Understanding your propeller

Transcription

1 gear Understanding your propeller Choosing the correct propeller for your boat involves complex formulae and an understanding of how a prop works but in the end getting best performance still boils down to some trial and error, as Pat Manley and Mike Taylor explain F or all craft fitted with an engine, whether it s a sailing yacht or a motorboat with a displacement or planing hull, having the right propeller is critical. The propeller should allow the engine to reach the maximum rpm recommended by the manufacturer in normal operating conditions: get it wrong and the engine will over-rev, causing possible damage to the motor, or alternatively will be unable to reach 66 Props_fjp.indd 66 its maximum power delivery because it cannot supply adequate torque to turn the propeller, resulting in poor performance, high fuel consumption and an unhappy owner. But for leisure boats in particular, propeller design remains a bit of a black art. There are books dealing with the subject that allow you to calculate, rather laboriously, the shape and size of a propeller for a given application, and now there s computer software to do the job effortlessly. What do they have in common? All the methods are based on trial and error, they use assumptions, and at the end of the day will get you a prop of roughly the correct design that only sea trials will verify. Not surprisingly, yacht owners and motorboaters look at engine and propeller criteria very differently: the sailor will normally use the engine in either a sails down mode, or as a motor-sailer to quicken the pace provided by the sails. The motor cruiser owner will also be thinking in terms of leisurely progress. In both instances the engine will be working at mid/two thirds throttle opening with proportional expectations on propeller performance. In contrast, a sportsboat enthusiast will often be using the engine at full throttle openings to gain maximum and acceleration, placing greater demands on the propeller. But in all cases the critical factors affecting a boat s performance under power are weight, engine horsepower and propeller design. Practical Boat Owner 527 October /9/10 11:40:21

2 Choosing the right prop HOW DOES A PROPELLER WORK? Newton s third law of motion says that for every force, there s an equal force in the opposite direction. We want to push the boat forward, so to achieve this we have to push the water backwards. When a propeller rotates, the turning blades push water backwards causing the boat to move. Also, as the propeller pushes the water back, the pressure reduces on the forward side of each propeller blade which helps to suck the boat forward. This pressure differential increases as the propeller spins faster. Thus, the pressure differential and the greater volume of water moved combine to increase the of the boat. Propeller design has managed to keep pace with most marine technology over recent years while more and more is being understood about a propeller s hydrodynamics. Naturally, one of the primary drivers behind all this research effort is effi ciency: to help engines use less fuel, and/or make the boat go faster. With the advent of computeraided design (CAD), much work can be done before a prototype prop is even manufactured and placed in the water. To work out what the diameter and pitch of a propeller should be, the designer needs to consider several other factors: other factors including the hull shape, all ending up in equilibrium. The benefi t of experience has taught propeller designers that the value of slip appropriate to smaller sailing yachts and displacement motor cruisers varies from around 40% to 55% when at maximum boat. Efficiency The effi ciency of a propeller is the ratio of the power that is making the prop turn compared with the PITCH AND SLIP EXPLAINED PITCH Pitch Pitch is the distance the prop screws forward in one complete revolution Diameter thrust that the prop develops. Major factors affecting the prop s effi ciency are the plan shape of the blade, the cross section shape of the blade, the ratio of the pitch to the diameter, whether the blade has correct twist and the angle the blade makes to the water entering the prop, which of course depends on the slip. The blade shape chosen will vary according to the and type of hull, and in the case of a sailing boat, whether performance under power or sail is paramount. Slip SLIP The thrust developed by the propeller is determined by its diameter, pitch, blade area and its of rotation. The of rotation of a prop is determined by the horsepower delivered to it. At a steady boat, the thrust from the propeller equals the drag of the boat, so the thrust depends on the shape of the hull, its through the water, and the propeller shape. Pitch angle ANOTHER WAY OF LOOKING AT A SLIP Angle of attack Slip angle Slip angle is the angle the blade makes to the water as the blade rotates Prop rotation Slip Slip is the difference between the pitch of the propeller and the actual amount the propeller pulls itself through the water for each revolution of the prop. With no slip, no water is moved backwards by the prop. With 50% slip, a water volume equivalent to the propeller disc area times half the pitch is moved backwards (ie 50% of the water is moved backwards). With 100% slip, the whole water volume would be moved backward. This is the case if you carry out a bollard pull measurement with the boat tethered to the dock. So if there were no