Guide to Choosing the Right Marine Diesel Engine for Your Boat

Marine diesel engines are crucial to powering both commercial and personal boats while out on the water. When you’re outfitting your boat with a marine engine, it’s crucial you select the right engine type for your vessel and the right horsepower. By choosing the right diesel engine for your boat, you can get the best performance out of your vessel, helping it reach top speeds and handle better.

To select the best marine diesel engine, learn more about the kinds of boat engines and how much horsepower your boat will require. You may also want to know more about picking the right engine size and when you should replace or rebuild your engine.


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Types of Boat Engines and Propulsion Systems

A few different boat engines and propulsion systems are available for customers to choose from. As you select an engine, you should be aware of the main types of boat engines and propulsion systems you can choose. These are the primary types of boat engines:

  • Outboard engine: An outboard engine is placed on the boat’s transom and rests outside the boat’s hull. These engines are used to steer and power the boat. One primary aspect of them is that you have to move the engine to steer the vessel. A smaller boat will typically employ a hand tiller to move the engine, while a larger boat will rely on a steering wheel to turn the engine.
  • Inboard engine: Inboard engines are installed inside the boat’s hull. These engines are typically four-stroke automotive engines modified for watercraft that feature a propeller connected to a drive shaft. In contrast to outboards, an inboard engine isn’t used to steer the boat. An inboard engine is typically paired with rudders behind the propeller, with the rudders adjusted with a steering wheel.
  • Sterndrive engine: A stern drive engine is sometimes referred to as an “inboard-outboard” engine as it comes with features from both of these engines. Like inboards, they are four-stroke automotive engines installed on the hull’s interior. They’re also similar to outboard motors, as they come with a drive unit that operators use for steering. Turning the wheel turns the drive unit turns as well and dictates the boat’s direction.
  • Jet drive engine: A jet drive engine uses the water surrounding the boat to steer and propel it. At the bottom of the boat, an intake draws in the water, and then a jet drive unit accelerates the water to push the boat forward. This engine assists with steering by changing the water stream’s direction when it leaves the boat.

Alongside the different kinds of boat engines, several kinds of propulsion systems can generate the needed power to propel a boat forward. One of the most popular types of propulsion systems is a diesel propulsion system. This kind of system uses diesel fuel as a power source, with two-stroke and four-stroke engines the most common.

You can also find diesel-electric propulsion systems in some commercial ships, with the system using electricity and diesel fuel together. Tri fuel propulsion systems are another top option, as they can burn heavy fuel, diesel and gas fuel. This kind of engine can also reduce a ship’s emissions.

How Much Horsepower Do You Need for Your Boat?

After selecting the right type of engine for your boat, you’ll likely wonder how much horsepower you need. If you have too little horsepower, you won’t go as fast as you’d like. However, engines with too much horsepower can damage your boat by placing extra pressure on it that it wasn’t designed to handle. Typically, you’ll want your engine to have one horsepower for every 25 to 40 pounds of the boat’s weight.

Below are some of the considerations for selecting a marine engine with the right amount of horsepower:

  • Check manufacturer requirements: One of the first places to look when you’re searching for the right horsepower is to check your owner’s manual. This manual will often include information about the maximum amount of horsepower the boat can handle, as well as explaining how much weight you can place on your boat safely. This manual may also have recommendations about the range of horsepower you should look for when purchasing an engine.
  • Calculate horsepower-to-weight ratio: When deciding on an engine, you’ll also want to calculate the horsepower-to-weight ratio. To calculate the ratio, you’ll take the boat’s weight and divide it by the engine’s horsepower, with the result representing the boat’s pounds per horsepower. You can compare multiple engines with these calculations, as the results can tell you which engine will be the most powerful. The lower the boat’s pounds per horsepower, the faster the boat will go.
  • Account for how many people you plan to usually carry: Since your boat’s weight plays a direct role in how much speed you can get out of your engine, you may want to factor in how many people you usually take out on your boat. If you’re planning on constantly having lots of people on your boat, you may want to choose a more powerful engine to account for the increase in weight.
  • Think about the boat’s use: Alongside considering how many people you’re going to carry, you may also want to consider the boat’s use. If you just want to cruise around with your family and friends, you’ll only need to think about the passengers’ weight. If you plan to tow materials or other watercraft behind your boat, you may want to select a more powerful engine.

What Size Boat Engine Do You Need?

Generally, when you’re trying to decide what size engine is right for your boat, it’s a better choice to select a large engine than a smaller one. Larger motors tend to have higher horsepower and will provide you with faster rides. At midrange speeds, a bigger motor will provide your boat with improved handling. This increased handling translates to better maneuvering and greater control while you pull heavy materials behind your boat.

Another benefit of using a larger motor is that it provides greater fuel efficiency.  While you might think that a smaller engine will use less fuel, it’s not the case. Since a smaller engine will have more difficulty powering a heavy boat, it will use more fuel, raising your fuel costs as a result. A bigger motor won’t use fuel as fast since it won’t struggle to power a heavy boat, saving you on fuel costs over time.

