Inboard Engine Sizing: Choosing The Right Power For Your Boat

Inboard engines are installed inside the hull of a boat, either in the bilge or in an engine room for larger vessels. They are four-stroke automotive engines that power a drive shaft connected to a propeller. The faster the propeller spins, the more the boat will accelerate. The size of the inboard engine you need for your boat depends on various factors such as hull shape, prop and weight. A common rule of thumb is 1 hp per foot of waterline length or 1 hp per 500 lb of total weight.

Inboard vs Outboard Engines

When choosing an engine for your boat, you have to consider the type of boat you have, your budget, and how you plan to use the boat.

Inboard motors are mounted inside the hull of the craft and are usually connected to a driveshaft. They are more commonly seen on larger vessels such as yachts, where large motors cannot be placed on the rear of the hull. Inboard motors are quieter and provide broader power ranges, but they are more expensive and less efficient. They also require a separate rudder to steer the boat.

Outboard motors are mounted outside the hull of the boat, usually on the stern. They are more commonly seen on fishing, recreational, and light commercial inshore boats due to their simplicity and ease of use. Outboard motors are easier to operate and navigate, making them a better option for beginners. They are also easier to service and maintain since they are outside of the boat. They are also more lightweight and can be tilted or lifted entirely out of the water, making them easier to store and transport.

Inboard/outboard (or sterndrive) engines are a hybrid of the two, with the engine housed inboard and the drive system mostly outside, exiting the back bottom of the hull.

Hull Identification Numbers

Since 1972, all boats manufactured in or imported to the United States have been permanently marked with a Hull Identification Number (HIN). Like a Vehicle Identification Number (VIN) given to automobiles, the HIN is used to identify each boat.

HINs are located at the stern of the boat, usually on the starboard side of the transom, or as close to that area as possible. It's used to register your boat and to track your boat's history.

The twelve-character HIN is made up of three letters, which indicate:

• A five-character serial number
• Month and year of production

It's important that you never deface, remove, or otherwise alter your boat's HIN.

Here's what the letters and numbers mean:

• ABC: This is the US Coast Guard-assigned manufacturer identification code (MIC).
• 12345: This is the serial number assigned to the hull by the manufacturer. This may be a combination of letters and numbers. The letters "I", "O", and "Q" are excluded because they could be mistaken for numbers.
• D: This is the month of certification, indicating the month in which construction began. "A" represents January and "L" represents December. In our example, "D" means April.
• 4: This is the year of certification. The number is the last digit of the year in which the boat was built. "4" in this case designates 2004.
• 04: This indicates the boat's model year.

When buying a used boat, take a rubbing of the boat's HIN using a pencil and paper to ensure it matches the seller's documents.

Engine Power Loss

Fuel Filter or Fuel Quality Issues

If you've ruled out the possibility of running out of fuel, a common cause of power loss is a dirty or faulty fuel filter. This can be easily fixed by replacing the in-line fuel filter with a spare that you should always carry on board. It's also a good idea to clear the filter element of any debris and drain any accumulated water. If the problem persists, it could be due to bad fuel or water in the gas, especially if the tank was left near empty for long periods. Consider adding a larger aftermarket fuel filter and always carry spare filters and elements.

Spark Plug Issues

Check your spark plugs, especially if you have an older engine. Carry spares, along with the necessary tools to change them, as this could be the reason for power loss.

Engine Overheating

If the temperature gauge on your engine is rising, it could indicate a lack of water flow in the cooling loop. Most small inboards use the water they are floating on to cool the engine, so an obstruction in the raw water intake, such as weeds, mud, or plastic, could be the culprit. Locate and clean the intake, and check for any loose hose clamps or damaged hoses. Regularly service and replace the impeller, and inspect the exhaust system for any signs of corrosion or blockage.

Propeller Damage

If you experience vibration or loss of speed when trying to increase the boat's speed, it could be due to propeller damage or distortion. A nick or gouge in the blade, fishing line or towrope wrapped around the shaft, or a direct hit on an object could cause this issue. Consider carrying a spare prop and the necessary tools to change it, and practice changing props before you encounter problems on the water.

Electrical Issues

Power loss can also be caused by electrical issues, such as a low or dead battery, a break in the ignition circuit, or corrosion on battery terminals. Always carry a battery charger and the necessary tools to clean and tighten connections.

Other Possible Issues

Other potential causes of engine power loss include transmission problems, such as lack of fluid or gear oil, belt breakage, or issues with the intake or exhaust system. Regular maintenance and carrying spare parts can help prevent or mitigate these issues.

