About Propellers

prop page 1As an outboard technician, consultant, and technical writer I am most often asked questions concerning boat performance which in turn is directly related to propellers. Sometimes it is a complaint about poor performance or fuel consumption and other times how to achieve more speed. With either inquiry 99.9% of the time the propeller is the underlying factor.

 

Evinrude publishes propeller booklets and videos, teaches correct propping techniques in the training centers, and engineers a selection of sizes and styles of props for almost any type of boat. Even with all these educational endeavors and prop choices, the same questions show up repeatedly online about propellers and boat performance by unhappy owners.

 

Very often a new boat customer is disappointed in his purchase because of the way the boat performs. The complaints are the same – “the boat has a hard time getting up to speed”, “I can’t pull up a water skier”, “why does the bow rise up so high when starting out?” Each one of these situations may be resolved in part by fitting a different size propeller on the engine. Sometimes the fault lies with the boat builder, at times with the boat dealer, and once in a while with the customer who unknowingly chooses the wrong prop.

 

Manufacturers and boat dealers often test run with very little fuel and only one or two people on board. From those sea trials they determine which propeller to ship with the rig. In the real world, people fill their gas tanks and then add speed-robbing items such as T-tops, thru-hull transducers, bottom paint, accessories, ice chests, extra batteries, and more people with gear. Now with the additional weight the original propeller will not allow the boat to get up on plane easily, plus the motor is lugging which affects drivability at all rpms. Oftentimes the motor is mounted too low on the transom which also slows the boat down and robs efficiency and fuel mileage.

 

rpm range chart

There has been a lot written about propellers so you can GOOGLE propellers then spend days or weeks reading a fraction of the 5 million results. The important thing to know is that an outboard motor does not have a multi-speed transmission like your car. It only has one gear in forward and it is the propeller than governs the maximum rpm and power output that your engine can achieve. An outboard develops its peak horsepower only when the throttle is wide open and only when running at a specific rpm. There is an area on either side of that point called the operating range; that is where the propeller comes in. A prop has to be sized to allow the motor to reach its top rpm within that operating range for best results. This affects the operation at all speeds as an outboard is designed to operate with a particular “load” on the engine.

 

prop page 2What is often overlooked or not explained well about propellers is that each one is designed for a limited range of boat speeds. Blade angles in addition to blade contours and shapes play a big part in how a specific propeller will function.

 

A fast bass boat requires a prop with a pitch to diameter ratio from around 1.5:1 to about 2:1. For example the high-performance Raker II propeller has a 14-1/2″ diameter and pitches ranging from 22″ to 28″.

 

On the other end of the scale a non-planing boat such as large pontoon, barge, or a houseboat requires a slow speed propeller but one that has a large amount of thrust. These types of propellers have a diameter to pitch ratio of 1:1 or less. Most of the props for those applications have a 15″ diameter with pitches of 15, 13, and 11 inches.

 

 

prop diameterprop pitchprop rake

 

 

Evinrude sends their dealers an optimum rpm chart to help with selecting a propeller best suited for the boating application. This is a narrower rpm range than the recommended operating range. The optimum rpm chart is for the best combination of speed, economy, and overall performance of a specific model and year outboard motor. For example referring to the 2012 RPM Quick Reference Chart, a 150hp E-TEC is optimally propped when it can turn 5500-5600 rpm with a normal load aboard. In the absence of a chart like this, the rule of thumb is to allow the motor to turn in the upper half of the recommended operating range.

 

rpm chartchart

 

What is a boat owner to do if he or she is not satisfied with their boat’s performance? The first thing is to find a knowledgeable dealer who will work with them to maximize their boat’s potential. That may be trying a series of propellers and/or changing the mounting height of the engine on the transom. Evinrude offers a loaner prop program to select dealers to allow customers to try various propellers before they buy.

 

Choosing a prop for the best all around performance often means a good amount of trial and error on-water testing. If a dealer is familiar with the boat and motor combination, they may already know which items to suggest. Study the informational links at the end of this article and with a little bit of effort, you can be assured of obtaining the best all around performance of your boat and motor. Remember, always go boating with safety in mind.

 

For more propeller and propping information, click on the following links:

 

Propeller Selection Guide

 

Propeller slip and speed calculator

 

Another Prop Calculator

 

Smart Phone and iPad propeller apps

 

Determine the Gearcase Ratio

 

VIDEO – Science of Propeller Selection

 

VIDEO – Choosing the Right Propeller