slip, there would be no thrust, as no water is moved backwards. In reality, it s the slip that creates thrust and thus we get propulsion. The amount of slip is determined by a balancing act of water density, blade shape, boat drag, engine power, gearbox ratio and No slip 100% slip 50% slip No water moved 50% of water moved All water moved No thrust This situation is normally not possible as this produces no thrust. It can occur when motorsailing when all the thrust is provided by sail This is how the propeller normally operates at maximum boat and engine power = Drag This is what occurs when the boat is moored to the pontoon with the engine running in gear. The boat is zero. In reality, maximum rpm will not be achieved and maximum thrust will be reduced Practical Boat Owner 527 October Props_fjp.indd 67 6/9/10 11:40:37

3 Gear HOW DOES A PROPELLER WORK? CONTINUED Blade area Up to the point at which blade loading makes cavitation (see page 70) likely, the blade area is not a big factor, as reduced blade area is made up by increased pitch. If the prop is working close to the cavitation point, extra blade area will reduce the likelihood of cavitation. So if the prop is working well within its limits, such as when cruising, a two-blade prop (of correct pitch) will produce a thrust similar to a three-bladed one. In fact, because each blade has more room, it is likely to be more effi cient than a three-bladed prop and deliver more thrust. However, when the two-blade prop is running under adverse conditions, such as rough weather, or under maximum acceleration, the three-blade prop will then deliver more thrust because its blade loading will be lower. Blade twist The at which the propeller blade meets the water varies along its length, because the tip of rotation is much faster than at the root. In order that the blade s angle of attack is the same all the way along, the propeller blade must be twisted. Water flow We might well expect that the water fl ow into the propeller disc is the same as the boat s forward, but unfortunately life s never that simple. Because of the friction between the hull and the water, the fl ow close to the hull is slowed down and the water s at the rear of the hull is reduced. Also, because the hull is curving in towards the stern, this exerts another slowing effect and if the prop is mounted in a cutout in the keel the slowing effect is exaggerated. Depending on the hull s shape and size and how close the prop is to the hull, this reduction of water into the propeller may be as much as 40%. So, when calculating the propeller size, we must use only a proportion of the boat in order to get the fl ow into the prop. Like slip, there are no hard and fast fi gures, so again experience supplies the numbers. Various experts use fi gures between 20% and 40% as the reduction in water into the prop for displacement motorboats and sailing boats. The more the prop protrudes into the fast fl owing water stream, the lower the number. BLADE AREA RATIO, TWIST AND SIZE 20% Typical two-blade folding propeller Blade through water BLADE AREA RATIO (BAR) 30% Typical two-blade fixed propeller 45% Typical three-blade sailing propeller 55% Typical three-blade displacement motorboat propeller BLADE TWIST same angle of attack all along the propeller blade Boat Blade tip Boat 2 3 diam Boat 1 3 diam The actual pitch of a fixed blade propeller varies from blade root to tip, so that the angle of attack is constant along the blade Specifying your propeller All of the factors mentioned are the concern of the propeller designer, so by the time you are considering buying a prop they have been distilled into two numbers diameter and pitch. These are the factors you must get right to match your prop to your boat. In addition, you will need to specify the propeller rotation, the material from which it is made and its type. Starting with a new installation, if you know the dimension and weight of your boat and the desired maximum working, you can then calculate the required prop size and the engine and gearbox necessary to drive it. Some propeller manufacturers even say you should decide on the propeller dimensions fi rst before choosing the engine and gearbox to match. If you suspect your current prop is not providing your boat with the best performance and economy or if you are overhauling your engine and gearbox unit, now is the time to check the prop dimensions are the best for the power delivery. For existing installations, you ll need to know the power output of your engine and the ratio of your gearbox: the manufacturers should be able to help. There is also often technical assistance available from the owners association for your type of boat. Failing either of these options, you can test the power yourself using a rev counter with the boat in the water. If the engine is able to reach maximum revs at only two-thirds throttle opening, for example, the propeller needs to be changed for one with a steeper pitch. If the engine fails to reach maximum revs altogether, a shallower pitch should solve the problem. As a rule of thumb, a 1in reduction in pitch will increase the engine s rev capacity by 200rpm. Many propeller suppliers offer a propeller specifi cation service, and there are several prop calculators on the internet which can help you to work out which is the right one for your boat. Even so, the software is based on graphical solutions incorporating approximations of the various factors. In the end, the calculations will