Inboard Propeller Diameter Notes
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Inboard Propeller Diameter Notes

Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed.  Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

Most of the Glen-L plans will have a list of recommended underwater hardware suited to the specific model.  This will include the strut, maximum propeller diameter for the strut, and propeller bore and diameter.  The propeller pitch is not specified as the pitch will be selected to suit the engine and transmission installation.

Most designs are light enough that a reduction gear is not required.  If you choose to install a down-angle transmission, be aware that they are not 1:1 ratio transmissions and have a slight reduction ratio.  The reduction ratio usually requires a larger diameter propeller to consume the added propeller torque produced by the reduction gear.

Most of the time, builders will add 'a little bit more prop pitch than a direct drive'.  This approach doesn't always work and the engine will exceed its recommended WOT (Wide Open Throttle) RPM even though the steepest pitch propeller has been installed.  The only solution to shed some RPM is to install a strut with more drop to allow enough clearance for a larger diameter propeller.

Before selecting a down-angle transmission, you may want to check the optimum propeller diameter for your transmission and make sure that the strut you plan to use has sufficient drop to accomodate the larger diameter propeller required by a reduction gear transmission.

The following screenshot demonstrates how to determine diameter from HP and RPM using Crouch's Diameter-HP-RPM Formula.  We've expanded the calculations to make it easier to follow along :


  • The recommended propeller diameter for a 400 HP engine rated at maximum 4500 RPM equipped with a 1:1 ratio transmission would be 13 inches.  Propeller torque is 453 ft/lbs.  If we wanted to fit an 8 degree down-angle transmission with a 1:21 reduction ratio, propeller diameter would have to be increased to 15 inches.  Propeller torque has increased to 548 ft/lbs.  This is a 95 ft/lb gain over the 1:1 ratio transmission. Notice that propeller torque and diameter increase as the reduction ratio increases.

    For example, if the recommended strut for the design we're building is the Glenwood Speed Strut 90-122 , the specs state that the strut drop is 7 1/2" which will accomodate propellers to 14" diameter.  A 13" diameter propeller with a 1" bore with pitch/rotation to suit engine is also recommended, but remember that this is for a 1:1 ratio installation.  We'll have to find a strut with more drop to provide the clearance for a 15" diameter propeller.

  • Increased propeller torque generated by the reduction gear will require a larger diameter propeller to load the engine so it operates within its maximum RPM range.  Fine tuning of max RPM and speed can be done by increasing or decreasing propeller pitch.

Rules of thumb

  • One inch in diameter absorbs the torque of two to three inches of pitch.

  • Every two-inch increase in pitch will decrease engine speed by 450 RPM, and vice versa.

  • At low to moderate speeds (35 knots or 40 MPH) the slower the shaft RPM and the larger the diameter the more efficient the propeller will be.  Smaller diameter, high pitch-ratio propellers are better suited to speeds above 35 knots.  Pitch ratio = Pitch / Diameter

  • For a cupped-bladed propeller, decrease the pitch by one inch or 5 percent, whichever is greater.

Propeller Shaft Diameter

If you're considering a reduction gear with a 1" propeller shaft and prefer to have a wider selection of propellers to choose from, you may want to increase the shaft size to 1 1-8".  Here's are links to several propeller suppliers :

ACME Ski Boat Props

OJ Propellers

Related Links

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Building the Glen-L Hot Rod :      Revised 05-APR-2013