Re: FAQ on prop selection?
See if this covers anything we're looing for....<br /><br /> "The Basics of Selecting A Propellor"<br /><br />This is a "general" rule guideline and in no way will indicate the "perfect propellor", as all applications are different in many ways.<br /><br />There are 3 basic items to consider for propellor selection:<br />#1 The engine manufacturer's recommended wide-open throttle RPM.<br />#2 The primary use of the vessel.<br />#3 The amount of money your budget will allow.<br /> <br />Propellors are manufactured out of several different materials, with 3 being the most common, they are:<br />#1 Aluminum.<br />#2 Composites.<br />#3 Stainless steel.<br /> <br />The most popular by far, is Aluminum, and here are some advantages of this material:<br />#1 Largest selection of sizes.<br />#2 initial cost and ease of repair and/or alteration.<br />#3 Durable and yet forgiving of hard objects as the blade will bend or break usually before major damage can occur.<br /> <br />Disadvantages of Aluminum:<br />#1 Thick blades, lowering efficiency.<br />#2 Some blade flex.<br /> <br />Advantages of composites:<br />#1 Cheap to purchase.<br />#2 Very forgiving of hard objects as the blades will break free.<br />#3 Very light-weight, easy to store in boat as a spare.<br /> Disadvantages of composites:<br />#1 Blades break quite easily.<br />#2 1-piece molded props cannot be repaired with the exception of the replacable blade models.<br />#3 Blade flex under load.<br /> <br />Advantages of Stainless Steel:<br />#1 Highest performance props, with many custom built to suit your application.<br />#2 Not easily damaged.<br />#3 Thinner blades creating much higher efficiency.<br /> <br />Disadvantages of Stainless Steel:<br />#1 Cost is considerably higher than all other materials.<br />#2 Much harder to repair, and cost more to do so.<br />#3 Less forgiving on hard-object impact.<br /> <br />The first rule of thumb is to know and understand your engine manufacturer's recommended wide open throttle (WOT) RPM.<br /> If you don't know the Max WOT RPM for your engine, contact your dealer or ask the experts right here on iboats under the Forum listing your engine manufacturer.<br /> <br />Select a prop based on the Type of boat you have and how the current prop is/was performing.<br /> <br />If you do not have a Tachometer, understand this is the most important instrument you can have.<br /> <br />The amount of work your engine is doing is directly related to RPM, not boat speed.<br /> <br />In selecting a prop, consider a normal or average load that you will generally have on board.<br /> <br />Select a prop that puts you "AT", or "AS CLOSE TO" Max recommended WOT RPM.<br /> <br />Some manufacturers give an RPM "Range". It has been found best for engine longevity, to prop for the "TOP" of that range.<br /><br />There are several different "STYLES" of props.<br />Your boat type, size and weight, with consideration of available Horse power, will determine the type of prop best suited to your needs as well.<br /><br />Prop styles include options such as number of blades, generally 2-5, with 3 blades being the most common.<br /> <br />These props come in variations of "Flat-pitch", "progressive pitch", "cupped", "un-cupped", "Raked", etc.<br /> <br />Progressive pitch, cupped props are the most common in use today. The progressive pitch allows the blades to pull the same load the full length of the blade. The cupping adds a certain amount of "Bite" to decrease ventilation to a minumum.<br /> <br />High-Rake, progressive pitch props are used mostly in Stainless Steel props. The best explanation of "Rake", is the amount of angle of the blade to the rear. High-Rake props are common on most high-performance hulls such as Bass Boats, select Flats-style Boats, and some Racing configurations.<br /> <br />Generally, the higher the Rake, the more Bow-lift, which is important on heavy, or nose-heavy type boats<br /> <br />Pitch Recommendations:<br />As a general rule, 2 inches in pitch change will result in a 350-400RPM change. EXAMPLE: A 15 inch pitch prop turns 5600 at WOT. A 17 inch pitch on the same engine/boat would turn about 5200RPM at WOT.<br /> <br />NOTE: Not all manufacturer's props listed as the same pitch will deliver the same RPM.<br /> <br />Diameter is another factor that plays a role in prop performance.<br /> <br />In most cases, as a particular prop style increases in pitch, the diameter decreases. EXAMPLE: 13-3/4X15, 13-1/4X17, 13X19, 12-3/4X21.<br /> <br />A larger diameter prop in one particular pitch will have more "Blade area" and tends to be a better heavy-load prop, but often times suffers "slightly" on top-end due to the increased drag of the larger blade area.<br /> <br />Smaller diameter props in a given pitch tend to be "slightly" faster for the same reason..LESS drag from less blade area. The smaller diameter props are usually better suited to "Lighter" boats.<br /> <br />An exception to this is in High-performance applications where the prop blades break the water's surface and a larger diameter prop is preferred for better performance.<br /><br />Choosing a prop with fixed-hub or consumer replacable type hub.<br /> <br />Three hub types are available in the propellor industry, and I will describe all types briefely.<br /> <br />#1, The Manufacturer installed rubber hub system, which has been the most common type for years. This hub type is designed to absorb "shock" in several forms including shifting and prop strikes. The hub in this style is "Intended" to slip free before damage occurs to the lower unit and it's components.<br />This type of hub is found in both Aluminum and Stainless Steel props.<br /> <br />#2, The consumer replacable hub system under such names as "Flo-torq", "XHS", etc., are hubs that are sold as kits to make many different manufacturers prop casings fit most any application. This system allows easy replacement of the outer prop assembly at a lower cost than replacing a "spun hub" in a "fixed hub" type prop.<br />It also allows the consumer to purchase propellor casings at a much lower price, and therefore makes it more affordable to carry different pitches. If a hub or prop is damaged, a simple hub change or casing replacement allows "new prop" installation at almost half the cost of fixed hub props.<br /> <br />#3, The "solid hub" type is a prop with the splines cast or machined directly into the material the prop is made from, and is actually part of the prop.<br />This type is used mostly in high-horsepower applications where the torque is too high for a flexible type hub such as those listed above, with the exception of Composite props, which also fall under the "solid hub" list, although the composite material will "give" before damage is done to splines or other lower unit parts. <br /> <br />V-drives and standard thru-hull inboards use this type of prop as well, although in these applications, the prop material usually consists of some aluminum, brass and nickle to allow a "softer" material in the case of a prop-strike in the V-drives and Inboards.<br /> <br />This is not a science, only a guidline to follow. The best set-ups have come from countless trials with different props and many test-runs. <br /> <br />It is up to the owner to fine-tune any application, and I hope this helps.
