Propeller FAQs

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1. Will a propeller change help my I/O installation?
2. Why won't my sterndrive perform as well as another sterndrive of comparable length, beam, weight, and I/O installation?
3. Does it make any difference where the outdrive is vertically positioned on my sterndrive?
4. My houseboat plows when the sterndrives are tilted to the furthest lock bar position. In an attempt to correct the bow-down attitude, the propellers cavitate. Why?

Will a propeller change help my I/O installation?

Generally the boat manufacturer exhaustively tests the various models to insure that the boat performs properly. If, however, the owner increases the boat displacement by installing heavy optional equipment, a different propeller will be required. In almost all cases, the propeller to be used would be comparable in diameter but have less pitch.

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Why won't my sterndrive perform as well as another sterndrive of comparable length, beam, weight, and I/O installation?

Differences in hull design, such as hull dead rise, position of center of gravity and transom angles can account for significantly different performance. Transom angle is important since it governs the maximum tilt setting of the sterndrive and consequently the shaft angle.

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Does it make any difference where the outdrive is vertically positioned on my sterndrive?

Generally it is best to mount the outdrive on the transom so that the cavitation plate is located approximately 1” below the bottom of the keel. (Or 1” below the hull if there is no keel.) Sterndrives on houseboats are generally located deeper in the water and may have strut extensions. High performance ocean racers will position the cavitation plate even with or above the hull bottom for best performance.

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My houseboat plows when the sterndrives are tilted to the furthest lock bar position. In an attempt to correct the bow-down attitude, the propellers cavitate. Why?

The bow-down attitude results from the longitudinal position of the center of gravity being forward of midships. Merely changing shaft angle will not correct the severe plowing that would be experienced. As the lower unit is tilted toward the furthest lockbar position, effectively changing the shaft angle, the depth the propeller is immersed is decreased and ventilation will occur causing a breakdown of thrust. This phenomenon is sometimes mistakenly termed cavitation.

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