PROPELLERS & PROPULSION SYSTEMS
CHOOSING THE BEST MARINE PROPELLER
There are many exotic ways of generating thrust in water: Caterpillar drives, fish-tails, paddles, thrusters, etc. But, when it comes down to it, a good propeller is more cost effective, reliable and available. So what is a good propeller? The answer to that is: the one for the job. However, there are so many applications one must choose carefully - prioritise. Most commercially available propellers only achieve 40-50% efficiency. Unfortunately, this is nowhere near enough to enable the Elizabeth Swann to tackle the high seas using only renewable energy.
Long before there was any practical application it was discovered that a flat plate pushed through the air at an angle created lift. Of course a flat plate is not very efficient. Marine propellers work on the same principles. Except that air is many times less dense than water and water is virtually incompressible. Credit for the invention of the marine screw propeller comes down to two men: Francis petit Smith and John Ericsson. In 1836, both Smith and Ericsson obtained patents for screw propellers of different designs. An accident led to the discovery a shortened Archimedain screw was more efficient where a collision on the Paddington Canal swept away half of Smith's propeller and as a result the boat actually gained speed. Originally, the wooden propeller had been two complete turns.
In 1839 I. K. brunel changed the design of the Great Britain to screw propulsion. This iron boat could acheive 11 knots with 1,500 horspower. Also in 1839 George Rennie patented his Conoidal propeller which combined the ideas of increased pitch and multiple threads. Then in 1869 C. Sharp of Philadelphia, Pennsylvania patented a partially submerged propeller with high pitch and cupped blades. Later the Wright Brothers built a wind tunnel to develop a practical wing shape. After testing hundreds of wing shapes they discovered an aerofoil section worked best and formulated an efficient chord to length ratio for their biplane wings, such as to enable the first genuine powered flight.
James Watt of Scotland is generally credited with applying the first screw propeller to an engine at his Birmingham works, an early steam engine, beginning the use of
a hydrodynamic screw for propulsion.
Marine propeller sizes are always specified by diameter and pitch (diameter x pitch) with the diameter dimension specified first. For example: a 15 x 12 propeller has a 15" diameter and a 12" pitch. Normally, these are the only dimensions given in a prop size, which is unfortunate because there are other characteristics of the prop you need to consider when selecting the size. You should always start your search with the diameter and pitch of the prop first then work from there.
Pitch is measured in inches and it is a
theoretical measure of how far the propeller
should move through the water in one revolution if
there was no slippage at all. For example, an
18" pitch prop would move 18" forward in
one revolution (provided there was no slippage).
Diameter is just what you think it is: the diameter of the spinning prop. You can quickly check the diameter by measuring from the center of your prop nut to the tip of one of your prop blades. Double the number you get to get your diameter. Diameter can influence the speed you get, but it has its greatest affect on your acceleration and thrust. Bigger diameter is like adding bigger tires to your car: more traction or more appropriately, less slip. Larger diameters put more load on your motor because they move more water though so don't over do it. A blade moving through water does experience drag. The less blade there is (less diameter), the less the drag will be. Provided your motor and prop produce enough thrust, when you decrease your prop diameter your speed will increase a bit because there is less drag.
NUMBER OF BLADES
How many blades should your prop have? Three blade propellers are the most common, but 4 and even 5 blade props are available. The immediate benefit to increasing your propeller's blade count is increased thrust and a smoother ride (Much like adding more cylinders to your engine). Theoretically, the more blades the lower the efficiency of the propeller.
However, increased blade count generally means that each blade no longer has to deal with as much horsepower and consequently the blades can be made a bit thinner which improves their individual efficiency. For example, on a three blade prop running on a 300HP motor, each blade has to handle a 100 hp each. On a 4 blade prop on the same motor, each blade only has to handle 75 HP! This allows the designer to build a prop with a thinner blade without sacrificing stiffness or strength. Unfortunately, the overall efficiency of a multi-bladed propeller is reduced when the blades are forced to run through confused waters - increasing drag. Equally, drag rises to the square (approximately) of speed. Also, a narrow blade, just like a thinner blade passes through the water with less drag. Ideally, therefore, we want a thin section, narrow twin blade, revolving as slowly as possible. Which is just not practical for most boating applications.
Cupping is added to most propeller blades to improve the propeller's bite on the water and decrease slippage. It is most commonly seen on the trailing edge of the blade. Usually the effect of cupping makes the prop perform like a higher pitch propeller, but it does enhance your thrust as well. In addition, cup reduces a prop's tendancy to ventilate or slip. Custom prop builders often use aggressive degrees of cupping to fix slippage problems on large diameter, low pitch props on certain boating applications.
Most props you will encounter are cupped. Most likely, you will only see an uncupped propeller in performance applications. On smaller outboards, which produce most of their power in the upper third of their RPM range, an uncupped prop with a semi-cleaver design often produces better performance. To get maximum performance, it is necessary to get the motor spinning up in this range as quickly as possible. An uncupped prop loads the motor a bit less, allowing the rpm to build more quickly. This enables you to operate your motor in the meat of its powerband where you'll get maximum power. You will notice faster hole-shot, quicker acceleration, and often times, higher top speeds with these semi-cleaver, uncupped style blades. If you are currently running a cupped style propeller and want to improve your overall performance consider trying a semi-cleaver design. Generally, when switching to an uncupped semi-cleaver style prop, you can up the pitch 2 inches and still have better hole-shot, quicker acceleration and often better top speed! Cup can also provide additional bow lift when utilized on the rake line of the prop.
