|Inboard Propeller Diameter Notes|
|*** WARNING *** This page is Under Construction !|
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 :
Rules of thumb
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 :
Building the Glen-L Hot Rod : www.boats.chelseacoachworks.com Revised 05-APR-2013