Got a call the other day from an old mate having teething problems with a new motor on his Southwind longboat, which he has had for a few years. Maximum rpm was just 4500, nowhere near high enough to allow the unit as a whole to operate efficiently.
He enquired back at the point of purchase of the outboard about this dilemma and the salesman advised him that “that propeller was the correct and only one that should be run on that motor as per the manufacturer’s specifications”. End of story.

My mate came away from that meeting none the wiser and with a boat that still operates poorly.
Following are some of the questions that should have been asked by the salesman:

• You have a particular boat: what sort of fishing or boating do you intend doing with it?
• When you take it fishing or boating, how much payload are you carrying? (Payload is made up of fuel, people, tackle or dive tanks, iceboxes, etc.)
• Do you need a slow troll speed?
• Do you run the boat in predominately choppy or rough conditions where a slow planing speed would be advantageous to the economy of the rig?
• If you need more power to get “out of the hole”, can you sacrifice some of your top end speed?
• What is the maximum rpm the motor can achieve when trimmed correctly as it is now?
• While outboard manufacturers tout particular propellers being suitable for a motor, they have no way whatsoever of predicting how their engine will be used and under what circumstances. They can only generalise. In reality there is plenty of scope for improvement.

Let’s look at the dynamics of a propeller to begin with…

PROPELLER SIZE BY

The size of the screw (propeller) is usually stamped on the hub or a blade of the propeller. It will be read as two sets of numbers, the first being the diameter and the second being the pitch. The diameter is measured from the centre of the hub to the blade tip times two, or the measurement across the circle the prop would make when spinning.

PITCH

This is the angle of the blades in relation to the hub on which they are moulded. Theoretically, pitch is the distance that the propeller moves forward after completing a full circle. In reality though, there is often a small amount of slippage inherent in this physical function so that distance is not always true to the stated pitch on the propeller.

Just looking at a propeller will give you an idea of its potential. Blades angled sharply so that they run more along the hub offer a high pitch. Look at a ceiling fan and note that the blades are near flat and at near 90-degrees to the central hub. This is because the ceiling fan relies on its diameter to get the job done. High pitch blades on a 48in ceiling fan would create a force 10 gale and the electrics would suffer premature burnout from the undue stress placed on them when attempting to turn such a contraption. An overpitched prop on a boat could give you the same result.

By understanding the mechanics, we can see, for example, why large houseboats can use small four-stroke motors to drive them through the water — they run large-diameter, low-pitch propellers that allow the motor to run efficiently at its correct rpm range.

BLADE SHAPE

Many blades are cupped at the point of manufacture. They have turned-up lips on their trailing edges that offer more bite or grip in the water, which equates to improved holeshot and the ability to trim the motor out so the propeller runs closer to the surface without ventilating. Ventilation is evidenced by increased rpm but falling boat speed. Propellers used for high-speed travel have dominant cup shaped propellers.

CAVITATION

This is often confused with ventilation. Cavitation is caused by the extreme reduction of pressure on the back of the blade, which causes the water to boil. Most propellers cavitate to a certain amount during normal operation but excessive cavitation can erode propellers. I have seen enamel paint burnt off propellers due to excessive cavitation, which can be caused by the installation of an incorrect propeller for the application, damaged blades or incorrectly-pitched propeller usage.

There are a number of types of propellers in the marketplace. Alloy and stainless steel are common; less common are plastic types that offer the ability to replace single blades should they become damaged.

Propellers are available in twin-, three- and four-blade. The latter is worth investigating if you have a rig that is low down in the stern or perhaps you traverse rough or choppy water much of the time. With so much “bite” in the water, the four-blade propeller allows the boat to hold a good attitude at a slower planing speed therefore making it more economical when travelling in those conditions.

And how is my mate’s boat now? Well, he’s still scratching his head but won’t be for long. I have put him on to a responsible expert: Steve Evans from Solas Propellers. Steve’s the man!

By Rick Huckstepp, as featured in Trailerboat issue 191.