Propellers on Sailing Yachts
Technical Feature by Chris Hares, Darglow Engineering
Ever since inboard auxiliary engines were first fitted into sailing yachts, it has been appreciated that, although the benefits of having an engine are considerable, the sailing performance is adversely affected by propeller drag. On early heavily built yachts with relatively low hull speeds this was of no consequence. Also the power available was low and hence propeller sizes were small, so drag was minimal. As hulls developed and became sleeker and faster under sail the adverse effect of propeller drag became increasingly obvious. Also engines improved, giving more power from smaller physical sizes. This required larger propellers in order to use the power efficiently, hence greater drag under sail.
Today’s modern yachts have enormous resources spent on them to improve hull, keel, rig and sails, all in order to increase potential sailing performance. Then the builder sticks a whacking great propeller onto an exposed shaft, effectively cancelling the benefit of that development. The result is like tying a bucket onto the stern rail and dragging it along when the yacht is sailing. Not only does it reduce sailing speed, but it also causes turbulence from the propeller resulting in unpredictable effects on the rudder surfaces making steering problematic in many instances.
The drag of a propeller is not easy to quantify but some generalisations are in order such as,
- The drag effect increases with speed – actually it increases with the square of the speed, so faster hulls are penalised much more than slower ones.
- The average sailing speed is reduced by about 15% by the drag of a 3 bladed propeller of a size suited to propel a particular hull. When there is a strong wind there is often sufficient power from the sails to overcome the drag of the propeller and hull speed is achieved. As the wind drops, the drag of the propeller becomes more significant and in light winds it can seriously reduce sailing speed.
The early attempts to overcome this problem saw 2 bladed propellers with thin narrow blades. These did help a bit, but also compromised the motoring performance particularly in heavy weather conditions. The propellers simply did not have enough surface area to efficiently transfer the power available into the water to create thrust. Also they were pretty hopeless in reverse so manoeuvring became difficult if not impossible. Many yachts are still like this – it is after all the cheapest possible way to reduce propeller drag!
As long ago as 1920’s folding propellers were invented with complicated operating mechanisms. When these worked, they were effective in eliminating drag, but they still suffered from limited blade area and poor propulsion. Also they were expensive and only produced in small numbers as a result. The first serious improvement came in the 1970’s with the development of reasonably efficient folding propellers with geared blades. Within limits these now had acceptable performances and are still the most cost effective way to eliminate propeller drag.
However they are not a complete answer as there are inbuilt limitations to the concept, the most serious of which is the problem of opening (and staying open) in reverse. The blades try to screw themselves closed and the only force holding them open is centrifugal force. To make them operate in reverse at all, it is necessary to increase the weight of the blades particularly towards the tips. This results in thick heavy blades, which consequently absorb a good proportion of the available power simply driving them through the water. So even after decades of development, folding propellers are still a bundle of compromises. Lately, 3 and even 4 bladed folding propellers have arrived on the market. This improves the efficiency of the folding propeller concept by increasing blade area, but still suffers the penalty of heavy blades that are needed to operate in reverse.
Another innovative propeller, unique in its own right, is the “self pitching” “Autoprop” propeller. It offers blades of a peculiar shape, each independently hinged to a hub with ball-bearings. These are designed to adopt the optimum blade angle for all hull and shaft speeds, though the resulting benefits are not easily defined. Under sail, the blades are free to adopt a trailing position in the water, thus reducing drag although some part of the blade is necessarily exposed to the flow due to the influence of gravity. The slower the yacht is sailing, the more the blade or blades hang down and the greater the drag produced. So, in lighter winds, just when you need the propeller drag to be least it is in fact becoming greater. At higher hull speeds this “hanging blade” can cause the propeller (and shaft) to rotate which results in the blades flying out from the trailed position, partly exposing the propeller to effectively drag once more. The propeller is also very sensitive to imbalance since each blade is independently free to seek its own pitch. Balance problems can then arise when marine growth fouls the propeller unevenly. Wear occurs on the bearings because they are constantly moving all the time so they need regular replacement.
Back in the 70’s self feathering MAXPROP propellers were developed, initially 2 bladed, only for the racing yachts, which thereby gained a small racing advantage. When a 3 bladed version was produced, the world of yachting finally had an answer to the problem of propeller drag, without compromise. At last a yacht owner could have the best of both worlds. Minimum drag under sail with each blade lined up fore – and – aft offering virtually the same miniscule resistance as a folding propeller, whilst still giving the full sized blade area and diameter required allowing the engine power to be fully utilised. Many yacht builders fit undersized, over-pitched fixed propellers in an attempt to slightly reduce drag. The MAXPROP having virtually no drag; can be of the larger, correct size for the engine. Motoring performance can be expected to therefore exceed that of a fixed 3 bladed propeller in these circumstances.
A fixed propeller is efficient in forward, with the concave blade shape giving best drive and the aerofoil section of the blade helping by adding hydrodynamic “lift” in the forward direction. In reverse this all falls to bits! The “lift” is still in the forward direction thus working against the screwing effect of the propeller, which is now trying to work with the load on the back (convex) surface of the blade- in itself a very ineffective shape. Hence fixed propellers are ineffective providing only about 50% thrust in reverse. This all adds up to problems of stopping and manoeuvring in reverse with excess engine revs, paddle-wheeling of propellers and general mayhem in crowded marinas. A MAXPROP propeller overcomes this disadvantage as it remains equally efficient in both directions..
Maintaining a Max-prop is easy because there is in fact very little wear taking place. The only time the bearings of the propeller move are when changing from forward to reverse or from motoring to sailing. Otherwise you can sail or motor for hours on end with no wear occurring to the propeller. The internal mechanism is enclosed and completely filled with grease which is topped up once a year by injecting grease with a grease gun. The only other maintenance is to change the anode as and when required.
The other great advantage of a Max-prop is the ability to adjust the pitch (angle of the blades). Fixed, folding or self-pitching propellers cannot be adjusted in any way. You rely totally on getting the pitch absolutely right first time. This is not all that easy in fact because all boats and more importantly their owners are different. Some are heavily laden and the owner wants top speed at maximum engine revs. Others are lightly loaded and the owner may prefer to optimise cruising performance and in this case could carry more pitch. After fitting a Max-prop and performing simple sea trials the owner can then decide if an adjustment of pitch would offer any advantage depending on what he or she is trying to achieve. The owner of a non-adjustable propeller does not have this luxury.
By eliminating propeller drag, a yacht’s sailing potential is greatly enhanced. In a good blow, there is enough power in the rig to overcome the drag and achieve the potential hull speed. In calmer conditions, where most of us prefer to enjoy our sailing, the effect is more pronounced. With a fixed 3 blade propeller the yacht may be reduced to 3 knots or so, whereas without the propeller it may still be doing 4 or 5 knots. Then, if you need to be at your destination quickly, you can still switch on your powerful diesel auxiliary and punch your way back to the marina against wind and tide using your efficient MAXPROP propeller. You may even be back before opening time at the pub!
