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Performance Tuning Propellers
  |  First Published: July 2006



No matter how careful you are, one day you are going to hit a sandbank or bit of flotsam in the water and do some damage to your propeller. Many boaties just ignore this because the boat will still get onto the plane, so in their minds everything must be all right.

Even new propellers aren’t always in tune – mass-produced propellers can have manufacturing defects, be poorly made or get knocked or bumped when the motor is being mounted.

With the number of custom boats on the market, high degrees of difference exist in the ways that particular boats ride in the water. Some have a little too much weight in the transom because of the fuel tank and have trouble getting up onto the plane. All these problems can be reduced by performance tuning your propeller to your boat.

Many of the problems that affect a boat’s ride and how it handles often result from the motor and the propeller. Having a propeller that isn’t perfectly in tune can cause all sorts of problems for your motor. Vibration will promote wear on the motor and could cause terminal damage. A poorly tuned propeller can have a negative impact on fuel consumption, which can be quite expensive in the long run.

Boats the struggle to get out onto the plane or cavitate when they turn or accelerate often have a poorly tuned propeller.

CAVITATION ALLEVIATION

Cavitation is caused when air bubbles form around the propeller and it loses its grip on the water. With the decreased load, the propeller will initially speed up and then bite in again as the water reforms around the blade. This can be taxing on the life of your motor, which could be a problem if you are crossing a bar and need to gun the motor to get out of a trouble spot. The delay in acceleration could be the difference between getting out of the trouble area or getting swamped. Cavitation is often caused because each blade of the propeller is not at the same angle or pitch. Cavitation is usually noticeable when the motor is under the most load such as cornering or accelerating onto the plane.

Performance tuning your propeller ensures that all blades are correctly in tune, which will remove problems with cavitation, vibration and poor fuel consumption.

PERFORMANCE PLUS

Most outboard motors are designed to reach around 6000RPM at full throttle. If your motor is running at a lot less than 5200RPM flat out, then you’re putting a lot of strain on the motor and the gearbox.

With higher RPM there is less pressure on all the drive gears and less wear. It’s like riding a pushbike up a hill in high gear – it takes a lot more effort to make the pedals turn and propel the bike forward. Once you drop into low gear the pedals turn more easily and the pressure on your legs decreases creating less wear on your legs (the fuel) and the mechanics of the bike.

If a motor isn’t running at a high enough RPM then the pitch (angle) of the blades needs to be decreased to make it easier for them to turn in the water. If the RPM is too high then the boat will have trouble reaching top end speed and will cavitate.

Tuning your propeller to suit the drive characteristics of your boat will increase performance, decrease fuel consumption and maximize the life of your motor.

How much each of these factors is affected will depend on the state of your propeller. One friend of mine used his motor for three years before I convinced him to have the propeller checked. The boat was using more fuel than I thought it should have and was cavitating in tight turns, which he thought was normal for heavy fibreglass boats. After having his propeller performance tuned the handling was better, cavitation was eliminated and there was almost a 25% saving in fuel. An average trip went from around 100L of fuel to 75L. With a saving of almost $30 a day it only took a few trips to recoup the money spent on the propeller tuning.

That propeller was performance tuned at Performance Propellers in Bulimba a few years ago. Recently, the propeller on my Yamaha 60hp 4-stroke was suffering a few wear marks on the blade from air bubbles caused by the small breather hole on the leg. These bubbles were getting caught in the camber (scoop) of the blade where the friction was wearing away the aluminium blade. Even after almost three years of use, these were the only pieces of paint off the blade and were a sign that something was wrong. I had been considering getting the propeller done for a while but the deciding factor came when I hit a sandbank at Rainbow Beach. Although the propeller wasn’t badly damaged, it was out of tune and the small increase in engine vibration was a definite indicator.

Colin Lough, from Performance Propellers, will do a free appraisal on your propeller to see if it has any faults and will ask you a few questions about top end speed and RPM, type of hull an any other contributing factors. If you keep a log of fuel usage and distance traveled before you have your propeller done then you can work out your fuel saving afterwards.

Colin provides a computer-generated Prop Scan EPS propeller report, which highlights any problem areas. To compile this report, the radius of each blade of the propeller will be split into tenths and measurements are made on the outer radii of each blade with 0.7 being 7/10 of the way out from the hub and 0.8 being 8/10 of the way out etc. A minimum of four radii out of the ten are measured on each blade and then compared. Each blade has at least 16 local areas checked to determine local pitch (pitch of one portion of the blade), section pitch (mean pitch of one radius of the blade) and blade pitch (mean pitch of one blade) measured. It’s all a bit technical but Colin will give you with all the written information so you can interpret the Prop Scan bar graph report you receive after getting a propeller checked. If you do decide to have your propeller performance tuned, you will receive another report afterwards so you can compare the before and after.

You won’t know how much better your propeller and boat performance can be until you have a Prop Scan report done. Sometimes dramatic changes need to be made and at other times it will just be a subtle adjustment. Badly damaged propellers, even with chunks missing, can be repaired at a percentage of the cost of a replacement propeller.

I was very impressed with the difference a performance tune made to my propeller and best of all it was explained to me so I could easily understand what the problem was and how it could be fixed. The results spoke for themselves. If you would like any more information then give Colin a call on (07) 38991501 or drop your propeller in to have a free appraisal done.

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