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Back 20 years ago there wasn’t any perceived problem when   selecting an appropriate propeller for a particular outboard motor on a specific boat and how things have changed.

Sure the old 9.9hp, 25hp, 50hp, 90, 140, 200hp have been around for over 40 years and coming from only three USA manufacturers   Chrysler, Mercury and OMC Evinrude/Johnson.

Since then the Japanese brands have become a major force and all the “horsepower” gaps have been filled in between these basic models.

Meanwhile, we’ve seen engine weights reduced, and marine electronics and ignition system made a lot more reliable.

The benefit of improved electronic fuel delivery systems can’t be over-stated in that it made for easy, all- weather start ups and greater confidence to the boat user.

These days with modern era engines we can go out fishing and not having to worry about the engine restarting after we’ve been out there all day.

Outboard motors are now available in 2hp, 3.5, 4, 5, 6, 8, 9.8, 9.9, 10, 15, 18, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 115, 120, 130, 135, 140, 150, 175, 200, 225, 250, 300 and 350hp.

God forbid we’ll see a 375hp in the future, but otherwise you could say the industry has the power band pretty well covered.

However, regardless of engine size the important thing to remember is horsepower needs to be delivered into the water and converted to thrust. And it all comes from a single, shaped rotating wheel called a propeller!

With a multitude of new hull designs and boat construction in a variety of materials the humble propeller has a tall task to perform. It has to perform across a lot of hull types in a wide range of water conditions and be ultra reliable. And this is after having to plough through mud and sand along the way.

VARIABLE DOES IT

The ultimate propeller for this multitude of uses is the “variable-diameter, variable-pitch, variable blade” computer controlled by the engines electronic onboard computer unit and other boating sensors.

Well, even NASA has been able to come up with a propeller like that. And even if it was possible to create such an all-singing, all-dancing propeller the cost would be prohibitive, especially considering the damage that can happen to boat propellers at boat ramps and rocky shallows.

Put simply, the ultimate propeller isn’t going to happen.

We have a saying in our business – “propellers, like life are a compromise.”

So how does a private, family business like Solas Australia tackle this challenge and give its customers the best/correct propeller   application for their needs?

The answer is to draw from a multitude of different blade geometries. And this is based on a historical database of many, many propeller sizes.

The names New  Saturns, Titans, High Rake Titans—4  blade, Lexors and Lexor plus-Scorpion represent a lot of research. The list goes on and all are designed for different boat/motor applications.

Each propeller design has different blade area, blade camber, diameter for pitch ratio and rake. Each design is then chosen for a particular boat/motor application.

The success of matching has progressively got better over the years from  simple trial-and-error to something quite scientific. Thanks to the huge database we have created we pretty accurately match the most effective propeller for the job.

To do this properly we need certain information from the client. We need the make and model/length of the boat. Most importantly we need to know the boat weight, engine brand/model and horsepower. Some engine brands use different gear ratios so it’s important to know exactly what motor we are dealing with.

HORSES FOR COURSES

Then we need to know the application of the boat. We need to know wither its being used. For example is the boat being used mainly social boating inshore, offshore fishing, water-skiing or commercial application with heavy payloads aboard.

If it’s an existing rig we need to know the current propeller   size and diameter/brand. And we need to know the boat speeds through the rev range currently been achieved.

It also helps us a lot to know the general boating environment you’re operating in. For example, deep water, a place where you are going through sandy, or rocky bar crossing. In other words, we need a good picture of the area the boat is operating most of the time.

Armed with knowledge a propeller style is chosen, say it might be an H/R TITAN 3 in say 17P to suit a Yamaha 150hp powered boat.

If the boat was one of those USA made fibreglass bowriders we might instead go for a 4 blade19-P Torquemaster Stainless steel propeller.

We at Solas know from experience that all the millions of dollars worth of boat/motor technology are worth a damn if you can’t   delivered the power to the water efficiently.

A classic example of this would be the new 60hp outboard group.

We all know that outboard manufactures are trying to outdo each other for your hard earned dollars.

The  ever  popular Yamaha 60hp four-stroke would swing in  many boats a standard 11 3/8 diameter x 13 inch pitch propeller. This would push the boat to a speed of 33-34 knots at 5800revs if the rig was something like the Quintrex Hornet Trophy.

Five years later the impressive new 60hp EFI Honda four-stroke with  “blast”  and   VTEC technology swing a much larger  propeller – like a 12inch  diameter  x 14 inch  pitch. This some 20 per cent larger than the propeller we would have used before.

Not  alone  but  new  model Suzuki  60hp  swings a similarly large propeller and offers superior load-carry capacity as well as better low speed planning ability, plus better overall speeds.

This is not unlike the situation we’ve seen in small cars. Sporty little cars like the Subaru now run 17 and 18inch wheels whereas ten years ago they were running 13 and 14in wheels. Pardon the pun, but we’re moving forward quite literally!

In the meantime at Solars we’re constantly working on improving the propellers as technology moves forward.

We are now one of the  largest  producers of  high-quality duplex   24/10 stainless steel  propellers. We also produce compression cast aluminium props as well as the  new high-torque Rubex rubber  bush  system suitable for motors up to 350hp, covered with an industry-first five-year  warranty.

Further information check out the Solas website  www.solas.com.au

BREAKOUT BOX

Cavitation is something you’ll hear a lot about when people discuss propellers and engine performance. Basically cavitation happens when there’s too much power being applied through the propeller.

 At high rotating speeds or under heavy load pressure on the inlet side of the propeller blade can drop below the vapor pressure of the surrounding water. This results in a pocket of water vapor developing also the blade, thereby making it ineffectual.

Cavitation results in wasted energy, and a noisy propeller but can led to serious surface corrosion of the propeller due to localized shock waves against the blade surface.

Some racing propellers (surface running) propellers actually use cavitation to their advantage but generally it’s not something you want in an ordinary outboard motor rig.

The other thing that can happen to a propeller is ventilation – literally sucking air from the surface of the water and causing lost of propeller thrust in the water. In either case you can often solve the propeller by setting the propeller deeper in the water.

In short, cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream.