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Piranha Boat Propellers: Glossary of Terms

Not sure what a word or phrase on our site means? Use this glossary to go from deck swabbie to captain in no time.

Blade-Count

This is simply the number of blades the propeller has. Boat propellers most commonly have 3 blades, but 2, 4, and 5 blade configurations are also commonly available.

Cavitation

If you heat water to 100C it will boil. If instead of heating the water, you drop the pressure quickly and sufficiently enough it will also turn to vapor. Propellers operate by generating pressure that pushes the boat. Unfortunately the pressure around the blades rise and fall very quickly under normal use. When the rise and fall is great enough. The water will turn to vapor. The small vapor bubbles usually collapse again quickly causing no problems. If the bubbles are large enough in size and number, they can decrease the density of the water the prop is pushing causing a loss of thrust. This causes the prop to spin faster which again makes the problem worse until the prop slips out of control or you back off the throttle. The results of exessive cavitation are very similar to ventilation.

Counter-Rotation

On boats with 2 outboards or sterndrives, it is common to have the propellers counter-rotate or turn in opposite directions. This arrangement cancels out the steering torqued that would be created if the props spun the same direction. The most common arrangement for the drives is to have the right-hand rotation unit mounted on the starboard side of the stern and the left-hand rotation unit on the port side. This arrangement minimizes the likelyhood that the prop wash created by the props will cause the propellers to ventilate. This is not the only arrangement in use though, so always make sure you know which prop goes on which drive before mounting your props.

Diam

When a manufacturer designates a propeller’s size, it is done by stating the diameter and pitch of the prop (Diameter x Pitch). The most important thing about the prop diameter isn’t the performance change, it’s whether it will fit your motor correctly! If the diameter is too big, the prop may hit your motor when it turns. This is one of the reasons Piranha may not recommend trying a certain blade size on your motor even if the blades size will fit your propeller hub. In most cases the propeller diameter will make a minor difference in the propeller’s performance. In cases where it is a significant concern, Piranha’s database has been programmed to account for this and select the best diameter available for the job

Dual Exhaust

This is an exhaust design seen on many 1989 – 1992 Force / US Marine outboard motors. The majority of outboard motors vent their exhaust gas out through the center hub of the propeller. Other motor designs feature an exhaust port which extends back past the propeller. On A few Force outboards, the design uses a modified combination of both concepts for exhaust venting.

GPS_Consideration

GPS is a wonderful tool when your boat does not have reliable instrumentation, or in the case of small boats, no instrumentation. You should observe a very important protocol though if you try measuring your speed when traveling on a body of water. If you are using the GPS speed reading to evaluate the performance of your propeller, remember that GPS measure land speed, not your on-the-water speed.
To correct for speed differences caused by localized water currents, you should always make at least 2 speed runs in opposite directions. After doing this and recording your results, taking the average of the runs will help correct for speed differences caused by wind and or water currents and provide more reliable info about your propeller’s true performance.

Left-rotation

Not every propeller is designed to turn the same direction when the boat travels forward. There are always two possibilities when the propeller is viewed from behind the boat: left-rotation(counter-clockwise) or right-rotation (clockwise). If replacing a propeller and you use the incorrect type, your boat will wind up going backwards when you try to drive forward. A left-hand rotation propeller is one that will move a boat forward when it turns counter-clockwise when viewed from the back. Motors that have 2 motors or outdrives commonly have one right-hand rotation unit and one left-hand rotation unit. Because the propellers turn in opposite directions, the torque they generate on the boat cancels out and allows the boat to track straight. Most single outboards and sterndrives have right-hand rotation propellers with the exception of many Volvo-Penta sterndrive made before 1994.

Material

Propellers are commonly made from several materials with most being made from a metal alloy of some kind. Each material type has different advantages and disadvantages and some of these will be a factor in selecting the correct propeller size if your switch the type of material your prop is made from. The propeller size selector that Piranha uses take this into account when suggesting a replacement Piranha Propeller size. Common propeller construction materials are aluminum, stainless steel, bronze, Nibral (Nickel, Aluminum, Bronze alloy), and composite. Of these composites are the newest, though composites are special combinations of different materials selected in such a way as to achieve very specific mechanical properties. Most commonly used “composites” are combinations of very high tensile strength ceramic fibers bound together by a polymer resin which gives the material shape and volume and provides the toughness impact strength needed by the end product.

