Why does my boat vibrate? What is cavatation? What is hull design? What is the diameter of a propeller? What is the propeller pitch? What is slip? What is ISO Standard 484/2? What is Propeller Rake? What is Blade Track? What is Propeller Efficiency? How many blades are best? What alloy should I use? What are Engine Loading Arms? What about the Propeller Bore? Constant Pitch vs. Variable Pitch? What is Galvanic Corrosion? What is D.A.R.? Do I want my propeller dynamically balanced? What about the installation or removal of a propeller? Why does my boat vibrate? This is typically caused by an out of balance propeller so that would be the first place to start (and the good news is that it’s the least expensive fix.) The next concern would be to check the clearance from hull to the propeller tip. This should be a minimum of 15% of the propeller’s diameter. Other considerations are a bent shaft, worn bearings, misaligned shaft, or worn engine mounts. Back to Top What is cavatation? When propeller RPM is increased to a point where suction ahead of the propeller reduces the water pressure below its vapor pressure, vapor pockets form. These vapor pockets interrupt the solid flow of water to the propeller. One of the most common causes of cavitations is excessive tip speed (propeller turning too fast for the water to flow through the blade contour.) Cavitations usually occur at tip speeds over 130 MPH and can result in the loss of thrust, vibration, or blade erosion. This condition can also be caused by damaged blades or incorrect pitch. Back to Top What is hull design? The three general hull type classifications are displacement, semi-displacement, and planning. Each of these hull types has different design features and operational characteristics that are suitable for specific vessel applications. a.Displacement Hulls are shaped so the bow cuts a wedge in the water which the fore body then moves aside to the extent of the midsection. Aft of the midsection the hull is shaped to allow the water to converge at the stern in a natural flow and reduce drag to a minimum. b.Semi-Displacement Hulls are intermediate between displacement and planning hulls, possessing the characteristics of each hull type. It is designed for economical operations at speeds falling between semi-displacement and planning hulls. The semi-displacement hull wraps from a pronounced v-shape at the bow to a flattish stern. c.Planning Hulls are designed to ride on top of the water at higher speeds. Its usually very flat stern is shaped to break cleanly from the water creating very little drag at high speeds and is therefore efficient at these speeds. Back to Top What is the diameter of a propeller? The diameter of a propeller is found by measuring the distance from one tip to the farthest tip to of the rotating propeller blades. It is also the first number stamped on the boss or hub of the propeller. Back to Top What is the propeller pitch? The propeller pitch is the angle the blade makes in relation to the centerline of the hub. It is normally expressed as the distance, in inches, that the blade would advance in one revolution assuming there were no slippage between blade and water. Most boats have slippage therefore the distance advanced is less than the design pitch. The pitch of the propeller is the second number stamped on the boss or hub of the propeller. Back to Top What is slip? Slip is defined as the difference between the theoretical advance and the actual advance of the propeller. Slip is not a measure of propeller efficiency. The amount of slip changes with boat speed. Slip occurs because water is not solid and is actually required to create thrust. Back to Top What is ISO Standard 484/2? There are four levels which are S, I, II, and III. ISO 484/2 has become somewhat of the industry standard accepted in 1981 prior to CNC machining and computerized measuring devices. Back to Top What is Propeller Rake? Propeller rake is the amount a blade slants forward or aft from a perpendicular line in axis of rotation. Positive rake slants towards the aft of the boat, negative towards the bow. Back to Top What is Blade Track? Blade track is the difference between minimum and maximum race of all the blades. Back to Top What is Propeller Efficiency? Propeller Efficiency is a measure of the amount of torque a propeller shaft that is transmitted into usable forward thrust. Propeller efficiency varies with speed. Back to Top How many blades are best? Two blade propellers are typically used on sailboats and hydroplanes. Two blade propellers rarely operate without vibration, except at very high speeds. Sailboats benefit from the reduced drag. Three blade propellers are ideal for smaller boats under 35 feet. They can be used on sailboats that require extra power to handle heavier weather or for maneuvering. Four blade propellers are used on commercial and pleasure vessels over 35 feet. They offer excellent towing capabilities for commercial vessels, and are used increasingly on higher speed planning hulls to reduce cavitations and improve performance. Five blade propellers can further reduce vibration for extremely smooth operation. The additional blade area is useful in highly loaded applications with higher horsepower typically found on larger custom yachts. Back to Top What alloy should I use? There are four grades of allow available: manganese, bronze, NI-BRAL, stainless steel, and aluminum. Bronze works fine for most displacement boats such as sailboats, trawlers, and work boats. NI-BRAL is best for planning applications such as high horsepower and high speed. Stainless steel has the same strength as NI-BRAL but is much more expensive. It is a must for aluminum boats that are not bottom painted. Aluminum is typically used on small runabouts with outboard stern-drive power. Back to Top What are Engine Loading Arms? The most common problem encountered when sizing propellers are engines that are not loaded correctly. If the propeller is too small, the engine turns well over the maximum rated RPM at wide open throttle and may be restricted by the governor. If the propeller is too large, the engine does not reach the maximum rated RPM at wide open throttle. If the engine RPM at wide open throttle needs to increase, then decrease the pitch. If the engine RPM needs to decrease at wide open throttle, then increase the pitch. Back to Top What about the Propeller Bore? The most common found in America today is the Society of Automotive Engineers (SAE) J755. The bore taper for most inboards is 1/16 or ¾” per foot of shaft taper. 1/12 is used for shafts 6” and larger and 1/10 is common in Europe (metric system.) Back to Top Constant Pitch vs. Variable Pitch? A constant pitch propeller is a propeller that has the same pitch at each section from the hub to the tip. A variable pitch propeller has different pitch values at each section. Generally the pitch will increase from the hub to a maximum at the 0.70 radius and then decrease slightly out to the tip. Back to Top What is Galvanic Corrosion? Galvanic corrosion takes place when two dissimilar metals are electrically connected and submerged in an electrolyte or sea water. The natural chemical reaction takes place and begins a circuit where the least noble metal (the anode) beings to break down, leaving the most noble metal (the cathode) intact. Remember to protect your cathode by installing adequate sacrificial anodes (zinc.) Back to Top What is D.A.R.? D.A.R. stands for “disc area ratio” and is the developed area of all the blades as a percentage of the total area of the circle described by the propeller. D.A.R. can also increase or decrease load on the engine. Back to Top Do I want my propeller dynamically balanced? Most premium grade propellers are dynamically balanced at the manufacturer. All other propellers are typically static balanced. Repaired or modified propellers can be dynamically balanced as a special order at an additional price. Back to Top What about the installation or removal of a propeller? Heat should never be used to install a propeller and therefore should rarely be used to remove the propeller. Use a certified propeller removal tool and follow manufacturer instructions. If after torquing down puller and the propeller down not break free, a gentle warming with a torch may enlarge the hub and the propeller will break free.