What is an InboardAug 24 '01 Write an essay on this topic.
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The Bottom Line To sum it all up, an inboard is a car engine, and an outboard is a motorbike engine.
What is an inboard motor? Well since no one else has said anything on this yet, and you read around all these reviews on boats, all referring to 5.0litre inboards etc, I thought I would talk about the types of inboards, and their uses in the marine trade. I warn you from the start, it is difficult to explain, and may get a little confussing in places So what is it? Well, an inboard motor, is a car engine. That’s probably the best way to describe one. The technical way would be it’s a fixed propulsion system, driving a drive system via a gearbox with external auxiliaries. But I think car engine explains it better. You all should know what an outboard engine is. That’s an engine that is directly connected to the drive system and the whole thing turns etc. Inboards are fixed, just like in a car. So what makes an inboard engine? Well, most inboard engines are built by either GM, or Ford. Not common knowledge from what I understand, but the most popular manufacturer of engine Is Mercruiser (Mercury) Or Volvo Penta (Volvo) Volvo being the most common diesel engine, and Mercruiser being the big boys in petrol (gas). But if you took a Volvo engine apart, or a Mercruiser engine apart, you will see, it is a GM engine (Vauxhall, Opel, depending on your country) In fact, you will find most car engines, with the exception of Japanese engine, are also made by GM. So all that means, there is only really one engine manufacturer on the water, there are others, such as OMC, but they have gone bust, at the time of writing this, so aren’t really players anymore. So in terms of choice of make, you are pretty limited. However, only the basic block is GM, the fittings and other parts or made or designed by the individual manufacturer. So how does an inboard work? Well, in the same way as a car engine basically. Most inboard engines are ‘raw water cooled’, Separate lubrication system, (don’t worry I will explain these later) any cylinder formation, be it straight 3, straight 4, V6, V8, racing. I say racing, as these are the big boys, and you wont find one in your everyday Bayliner, just like you wont find a Ferrari Formula 1 engine in a Honda civic. OK cooling is the first thing that defines an inboard engine, or any marine engine for that matter. They use what is known as ‘raw water cooling’ or ‘Open circuit cooling’. This means they are cooled internally like any water cooled car engine, but instead of having an expansion tank and a fresh water supply that is circulated and cooled via a radiator, they take their water straight from outside. The water is pumped up via the water pump, through a skin fitting in the hull, or in the drive leg, or an external supply of some form, racing engines for instance have a hose pipe, basically, that hangs down behind the drive. This water is pumped up and circulated around the engine, and then enters the exhaust manifold, where it is mixed with the exhaust and exits the boat via the exhaust system. This cools the exhaust, and also acts as a silencer for the exhaust, If you look at your boat, you wont find a silencer (muffler I think they are sometimes known as) like you would on a car. The advantage of this system is it has a constant supply of cold water, where as a ‘closed circuit system’ relies on the radiator to cool the water. This means the engine is always cool even when idle or at slow speeds. Also because it is often not possible to fit a radiator to a boat, the lack of need for one, is an advantage. Disadvantage with this system, is the boat has to be in the water before the engine can run, unless you attach an external source to the engine, via ‘muffs’ attached to a hose pipe, or run it in a testing tank, not really practical in an inboard engine. Also because the water temperature cannot be controlled, the engine doesn’t always run at optimum temperature, where as in a car the temperature can be controlled by opening pressure release systems on the expansion tank. A boat engine can only rely on a thermostat to control the amount of water entering the engine. Another disadvantage is corrosion, especially in salt-water use. When running the water is in and out quickly and so doesn’t have a chance to corrode, but when stationary, for a period of time, the salt water remaining in the system can start to corrode the system, as can fresh water. Because obviously you cant add chemicals like antifreeze. If you look around the water pump, and hose connections, you will see a white crystal form of corrosion this is where it has started to corrode. This itself is not a problem, because the system has been designed to last longer in salt water, but if left over time, it will block up the system, and then when used next, water will not be able to circulate and over heating is inevitable. However a closed circuit system will work in some applications, and will work to an advantage. I have designed a system that uses both closed and open circuit systems to cool the engine. This means any car engine can be used for instance, due to the clean water being used, as well was meaning the engine can be run out of the water, for testing, maintenance etc easily. Anti freeze etc can be used which is helpful in winter due to freezing as well as corrosion protection. Although my design is intended for racing applications, and this cooling system