TEAGUE TECH [continued from page 8] rake. Generally speaking, cleaver style propellers create stern lift and round ear propellers help to lift the bow. Surfacing propellers have less leverage influence on boat attitude compared to propellers that are running deeper. Assuming that vee hulls and catamaran sponsons have bottoms that are on an angle (deadrise), this is what creates the propeller rotation influence. It is the “clean” water that is hitting the propeller blades that influences hull attitude. If the propeller is rotating out, the downsweep of the propeller blades “sees” more clean water that the upsweep. This causes stern lift. On the other hand, when the propeller is rotating inward (starboard: left, port: right), the upsweep of the blades “see” more clean water which pulls the stern down and results in bow lift. Are you confused yet? Let me provide some common scenarios. With conventional (non-stepped) veebottoms, most were originally built with the drives running pretty deep with the propellers turning out. Most of these boats are side-by-side installations with the drives about 36 inches apart. This configuration helps the boat to get on plane by lifting the stern and results in predictable maneuvering around the docks. A typical boat with this application would be like a Baja Outlaw, an older Formula or Scarab. In some cases, changing the propeller rotation on these boats results in better top-end performance, especially if shorter drives are also installed. Stepped bottom vee hulls traditionally have the drives installed higher and the boats are designed to run with a flatter attitude. Included would be boats built by Fountain (later models), Outerlimits, Nordic, Nor-Tech, Formula FasTech, some Cigarette models, to mention a few. Most of the hulls work better and are faster with the propellers turning in for optimum performance and handling. This is partially because the propellers are lifting the entire hull and the aerated bottoms are reducing the wetted surface. Catamarans are a whole different set of circumstances. But the fact remains that in most cases, propellers spinning in will create bow lift and propellers spinning out will create stern lift. Propeller rotation can also influence turning attitude. For speedboat.com
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example, most SuperCat race boats turn the propellers outward for maximum top speed and better turning in racing conditions. Catamarans without center pods such as Skater, MTI, Mystic, and others work well with either rotation depending on the intended use and depending on the propeller blade rake. Catamarans with center pods usually run with the propellers turning in to create more air pack in the tunnels and minimize midrange porpoising.
So, what is rake? Propeller blade rake is essentially the angle that the blades are swept back in relation to the propeller hub. Most “Bravo” style propellers are only available in the rake associated with the particular propeller model. Racing cleaver propellers are available in standard rakes from 15 to 21 degrees. In general, the higher the rake, the more lift a propeller creates. If the propellers are turning in, it will be bow lift, and if they are turning out, it will be stern lift.
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