Applying cup to the trailing edge of the prop along the pitch line will increase the effective pitch of the propeller. A standard cup will typically result in a decrease of 200 to 400 rpm's. This usually means a decrease in pitch of 1 to 2 inches is required to run a cupped propeller in place of an uncupped wheel.
CONTRA ROTATING IMPELLERS - Traditional single propeller systems generate thrust by accelerating water in the axis of the propeller shaft. However, the water discharged by the propeller also rotates. This is due, in part, to the friction between the surface of the propeller and the water. This rotational component does not produce thrust, but it does consume energy. By turning the propellers in opposite directions, the efficiency of the a thruster becomes 10 to 15% greater than that of single propeller systems. This has to be offset by frictional losses in the bearings and gears, but overall there is a gain with additional cost and complexity. Another method used to increase the efficiency of water (jet) pumps is a fixed stator. Torpedoes that have to travel at high speed use contra-rotating propellers for this reason.
Blade Rake represents the angle of attachment of the blade to the hub of the propeller. This is not to be confused with the pitch, which is a measure of the twist or screw progression. The amount of rake built into a propeller blade is not something many people consider when buying a new prop, but it can be just what the Dr. ordered in some cases. Rake angle isn't an immediately apparent thing to the untrained eye, but if the propeller blade is cut down the center, it is readily apparent. The biggest benefit of a high rake design is a greater resistance to ventilation. This allows you to trim the outdrive up toward the surface more without the prop ventilating as readily during tight manuevering. You should seriously consider using a high rake propeller if ventilation is a problem for you.
If you have a large boat like a house or deck boat a high rake prop may not be the best choice because it does not produce as much reverse thrust. This may reduce your ability to manuever your vessel at low speeds making your boat more difficult to dock.
Higher rake normally improves performance in ventilating or cavitating situations (high engine elevations and high trim angles). Additionally, higher rake can provide higher bow lift, which will frequently improve speed. Low rake blades are typically used on motors with propellers running fully submerged, typically carrying moderate to heavy loads. The rake angle can either be straight or the average angle of a parabolic curve.
Not many manufacturers quote the efficiency of their propellers and even fewer customers ask. The efficiency of a propeller is defined as the power coming out of a prop divided by the power going in:
-------------- X 100 For a solar powered boat, efficiency is probably the most important statistic.
BLADE THICKNESS (thinner blades cut better)
Why is Stainless Steel the first choice for high performance props? A: It is much stronger and far stiffer than aluminum, but that's not the whole answer. The higher strength of stainless means that the blades can be made thinner and that is where the performance benefit comes from: thin blades slice through the water more efficiently than thicker blades. As a result stainless props produce a couple more mph over even the best aluminum designs.
This Design concept is part of the reason our 4-Blade propellers and our uncupped "XB" style 3-Blade propellers outperform our Standard 3-Blade Designs. By incorporating a thinner blade cross-section, those props cut through the water with less resistance. There is a compromise to be made though and that compromise is in durability. The thinner blades are faster, but they are not as durable as our thicker blade designs. If you run up and down a shallow river and hit things frequently, you may be happier sacrificing some performance to run the tougher blades. If you typically run in the ocean with little chance of a strike, you may prefer the added speed of our thinner blades.
Thick Nickel Leading Edges: Nickel is twice as hard as stainless steel, and has superior abrasion and impact resistance. A protective metal leading edge can save composite propeller blades from destruction should driftwood, etc, go through the propeller -- in this case a 5/8" wrench. This blade was able to be repaired to like-new condition for just under $100. Foreign object damage like this would destroy most any other composite propeller.
Unusual variable pitch surface piercing propeller on the Turanor PlanetSolar
ELIZABETH SWANN'S DESIGN COMPROMISE
As mentioned above, the propellers on the Elizabeth Swann must be efficient above all. We are looking to achieve 70% efficiency. So far our tests have exceeded our target figure up to about one third full size. Hence we are quietly confident. Having said this our technical members will not be completely happy until a full size propeller demonstrates the same efficiency.
The basic theory of propellers tells a single bladed prop is theoretically the most efficient. In practice a twin blade is the most practical to reduce vibration. The more blades the more each other blade introduces turbulent flow into the equation. The slower a blade moves through the water the less the drag and the smaller a blade's surface area, (narrow) the less the frictional drag.
It would seem from the above we are looking for a slowly revolving propeller with low drag and a small surface area. Provided we know the speed our vessel will cruise and the thrust needed to overcome hull drag, we can then calculate for the most efficient propeller. Again, this is not quite so straight forward because there are size and other mechanical limitations. For this reason commercial propellers usually have three small blades revolving at quite high speeds of 3,000 rpm or more. On the other end of the scale we have huge Tug propellers revolving @ only 500 rpm.
The material a propeller is made from can seriously improve its performance. For example, a cast aluminium propeller will corrode more and have thicker sections than a stainless steel prop. Then again the stainless prop will cost more. In some applications carbon fibre is cost effective, as in human powered craft.
Assuming cost will not prevent the use of exotic materials, the blade section, pitch and swept area are important to producing the right propeller for the job. No doubt we will experiment with quite a few full size props in deciding on the final pair.
LINKS & REFERENCE
HAWTS - A proposed design for a solar and wind powered ocean sampling machine, with especial targeting of the marine litter and plastics that is choking our seas and killing marine life.
A taste for adventure
A heartwarming adventure: pirate whalers V conservationists
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