Choosing between an inboard and outboard motorboat affects much more than where the engine sits. The propulsion system can change how the boat handles around a marina, how much usable space you have aboard, what maintenance involves and whether the boat suits the type of trips you have planned.
Neither is automatically the better choice. A small boat used for day trips around the Solent has very different requirements from a larger motor cruiser intended for longer coastal passages.
Understanding those differences makes it much easier to narrow down the type of boat that will work for you.
What is the difference between an inboard and outboard motorboat?
An outboard engine is mounted externally on the transom. The engine, gearbox and propeller form a single unit and the whole unit turns when you steer.
An inboard engine sits inside the hull. However, not all inboard arrangements are the same.
A shaft-driven boat uses an engine inside the hull connected to a propeller beneath the boat, with a separate rudder providing steering. A sterndrive also has its engine inside the boat, but power is transferred through a drive unit mounted at the transom.
That distinction is important. A shaft-driven motorboat and a sterndrive motorboat can behave quite differently, even though both have inboard engines.
How do they feel different around a marina?
One of the biggest differences becomes apparent at low speed.
With an outboard, turning the wheel changes the direction of the propeller thrust. This can give the helm a fairly direct response when manoeuvring into a berth.
Twin outboards provide even more options. Using one engine ahead and the other astern can turn the boat within a relatively small area, although wind, tide and the boat’s shape will still affect what happens.
A single shaft-driven inboard behaves differently. At very low speed, there may be little water flowing over the rudder, reducing its effectiveness. Short applications of ahead and astern, together with an understanding of the boat’s momentum, become more important.
Prop walk can also affect how a shaft-driven boat moves astern. Rather than treating it as a problem, owners can learn how their particular boat responds and take it into account when approaching or leaving a berth.
A sterndrive steers by turning its drive leg, directing the propeller thrust in much the same way as an outboard, so it generally responds more like an outboard when manoeuvring at low speed.
Whichever system is fitted, close-quarters boat handling becomes easier once you know how the boat responds to ahead, astern, steering, wind and tide.
Does an outboard give you more space?
Often, particularly on smaller boats.
Because an outboard is mounted on the transom, more of the hull can be available for seating, storage and other uses. This is one reason outboards are common on RIBs, sports boats and smaller day boats.
The trade-off is that the engine occupies space at the stern. Depending on the boat’s design, that can affect access to a bathing platform or make getting in and out of the water less straightforward.
An inboard takes up space inside the hull, but it can leave the transom clear. On larger cruising motorboats, this may allow for a full bathing platform and a layout better suited to spending longer periods aboard.
When comparing boats, look beyond the brochure layout. Imagine people moving around with bags, fenders, lines and other equipment aboard.
Which is easier to maintain?
There isn’t a simple answer because the maintenance requirements are different.
Outboards usually provide relatively straightforward access for routine servicing and can often be tilted clear of the water when the boat isn’t being used. If an engine eventually needs replacing, replacement can also be less involved than removing an engine installed deep inside a boat.
However, an outboard is exposed to the elements and its external position can make security an important consideration.
Inboard installations have their own maintenance requirements. Access is particularly important. A large engine hatch might look impressive, but what matters is whether oil, coolant, filters, belts and other routine service points can actually be reached easily.
On a used motorboat, the condition and service history of the engine matter just as much as whether it is mounted inside or outside the hull.
What about fuel use?
Engine position alone doesn’t tell you which boat will use less fuel.
Boat size, weight, hull design, engine output, cruising speed, loading and sea conditions all influence consumption.
A lightweight planing boat with a modern outboard has very different requirements from a heavy shaft-driven cruising motorboat. The difference between displacement and planing motorboats therefore matters just as much as the location of the engine when considering performance and fuel use.
Think about the complete running cost as well. Servicing, fuel, berthing, insurance and lift-outs can all make a difference over a full season.
Which is better for day trips?
Outboards are particularly common on boats intended for shorter trips, watersports and days spent exploring the coast.
They work well with smaller, lighter boats and can provide responsive handling without taking up valuable internal space. Being able to tilt the engine can also be useful for boats kept in shallow water or regularly launched and recovered.
That doesn’t mean an outboard boat is restricted to short trips. Modern multi-engine outboard installations are found on increasingly large and capable craft.
The more useful question is whether the whole boat suits the trips you intend to make.
Which is better for longer cruising?
Larger inboard motorboats are a familiar choice for coastal cruising because they can provide more accommodation, substantial fuel capacity and a clear bathing platform.
Shaft-driven diesel installations are particularly common on heavier cruising boats.
But an inboard engine doesn’t automatically make a boat a better cruiser. Hull design, range, accommodation, weather protection and sea-keeping all matter just as much.
If most of your boating will involve short trips around the Solent, buying a boat designed around occasional long-distance cruising may leave you paying for space and capability you rarely use.
What should you consider before choosing?
Start with the boating you’re actually likely to do.
Think about:
- Where the boat will be kept
- How many people will normally be aboard
- Whether you want overnight accommodation
- How often you’ll be manoeuvring in a marina
- Whether swimming or watersports are important
- How far you expect to travel
- Whether the boat will be trailered
- Access for routine maintenance
- Fuel and servicing costs
Then look at the propulsion system as part of the complete boat rather than deciding that you must have an inboard or outboard before you start looking.
A well-designed outboard boat may suit your plans better than an inboard alternative, or vice versa.
Get to know how your boat responds
Whichever propulsion system you choose, allow time to learn how that particular boat behaves.
Find out how quickly it stops, how the stern moves when you select astern, how much momentum it carries and how strongly wind affects it at low speed. If there are two engines, practise using them independently somewhere with plenty of room before relying on them in a tight berth.
The differences between inboards and outboards matter, but they’re only one part of choosing a motorboat. Hull design, size, layout, visibility and where you intend to use the boat all have just as much influence on whether it is the right choice.
FAQs about inboard and outboard motorboats
Does an inboard or outboard motorboat have more draught?
The propulsion system can affect underwater clearance, but the overall draught depends on the complete boat design. Shaft-driven boats have fixed underwater gear, while many outboards and sterndrives can be raised when appropriate. Always use the specific draught figures for the boat when planning shallow-water passages.
Can you flush an outboard engine after using it in salt water?
Many outboards have a flushing arrangement intended to help remove salt and other deposits from the cooling system. The correct procedure varies between engines, so owners should follow the manufacturer’s instructions rather than assuming the same method applies to every outboard.
Does an inboard or outboard engine affect noise aboard?
It can, although installation and boat design make a considerable difference. An inboard is enclosed within the hull, while an outboard sits externally at the stern. Insulation, engine speed, hull construction and the position of passengers can all influence how much noise and vibration are experienced aboard.
What happens if one engine fails on a twin-outboard motorboat?
The remaining engine may still provide propulsion, but the boat’s speed and handling can change significantly. Manoeuvring may become less predictable, particularly at low speed, so the helm should allow more room and consider whether assistance or a simpler berth is appropriate.
Can you change a motorboat from an inboard to an outboard engine?
Such conversions are possible on some boats, but they can involve substantial structural, weight-distribution and system changes. It is not simply a matter of removing one engine and fitting another, so any proposed conversion should be assessed properly for the particular hull and installation.