Non-thru

This type of propeller is predominantly found on low horsepower outboard motors where the smaller size and simplicity of the propeller gearcase combination is more practical. By not having to put exhaust ports in the prop, the prop hub diameter can be smaller as well as the corresponding gearcase torpedo diameter. This makes for a simpler unit. In decades past this design was not uncommon on horsepowers as high as 50, but these days they are usually only found on motors with less than 8 horsepower. Another common element of the prop design is that these props often use a shear-pin instead of a splined drive to drive the prop. Again a shear pin is a simpler to manufacture arrangement. Piranha does not make very many propellers that can be used with non thru-hub exhaust motors. If the prop is question is a pin-drive prop, then Piranha DEFINITELY will not have a prop available. Otherwise a Piranha may still be applicable. Please call for available applications.

Pin-drive

Pin-drive systems are not as commonly used as splined shaft drives of more that 9.9 HP. This type of propeller drive was more commonly used 20+ years ago, but even then, they were commonly used on engines less than 75 Hp. One exception to this were OMC sterndrives built before 1977. The pin-drive uses a smooth surface propshatf with a hole drilled through it , perpendicular to the axis of rotation. A short pin is then inserted into the hole so that it protrudes equally out the sides of the propshaft. This protrusion then engages cutouts in the prop so that the pin drives the propeller around as the shaft rotates. Pin-drives also tend to be non-thru-hub exhaust units as well. This is seen when the propeller is removed from the gearcase and no exhaust ports are observed between the propeller and the gearcase. Propellers for pin-drives have slots cut into the propeller perpendicular to the prop shaft hole. These slots engage the shear-pin which drives the propeller around. These slots are highlighted in yellow in the picture above.

Pitch

Pitch is one of the fundamental characteristics of a propeller used to designate its size. Props are usually sized by their diameter and their pitch. Pitch is the angle the blade makes relative to its plane of rotation. Pitch isn’t actually the angle itself. As a result the prop acts somewhat like a screw being driven into a solid material. As it turns it moves forward. How far is determined by the pitch. Therefore, pitch is represented as the distance the prop would travel forward in one rotation, measured in inches.

Ratio

The speed a propeller can propel you depends, at a basic level, on two factors: its pitch and how fast it is turning. The RPM you read on your tachometer tells you how fast the engine itself is spinning, but it does not tell you how fast the propeller is turning. To determine the propeller speed you must know what the gear ratio of your boat’s transmission is. Propeller rpm is calculated as follows:
Prop RPM = Engine RPM / Gear ratio Gear ratios are commonly reported as something-to-1 (something : 1). In most marine applications, the “something” is usually between 1.5 and 2.5 (1.5:1 to 2.5:1). If our boat’s gear ratio is 1.86:1 (a common value on large Mercury Outboards), and the rpm reported by the tachometer is 4800, our propeller rpm would be 2580rpm: 2580 rpm = 4800 / 1.86

Remove TW

The stock thrustwasher prevents the propeller from contacting your gearcase and allows the prop to turn freely without binding. Some Piranha propeller hubs have the thrustwasher built in. Failure to remove your original thrustwasher in these cases causes the propeller to be “Double spaced” and you will consequently have less prop shaft sticking through the prop. This might make it tough to get the prop nut tightened. Also, this can cause exhaust to leak between the prop and the gearcase, causing the prop to slip excessively.

Rev-Range

Every maker of marine engines specify a recommended Full-throttle RPM for each model it produces. This information is somewhat like the “redline” rpm that automotive manufacturers specify, but in the marine case, it carries far more ramifications than a simple “redline” that one must stay under. Unlike automobiles that have multiple forward gears, marine transmisions have only a single forward speed. In a terrestrial vehical, if your rpm is to low or too high, the driver can change gears so that the engine is not over loaded or over-revved. In a boat, the only way to alter the load being put on the motor is to change the propeller size. To allow you to know when the propeller you are using is either over-loading your engine or allowing it to over-rev the maker specifies the allowable full throttle rpm as a range.

