line. Line discarded in the water can become wrapped around the shaft and will eventually damage the prop shaft seal. This can lead to water entering the gearcase and causing catastrophic failure and an expensive repair bill. Check your owner's manual for instructions on removing the prop and inspecting the shaft.

Engine Flushing

Flush out the engine after every outing, whether in saltwater or freshwater. Start up the engine and let the water pump do its job. While flushing the motor, check the water pump to ensure it has good water flow. Carefully put your finger through the stream of water; it should be warm, but not hot. If the output is weak, you may have debris stuck in the outflow tube. Shut down the engine immediately to prevent damage, insert a small piece of wire into the flow tube, and then restart the engine and check the output.

Fuel Line and Tank Maintenance

After flushing the engine, disconnect the fuel line and allow the engine to burn all the fuel in the carburetor. Turn off the key and the battery switch. Take off the engine cowling and check for fuel or water leaks. Wipe everything down and spray with an anti-corrosive like WD-40. Lubricate all moving parts, such as the shift and throttle cables. Replace the cowling and keep it covered between trips. Always use fresh fuel and drain your tanks at the end of the season. Periodically check the fuel line for cracks and worn spots. Make sure the fuel primer bulb is not cracked and is pliable, and that the fuel line fittings don't leak. Check the fuel tanks for damage and corrosion, and the tank vent to ensure it aspirates properly. Check regularly for water in the fuel.

Engine weight

When considering engine weight, it's important to note that diesel engines tend to be heavier than gasoline engines. Diesel engines are often used in larger boats over 45 feet in length, while smaller boats up to 35 feet typically use gasoline engines. The weight of the engine also varies depending on the type of engine, such as outboard, inboard, or stern drive. Outboard engines are mounted on the transom of the boat and are used for both power and steering. Inboard engines, on the other hand, are located inside the boat's hull and power a drive shaft connected to a propeller. Stern drive engines, also known as "inboard-outboards," combine features of both outboard and inboard engines.

The weight of the engine is a crucial factor in determining the overall weight of the boat and its power-to-weight ratio. A heavier engine can impact the boat's speed, fuel efficiency, and handling. Additionally, the weight of the engine can affect the boat's flotation, which is calculated using tables that take into account the engine weight, boat length, and other factors.

When repowering a boat or choosing a new engine, it's important to consider the weight of the engine and its impact on the power-to-weight ratio. For example, a single 300 hp engine may weigh less than two smaller engines with the same combined horsepower, affecting the boat's speed and performance. Engine weight should also be considered in relation to the boat's design and intended use, as it can impact the overall balance and handling of the vessel.

Engine life

The life of a boat engine is typically defined as "time between overhaul" (TBO). While engine life depends on a variety of factors, there are some general guidelines and best practices to ensure longevity.

Factors Affecting Engine Life

Several factors influence how long a boat engine will last, including the type of fuel, hours of operation, and years of usage.

• Fuel Type: Diesel engines generally last longer than gasoline engines. This is because diesel engines operate at slower speeds, resulting in less wear and tear. Diesel fuel also contains oil, providing lubrication in addition to fuel for combustion.
• Hours of Operation: Boat engines are typically measured in hours of usage rather than miles travelled. On average, gasoline engines will run for about 1,500 hours, while diesel engines can go for approximately 5,000 hours before requiring major maintenance or overhaul.
• Years of Usage: The availability of replacement parts and general deterioration due to constant exposure to moisture also play a role in engine life.

Extending Engine Life

To ensure the longevity of your boat engine, it is essential to follow proper maintenance procedures and take proactive measures.

• Regular Maintenance: Adhere to the manufacturer's recommendations for oil changes, coolant replacement, and inspection of the engine cooling system.
• Reduce Overheating: Overheating is a significant threat to engine life. Monitor coolant levels, replace sacrificial anodes, and keep an eye on raw-water strainers and temperature gauges to prevent overheating.
• Saltwater Exposure: Saltwater can cause corrosion in metal components. Regularly inspect your engine for corrosion and, if found, remove it down to the bare metal, prime the area, and paint it with manufacturer-approved paint.
• Preventative Maintenance: Consider replacing or repairing parts before they fail to avoid sudden breakdowns and major damage.
• Throttle Back: Since the amount of fuel burned is a reliable indicator of engine life, throttling back to efficient cruising speeds can extend engine life.
• Proper Lubrication: Ensure that lubricating oil is warm before putting a load on the engine, and allow the engine to idle for a couple of minutes before shutting it down to prevent wear.

By following these guidelines and best practices, you can significantly extend the life of your boat engine, ensuring its reliability and performance for years to come.

Engine type

There are four main types of boat engines: outboard, inboard, sterndrive (or stern drive), and jet drive.

Outboard Engines

Outboard engines are mounted on the transom of the boat, outside of the hull, which is why they are called "outboards". They are used to both power and steer the boat. On smaller boats, this is often done using a hand tiller, while larger outboards use a steering wheel to adjust the direction of the engine. Today's outboard motors range in horsepower from 2.5 HP to over 500 HP per engine.

In the past, outboards were typically conventional two-stroke engines, but manufacturers have since switched to environmentally friendly low-emission engines, including direct fuel injection (DFI) two-stroke and four-stroke designs. Modern low-emission marine outboard engines run quieter and smoother, with virtually smokeless exhaust compared to older models.

Inboard Engines

Inboard engines are located inside the boat's hull. They are four-stroke automotive engines that have been modified for use on the water. These engines power a drive shaft that is connected to a propeller. Inboard engines do not steer the boat; instead, they have a rudder or rudders located behind the propeller and controlled by a steering wheel.

Sterndrive Engines

Sterndrive engines, also known as "inboard-outboards", combine features of both outboard and inboard engines. They use a four-stroke automotive engine mounted on the inside of the hull to power the boat, with a drive unit used to steer the boat. Sterndrives are very common in pleasure boats.

Jet Drive Engines

Jet drive engines use water to propel and steer the boat. Water enters through an intake on the bottom of the boat and is accelerated through the jet drive unit at the transom, thrusting the boat forward. Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. It is important to note that jet drive engines may lose some steering control at low speeds.

Frequently asked questions