Re: help on props
Sure, its pretty simple really, but of course if no one has every explained it to you it would be a mystery.<br /><br />Think about your prop as being a screw. For every single turn of the screw it will move forward some distance. That distance, expressed in inches, is the pitch of the prop.<br /><br />Props are described by their number of blades, their pitch, and their diameter. Those numbers are often used in the part number of the prop (different manufacturers do it different ways, but mostly they are similar). As an example a prop might be described as being 14.25 x 19. That would mean that a swath cut by its revolving blades would be fourteen and a quarter inches in diameter and its theoretical forward motion for a single revolution would be 19 inches.<br /><br />A certain amount of power is required to push a boat forward a certain distance in a set amount of time. Think of it like this: T*P=E where T is time, P is pitch, and E is the energy (horsepower) required. The time is a function of engine RPM and the energy is the power of the engine which is fixed for any given RPM. So it becomes pretty clear that the only variable you have much control over is the pitch of the prop. If you increase the pitch you will move farther forward for a spin of the prop and because the time remains constant it means you will move faster - as long as theire is enough power to do it. At some point in the game of increasing pitch you hit a point where there is not enough power to increase speed. In fact at some point increases in pitch will demand so much power that the engine is not able to accellerate. Remember, you can only accelerate as long as you have a reserve of untapped power. Too much pitch will hold the RPM down. Because most engines put out their maximum power at some RPM close to their design maximum it is desirable to find a prop that allows you to achieve the manufacturer's recommended maxim RPM for the engine when its on your boat. I say it that way, and there are such a wide variety of props available, because no two boats are the same.<br /><br />Anyway that stuff is why you so often see folks say that you need to find a prop that allows you to achieve the manufacturer's recommended top RPM when you are running at Wide Open Throtle (WOT as it is so often called). If you over prop it is roughly equal to trying to climb a very steep and continous hill in your truck with it in overdrive all the time. It basically lugs the engine and puts great stress on the bearings and much more importantly it drives up cylinder pressures greatly. This is very hard on an engine and WILL lead to preamature failure.<br /><br />Now, as long as we're on the subject, we might as well talk about diameter and blade count as well.<br /><br />These are so much simpler. As props push through the water they do so with some level of efficiency. That is to say that although the angle of the prop blade may cause the theoretical forward motion to be some distance, as expressed by the pitch, but in the real world the boat won't go quite that far. There will be some slippage, usually up to about 10% of the theoretical pitch, sometimes even more. Prop blades come in all sorts of sizes and shapes but as a general rule the larger the surface area of the prop the less it will slip. Unfortunately the larger the surface area of a prop the more drag it presents as well. So in deciding how large to make a prop, aside from concerns about clearance of the other parts of the engine, the manufacturer has to take into account how much drag is induced simply by the size of the blades. So a prop that is larger in diameter will have less slip but will also create more drag. The proper sized of blade is a compromise between thouse two concerns. I think it should be pretty obvious that one way to get a lot more surface area in a hurry is to add more blades. The more blades a prop has the less slip it will have and that is why you find 4-blade props used in applications where slippage is important. Think about trying to rapidly pull a water skier up out of the water and you will see an example where slip should be kept to a minimum.<br /><br />Another place where slip should be kept to a minimum is in boats that operate in choppy water, In this case the blade tips may at times actually be out of the water and so haveing more blades gives a better bite.<br /><br />At the same time there is another source of drag that is important. Props have some thickiness and that cutting face of the prop blade causes drag too. This one is directly effected by the number of blades but a lot less by the diameter of the prop (a function of the absolute length of each individual blade). So, 4-blade props which give better grip (or think about it as traction if you like) also provide more drag. That is why they tend to reduce top speed to some small extent in many applications.<br /><br />There are other prop design consideratons too, chief amoung them, other than material considerations, are the rake of the leading and trailing edges of the prop blades. We won't get into these other than to say that they have a strong effect on bow or stern lift. Generally speaking more rake results in more bow lift. We'll leave it at that simply because I'm really not well versed in rake and its effects, or at least while I understand the effect I don't understand the cause.<br /><br />Finally there is what is called cup. Cup refers to the inner face of the prop tips being somewhat concave. You can see it very well in a cupped prop (and almost all outboard props are cupped to some extent or another) out at the end of the blades. They will look a bit like the curve of the palm of your outstretched hand. Cupping has the effect of artificially increasing pitch. It also increases prop efficiency to some extent, particularly in the mid RPM range, which is to say it reduces slip.<br /><br />There, that help any?<br /><br />Thom
