Ventilation

AIR FROM THE water surface or exhaust gases from the exhaust outlet drawn into the propeller blades causes ventilation. When this situation occurs, boat speed is lost and engine rpm climbs rapidly. Ventilation is often mistakenly called cavitation, see right, above. Ventilation occurs most often with engines mounted high on the transom, when engines are over-trimmed or during sharp turns.

So slow boats running low-revving engines – think displacement craft – tend to swing large propellers while highspeed planing craft, including outboard-powered trailer boats, run smaller diameter props.

The prop size increases with engine horsepower. For example, the propeller on a 250hp outboard is considerably larger in diameter than the propeller on a 50hp outboard.

Outboard manufacturers fit or recommend a certain diameter propeller for each of their engines, so it’s usually

the propeller pitch with which people experiment.

In the same manual Mercury defines pitch as “the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood”. For example, a 21-inch pitch propeller would move forward 21 inches in one revolution.

A good boat propeller pitch analogy is a motor car’s final drive ratio. The lower the ratio, the more pulling power – or torque – the car has from a standstill. The same is true with a prop: the lower the pitch, the better your boat’s hole-shot, but at the cost of top speed. The lower pitch allows the engine to reach maximum rpm at slower speeds.

Conversely, a higher pitch means a higher top speed, but slower acceleration. However, if you use a propeller with too large a pitch, the motor simply doesn’t have enough horsepower to turn the propeller adequately so the revolutions per minute drop. This is called over-propping; overloading the engine this way places excessive wear on the engine.

The opposite is also true. Under-propping – fitting a propeller with too low a diameter and/or pitch – can damage your engine through over-revving. It will also adversely affect fuel consumption.

Ideally an outboard engine under load should reach the manufacturer’s recommended maximum revolutions at wide open throttle ( WOT). Maximum revolutions at WOT is usually given as a range spanning a few hundred rpm. The trick is to find a propeller that delivers acceptable acceleration and top speed while allowing your outboard to reach its recommended revs at WOT.

CHANGING YOUR PROP

With a few simple tools the mechanics of changing the propeller is easy, but choosing the right replacement is an art.

Outboard manufacturers supply a range of propellers to suit their engines, as do independent manufacturers. But with so many propeller options, it can be tricky to choose the right one. Every boat is different, and every operator has different performance parameters, so getting it right can involve numerous propeller changes. Sometimes it’s best left to the experts.

If, however, you’re keen to change the boat’s propeller yourself, there are a few things you need to know: of blades, its diameter and pitch. Diameter and pitch are usually stamped somewhere on the prop.

You’ll also need to run the boat with a typical load, trimmed for best speed, to discover the engine’s rpm at WOT.

In general you are looking to change the propeller pitch. According to the experts, there’s a simple rule of thumb to follow when experimenting with pitch.

At WOT, increasing the propeller pitch reduces engine revolutions by roughly 200rpm per inch of pitch. In other words, when switching from a 23 to a 25-pitch prop, the maximum revs will drop approximately 400rpm. The reverse is true when going down in pitch size.

If you want more torque for better acceleration and faster hole shots, or your current propeller is not allowing the engine to reach its recommended rev range, look to reduce the propeller’s pitch.

On the other hand, if the propeller you use is allowing the engine to over-rev, fit a propeller with a higher pitch. The correct propeller choice will prevent the engine from over-revving, yet allow it to reach the minimum revolutions where maximum horsepower is produced.

Incidentally, fitting a lower pitch propeller to an over-propped engine that’s not achieving its recommended maximum revolutions can sometimes result in a higher top speed as well as better acceleration, since the re-propped motor can now reach those revolutions.

ALUMINIUM OR STAINLESS STEEL?

Many outboards are shipped with cheaper aluminium propellers, but most larger engines come with stainless steel propellers.

Aluminium propellers are fine for smaller outboards and a good choice where frequent propeller damage is likely. Aluminium propellers deform more easily so striking an object with the propeller is less likely to damage the engine’s drive chain because the relatively soft propeller absorbs most of the shock.

But aluminium propellers have their limitations. The blades can bend so much when the propeller is in use that the overall size of the propeller reduces by approximately one pitch size.

Propellers that flex are less efficient, which impacts on vessel performance, making aluminium propellers less desirable for high-powered engines.

Stainless steel is a stronger, more durable material than aluminium, so stainless propellers flex much less, delivering better performance. Stainless propellers provide more bow lift than equivalent aluminium propellers, giving a smoother high speed and rough water ride, and come in larger pitch sizes.

THREE BLADES OR MORE?

Most outboards are fitted with three-bladed propellers, which are perfectly adequate for most trailer boat applications, but outboard propellers come in four and five-bladed versions too.

These were developed for modern, high horsepower outboards which tend to be mounted higher up on the transom than smaller motors. With the propellers running close to the water’s surface, the extra blades help to provide bite and stability at high speed.

Propellers with extra blades generally provide quicker take-offs and hold the boat on the plane at lower revs, which is useful in many boating applications. The trade-off is top-end speed: a good three-blade propeller is normally a couple of knots faster at the top end than a multi-blade propeller of the same pitch.

DAMAGE AND REPAIR

Propeller damage – nicks, chips, rolled edges, bent blades or distortion of any sort – adversely affects boat performance. Blade erosion from cavitation or wear that reduces blade size will allow the outboard motor to rev beyond its recommended operating range, risking engine damage. In addition, distorted blades cause vibrations that can be harmful to the engine and drive train.

Accidents happen and propellers regularly sustain damage by striking objects in the water or contacting the bottom. In extreme cases, catastrophic engine and/or gearcase damage can result, but usually it’s just the propeller that carries the scars of the encounter.

Unless distortion is unusually severe, aluminium and stainless steel propellers can be repaired. But it’s a skilled job and in many cases, it’s cheaper to fit a new one than to repair the old – particularly with aluminium props which cost less than half the equivalent stainless steel propellers. B