When buying a used trailer boat, you can be faced with a couple of options which may not make a lot of sense to start with. What does “sterndrive” or “outboard” mean? What are the advantages and disadvantages of them? To give a fair comparison, we need to clarify that we are specifically talking about trailer boats that permanently live on a trailer, and are only put in the water at each outing. Comparisons about reliability cannot be compared when a boat lives permanently in the water, as mooring a boat can be a much more taxing environment.
What is a Sterndrive?
A sterndrive, as the name might suggest, is a vessel that has a propeller at the stern (back) of the boat. This is achieved by mounting the engine at the rear, and having a gearbox to transmit the power through the back and into the water. Sterndrive units typically have single or twin (duo) propellor configurations, and can be found in a number of different brands. Volvo Penta and Mercruiser are the two prominent brands in Australia.
What is an Outboard?
An outboard powered vessel is an engine that is mounted “outboard” or “outside” of the boat. Outboard motors are bolted to the transom, and hang above the water at the back of the boat. They still have a small gearbox to transmit the power to the water, and typically only have a single propellor although some rare models can be found with duo propellors.
What Are The Advantages?
Outboard powered boats have the advantage of being able to include more space inside the boat, whether it be for storage, seating, or other features the manufacturer may wish to include. Outboard engines typically weigh less than their sterndrive equivalent. For example, a 2019 220hp Mercruiser V6 MPI sterndrive weighs 329kg without a transmission, while a 2019 225HP Yamaha outboard weights 253kg including a transmission. This weight reduction in turn usually equates to slightly better fuel economy. The pricing is very similar new, with a 225hp Yamaha coming in at approximately $25,000 and a Mercruiser 200HP 4.5L V6 coming in at $19,000 with an Alpha gearbox (less heavy duty), or $25,000 with a Bravo gearbox (more heavy duty option). These prices were obtained from a quick Google search March 2020. An outboard is also quicker and easier to install on a boat than a sterndrive.
Outboard engines are generally easier to work on because the access is not as limited to the bilge, and there are less moving parts than their sterndrive equivalent. This makes outboards cheaper over a longer period of time for servicing. Outboards also have less perishing parts. For example, there are no bellows, and can be trimmed completely out of the water.
From a performance perspective, the two units are pretty close as they are both mounted in a similar position. However, you will also benefit from some slight fuel economy savings, because outboard engines are lighter. At the time of writing this, there are a handful of diesel outboard options, but this has not really taken off and become popular, so it’s rare to see a diesel outboard. Sterndrives have an array of diesel options, although at a much higher price point.
Vibration and noise is generally lower with an outboard, although this is relative to the construction of the hull, insulation used and whether the motor runs on petrol or diesel.
The advantages to a sterndrive are the vessel design and ability to offer larger horsepower options. With a lot of fishing boats, the rear deck space and access is important. For those looking to do water sport, it can be beneficial having a rear swim platform without a motor protruding in the middle. A sterndrive also opens up the design options, making it possible to have a rear sun bed. Having an outboard can prove to be in the way, although there are plenty of excellent outboard fishing boats. Sterndrives, up until recently, used to be capable of larger horsepower output, although in the last 10 years the technology has come a long way. The largest outboard has gone from 350hp to 450hp (in Australia), with one particular model boasting 627hp, although I’ve never seen one fitted to a boat in Australia, and you probably won’t find it on a trailer boat!
As with both outboards and sterndrives, the age of an engine that has been used in saltwater can increase the toll it takes on its components. Not all second hand boats are guaranteed to have been flushed out after use, and this will start to show at the 5 to 10 year mark.
Outboard motors have small water galleries and water jackets designed to cool the compact motors, which can clog up with salt. In most water jackets, there are anodes designed to decrease the effects of corrosion. Outboard motors also have thermostats and poppet valves, which means more mechanical components that can seize up.
Sterndrives usually have exhaust elbows or manifolds and risers, which can be saltwater cooled and need replacement at regular intervals defined by the manufacturers. Sterndrives also use rubber bellows which house driveline components that protect them from the saltwater, which can perish overtime and require replacement to avoid water ingress. Lastly, depending on the style of engine fitted, there can be freshwater cooling components fitted such as heat exchangers, or oil coolers and thermostats that will need servicing after periods of saltwater use.
In a trailer boat environment, you can’t go wrong with either option provided that budget is not a concern. Both options offer a similar amount of reliability at the same age, provided correct maintenance protocols have been adhered to. Sterndrives cost more to maintain, but they offer more access to the rear of the boat. On the other hand, outboards are slightly cheaper to maintain and offer better economy due to being a lighter package, and have more space inside the boat. Be aware, if the boat is moored permanently on the water you are faced with a different set of issues due to saltwater damage. Personally, I would pick a nice four stroke outboard option (Yamaha) from one of the Japanese manufacturers, unless I definitely wanted the layout that can’t be provided with an outboard or a diesel engine. This provides cheaper servicing, economy and a quiet operation while maximising space inside the boat.
Is an outboard or sterndrive cheaper to maintain?
Sterndrives incur higher maintenance costs due to more perishing parts over time and limited access for servicing.
Are sterndrives reliable?
Yes, provided correct maintenance procedures have been carried out at the required intervals. Sterndrives on trailer boat applications are more reliable than permanently moored sterndrives. Most issues arise around neglect and improper servicing, although there are a small number of faulty sterndrive models which should be avoided.
Are outboards more fuel economical?
Generally speaking the fuel savings are due to to an outboard being lighter than a comparative sterndrive counterpart, so the short answer is yes.