Understanding Propellers

by admin

Of all the items on an outboard motor, seemingly the most unpretentious and uncomplicated, is the propeller. Yet amazingly, despite the fact they are such an integral part of an outboard motor, propellers are often a very neglected item. It is just so important that we understand the basics of the propeller – the importance of it’s operation, the principles, and how to gain the maximum benefit not only in performance, but also in efficiency – at whatever form or level of powerboating you participate in.

Different boats with different uses require different propellers. So don’t expect one propeller to do all. For example, a propeller that is suitable for skiing will not necessarily be suitable for long distance or high-speed work. It may well do the job, but definitely not effectively or indeed efficiently. To explain that further, and most importantly simply, one must first understand thebasic concept, and some of the associated jargon. 

A propeller consists of anything from two to eight blades; that are attached or moulded, onto a central hub. 

These blades are attached and shaped at such angles that with each revolution the boat is propelled a certain distance forward. This amount is determined by what is known as the PITCH of the blade. For example, a 21-inch pitch propeller would propel a boat 21 inches forward with each revolution it made, less an allowance for what is called Dzslippagedz. Slippage is usually expressed as a percentage of the pitch. A propeller with 20% slip can in some instances be quicker than one with 15%. Slip varies through the range but a general rule of thumb puts displacement boats at about 30%, planing hulls at 20-25%, twin rigs at 15-20%, and faster sports and performance boats at 10-12%. The ultimate speedsters like racing hydroplanes have propellers with little slip, that gets down as low as 3-5% in some instances. More mathematically inclined readers may care to calculate the slip at full throttle on their own boat. Here’s an example: A top speed of 47.5 knots is achieved at 5800 rpm with a 1.87:1 lower unit gear reduction and a 21-inch pitch prop. If there was no slip, the speed would be (5800/1.87) x 21 inch/minute x {1 ft/12 inch} x {1 nauticalmile/6080 ft} x {60 minute/1 hour} x {1 knot/1 nautical mile/hour} = 53.56 knots. Note
that by inserting the various conversion factors, in {} brackets, the units of inch, minute, ft, nautical mile and hour cancel out, leaving the speed in knots. Thus the slip is 6.06 knots, which is 11.3% of 53.56, so the slip is 11.3%.

Right Operating RPM

The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator’s manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).The goal in prop selection is to determine what propeller style and size will maximise performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from going beyond its maximum rated RPM, yet allow it to reach the minimum rated RPM where maximum horsepower is produced. Run the boat/motor at w.o.t. under normal operating load to determine the maximum RPM you are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle for the optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded. If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 1″ will result in approximately a 200 RPM drop. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the minimum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing 1″ pitch will increase your RPM by approximately 200 RPM.

Types of Pitch

To further complicate the situation, there are two types of pitch – CONSTANT andPROGRESSIVE. Constant pitch is a true or flat pitch, which is the same at all points, from the leading to the trailing edge. Progressive pitch, more usually found on high-performance stainless steel props, is an average of a low reading at the leading edge of the blade, and a higher reading at the trailing edge. Propellers act in much the same way as an aircraft wing, in that the motion of a propeller through the water generates a greater average pressure on the rear surface than on the forward surface. This difference in pressure, multiplied by the blade area, provides the thrust force that drives the boat. In order to produce the thrust necessary to get a boat up on to the plane, there is no substitute whatsoever for blade area.

Without sufficient blade area, it becomes impossible to generate sufficient pressure difference to produce the necessary force, and the propeller simply Dzdigs a holedz and goes nowhere. Of course, choosing a propeller with more blades is one way to gain blade area, but there is a limit, after which the only way is to go for a bigger diameter. But with outboards and sterndrives, that is not possible. That’s why even twin sterndrives are seldom used on boats over 5 tonnes. In a nutshell then, blade area gets you going and the pitch gives you the forward impetus.

Performance Aspects

There are two other important performance factors, albeit more traditionally associated with high-performance propellers. One is RAKE, which is the slanted angle aft (at the tip) from the perpendicular to the shaft axis. Standard production propellers normally run around 15 degrees of rake, whereas performance versions can run anything up to 30 degrees at the blade tip. The second performance factor is CUPPING. We have all heard at some time or other of the racer who sits his propeller over the tow-ball on his car and proceeds to ‘cup’ it with a ball-peen hammer. What this does is increase the effective pitch of the propeller, thereby increasing top speed. In some instances too, it is equally as effective in reducing the revs of an over-stressed engine. The last common blade ‘peculiarity’ is SKEW. Instead of being radially symmetrical, in this case each blade is ‘swept back’ in shape, when viewed along the propeller shaft axis. Skewed propellers are ideal for lower horsepower domestic or work engines, where weed or other obstacles are a factor (this shape of blade sheds weed easier), the higher skew in a surface-piercing or high speed application will also greatly reduce the shock sensation as the blade re-enters the water.