Right-Rotation

Not every propeller is designed to turn the same direction when the boat travels forward. There are always two possibilities when the propeller is viewed from behind the boat: left-rotation(counter-clockwise) or right-rotation (clockwise). If replacing a propeller and you use the incorrect type, your boat will wind up going backwards when you try to drive forward.

Shaftlength

Volvo sterndrives produced before 1994 did not all have propeller shafts of the same length. The drive type, 270, 280, 290, etc did not determine the length of the drive shaft exclusively. Volvo sells replacement props for those drives and designates each as a long hub or short hub propeller. When shopping for a new prop you need to know which you need. Short hub props are most commonly used on 4cyl motors while 6 and 8 cyl models more commonly use the long hub props. Even so, the most reliable way to find out is to break out your tape measure. **NOTE Piranha 4-blade propellers can only fit on units that use the long hub propellers. You will never get the prop nut on, on a short hub propeller propshaft with a 4-blade Piranha prop hub.

Single Exhaust

Force motors made after 1994 are not of this type by definition, even though those motors only have one exhaust outlet. Force / US Marine motors made before 1994 will require you to determine if you have a “Single” or “dual” exhaust unit when shopping for a prop. The majority of outboard motors vent their exhaust gas out through the center hub of the propeller. Other motor designs feature an exhaust port which extends back past the propeller. “Single Exhaust” Force outboards, utilize this separate, non-thru-propeller exhaust method.

Splines

Splines are the teeth on the drive shaft that match the corresponding teeth in the propeller hub. The teeth lock the propeller to the prop shaft so that it does not slip.

Thru-hub

Propellers can be classified 2 ways regarding exhaust gas flow: Thru-hub exhaust and Non thru-hub exhaust. Thru-hub exhaust propellers are commonly found on “outboard” and “sterndrive” marine systems and are never found on “inboard” applications. This propeller type is part of the boat’s exhaust system as the exhaust actually exits the engine through the propeller (hence the name). This adds some complication as improper exhaust flow can cause a propeller to perform poorly if a prop is incorrectly installed or damaged. A Thru-hub exhaust prop is easily identified by the large open exhaust ports located in the prop around the prop shaft. As a result the diameter of the propeller hub area tends to be considerably larger than it is on Non thru-hub exhaust propellers.

Top-speed

Most people know this term without doubt. It is the maximum speed your boat will reach at full throttle. It is crucial piece of information to have when selecting a new propeller, especially if you do not know what size propeller you presently have. Many boaters dismiss top speed when selecting a prop because they never run their boats wide open at full throttle. A propeller’s suitability to a particular application must be assessed based on Top speed and the RPM that is achieved at that speed. Other than your set-of-the-pants preference, there is no other available information that can tell you whether your propeller size is correct, or more commonly, just how FAR off it is. Just as a Doctor will evaluate your personal condition by running you on a treadmill (even if that’s not how you use your “equipment”), so too must a prop technician know how you boat is working when pushed to it’s limit.

Ventilation

usually comes from one of two sources: exhaust gas or surface air. The propeller generates thrust by pushing the water and the higher the density of the water, the more force it can exert on the propeller. If air or exhaust gets mixed with the water where it meets the propeller, the density of the fluid will be reduced and the generated thrust will drop. As this happens the resistance or drag on the prop decreases and it over-revs the engine. Common causes of the problem are having the tilt or mounting of the motor or drive too high, having a broken diffuser ring on the prop, or by having a poor hydrodynamic exhaust seal where the prop fits into the gearcase.

WOT

This stands for “Wide Open Throttle “. This simply means that the motor is running as fast as it possibly (safely) can with the current propeller size and payload in the boat. When the proper propeller size is used on the boat, the WOT rpm should fall within a particular range specified by the motor’s maker. This information is very important when trying to determine the suitability of a particular propeller size for your application. This is true even If you never run your boat that hard or that fast.