get you into the correct ball park but fi ne-tuning may be needed after sea trials, so it s best to let the engine/propeller supplier carry out the calculations. That way, if the prop is not correct, at least there s some comeback. Look at your existing propeller and you ll fi nd stamped on the hub something like 17 x 11 LH. The fi rst number (17) is the diameter of the propeller in inches. The second number (11) is the pitch of the prop, also in inches, and the LH denotes left-handed rotation. Diameter A slow-turning, large-diameter prop is preferable to a smaller, faster-spinning version. Exceptions occur when the prop reaches a size that places signifi cant twisting moments on the shaft, leading to severe vibration, or in the case of a surface-piercing prop. Increasing prop diameter increases thrust, but you need a minimum of 15% of the prop s diameter as clearance between the prop tip and the hull. But in general, go for the biggest prop you can. A 1in increase in diameter will allow you to reduce pitch by around 2-3in for the same load. Pitch This is effectively the angle of attack of the blade to the water (see diagram, page 67). A propeller with a high pitch angle will move greater volumes of water than one whose angle is less. This in turn is affected by the shaft rotation and is itself governed by the boat s engine and gearbox design. The pitch is defi ned as how far the propeller would push itself 68 Practical Boat Owner 527 October Props_fjp.indd 68 6/9/10 11:52:26

4 Choosing the right prop Types of propeller A PROP S OPTIMUM OPERATING RANGE Fig.1 Fig.2 Slip angle Pitch angle Blade angle of attack Prop shaft Propeller thrust Propeller drag* *The greater the prop drag, the more the engine torque needed to turn it The best operating range for a propeller will be found with a pitch somewhere between figure 1 and figure 2 where there s the best compromise between the thrust produced by the prop and the torque (power) required to turn it. PROPELLER SIZED FOR MAX SPEED, POWER AND RPM Resultant waterflow onto blade due to prop rotation and boat Boat of 6 knots Boat of 3 knots Boat of 0 knots (boat stationary in water) Fixed pitch propellers A fixed pitch propeller is the correct size only for one set of conditions. Normally it s designed to be correct at the boat s designed maximum and with the engine developing maximum power at the rated (maximum) rpm. Under any other conditions it s the wrong size! This is because the angle at which the blade is meeting the water changes as the forward of the boat changes (see diagram, left). A fixed blade prop has very good propulsive efficiency at the for which it s designed when under power. In astern, the leading edge becomes the trailing edge because the direction of rotation is reversed and thrust is reduced by as much as 50%. Waterflow due to rotation of propeller Blade angle of attack Optimum angle of attack: maximum boat, maximum engine power Some props can be no larger than the constraints of their surroundings Boat stationary, very large Less through water, greater angle of attack. More angle of attack. Insufficient engine power to turn power required to turn prop. prop=reduced rpm Pitch too big through the water with no slip. For example, a simple woodscrew that moves forward into the wood by 3mm each revolution has a 3mm pitch. But there the difference ends. The woodscrew works with no slip because the wood is solid. The water screw pulls itself forward only because it can exert a force on the water, working like an aeroplane s wing, so only a small part of the water is accelerated by the propeller. Increasing pitch increases thrust, but only up to the point that the propeller blade stalls, when there s a dramatic fall in thrust and a large increase in blade drag. Prior to reaching this point, the pressure on the upper side of the blade is reduced to such an extent that cavitation occurs. If you specify the wrong pitch, a propeller specialist will be able to adjust the pitch of a fixed pitch prop by up to 2in to tune the prop to the boat if it s not quite right. Rotation A right-handed prop turns clockwise when viewed from astern in forward gear, a left- handed propeller turns anticlockwise. This is dictated by the engine and gearbox combination on your boat, so is rarely a matter of choice. Materials It hasn t just been propeller design that has been affected by the march of modern technology. While bronze retains its popularity, particularly in the sailing market, there is now a range of other materials for propeller manufacture. Aluminium propellers are in common use on motorboats, while stainless steel, though pricey, is stronger than other propeller materials, offering longer life and high hydrodynamic efficiency derived from stiffer blades for the same given blade thickness. Composite or plastic propellers are also expanding from their niche as small outboard propellers, with Kiwiprop making a hybrid design with replaceable plastic blades, or various manufacturers producing composite props for use in an emergency when your main one is damaged. Practical Boat Owner 527 October Props_fjp.indd 69 This prop is mounted clear of the hull on a P-bracket Under sail the drag is very high and for a three-blade prop a reduction in of 15% (0.9 knots at 6 knots) is possible when compared with a feathering prop. Typically a fixed three-blade prop will have a Blade Area Ratio (BAR) of about 50%, ie the area of the blades will be 50% of the area of the circle defined by the diameter. A two-blade prop will have a BAR of about 33% so it would have less sailing drag than the three-blader. Slim two-blade props of 25% BAR are also available. A typical three-bladed bronze propeller 69 6/9/10 11:41:19

