Reviewed: May 2009
Author: Brett Delaney
Fitting propellers is relatively easy and you will never realise the full potential of your boat until you have the correct prop.
Ignore the salesman’s pitch and study your propeller’s instead.
Boating Article by Modern Boating
You have just bought a flash new boat. You have spent countless hours researching which hull shape suits your needs, obsessing over the deck layout and picking the correct power plant. But what about the prop? If you fit the wrong propeller to your motor, your seemingly well-considered package will under- perform. If you fail to give due consideration to this vital element, all of your hard work could be undone.
Differences in both propeller styles and pitch size can dramatically affect the performance and handling characteristics of your boat. Fuel economy, top speed, hole shot, planing speed, turning performance and ability to run through chop can all be influenced by the type and size of prop you are running. The bottom line is that you will never realise the full potential of your rig until you have the correct prop fitted to your motor. It is all well and good to hear the theory regarding props and boat performance, but we wanted to roll up our sleeves and experience the effects of using different props firsthand. To this end, our esteemed editor organised a prop testing session using the Modern Boating rig so we could put our theory to the test in the field by obtaining hard data that would allow for direct comparisons of the performance characteristics of different props.
When it comes to choosing a prop for your boat, you have two basic decisions to make. The first is what style of prop suits your needs. Propellers for trailerboats are usually made of either stainless steel or aluminium, have either three or four blades and may also have a number of other performance characteristics such as porting on the blades. The combination of these factors will make a particular model of prop more suited to some uses than others. The second decision is to choose the propeller size in the chosen model that will optimise your boat’s performance. When people talk prop sizes, they commonly refer to the prop’s ‘pitch’ size (for example 17in), which refers to the distance the prop will travel in one revolution moving forward.
The right prop for your boat will be highly dependent on the individual characteristics of your boat and what your priorities and uses are. For instance, a waterskier may value hole-shot performance and cornering over top-end speed, whereas a competition bass angler will be looking for maximum top-end speed above all other considerations. Most fishing and cruising boaters are looking for a good blend of performance and economical fuel usage.
Stainless steel props offer superior performance to aluminium props due (among other factors) to their lack of flex, which provides greater thrust. However, this same lack of flex can spell big damage to your gearbox if you collide with an immovable object. People operating in remote locations may prefer aluminium to minimise engine damage if they hit something. Aluminium is also a lot cheaper to replace.
Modern Boating’s boat is a 6.1m Sea Fox 206CC centre console, powered by an evinrude e-teC 150hp 60° V6 outboard motor. We were looking for a prop that provides good performance and fuel economy. The boat is fitted with a 17in evinrude Viper, which is a versatile three- bladed stainless steel prop designed to offer all-round performance. At the end of our testing, this still proved to be the best prop for the boat.
The test venue was Sydney’s Middle Harbour. The water was smooth and the tide was dropping. Tidal flow and water conditions affect test results, so it is important to conduct each prop test in similar conditions. We had half a tank of fuel and two people aboard—remember that any dramatic weight changes will affect your results.
I had Arpi Szoboszlay from BRP on board to provide the technical know-how and a selection of different BRP prop models in a variety of pitch sizes. We decided to test three models in equivalent pitch sizes to determine the prop style that optimised the boat’s performance. We would then play around with pitch sizes in the chosen model until we found the best top-end speed and all-round performance.
We conducted the tests by following the same course and recording data at 500rpm intervals from idle to wide-open throttle (WOT). We used 50 percent trim for most of the rev range and moved to 100 percent trim for WOT. at each interval, we recorded speed (miles per hour—mph), fuel flow (litres per hour—lph) and economy (miles per litre—mpl). We repeated this for each prop and compared the numbers. We also put each prop through some high-speed turns and took note of general ride characteristics. I have only noted the WOT readings below in order to highlight top-end speed.
However, if you mostly cruise in your engine’s middle range, it makes sense to analyse those numbers more closely.
Our Boat: Sea Fox 206cc
Engine: Evinrude E-TEC
Conditions: Smooth Water
Wind: 5-10 Knots
The 17in Viper was the prop already fitted to the motor. This three-bladed prop is designed to give great all-round performance—and this is what our tests showed. WOT (5700rpm) produced 46.9mph, fuel flow of 61.9Lph and an economy rate of 0.75mpL. This prop cornered reasonably well, gave acceptable hole shot and produced great top-end speed at a good economy rate.
Now this was an interesting test! The Cyclone is a four-bladed prop and you could certainly feel the extra bite when this prop was fitted.
The handling of the boat was very different to the Viper. The bow lift and hole shot were whiplash inducing, the ride was smoother, the boat tracked straighter through chop and made faster and more stable turns. However, all of this came at a cost—namely a reduction in top-end speed. WOT (5200rpm) achieved 44.9mph but the engine failed to reach an optimal rpm range at this pitch size.
The Rebel SSP is a large diameter, entry-level, stainless steel propeller designed to offer efficient cruising and all-round performance. We found that this prop provided better hole shot than the Viper and was fast planing, which provided good fuel economy at cruising speed and middle range rpm. However, the Rebel had the lowest top-end speed of the test, giving us 43.4mph at WOT (6200rpm).
We decided that the Viper provided us with the best mix of performance and economy for the Sea Fox. We then tried this prop in a larger 19in pitch size. We found that at this larger pitch size, the boat had a much slower hole shot but achieved a slight gain in top-end speed and fuel economy. Top speed at WOT (5200rpm) was 47.2mph, fuel flow was 57.4Lph and economy was 0.81mpL.
At the end of the day, the results declared that the boat’s original 17in Viper was our winning prop as it offered the best mix of hole shot, top-end speed and economy.
This series of tests was carried out using only BRP propellers and the results are very boat specific. However, the results do highlight a number of findings that apply across the board and should be kept in mind when you are trying to obtain the best prop for your needs.
As a general statement of principle, an increase in pitch size will result in a decrease in rpm and hole shot performance but an increase in top-end speed. Conversely, a decrease in pitch size will increase rpm and improve hole shot performance but will decrease top-end speed. If fuel economy is your main focus, you want a prop that achieves the speed at which you spend the majority of your time with the lowest rpm reading.
Different styles of props offer different performance characteristics when compared to other similarly sized models. However, the above ‘propeller principles’ should still apply when you increase or decrease the pitch size of the prop within a particular model.
Of course, these rules have limits and I have not even touched on the issue of how engine-mounting configurations can affect performance.
But there is a limit to how much one person can prattle on about propellers in one sitting.