30 chopper 2 propellor2/24/2023 ![]() ![]() Slip is inevitable, whereas both cavitation and ventilation, when they occur, are detrimental to an engine’s performance and well-being. Many people confuse slip with cavitation, or ventilation, when in fact they are very different kettles of fish. Skewed propellers are ideal for lower horsepower domestic or work engines, where weed or other obstacles are a factor (this shape of blade sheds weed easier), the higher skew in a surface-piercing or high speed application will also greatly reduce the shock sensation as the blade re-enters the water. Instead of being radially symmetrical, in this case each blade is ‘swept back’ in shape, when viewed along the propeller shaft axis. The last common blade ‘peculiarity’ is SKEW. In some instances too, it is equally as effective in reducing the revs of an over-stressed engine. What this does is increase the effective pitch of the propeller, thereby increasing top speed. We have all heard at some time or other of the racer who sits his propeller over the tow-ball on his car and proceeds to ‘cup’ it with a ball-peen hammer. The second performance factor is CUPPING. Standard production propellers normally run around 15 degrees of rake, whereas performance versions can run anything up to 30 degrees at the blade tip. One is RAKE, which is the slanted angle aft (at the tip) from the perpendicular to the shaft axis. There are two other important performance factors, albeit more traditionally associated with high-performance propellers. Decreasing 1″ pitch will increase your RPM by approximately 200 RPM. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the minimum rating given by your engine manufacturer, you would need to decrease pitch. Increasing the pitch increment by 1″ will result in approximately a 200 RPM drop. If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded. ![]() Adjust the motor trim angle for the optimum performance. under normal operating load to determine the maximum RPM you are able to obtain. The correct propeller will prevent the engine from going beyond its maximum rated RPM, yet allow it to reach the minimum rated RPM where maximum horsepower is produced. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).The goal in prop selection is to determine what propeller style and size will maximise performance for your boat, while allowing your engine to operate in the recommended RPM range. The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator’s manual. Thus the slip is 6.06 knots, which is 11.3% of 53.56, so the slip is 11.3%. If there was no slip, the speed would be (5800/1.87) x 21 inch/minute x brackets, the units of inch, minute, ft, nautical mile and hour cancel out, leaving the speed in knots. Here’s an example: A top speed of 47.5 knots is achieved at 5800 rpm with a 1.87:1 lower unit gear reduction and a 21-inch pitch prop. More mathematically inclined readers may care to calculate the slip at full throttle on their own boat. The ultimate speedsters like racing hydroplanes have propellers with little slip, that gets down as low as 3-5% in some instances. Slip varies through the range but a general rule of thumb puts displacement boats at about 30%, planing hulls at 20-25%, twin rigs at 15-20%, and faster sports and performance boats at 10-12%. A propeller with 20% slip can in some instances be quicker than one with 15%. ![]() Slippage is usually expressed as a percentage of the pitch. For example, a 21-inch pitch propeller would propel a boat 21 inches forward with each revolution it made, less an allowance for what is called Dzslippagedz. ![]() This amount is determined by what is known as the PITCH of the blade. These blades are attached and shaped at such angles that with each revolution the boat is propelled a certain distance forward. A propeller consists of anything from two to eight blades that are attached or moulded, onto a central hub. ![]()
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