5 Gear Folding propellers Under sail the blades come together to offer minimal drag. Folding props have considerably less drag than a fixed prop when sailing but some tend to slam or jerk as they open and there s some lag as the throttle is opened. They tend to be pretty poor in astern because they don t open fully, and may also have less than the optimum amount of twist and hence are less efficient. Some are more complex than others and this is reflected in their price. Folding two-blade props have a small blade area (around 25% BAR), so end up with high blade loading and a risk of cavitation. Folding three-blade props have more blade area so run at a satisfactory blade loading but still have little twist. They need room behind them to allow the blades to fold backwards. An interesting variation on the folding prop is the Gori three-blade propeller, which features an overdrive mode allowing you to move between a fine pitch for maximum power and a coarse pitch for cruising. A Brunton Autoprop Folding props offer reduced drag for a yacht appropriate pitch for your boat. They come in two- and threeblade forms and have a greater blade area (similar to a fixed prop) than their folding brethren so have a reasonable blade loading and are less susceptible to cavitation. However, they have no twist at all so lose out on propulsive efficiency, but are much better than a fixed prop in astern because the leading edge of the blade remains the same as the Feathering propellers With the engine switched off, the blades of feathering propellers turn at their roots instead of folding so that the blades are edge on to the water flow past the prop. Under power, they swivel to an end stop, which is pre-set to an blade flips round when going backwards. Feathering props need room for the blades to swivel, but not a lot more than a conventional prop so are often appropriate for longkeeled boats with limited propeller diameter clearance. Self-pitching props Brunton Propellers Autoprop cleverly balances centrifugal force, water load on the blade and engine power using swivelling blades, to give a continuously variable pitch matched to the load. This means that over quite a large boat /engine power band, the prop is always the right size, MAINTENANCE AND CORROSION A composite variablepitch Kiwiprop A CAVITATION, VENTILATION AND STALLING C avitation, ventilation and stalling are sometimes confused, although they re not the same thing. Cavitation can occur at any depth of water, and is caused by the reduction in pressure at the propeller blade surface as the blades spin faster. The faster the blades move, the lower the pressure around them will become, until it reaches vapour pressure the point where the water vapourises, forming small gas bubbles. As the bubbles collapse, they release very strong, localised shock waves, which can cause severe pitting damage to the blades. Stainless steel propellers can withstand cavitation better than aluminium or bronze ones. Ventilation occurs when surface air or exhaust gases are drawn onto the propeller 70 Props_fjp.indd 70 Cavitation Turbulence due to stalled prop blade Once a propeller begins working outside its optimum operating range forces such as cavitation, ventilation and stalling can create problems. blades. The load on the propeller is reduced by these gases mixing with the water causing the engine to over-rev. On outboard engines this problem is often overcome by the installation of an antiventilation plate cast into the gear case housing with claimed advantages of better fuel efficiency, more thrust and lower cruising rpm. It s also said to work well when motoring in rough weather. Sailing drag is low, only a little more than a feathering prop, and like the feathering prop, the blades flip round in astern so give good reverse thrust. Additionally the blades are properly twisted to maintain a constant angle of attack. Annual maintenance is required to lubricate the ball races and it would be as well to invest in the special prop puller to avoid damage if you have to remove it. This type of prop needs room both in front of and behind the propeller to allow the blades adequate room to swing. directly above the propeller. Stalling occurs when the angle of attack of a prop s blades to the incoming water is too great, just as an aeroplane s wings will stall if it climbs too steeply. If the blade pitch is too steep, slip becomes too high and the propeller loses all thrust. prop should be checked annually for condition and possible damage. Even a slight kink or a blade with a small section missing from hitting a submerged object can cause the screw to be out of balance resulting in a vibration, putting stresses on the shaft bearings and seals. If the prop is made from bronze, once the retaining nut and washer have been removed it should be just a matter of a tap with a mallet and a suitable block of wood to loosen it from the shaft. Modern aluminium props may well need encouragement with a heat source and a professional prop puller to overcome sea salt corrosion. The more metal there is in the prop, the more anodic protection it needs. Some folding and feathering props have Props sometimes need persuasion to release them from their shaft anodes incorporated, but these are relatively small and may not last a full season. Newer Volvo Penta saildrive legs are electrically isolated from the engine and rely solely on the anode at the bottom of the leg. Their heavy three-blade folding props can exhibit signs of surface corrosion, so timely replacement of the leg anode is important. Practical Boat Owner 527 October /9/10 11:41:40