Engine Placement

Firstly, inboard engines are located inside the boat's hull and are typically four-stroke automotive engines modified for marine use. They power a drive shaft connected to a propeller, and unlike outboard engines, they do not steer the boat. Instead, inboard engines rely on a rudder or rudders located behind the propeller and controlled by a steering wheel. This configuration allows for more precise steering and manoeuvrability, making it a common choice for various boat types.

When deciding on the placement of an inboard engine, it is essential to consider the weight distribution and balance of the boat. Placing the engine towards the centre of the boat can improve seakeeping ability and stability, especially in larger "blue water" vessels. This configuration, known as a mid-centred inboard engine, uses a torque tube or jackshaft to connect the engine to a stern drive unit. While this setup is less common, it can provide excellent slow-speed steering and is suitable for boats with planing hulls.

Another factor to consider is the ease of maintenance and accessibility of the engine. Inboard engines can be mounted in different locations within the hull, depending on the design of the boat. Some boats may have the engine placed further forward, while others may have it positioned towards the rear. Ease of access to the engine for maintenance and repairs should be considered, as well as the impact of engine placement on the overall weight distribution and handling of the boat.

Additionally, the size and type of propeller used in conjunction with the inboard engine will also influence the placement. Different propeller designs are available, and the choice will depend on factors such as the engine's RPM, the weight and size of the hull, and the intended use of the boat. Ensuring proper alignment and placement of the propeller relative to the engine is crucial for optimal performance and efficiency.

Finally, it is important to consider the cooling system of the inboard engine. Inboard engines can be either air-cooled or liquid-cooled. Air-cooled engines may require additional airflow within the hull to prevent overheating, which can influence the placement of the engine and the design of the boat's ventilation system. Liquid-cooled engines, on the other hand, may have more flexibility in terms of placement but will require a closed-loop cooling system with heat exchangers to maintain the engine's operating temperature.

Engine Maintenance

To ensure the longevity and performance of your inboard engine, regular maintenance is required. Here are some essential tasks to keep your engine in top condition:

Regular Engine Checks and Oil Changes

Conduct regular visual inspections of the engine, checking for any signs of wear, loose connections, or damage. Pay close attention to belts, hoses, and electrical connections. Consult your boat's manufacturer for specific recommendations, but generally, oil changes should be performed at least once a season or every 50 hours of operation. Remember to replace the oil filter during this process.

Inspecting and Maintaining the Propeller

Regularly inspect the propeller for any visible damage, such as dents, corrosion, or bent blades. Ensure the propeller shaft is correctly aligned and securely attached. Even minor propeller damage can significantly impact your boat's performance. Remove the propeller periodically for a thorough cleaning and inspection to extend its life and reduce the risk of unexpected breakdowns.

Cleaning and Flushing the Cooling System

Clean and flush the cooling system regularly to maintain optimal engine temperature and prevent damage. Inspect the system for blockages, such as debris or algae buildup, and clean the raw water filter to ensure it is functioning correctly. Periodically flush the system with fresh water to remove salt and impurities. This preventive maintenance will extend the life of critical engine components.

Checking and Replacing Fuel Filters

Fuel filters are essential to prevent contaminants like dirt, water, and debris from entering the engine's fuel system. Identify the location of your boat's fuel filters and regularly check and replace them as needed. Keep spare fuel filters on board for quick replacements while on the water.

Battery Maintenance and Care

Inspect the battery regularly for signs of corrosion on the terminals and cables. Clean any corrosion using a mixture of baking soda and water, and ensure the battery connections are tight and secure. Check the battery's water levels if you have a traditional lead-acid battery, and maintain the water level as recommended by the manufacturer. If your boat has a maintenance-free battery, ensure it is charged and replaced when necessary.

Hull Inspection and Cleaning

Visually inspect the hull for any signs of damage, such as dents, cracks, or scratches, especially around the waterline and the boat's bottom. Promptly address any issues found. Clean the hull regularly to prevent the accumulation of marine growth, such as algae and barnacles, which can increase drag and reduce fuel efficiency. Use appropriate cleaning products and techniques recommended for your boat's hull material.

Monitoring and Lubricating Steering Components

Inspect the steering system, including cables, connections, and the steering wheel, for any signs of wear, corrosion, or loose fittings. Lubricate the steering cables and any moving parts with a suitable marine-grade lubricant following the manufacturer's recommendations. Regular lubrication will extend the life of the steering components and ensure precise control of your boat.

Frequently asked questions