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Glossary of Terms and Materials Used

The following list is intended to help the non-specialist understand the terms and materials used within this website and in general relating to propellers, it is not intended to be a definitive work on propeller terminology or metallurgy. If you cannot find a specific term or material, please do not hesitate ask us via our contact page

Please select the item that you are interested in:
        Copper Manganese Aluminium Bronze
        Manganese Bronze
        Nickel Aluminium Bronze
        Silicon Bronze
    Plastic & Composites
    Stainless Steels
        316L Stainless Steel
        Duplex 2507
    Blade Back
    Blade Face
    Blade Root
    Blade Tip
    Cavitation Burn
    Continuous Rated Output
    Cushion Hub
    Feathering Propeller
    Fixed Propeller
    Folding Propeller
    Geared Folding Propeller
    High Pressure Side
    Hull Speed
    Leading Edge of Blade
    Left-Hand Propeller
    Low Pressure Side
    Prop Walk
    Propeller Diameter
    Propeller Hub
    Right-Hand Propeller
    Sacrificial Zinc Anode
    Saildrive (S-Drive)
    Sea Trials
    Shaft Alternator
    Shaft Lock
    Tip Clearance
    Tip Speed
    Trailing Edge of Blade
    Variable Pitch Propeller
    Wetted Surface
    Zerk Fitting

Materials Back to Top
The following are the most common materials used in the manufacture of Propellers.Note there are many variations of Bronze and Stainless Steel and the characteristics can vary greatly within each generic type. As an example, simply saying 'Marine Grade Stainless Steel' means nothing, it is the actual grade of stainless steel that is important. '316' stainless steel is a very broad category, whereas '316L' is a low carbon variation that has a much higher resistance to corrosion in a marine environment.
  • Aluminium – Light and reasonably strong, but has a low resistance to corrosion when used in the marine environment, tends to be used only for smaller powerboat propellers.
  • Bronzes – A generic term for alloys with a high copper content. Common bronzes used in propellers are...
  • Copper Manganese Aluminium Bronze – Specifically developed for propeller manufacture, with mechanical properties superior to Nickel Aluminium and Manganese Bronze for propeller manufacture although tend to be difficult to repair.
  • Manganese Bronze – A reasonable strength material for low to moderate performance propellers but tends to be vulnerable to erosion by electrolysis.
  • Nibral – Refer to Nickel Aluminium Bronze.
  • Nickel Aluminium Bronze – Higher strength and more expensive than Manganese bronze, but with a higher resistance to cavitation erosion, stress cracking, seawater corrosion and easily repairable.
  • Silicon Bronze – An essentially non-magnetic bronze suitable where low magnetic influence is required.
  • Plastic & Composites – Traditionally used for small outboard and trolling motors. More recently developments in these materials have increased the application in the propeller arena. Most plastics and composites are light and are fairly flexible. Tend to lack rigidity when used in larger propellers.
  • Delrin® – An Acetal resin used by SEAHAWK for the manufacture of bushings used in AUTOSTREAM propellers. Excellent dimensional stability, stiffness, with high fatigue and wear resistance.
  • Stainless Steels – The generic term applied to iron alloys with a high Chromium content, generally with a high resistance to corrosion. SEAHAWK currently uses...
  • 316 – A readily available, easily cast and machined marine grade stainless steel with good strength and corrosion resistance.
  • 316L Stainless Steel – A low carbon variant of 316 stainless steel with a higher resistance to corrosion in a marine environment.
  • Duplex 2507 – As the name implies this stainless steel is a combination of two stainless steels. Austenite, a non-magnetic, chromium and nickel based alloy, with high strength and Ferrite, a magnetic, chromium based alloy that has a very high corrosion resistance. This material allows high strength from smaller sections and excellent corrosion resistance, tends to be difficult to cast and machine.
  • Zinc – A soft metal with low resistance to surrendering electrons when subject to galvanic corrosion (electrolysis). Used as a sacrificial anode, commonly referred to as a 'zinc'. When two connected, but differing metals are exposed to an electrolyte, such as seawater, they in effect become a battery. The weaker metal loosing electrons to the seawater creates an electrical charge that also erodes the metal. As zinc has a low resistance to surrendering its electrons the zinc 'sacrifices' itself in order top protect the metal it is attached to.