6 Choosing the right prop WHICH PROP FOR WHICH BOAT? Normally propellers are sized for maximum, so at cruising power the prop will have too fi ne a pitch. Size the prop for cruising revs and the pitch will be too great at higher rpm and the engine won t deliver maximum power and the boat won t reach its design. Many owners, given the choice, will opt for the cruising prop and accept a lower top. This gives more relaxed cruising and better miles per gallon. However, at maximum throttle there s likely to be dark smoke at the exhaust and the engine may become overloaded leading to premature failure. This is the thinking behind the Gori folding prop and Brunton Autoprop to give the best of both worlds by changing pitch. When sailing, we would like our boat to have minimum drag. For this we need a small blade area propeller or even better, a folding or feathering one. Once you try and move away from a fi xed prop, you may need room for the blades to fold or swing and lack of room may dictate that only a certain type of prop will fi t. If your prop size is restricted, be wary of fi tting too big an engine or you may not be able to utilise all the engine s power because the prop will cavitate. Doing the sums The sailing yacht owner needs to consider several factors. Is under sail paramount? Is cost all-important? Is thrust under power the primary concern? For a 10.6m (35ft) sailing boat, powered by a 30hp engine, the accompanying graphs give some idea of the effect of the propeller parameters on drag and hence sailing performance. It s important, however, to read the propeller performance curves with a certain amount of caution. What you get with a given prop Forward thrust of all the props except the Brunton Autoprop follow the propeller law principle of being matched to the engine power only at the design. The Brunton follows the engine power output much more closely over a wide range. depends on what boat it s used on. The same prop will give a different performance on two different boats, which is why the drag is not quantifi ed in the following graphs. The data to draw PROPELLER COMPARISON DATA Drag of hull compared to drag of prop Drag Hull drag Speed (knots) Fixed 3-blade prop drag At low s the sailing drag of a three-blade fixed prop is very significant, but this reduces at higher hull s when wave drag of the hull takes effect. Forward thrust 100% 75% 50% 25% rpm Reduction in drag without a propeller Drag This curve allows us to estimate the increase in sailing that can be gained by removing the drag of the prop. The improvement is best at lower s. Reverse thrust 0.6 knot 0.7 knot 100% 75% 50% 25% Hull and prop drag 0.4 knot 0.5 knot Speed (knots) rpm Key to graphs Hull drag (no prop) Fixed 3-blade prop drag Reverse thrust at 3 knots forward 3-blade fixed prop 3-blade folding (Volvo) 3-blade feathering them has been derived from several different sources. The curves really just give an indication of what the different props are capable of to give you a starting point. Propeller drag by type Drag Speed (knots) The fixed three-blade prop has significant drag, followed by the fixed two-blade prop. All the feathering and folding props have significantly reduced drag. Normally you don t go rushing backwards at 6 knots, so I haven t shown maximum power in astern. To indicate the stopping power of the prop, I show performance of the prop in astern (see graph, left) with the boat moving forward at 3 knots and the rpm increased as you would when approaching a dock. The two-blade folder is particularly poor and the Brunton, although poor at very low rpm, outperforms the others as rpm is increased and it adjusts its pitch. 2-blade fixed Brunton Autoprop 2-blade folding GEAR RATIOS The gearbox is the mechanical interface between the engine and the propeller converting the engine s revs, power and torque curves into dimensions the propeller can transmit to the water, combining both gears and a simple clutch device allowing the drive to be engaged and reversed by the movement of, usually, one lever. A typical ratio for a small boat would be 2:1. The choice of gear ratio is an integral part of choosing the correct propeller, especially where the diameter or the blade is restricted. This fact often seems to be ignored when installing a replacement engine, especially if engine power is being increased over the original. If the prop diameter were restricted, gear ratio would become important to avoid overloading. The relationship between the engine, gearbox and propeller is critical if the drivetrain is to work at its best performance. Generally the torque curve of a diesel engine is flatter than a petrol engine, the unit delivering its maximum power at lower revs (say 2,500-3,000rpm) while a high performance marinised petrol engine will rev considerably higher to create its maximum power. More reading Diesels Afloat by Pat Manley, 19.99, Wiley, includes 17 pages of information on propellers. ISBN Dave Gerr s Propeller Handbook, 14.99, Nautical Books, covers propeller theory in an easy-to-understand d format. ISBN Practical Boat Owner 527 October Props_fjp.indd 71 6/9/10 11:41:55