Boat Propeller – Mystery and Science
What goes on down under the stern of a boat is a bit of a mystery to most people. Propellers are considered to be expensive, mysterious and fragile. But props are not mysterious at all. However, sometimes even a carefully considered change of prop can make boat performance worse rather than better and this can leave boat owners puzzled.
Before rush off to the boat dealer with a fully loaded EFTPOS card and buy an alternative new prop you need to do some research and gather some ‘data’. If you don’t you are likely to make a poorly informed prop choice because you can’t always just think it through.
Most outboard motors are purchased and used as is. The outboard motor propeller supplied may be suitable for “average” use on an “average” boat but the same prop is sometimes supplied across a range of engine sizes; say 5 hp, 6 hp and 7 hp. Presumably, the 7 hp could handle a slightly bigger prop that a 5 hp outboard?
Smaller outboard motors are often sold with an “Auxiliary” or “High Thrust” propellers as they are usually intended to be used on small displacement hull boats – that is a boat which does not have a planing hull – such as a dinghy. These propellers can shift a bigger boat easier than a similar size prop with less pitch. This makes them useful as auxiliary motors and they also perform well when reversing.
If you want to increase of speed of your boat you need a propeller to suit the motor. Often the outboards engine power is too much for the propeller as supplied but not necessarily too powerful for the boat. This will allow the engine to rev higher than it should. Ideally, the propeller should more or less exactly suit the engine power, regardless of the boat, and drive the boat while the engine is running at its most efficient engine speed.
Example: An outboard motors recommended maximum engine speed is 5000 rpm but it easily revs to 6000 rpm due to propeller being too small a diameter or having too little pitch. This shows that the engine does not have enough work to do as it is able to rev freely beyond what is good for it because it meets with minimal resistance due to lack of load. It’s a little like revving a car engine to the red line with the car in low gear. Changing a prop is just a way of changing the gear of your outboard.
To get the engine revs down you can fit a propeller with either a bigger “pitch” or an increased diameter. A larger diameter is not always possible because of lack of space between the cavitation plate and the edge of the prop on outboards so the only easy option is to change to a prop of the same diameter, but with a bigger pitch. i.e. change from an 8-inch pitch to a 9-inch pitch. Another less common option is to curve the trailing edge of each propeller blade inwards which is more or less the same as increasing the pitch – but that is an experts job.
It remains a matter of doing some tests to find the best propeller for your outboards engine power and what works best on a particular boat.
Do some trials
You need a rev-counter on your motor and a speedo on the boat before you can get meaningful information. ( A smartphone speedo App should work well.) What you are looking to achieve is to get the maximum boat speed with the engine running at a particular engine speed. So, for example, run the engine at 4000 rpm and record the boat speed. Then change the prop and run the engine again at 4000 rpm and note the speed. The boat will either be faster or slower depending on the prop.
Note: Propeller size is usually stamped on the back of a blade.
A propeller marked as 3 x 9″ x 7″ means it has three blades with a 9″ overall diameter and a 7″ pitch on each blade. If you increased the pitch of the propeller to, say, 8 inches (This is always measured in inches and not is metric.) the engine speed, at full throttle might drop by something like 250/300 rpm because the engine is under more load and may not rev higher.
What ‘pitch’ actually means is that, in theory, for every one revolution of the propeller, the propeller will move forward eight inches – for a prop with an eight-inch pitch that is.
If a boat engine is racing easily to full revs it is likely “under propped”, meaning it should handle a bigger prop or one with more pitch.
With an optimal prop pitch, the prop will push harder if the engine has sufficient power to run at its optimal revs. It will simply push more water out behind it. Thus the boat should move through the water faster and it may even use less fuel as the engine is not over revving. Theoretically, the engine will also last longer running at its correct speed.
If you increased the propeller pitch, or prop diameter, by too much the engine may not have the power to rev to its optimal most efficient power rev range and may be overloaded. In this case, you would have an engine that struggles to reach its ideal operating speed, or it may not reach it, and performance will be poor with increased fuel consumption.
You must record both engine speed and boat speed for each prop you try. Once you have this written down you will soon come to a conclusion after a couple of test runs. After all, changing the prop is just the same as changing gear in a vehicle so you need to remember the boat speed and engine revs accurately otherwise there is no point in proceeding with the tests.
You can start with a guess though. For example, if you outboard is easily revving beyond its maximum recommended engine speed you know the prop is either too small a diameter or does not have enough bite. Therefore, the first prop you should try would be a prop with more pitch. (Assuming you can’t fit a larger diameter prop to your outboard.)
Unfortunately, it is expensive to buy props and seldom possible to borrow a range of props so you can conduct some trials but some boat dealers will help you. Most boat owners are stuck with the prop that came with the outboard because they can’t test alternative props or don’t want the bother of changing props. They just opt to put up with less than optimal performance.
The thing that may skew the result is slippage. Because a propeller is winding its way through the water in the same way a screw winds its way through wood, there is inevitably some inefficiency. Also, things like cavitation or exhaust gasses, or even air, getting sucked in around the prop will also reduce the performance of a propeller. Prop design and condition will affect performance also.
Boat hull shape and boat weight will affect boat performance but by using the same engine on the same boat in similar conditions, but with different props, you will get a meaningful result.
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Thanks to Ray F for getting me started on this.
(This page is published for general information only. We do not sell propellers.)