Boat Engines Explained: Different Types, Outboard Engines vs. Sterndrive and More

Posted Date: 04/25/2019

Boat engines are an important topic whether you are buying, selling or maintaining a boat. Like the type of the boat and the amenities it offers, boat engines are an important consideration when thinking through your various boating use cases and when examining initial and long-term costs. In this article, we will go over the various types of boat engines, the pros and cons of each, consider more deeply the debate between outboard motors and sterndrive motors, and finally, discuss some of the newer technologies hitting the market with regards to boat propulsion.

Different Types of Boat Engines

  • Outboard engines
  • Inboard engines
  • Inboard/Outboard (Sterndrive)
  • Jet propulsion
  • Surface drives
  • Pod drives

Outboard Engines

The growth in recreational boating has had a corresponding growth in popularity and advancements in outboard engines. Fueled partially by a growing popularity in pontoon boats and saltwater fishing vessels, sales of outboard engines have been growing steadily in recent years. According to the National Marine Manufacturers Association (NMMA) Outboard Engine Sales Trends report, outboard engine retail sales increased for the 7th consecutive year in 2018 to a 12-year high.

Image 1560: outboard engines

An outboard motor is a self-contained unit that includes the engine, gearbox and propeller, mounted on the boat’s transom sitting outside the hull at the back of the vessel. In addition to providing propulsion, outboard engines also provide steering control by pivoting and controlling the direction of the thrust.

Because the self-contained outboard engine is mounted to the transom, maintenance is quite easy. An additional benefit comes with the ability to tilt the engine up when traveling through shallow waters and thus protecting the equipment.

A drawback of this design is the fact that a boat with an outboard motor cannot have a full swim platform (may have twin swim platforms flanking the outboard engine). The design, however, means the boater has better transom access which can benefit activities such as fishing (it’s no surprise that outboard engines are typically the primary choice for fishing boats). As we can already tell, the planned use cases for your boat are a major component of deciding upon the best boat engines.

Inboard Engines

Inboard engines are mounted inside the hull’s midsection and are connected to a drive shaft that runs through the bottom of the hull. The drive shaft is connected to a propeller. An additional rudder positioned behind the propeller is used for steering.

Straight shaft inboard engines are quite common propulsion systems. In this system, the rear facing engine is connected to a driveshaft that comes straight out of the bottom of the boat. The direct drivetrain can result in minimal loss of power from the engine, and the simple setup results in a high degree of reliability. However, on occasion the engines may need to be placed further forward which can compromise cabin space or seating options.

The V-drive inboard system differs from the straight shaft in that it has a forward facing motor with a transmission that redirects the drive shaft toward the back of the boat. The running gear on the bottom of the boat resembles that of the straight shaft inboard system. More interior cabin space or additional seating is often enabled in this arrangement as a result of the  engine being placed further back. However, some efficiency is often lost from the motor to the propeller due to the direction change of the V-drive.

Image 1566: boat engines inboard

Inboard motors are popular with boats where watersports are a priority. Inboard boats often create consistent, nice ramps and track true for skiiers and wakeboarders. Moreover, the propeller positioned under the transom is a safer position for, say, wakesurfers that are riding in relative close proximity to the boat.

Inboard engines are typically heavier than outboard engines, and compared to the outboard, the accessibility of the inboard is much less. Maintenance and repairs in the cramped inboard compartment is more difficult.

Inboard/Outboard (Sterndrive)

Stern drive engines, also called inboard/outboards (I/O) are a combination of the inboard and outboard engine. The motor is in under the transom of the boat and the drive unit is tucked beneath the swim platform. The motor, like most inboards, is a marine adapted automobile engine mounted inside the boat. The engine is attached through the transom to a drive unit (sometimes referred to as an “outdrive”) that looks quite similar to the lower portion of an outboard motor. The outdrive unit swivels similar to an outboard engine to direct the propeller thrust and provide steering.

Sterndrive systems can be advantageous for shallow waters due to the ability to trim up the drive unit and the absence of running gear at the bottom of the boat. Compared to an inboard, there are more components of the sterndrive engine exposed which can lead to damage or corrosion.

As with inboard operation, the use of a blower is imperative prior to starting the sterndrive engine in order to reduce fire risk associated with buildup of fuel vapors within the engine compartment.

Jet Propulsion

Many boaters associate jet drive propulsion with personal watercrafts from years past, but recently, many boat builders have included jet drive power in a number of runabout models.

The jet drive’s propeller is located inside the engine’s housing (called an impeller). The impeller pulls in water and forces it out the back which provides thrust for the boat. The strengths of the jet drive system are: shallow draft, more space available for boat builders to utilize (whereas a sterndrive typically needs to be covered by a sun pad), no exposed propeller blades, and high performance.

Image 1562: boat engines jet drive

On the other hand, jet drives are not great for low speed environments as the maneuverability suffers with less thrust. Additionally jet drives can often be noisy and less fuel efficient compared to other options (however, newer versions have improved in these areas).

Surface Drives

If speed on the water is what you seek, surface drives will do the trick. This propulsion type is an inboard engine with a propulsion unit that extends out the back of the boat as opposed to the bottom. The propeller is partially out of the water while running which reduces cavitation and provides high levels of fuel efficiency and performance.

Surface drives can often come with higher maintenance costs due to the large number of moving parts.

An early developer of the drive was Howard Arneson. Arneson developed surface drives primarily for catamarans and V-boats. Pershing Yachts make full use of the Arneson drive. Pershing yachts can often be identified by the rooster tail kicked up by its surface drives.

Pod Drives

Pod drive propulsion systems have been around since 2005, and an increasing number of cruising yachts have been outfitted with pod drive systems. Instead of a traditional shaft-based propulsion system, pod drive systems have one or more self contained pods mounted to the bottom of the boat that transfer engine power to thrust.

Pod drives tend to offer increased fuel economy, handling and performance. Additionally, position fixing and joystick docking are a breeze. Pod drive systems tend to cost more and finding the appropriate technicians to maintain and repair such systems might be slightly more difficult than traditional propulsion systems depending on your location.

Outboard Engines vs. Sterndrive Engines

Choosing between an outboard and a sterndrive engine is a common question for recreational boats, perhaps most commonly with bowriders. While in the past, most bowriders came standard with a sterndrive engine, boat manufacturers today are giving most bowriders the option for outboard power.

Image 1565: cobalt outboard

Here are some things to consider when choosing between outboard and sterndrive engines:

Architecture & Aesthetics - Typically sterndrive boats will offer a full sunpad and come with the option of a swim platform whereas the outboard powered boat may have twin swim platforms that flank the outboard engines. Outboards enable a more fishable transom and while you lose space on the back, you typically gain a more spacious cockpit for storage or seating. Aesthetically, many boaters like the clean lines of a sterndrive design with a sun pad and swim platform. However, based on personal preference, some enjoy the look of the outboard motors.

Performance - The power-to-weight ratio is an important component when analyzing the performance of outboard engines vs. sterndrive engines. With recent advances in outboard engine performance, horsepower can often be similar when comparing outboard engines with sterndrive engines. Outboard engines win by a significant margin on the weight factor. Hence, the power-to-weight ratio of an outboard powered boat is going to be better than a sterndrive boat. While a lighter boat is better in general for performance, it’s important to analyze the waters you’ll be cruising in, as more weight can give you a sturdier ride when dealing with rough seas.

Fuel Efficiency - Because outboards typically mean a lighter boat, this translates nicely into fuel efficiency - even while outboards tend to run at higher RPMs. If the amount of fuel you burn while boating is important to you, then an outboard engine might be the best bet.

Cost & Maintenance - In general, outboards tend to cost slightly less upfront than sterndrive motors, but there are many variables that can affect the cost. Maintenance is definitely easier with an outboard as the self-contained unit is quite accessible. Additionally, the sterndrive has more moving parts which tends to lead to more maintenance over time. If winterizing is relevant, winterizing is also more simple when dealing with outboards.

While performance, fuel efficiency and maintenance are important considerations, many boaters choose between the outboard engine and a sterndrive system simply based on planned use cases of the vessel. After all, most boaters aren’t really buying fuel efficiency and performance when purchasing a boat. Instead, they’re buying the fun and enjoyment that the boat represents.

The below photo shows a family enjoying the swim platform on the sterndrive powered Cobalt R35

Image 1559: Sterndrive Cobalt

Talk to a trusted salesperson or broker about the specific uses you plan for your boat. Fishing vs. watersports, how many guests will typically be with you on the boat, what waters do you plan to navigate? These considerations will likely drive your decision more than anything, and in most cases, you can match a suitable outboard or sterndrive engine with appropriate levels of performance and cost.

Future Boat Engines & Technology

Boat engine technology continues to march forward. Not too dissimilar to the automotive sector, much of the innovation surrounds electric power and propulsion in boats. The idea of “going electric” is a popular trend in many sectors today, and a number of companies are pursuing just that when it comes to boating.

Torqeedo is a German company that specializes in alternative propulsion systems including electric outboards, electric inboards, electric pod drives and hybrids.

Volvo Penta, a world renowned brand, is developing hybrid solutions with electric-only modes. Volvo expects to see their technology commercially available by 2021. The hybrid variant will operate with zero emissions, including lower noise and reduced costs. The hybrid system is currently in beta development.

While these companies (and others) continue to develop electric and hybrid based propulsion systems, advances in this area will be helped by improvements in battery technology (by companies such as Tesla and Panasonic) and improved infrastructure (e.g. charging stations at marinas).

Additionally, other advances in marine propulsion and environment technology may find their way eventually into smaller watercraft. Here are some more examples of innovation that may eventually find its way into recreational boating.

Maersk, a Danish shipping conglomerate, is outfitting their tankers with rotor sails to save fuel costs by up to 10%. Rotor sails rely on the magnus effect (a spinning object moving through air experiences a sideways moving effect). Maersk is outfitting vessels with 10 story high cylinders turned by small engines to help power the tanker, using the magnus effect.

Eco Marine Power is a Japan-based company that develops solutions for wind and power on tankers, yachts and other large vessels. Products already available are solar power systems with integrated computer management and solar panel systems that can be adapted for a range of ships and boats. The company also sells a full line of marine battery packs.

Do you have any additional questions regarding boat engines or propulsion for any size boat or yacht? The yacht brokers and sales personnel at Tom George Yacht Group are available to answer your questions and help you match the perfect boat with you and your family’s needs.

Frequently Asked Questions

What are the different types of boat engines?

Popular engine types for boats include Outboard engines, Inboard engines, Inboard/Outboard (Sterndrive), Jet propulsion, Surface drives and Pod drives.

Why are outboard engines so popular?

Outboard engines are self-contained units mounted to the transom. The location and accessibility makes maintenance easy. In recent years, outboard engine performance has increased. Since outboards weigh less than an inboard, the power-to-weight ratio is excellent in boats with outboards. This helps both performance and fuel economy.

When do boaters not want an outboard engine?

When swimming and watersports are the focus, many boaters like the clean look and design of an inboard or sterndrive engine. This enables a sunpad or swim platform.