An I/O engine is considered a mix of an inboard and an outboard because the engine is inside, but the propeller/rudder is outside. In a true inboard, the propeller is in a fixed position and cannot go up or down.įor a boat with an I/O engine, the engine will be placed towards the back of the boat (still fully concealed), but the propeller part of the engine will stick out.
Instead, there is a separate rudder, which controls the steering. When a boat has a true inboard engine, the engine will be placed in the center of the boat (inside the boat) and have a driveshaft connected to a propeller, which will give the boat power. I/O engines are more practical for families or boats that want more flexibility. True inboard motors will be found on boats designed for skiing as they produce less wake and drive much smoother. A true inboard is fully inside a boat and an I/O has components within and outside of the boat. There is a distinction between a true inboard engine and an inboard/outboard engine we need to discuss. The inboard engine is connected to a driveshaft, which controls the direction of the propeller, steering the boat. Īn inboard engine, sometimes referred to as an inboard motor or as sterndrive, is a boat propulsion mechanism that sits inside the hull of the boat permanently. If you are not quite ready to talk about engines but are interested in learning more about boating and what types of boats are out there, take a look at our Guide to Buying a Boat and when you’re ready to buy a boat, check out our guide to how much a boat costs. In this article, we are going to discuss more about what inboard and outboard engines are, the pros and cons of each type of engine, and how much they cost on average. When selecting an engine, there is one important choice you need to make first: inboard or outboard? The difference is actually within the words an inboard engine sits within the structure of the boat and is not visible whereas the outboard engine sits outside the structure of the boat and is clearly visible at all times. It also means you have some room for customization and can choose exactly which engine you want. It means you need to budget above the MSRP of the boat and plan to pay more for an engine (or two). Boats are almost always sold without the engine. If buying a boat is on your radar, one question you will need to answer is the type of engine you want. What Boats Usually Have an Outboard Engine?.What Boats Usually Have an Inboard Engine?.What is the Difference Between Inboard and Outboard Motors?.Even putting a fine pitch prop on, the rotation speed is too high to be optimum, so there are higher losses converting rotary motion to thrust. Gearbox has smaller gears to be compact, running faster so greater lossesīut after all those, this is the big one.outboard engines are designed to push a light boat fast, inboard a heavy boat slowly, so the prop shaft speed on the outboard is faster. Outboard engines have to be compact so smaller sized air input & exhaust pathways than optimum. Outboard power measured at prop, inboard at output shaft before the gearbox, so comparison of equal power not quite true to detriment of outboard Outboards higher revving so increased friction losses Here are some reasons why outboards of the same power use more fuel than inboards.Ģ strokes lose a significant proportion of their fuel unburned, but that's well known, so I'll look at losses from 4 stroke outboards.ĭiesels fuel injected, small outboards carbureted, so fuel metering not so goodĬylinder capacity of equal power outboard is smaller so less thermally efficient because of surface area to volume ratio of combustion chamber greater But the 10hp Nanni does it for about £6 on red diesel with no bleeding ears, far fewer pot lines more reliability No explosion risk and very little fire risk, better electricity generation - just, better weight distribution, easier and legally stored fuel and more prop drag.įurthermore, being quite light, it's only a tiny bit more hassle to remove it from the boat compared to the o/b in a well with controls connected to the boat. The 9.8hp Tohatsu 4T would do that same journey for about £18. Then when you consider that it took me some eight hours to motor from Shoreham to Bembridge, that worked out at about 6.5 gallons with my 8hp 2T, costing today about £33. A rule of thumb I use is 10hp 2T o/b uses a gallon per hour, a 10 hp 4T o/b half that and a diesel inboard half that.
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