Propeller ventilation & Propeller cavitation are covered on the RYA Powerboat Advanced Course. Propeller Ventilation is also covered briefly at Powerboat Level 2 when we teach planing speed handling. It is a subject that often comes up on RYA Powerboat Instructor Courses. The two phenomena are often confused with each other.
Propeller Ventilation (aka Aeration)
An outboard engine running at Wide Open Throttle (WOT) will typically operate at 4,700 to 5,500 rpm, the engines gearing will reduce this to something more like 2,000 rpm at the propeller. If you were to run the same engine out of the water, the prop would spin much faster as it would not have the friction of the water slowing it down.
When our propeller travels through aerated water the same thing happens. Aeration is commonly caused by exhaust fumes from your engine, driving with the engine leg trimmed near the surface, driving through a rough sea state or wave jumping. Powerboats that make sharp turns and try to cross their own wake sometimes suffer from ventilation. Simply put, ventilation is a propeller spinning in air rather than water and can be compared to a vehicle not having traction with the road as it tries to grip on ice. It’s not unique to powerboats either. I have been on plenty sailing yachts, motor sailing to windward in less than desirable conditions and the aerated water combined with heel has caused ventilation (the solution is to reduce heel or just sail). The rest of this article however refers to powered craft.
When ventilation occurs you will hear the engine rev faster and will feel the speed drop suddenly as the propeller looses grip in the water. This can be solved easily. The first step is to drop the revs and let the propeller bite in the water again. If ventilation occurs repeatedly then trim the engine leg lower, drive a little slower and try to avoid getting airborne. Repeated ventilation is often a sign of poor helming. Remember to trim the engine down before cornering in rough seas.
On very small displacement craft simply sitting all your passengers too far forward can lift the propeller too close to the surface and cause ventilation.
Outboard engines and sterndrives are fitted with a horizontal plate above the propeller which reduces the amount of air being drawn down from the surface. I call this plate the anti-ventilation plate. However it serves other purposes and also reduces cavitation (explained below), some manufacturers refer to it as the anti cavitation plate, anti-cav plate, cavitation plate or cavitation fin. The anti ventilation plate usually runs from forward of the leg to aft of the prop. The plate is highlighted in the image below by the stick resting on it.
Some boats have two plates, the higher one covers the forward 2/3 of the leg and the lower one the aft 2/3 of the leg so they overlap.
The image below shows an anti ventilation plate fitted to an E Propulsion engine.
The image below show a pair of fins (aka wings) that have been added to the anti ventilation plate in order to encourage early planing and therefore better fuel economy. I have tried them on two of my boats (an Artic 22/Yamaha 250HP and a XS700/Mercury 200HP). The fins did not appear to help on either so I removed them. One of the pairs I gave to a friend with a XS 650/150HP and he swears it has helped his fuel burn, so like many propeller modifications its a case of trial and error and what works on one boat might not work on another.
If you have followed the earlier advice (trim down and drive sensibly) and are still suffering from ventilation then there may be something wrong with your set up and the engine may be hung at the wrong height or may be the wrong leg length for your boat. The image below shows the correct height that your engine should be hung. The anti ventilation plate (the top one if you have two) should be inline with the keel of the hull or very close too it.
Propeller Cavitation
While ventilation is pretty simple to understand, cavitation is slightly more complicated. It occurs when the spinning propeller creates an area of low pressure (of water). The boiling point of water drops in lower pressure so water is literally boiling/evaporating and small vapour bubbles are formed. The vapour bubbles are transported by the flow of water to an area of high pressure where the boiling point is raised so the bubbles start to condense and literally implode. As these bubbles meet the blade of your propeller and implode they give off a shock wave of energy which can cause damage to the surface of the propeller blade. The damage is often described as pitting and can feel and appear a bit like the surface of orange peel.
For those after a bit more technical detail. As every GCSE school student will tell you the boiling point of water is 100°C, however they may be unaware, that assumes one atmosphere of pressure. So at “normal” air pressure (1013.25 mB), water evaporates at 100°C, however at a higher pressure the boiling point goes up and conversely at a lower pressure the boiling point (evaporation temperature) goes down. Thus in the low pressure scenario that we are discussing the pressure could for example be 23.7 mB. At this pressure the boiling / evaporation point of water will have dropped to just 20.5°C.
Propeller cavitation is in theory solved by your propeller designer who selects the correct pitch to eradicate it and maintains an even pitch across the blade (or a carefully designed change in pitch), however various things can bring cavitation back into play. Poorly positioned skin fittings in the hull can contribute to cavitation as they cause a disturbance in the flow of water (and therefore pressure). Likewise a damaged propeller with now varied pitch can cause it. The anti ventilation plate mentioned above helps by reducing the low pressure on the top side of the propeller.
Cavitation can be detected by the experienced driver, it will often be recognised by noise and vibration at a particular rpm. The temporary solution is to not drive at that RPM but the long term solution is to identify the source, whether it is a damaged blade, a damaged plate or something upstream of the prop such as skin fitting. I previously managed a motor boat that suffered very costly cavitation to the starboard prop. We solved it by moving a large sacrificial anode which the former manager had fitted just forward of the propeller. One proven method of avoiding cavitation is to keep your prop(s) in good condition.
The image below if of a PWC (Jet Ski) impeller and the pitting on the back edge of the blade is the result of cavitation. The water flow was disturbed as the water jet intake was damaged.
The propeller below shows considerable cavitation pitting.
The image below shows a broken anti ventilation plate. This was caused by the owner hammering on the plate when trying to remove the lower leg for a service. Interestingly the boat then suffered both propeller cavitation and ventilation. The plate can be repaired, the simplest way is often to attach the fins described above.
Doug Innes
RYA Advanced Powerboat Trainer