Cavitation or Ventilation

Many people confuse slip with cavitation, or ventilation, when in fact they are very different kettles of fish. Slip is inevitable, whereas both cavitation and ventilation, when they occur, are detrimental to an engine’s performance and well-being. VENTILATION or BLOW-OUT is basically loss of grip or bite by the propeller, caused by aeration either from the water itself or from the exhaust, that interrupts the flow of ‘good’ water to the propeller. This can be caused by a dirty, scratched or chipped bottom, or an item such as a water intake or transducer that interrupts the flow of water to the propeller. Ventilation or blow-out virtually always progresses on to become CAVITATION, which defined simplistically is in effect the boiling of the water along the lea ding or front edge, of the propeller. There the pressure is low enough that the water actually boils, forming water vapour. This vapour collapses as it moves along the blade and back into a higher-pressure area, thereby condensing back into a liquid form. This ‘implosion’ causes pitting of the propeller’s surface, also known as “cavitation burning”, and if allowed to continue this can cause a propeller to disintegrate. Ever seen a propeller with hundreds of little pin -holes across part of its surface? That’s cavitation burning. Fortunately today’s better designed and built propellers, with more cupping and rake, resist cavitation. So if you have a problem it is more likely to be caused by incorrect engine set-up (too high on the transom or trimmed too far out), or hull aeration caused by something disturbing the water flow to the propeller. Or, the propeller itself may have been damaged.

Appropriate Size

Armed with a basic understanding of the workings of a propeller, the choice of the appropriate size to run becomes a little easier. If you predominantly tow water skiers, you will want a lower pitched (just like a lower gear ratio on your motor vehicle) propeller that will give you more punch out of the hole. If  you do a lot of high-speed work, the rule of thumb dictates a ‘higher-geared’ propeller pitched at a minimum of two inches higher. One standard-issue propeller, unless you have massive horsepower in a lightweight boat, will seldom do both jobs satisfactorily, so the ideal solution if you want both acceleration and top speed, is to have two propellers. Or alternatively, you can either go to a removable-blade propeller (either situation would also be handy if you damage a propeller while at sea) that allows you to change to a different pitch in minutes, or spend the extra and go to a stainless steel high-performance example.

The above scenarios may seem excessive in the context of a quiet day out for the family, but if use your boat a lot for one particular application, then it is worth the effort. Grab a pen and paper and try working out the potential fuel savings in a year, and you may well come to the realisation it doesn’t take too long at all to recoup the extra expense.

Prop Construction

Having decided on the correct size of propeller required, the next decisions to make are whether to go solid or rubber bush, the number of blades required, and whether to go aluminium, stainless steel, or even composite glass-reinforced plastic. Before you begin this facet of the ‘choice’, remember that you don’t necessarily have to run the respective manufacturer’s propeller; Mercury for instance have propellers to suit other brands of motors, and there are many good after-market propeller manufacturers like for instance Solas and Michigan. Solid hub aluminium or metal propellers, without a splined rubber bush down the centre of the hub, are predominantly for competition and are to be avoided at all costs, because, there is no ‘shock absorption’ whatsoever – both when putting the engine into gear and when the boat is running at speed. 

Vibration also becomes a major problem without the shock-absorbing qualities of the bush. The number of blades really should only be an issue in high performance or sports applications; otherwise the standard propeller that comes with your engine is more than adequate. The extra expense of more or fewer blades is often not warranted, or justified. And it is the same situation with the stainless steel versus the rest argument. Many dealers (with ulterior motives), or self-professed experts, will tell you the stainless propeller is the only way to go: better performance, stronger blades that don’t warp under load (aluminium ones predominantly don’t either, of course), better cupping, more efficient shape, and so on. All true, certainly, when you get to the upper end of the performance scale, but for me, the rule of thumb is: run what the manufacturer suggests. If it is fitted with a stainless steel prop as standard, fine. If it is fitted with an alloy prop – still fine. Only if you want the best possible performance, and are prepared to pay a premium to achieve it, should you consider using stainless steel high-performance propellers. In saying that though, there is in fact one exception to that statement, where the additional expense and effort would most certainly be worth-while. Purely from an economic perspective, it may be that you have a particular scenario, be it a heavy rig, an inefficient hull, or any number of other contributing factors, then a stainless steel or multi-bladed version would certainly be more efficient. All of a sudden the extra economy you would get, would far out-weigh the additional purchase cost. If however you are looking for the ultimate in performance, then engage the help of an expert, generally a propeller manufacturer or supplier in business for themselves. There are many self-confessed experts out there though, so beware – a full-blown race prop that you may not necessarily need, could cost upwards of $5000 to $10000. A good place to start then is at your local outboard dealer or a specialist propeller shop. They have props that lift the bow, ones that lift the stern, surface riders, choppers, cleavers –ensure you end up with the right prop, for your application! The most important point when making your propeller purchasing decisions then is to provide the engine dealer or the propeller manufacturer with as much information as possible on the size of your engine and boat, what you expect from it in the way ofperformance, and its predominant use. They will know the correct sizes to run. Then, if your dealer is receptive to the idea, borrow a second-hand propeller of that size and specification (or a selection of them), to try out. Once you have purchased your new prop don’t just bolt it on, suffering under the illusion that it’s bullet-proof – respect it and look after it. If you have been unlucky enough to hit an object at sea, send your propeller in straight away for a check-up. A damaged propeller could well be sending vibrations through the motor and boat, causing undue stress and wear and tear. And it will certainly be costing you performance, which ultimately equates to an increased fuel bill. A recent field test with an only slightly damaged propeller revealed a drop in top speed of 13.7%, a drop in acceleration capability of 37.7%, and a drop in optimum cruise economy, of 21%. A $50 or $60 repair bill all of a sudden pales into insignificance.

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