  • Terms Back to Top
    The following is a list describing the more common propeller terminology in laymans terms.
  • Anode – See Sacrificial Zinc Anode
  • Blade Back – The back-side of the blade relative to the propellers rotation, or the side that faces ahead on the boat. Compare to Blade Face
  • Blade Face – The front side of the blade relative to the propellers rotation, or the side that faces astern.
  • Blade Root – The very base of the blade where it joins the propeller hub.
  • Blade Tip – The outer extremity or end of a propeller blade.
  • Cavitation – In the case of propellers this is the action of water boiling or vaporising in the extremely low pressures generated on the back of a propeller blade, if water is unable to fill the void created by the push of the blade through the water. Can be caused by poor blade design, damaged blades, excessive rpm’s or simply too much power for too small a propeller. In severe and prolonged cases this can erode the metal from the propeller blade.
  • Cavitation Burn – Erosion or marking on a propeller caused by prolonged cavitation.
  • Continuous Rated Output – A specific engine power output at a specific engine speed. Specified by the engine manufacturer as the approved maximum output for continuous operation. This figure is less than the maximum output available for intermittent operation.
  • Cushion Hub – A propeller with a rubber insert installed between the shaft spline and hub outer, designed to reduce the shock passed to a saildrive or outboard gear system when changing direction or if the propeller blade strikes an object. Most saildrive and outboard manufacturers stipulate a cushion hub as a warranty requirement.
  • Electrolysis – When two connected, but differing metals are exposed to an electrolyte, such as seawater, they in effect become a battery. The weaker metal looses electrons to the seawater, which creates an electrical charge. This process rapidly erodes the metal supplying the electrons. Protection is achieved with the use of sacrificial zinc anodes.
  • Feathering Propeller – A low drag sailboat propeller where some form of mechanism is used to turn the blades edge on to the water flow (feather) to minimise the drag while under sail. The blades are rotated back into position once power is applied to the propeller. These propellers have low drag while under sail, and due to the blade being orientated for forward or reverse operation are the most efficient propeller in reverse versus fixed or folding types. Performance is close to a fixed propeller in forward and superior in reverse, usually no special operating technique is required in its use.
  • Fixed Propeller – A propeller where the blades are generally designed for optimum forward propulsion. Generally the highest drag when under sail. Reverse characteristics are fair due to lower efficiency when working backwards
  • Folding Propeller – A low drag propeller used on sailboats where the blades of the propeller are hinged and allowed to fold backwards under water pressure when not in use and relying on centrifugal force to open the blades up when under drive. They have a tendency to reverse poorly as the reverse thrust from the blades attempts to close the blades back up which counteracts the centrifugal force opening the blades, the lower the gear reduction (slower propeller speed) the worse this problem becomes. Generally offer the lowest drag with good forward ability, but tend to require a certain amount of ‘technique’ to be learnt to use effectively in manoeuvring.”
  • Geared Folding Propeller – A folding propeller that utilises gears between to blades to synchronise the opening and closing of the blades for smoother operation. Also assists the blades to fold back fully under the influence of water flow.
  • High Pressure Side – Refer Blade Front, the part of the blade that has high pressure when providing propulsion, in effect pushing the boat forward.
  • Hull Speed – The maximum theoretical speed that a given hull length and displacement will travel through the water in a displacement (non-planing) boat.
  • Leading Edge of Blade – The edge of a propeller blade that cuts through the water first, the part that is closest to the front of the boat. (Compare to Trailing Edge)
  • Left-Hand Propeller – A propeller that rotates anti-clockwise when driving ahead and viewed from astern.
  • Low Pressure Side – Refer Blade Back, the part of the blade that has low pressure when providing propulsion, in effect sucking the boat forward.
  • Over-Propped – The term used for a boat that has an excess of propeller blade area and/or pitch for the available engine power. The engine is not able to achieve maximum rpm’s before the load is greater than the engine can provide. Potential exists for engine damage due to overloading
  • Pitch – The distance that a propeller would theoretically screw itself through a soft solid like a screw as it was rotated through one revolution. The larger the angle of the blade the further it will screw itself along and the higher the pitch. This theoretical distance is offset by the fact that the propeller will slip slightly as it pushes the water.
  • Prop Walk – The tendency for the stern to pull to the side when beginning to reverse off. Generally the higher the pitch or lower the efficiency of a propeller, the greater this tendency becomes.
  • Propeller Diameter – The distance across a circle made by a rotating propeller, twice the distance from the centre of the shaft to the tip of one blade.
  • Propeller Hub – The centre part of a propeller that holds all the blades together and attaches them to the propeller shaft.
  • Right-Hand Propeller – A propeller that rotates clockwise when driving ahead and viewed from astern.
  • Sacrificial Zinc Anode – As zinc has a low resistance to surrendering electrons when subject to galvanic erosion (electrolysis), it is used as a ‘sacrifice’ to electrolysis. As zinc is a soft metal that easily gives up its electrons, it is attached to metals exposed to potential electrolysis that we wish to protect. This zinc then ‘sacrifices’ its electrons instead of the more important component it is protecting.
  • Saildrive (S-Drive) – The term used to describe a drive system that incorporates the gearbox and two right angle drives. This forms a leg that runs directly out through bottom of the hull, and then faces back towards the stern. The propeller is mounted directly on this leg without the need for further shafts or supports.
  • Sea Trials – Where the gap between theory and reality is identified. Small adjustments to the theory based, on experience, can be used to minimize any discrepancy.
  • Self-Pitching – The term applied to propellers that have the ability to increase their pitch as load is reduced on the propeller. Allows a lower engine rpm to be used when motor sailing.
  • Shaft Alternator – A low speed alternator driven by the propeller shaft when under sail. The power to drive the shaft is provided by the water flow over the propeller.
  • Shaft Lock – A mechanism where the propeller shaft is physically locked to prevent it from turning. Most commonly needed to protect the gearbox if insufficient lubrication is available with the engine stopped and the propeller tends to rotate the shaft under sail.
  • Slip – The difference between the theoretical distance that a propellers pitch would indicate that a propeller should screw itself through the water and the actual distance that it does as it is screwing through a fluid that displaces slightly as the propeller screws through the water.
  • Tip Clearance – The distance between a boats hull and the tips of the propeller. Generally considered to be minimum of 15% of the propeller diameter. Experince shows us that in a slower turning sailboat application 10% is usually not a problem.
  • Tip Speed – The speed in feet or metres per second that a propeller blade passes through the water as measured at the propeller blade tips.
  • Trailing Edge of Blade – The last edge of the propeller blade to cut through the water, the edge farthest from the front of the boat. (Compare to leading edge)
  • Under-Propped – The term used for a boat that has insufficient propeller blade area and/or pitch to absorb the available power. The engine will achieve maximum rpm’s without applying all available power to the water as thrust.
  • Variable Pitch Propeller – A propeller that has a mechanism attached to and running down the propeller shaft to allow the pitch of a propeller to be changed by the operator. Usually has a high price and not usually considered on typical recreational sailboats.
  • Wetted Surface – The surface area exposed to water resistance.
  • Zerk Fitting – Alternative term for a grease nipple
  • Zinc – Refer to Sacrificial Zinc Anode