POSTED July 23, 2015
Choosing A Boat – 4 Major Drive Types Explained
One of the most common questions I get asked is “Should I buy this boat for X amount?” Before I can give a good answer some basic details need to be figured out. First, you need to determine the main purpose of your boat (for example: fishing, cruising, watersports etc.). Second, where you’re going to keep it. Are you going to moor it, or trailer it? Of course, the size of the boat may dictate this, but not always. Where you’re planning to store it may also affect the type of engine setup that you choose and vice versa. How do you know what type of engine and drive you should get? Here’s a rundown of your options to help you figure out whether to moor or trailer your boat and what sort of engine and drive to go for.
Moored or trailered?
When selecting your first boat you need to think about how you will get to your boat, or how to get your boat in the water.
- Do you have a big enough car to tow it?
- Will you have a berth or a mooring?
- If you have a mooring, how will you get out to the boat?
- Will you use the boat regularly enough to keep batteries charged?
- If you have a waterfront property with a slipway, is it set up to accommodate your choice of boat and if not, what else needs to be done?
Maintenance costs vary greatly depending on whether the boat is kept permanently in the water or not.
When you have a moored boat, you will need to have it slipped, cleaned and antifouled every 6-12 months. The reason for this is that over time the part of the boat in contact with water will accumulate barnacles and moss. Antifouling reduces the amount of growth and it also becomes easier to clean off when you have a protective coating. You will also need to change corrosive anodes to stop metal parts (e.g. stern drives, shafts, propellers etc.) of your boat corroding away.
When you store your boat on either a trailer or dry-dock, you no longer need to antifoul the boat as it isn’t in contact with the water. This will minimize maintenance costs, but you will still need to consider:
- Storage – garage, warehouse, off-street or on-street?
- A suitable car to tow – for instance, once a boat and trailer combo weighs over 2 tonne you’ll need to carefully consider your tow vehicle.
Engine and Drive Setup
How a boat is powered and the type of transmission attached can also have a large impact on maintenance costs. Let’s look at the different drive types, and review the pros and cons of each.
There are four major drive types you will come across when buying a boat:
- Inboard Stern drive,
- Inboard Shaft drive, and
- Pod Drive
There are other setups available but these are less common in Australia.
Inboard Stern drive
Stern drive is a transmission and engine package, which combines outboard drive with an inboard engine. They come in all different combinations: large stern drives coupled with diesel engines or smaller ones coupled to petrol engines. They are also available from a number of manufacturers but the two most common brands in Australia, for example are Mercury and Volvo.
Example of a Volvo Penta Diesel & Stern drive setup
Inboard Shaft Drive
A boat with a shaft drive has the engine mounted inboard, with a shaft through the hull driving a propeller. Shaft drives may have the engine mounted in a few different positions, including (but not limited to) middle and rear. A rudder behind the propeller steers the vessel.
Transparent view showing a mid-mount shaft drive
Outboards have the engine attached to the stern, on the outside of the boat. It is a single unit, which includes the engine and transmission in the one package.
Yamaha 80hp Four-Stroke Outboard
Vessels fitted with pod drives have the engine mounted either in the mid or the rear of the boat, with a “pod” incorporating the transmission, propellors and outdrive straight through the bottom. A pod can steer left to right, and the two most popular brands are Volvo IPS and Cummins/Mercruiser Zeus drive.
Advantages and Disadvantages
Typically the main reason people buy second hand boats fitted with stern drives are because they are common, cheaper than other types and provide good handling characteristics. They allow you to trim up and down to shift the planing position, and the whole stern drive physically moves when you steer. Stern drives are a great option for a trailer boat, especially as you won’t have any barnacle growth on them. They need to be serviced yearly, but are a good all round option.
When a stern drive is used on a moored boat it becomes the less desirable option. Stern drives usually have rubber bellows that house drive components and stop water entry. These crack with age, and if you leave your boat moored over time it will grow barnacles. When the leg is turned to steer the sharp barnacles can pierce the bellows, allowing water entry. The water can then cause damage to bearings and other components and if left unattended it may even cause the boat to sink.
Another thing to be aware of is the water pickups, which cool the engine. These are mounted on the stern drive from factory. In the picture below they are covered in barnacle growth, which would cause the engine to overheat under load.
Example of Mercury stern drive with growth from sitting in water for a long period
Shaft drives are a much better option if you’re intending to moor the boat. Barnacles will still grow on them but there is no rubber bellows to change, as they don’t have to steer. They have shaft seals that can wear and leak over time, but don’t cost as much to maintain.
A downfall of a shaft drive is that they draw more water, so you cannot enter shallow waterways. They do need anodes fitted yearly, but so does a stern drive. As for handling, a single engine shaft drive can be more difficult to steer compared to a single engine stern drive. If you have twin engines, then shaft drives are an easier boat to maneuver. Twin shafts allow you to split the controls, using one engine in forward and one in reverse, spinning the boat on the spot – this works too on a stern drive but to a lesser effect.
Outboard powered boats can sometimes have the best of both worlds with regards to maintenance costs. They are easily removed/replaced if a major repair needs to take place, and can be trimmed upwards to keep them out of the water, hence not needing to be antifouled. They also provide the handling benefits of a stern drive, in that they can be trimmed up and down and steer left and right. They offer more useable boat space, because the motor isn’t mounted inside.
The disadvantages of outboards are when you need certain weight distribution, bigger horsepower, or diesel power. Some people prefer shaft drive or inboard for fishing, so they can fish off the back too.
Pod drives are exclusively paired with diesel engines, and provide a number of benefits. All pod drives are coupled to a joystick, which enables control of the boat without manually engaging gears and steering. This makes the learning curve for docking the boat much easier. Pod drives are also between 10-30% more fuel efficient than a traditional shaft drive, as they can propel the vessel with less drag. Zeus pod drives offer inbuilt trim tabs and have rear facing propellors, where as Volvo IPS do not offer trim tabs and have forward facing propellors.
The disadvantages of pod drives are that they cost more than a regular shaft drive to maintain. They do not have rubber bellows like a traditional sterndrive, but use expensive synthetic oils that need to be changed regularly, along with anodes. Pod drives are very sensitive to water entry, and certified technicians are required to work on them. This means there are less people available to fix them should you have issues in remote locations.
Inboard Stern Drive
|Maintenance costs when moored
Inboard Shaft Drive
Maintenance when moored vs. stern drive
Engine output potential
Ease of use (joystick control)
Draft required in shallow water
So what do I buy?
Everyone’s situation is different and needs to be taken into consideration. You need to assess your intended use for the boat, your storage options, your budget and your ability to maintain the boat.
Some people can be quite flexible and would be happy with any of the above options, where as others have specific requirements. Feel free to email us and ask us any particular questions you might have, we love to chat!