is just to overcome another problem, it shows that raw water is not gospel for boat engines. Lubrication is done via a closed circuit system. Incorporating an oil sump, oil pump, and oil ways, sometimes an oil cooler, but rare on most standard boat engines. Oil is pumped around the engine just like in a car engine, and is re-circulated. You may have heard of 2-stroke engines, where the oil is mixed with the fuel and is burnt after use. This closed circuit system means all moving parts are constantly oiled and will last longer. However obviously the oil will need replacing eventually, as does all oil. This can often be difficult in a boat, because you don’t have much clearance under the sump in order to work, and drain the oil, like you do in a car. Also the oil can get very hot, in the confined space, and minus the oil cooling you often see in cars. Again I have incorporated an oil cooler in my design, but on most standard applications, oil is left to fend for itself. You also lose a bit of power with closed circuit systems, which is why you will fine a 2-stroke engine to be more powerful than a 4-stroke. Because you are having to drive auxiliary equipment, such as the oil pump, which all takes up useful energy. You would be surprised how much. Type of cylinder arrangement. Without doubt, the most common form of engine in a boat today is a V-engine, usually V6, because of the confined space they can be fitted to. Also because of the increased torque they produce over a straight cylinder. Also you won’t find an engine much smaller than maybe a 3litre in your everyday powerboat. Yet you look at the top speed of a Bayliner, with maybe a 5.3L V6 fitted, maybe push 50mph on a good day (don’t listen to manufacturer hype) where as a 5.3 V6 in a car would probably push 130mph+ doesn’t make sense does it. This is because, of gearing and the way the power is transferred to the water. A car can use a gearbox, with various gear ratios, to effectively transfer the engine output into power on the road, through the wheels. Where as a boat cant, because you are using a propeller, instead of wheels. When you change gear, the drive needs to be released while gear is changed. In a car you have the momentum of the car to keep the wheels turning at almost constant speed, but in a boat, the prop will just stop, then when engaged again, will be slammed into gear. Not good if reliability is important. Also wheels can transfer power much easier and more effectively than a prop can, Because of the friction they have compared to that of a prop. Why not fit an automatic gearbox? Well it has been done before, and worked well I believe in race conditions, because acceleration was increased from the start, the problem is, once they reach top gear, they stay there, and don’t change down again. In a car you have a greater resistance on the wheels etc, all this contributes to how an automatic gearbox decides which gear it needs to meet the speed it is being asked to produce at that particular time. A boat however doesn’t have that, and so the gearbox doesn’t know what gear it should be in, because it isn’t meeting an increased resistance, like going up hill for instance. So if individual gears are required, the best way is with a manual gearbox, or electronic gearbox. This then produces the lack of prop momentum when changing gears problem. Which is what my design overcomes, but as of yet, I don’t know of a manufacturer who has produced a successful gearbox for a boat. So basically, you have 3 gears, forward, neutral and reverse. All at a set ratio set up to give maximum performance and efficiency for what it is intended. And because of a single ratio, top speed is lost because some of that performance is put into acceleration, else you would never get going at all. And larger engines are needed in order to achieve a max speed. Complicated isn’t it? So what makes an inboard different to an outboard? Not a great deal, apart from obvious things like size. Again a comparison would be a car engine, and a motorbike engine. A 900cc motorbike can produce in excess of 150 bhp, where a car engine needs maybe a 2.0litre engine to produce that. Speed is not a comparison, because obviously weight plays a huge factor in overall performance. But the same applies to outboard and inboard engines. Outboards can produce anything from 1hp, up to 300+hp and yet, still remain 2 cylinder to 4 cylinder, you won’t usually find more than 4 cylinders in an outboard. The main difference is probably the drive system. An outboard has a fixed driveshaft that runs to a gearbox, located in the lower end of the leg, just behind the prop, and steering etc is done by turning the entire engine, where as an inboard is done via driveshafts and universal joint, usually powering a straight shaft with external rudder, or a sterndrive which is like an outboard leg attached to the engine. Because of the large weight and size on inboards, it isn’t practical to attach them to the drive system fixed, like an outboard. Arneson drives for instance are different to most applications, where steering and trim is done by the shaft, the prop is fixed to a straight shaft, which is attached to hydraulic rams which move the shaft left or right up and down in order to steer etc, again, not found on standard boats. To sum it all up, an inboard is a car engine, and an outboard is a motorbike engine. With the differences mentioned above. That’s probably the easiest way to describe them.  |
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