Of course, you don’t want to go too large, as a too powerful engine can damage your boat and putting you at risk of breaking the law. The boat manufacturer should provide you with information about the maximum horsepower the boat can handle. Using this information, you can ensure you don’t select an engine that’s too powerful or large for your boat.

You can also determine the right size for your engine using the 25 to 40 pounds per horsepower rule. If you stick to 25 pounds per horsepower, you can calculate the most horsepower that can be used safely. When you’re trying to have the most speed and best handling, you’ll want to find an engine providing you with the maximum horsepower your boat can safely handle.

Rebuild vs. Replace Marine Engines

When an issue arises with an engine, you can often repair it to get everything working right again. Some common repairs include fixing faulty valve-cover gaskets and replacing ancillary systems or corroded manifolds. However, sometimes you can’t make a simple repair to fix the engine. When something is going wrong inside the engine and the repair will be too complex to handle while the engine is on the boat, you’ll need to decide whether you want to rebuild or replace it.

Rebuilding an engine will involve removing the engine from the boat and transporting it to a boatyard machine shop where a professional can take it apart. Then they can repair the issue and clean all parts. After repairing and cleaning relevant engine parts, the mechanic will put the engine back together and reinstall it in the boat. In contrast, replacement involves simply swapping out an old engine for a new one.

Here are some of the main factors you’ll want to consider to know when to replace or rebuild your marine engine:

  • Cost-savings: One of the big factors in deciding between replacing or rebuilding your boat comes down to cost. Typically, rebuilding an engine will cost less than replacing it entirely, especially if you decide to purchase a newer engine. There can be some extra costs to rebuilding, however, so you should always check with a professional and get a quote before you decide to rebuild.
  • Time: Depending on the complexity of a rebuild and your mechanic’s capabilities, you may be looking at a long rebuild process. This long process can end up grounding your boat for a significant time before you can take it on the water again. If you need to get back on the water fast and already have a replacement engine in mind, it may be a better choice to replace the engine rather than wait for a specialist to rebuild your old one.
  • Upgrading: If you’ve wanted greater horsepower for your engine or realized you need a different type, you may want to replace your engine than rebuild it. While rebuilding an engine is largely about repairing or replacing broken parts, a replacement engine can provide you with more power and improve your boat’s performance. If you’ve already been thinking about getting a new engine, it might be time to replace your old engine rather than spend money on rebuilding it.

Benefits of Choosing a Cat® Marine Diesel Engine

Selecting a Cat® Marine Diesel engine for your boat comes with many advantages, especially when purchasing one from a reputable dealer like Lousiana Cat. When you invest in a Cat marine engine, you can expect to receive a durable engine with advanced technology, ensuring you get the most life out of your engine and the best performance. Additionally, Louisiana Cat provides all of our clients with several engine options and ongoing product support.

Learn more about the following benefits of Cat marine engines:

1. Exceptional Durability and Reliability

Many clients love the reliability and durability of Cat marine diesel engines. Since we know that engines and propulsion systems are only as strong as their weakest link, we only use the best materials in our products, building them with high-quality propellers, control systems and other components. Paying attention to the quality of every part we use ensures our engines last a long time. This engine longevity helps save you money in repair costs and keeps your boat on the water for longer before you need a repair.

2. Ongoing Product Support

When you purchase a Cat marine engine from Louisiana Cat, you’ll get access to ongoing product support throughout the engine’s life. This support includes service, replacement parts and rebuilds. Our 24-hour emergency service ensures you’ll have a professional ready to assist you at any time if a part breaks down and you need a repair. Additionally, our commissioning and field technicians are highly trained, with a background of offshore experience to assist with any support you might need.

3. Advanced Technology

Our engines include advanced technology to ensure they run at their best and you have more data on hand. For example, Lousiana Cat’s engines come with Product Health Connect®, a display and monitoring system that showcases crucial data about your engine. Another great feature is a touch screen management portal called Product Health Command®. This portal gives you control over your engine’s maintenance and on-board performance, allowing you to view performance and maintenance data online.

4. Variety of Engine Options

Companies and boat operators can select from several Cat engines available. At Louisiana Cat, you can find a diverse selection of engines, such as commercial propulsion engineshigh-performance propulsion engines and auxiliary engines. You can also find marine generator sets and high-performance propulsion and maneuvering solutions. With all of these options, you’re sure to find an engine fitting your boat or fleet’s requirements.

Contact Lousiana Cat for Marine Diesel Engines

As a result of all of the benefits of choosing a Cat marine diesel engine, you may be interested in finding a dealer that can supply you with the right products and services for your requirements. At Lousiana Cat, we’re ready to provide you with the best marine diesel engines available today and exceptional ongoing product support after purchase. We carry several marine power systems to make it easy for you to find the ideal engine for your boat or fleet.

Find a dealer location near you to purchase a marine diesel engine for your boat. If you have any questions not answered in our guide to marine diesel engines, contact us online or give us a call at (866) 843-7440 for more information.