How a Shaft Seal Works

A mechanical shaft seal depends on a surface that is able to turn or rotate in order to create contact with a non-moving surface.  In many quality marine shaft seals, the non-moving surface consists of a carbon flange.  Against this is turned a rotor made of stainless steel.  Because the seal is held closed by water pressure, vibrations caused by the engine will not affect the seal.

The Carbon Flange

To construct a quality marine shaft seal, a simple square of carbon is insufficient.  Instead, a carbon or graphite flange is manufactured from advanced composite materials and precision-machined to the exact shape necessary.  In operation, the face of the flange will slightly abrade the face of the stainless steel rotor during the first few minutes the two are in contact.  This action leads to a “polishing” of the rotor face that enables a perfect seal to be formed.

In this technique, the metal to be protected is paired with another metal known as the “sacrificial metal.”  The two need to be connected in a way that will establish them as an electrochemical cell.  Once this is done, the sacrificial metal will decay rather than the protected metal.  This is accomplished by making sure that the sacrificial metal is, in fact, more prone to corrosion than the other metal.

With cathodic protection in place, the sacrificial metal will steadily corrode until it is completely gone.  During this process, the other metal will not corrode.  However, as the sacrificial metal reaches the end of its useful life, it will need to be replaced in order for its protective function to continue.

Uses for Cathodic Protection

This technique is often used in marine applications.  For example, piers piles made of steel as well as offshore drilling platforms may use sacrificial metals to protect the integrity of the structure.  Ship hulls and propeller/engine combinations may also use cathodic protection.  In order to avoid creating a “dragging” force that will interfere with speed or navigation, sacrificial metals are often shaped in special ways that allow them to integrate well with boat design.

Pitch, as stated in propeller specifications, refers to a theoretical distance.  This is because conditions in the real world are rarely ideal.  The vagaries of weather and water conditions will normally mean that propeller pitch as actually experienced will usually be less than the pitch number advertised for that propeller.

Many boaters have misconceptions about propeller pitch.  They may think that a larger pitch number is always preferable since it will mean that the propeller, and hence the boat, moves forward more in a single revolution.  This in turn, they believe, will make the boat able to go faster.  The truth, however, is that every motor has a limit.  Using a propeller with a pitch that is too large will only stress the motor, causing the boat to go slower.  It can also cause a dependable engine to wear out long before its time.

The saying is true, however, in more ways than mere longevity.  Stainless steel props also offer a high level of performance and are generally regarded as providing boaters with higher performance than both bronze or aluminum propellers of similar size and dimensions.  Part of this facet of stainless steel propeller derives from the fact that casting such propellers is a process that allows for very finely graded modifications.  This allows propeller manufacturers to “tinker” over time until they achieve the perfect contours for the hub and blades of any given stainless steel propeller.

Some of the main types of stainless steel propellers on the market today are Ballistic, Apollo, and Michigan Match propellers.  All of these are manufactured by the Michigan Wheel Company, a producer of fine stainless steel propellers for many years.  Boaters can find the appropriate propeller for their particular hull and engine quite simply by using a propeller selection tool – this is a good idea, because the sheer variety of stainless steel propellers on the market can be staggering at first glance.

Cost aside, however, there are other reasons why aluminum boat props stand out.  Since they are very lightweight, they are an appropriate choice for small boats.  Their lack of weight also makes them quite easy to carry about; an aluminum propeller is much more portable than one made of a heavier, denser kind of metal.  When paired well with an engine and boat, they can deliver high performance.  Best of all, perhaps, is their availability.  Many different kinds of aluminum propellers are available for sale today.  For example, both Vortex and Michigan Match propellers are manufactured in aluminum by the Michigan Wheel company.

Like all propellers, aluminum boat props are measured mainly in the two dimensions of diameter and pitch.  Boaters will need to be sure that they choose a propeller that is sized appropriately for their watercraft.  Another decision to be made is three blades versus four blades, as aluminum props come in both configurations.

Some boaters know that they want to move from a three-bladed propeller to one with four blades, or vice versa.  A propeller selection tool can be invaluable to guide them during this process so that the propeller eventually selected functions well with an existing propulsion system.  Another reason to use a prop guide is to find a propeller that will enable a boater to stay within the recommended revolutions per minute range for his or her engine – and do it while the boat is being pushed to full throttle.  A propeller selection tool can take into account several variables at once so that the boater receives an ideal solution for his or her own particular needs.

Propeller selection tools can also assist boaters who are happy with their current prop to find a new one that matches in all specifications.  This can be quite helpful for boating enthusiasts who want to have a spare propeller on hand or for those who have noticed that their existing propeller is showing signs of wear or damage.  Replacing a well-functioning propeller with one that is identical in all-important particulars is an excellent strategy in such cases.

In order to provide effective propulsion, a propeller must match not only the engine to which it is attached, but also the hull and the gearing involved.  Engines, of course, produce horsepower.  This power goes through the gears and bearings to reach the output shaft.  The purpose of the gears is to reduce revolutions per minute to a speed that is appropriate for the propeller that has been installed.  During this process, a small amount of the power generated by the engine ends up being lost to the force of friction.  Typically, this loss hovers somewhere around 5%.

A properly sized propeller will absorb whatever power is remaining from the engine with high efficiency.  This will enable the boat to reach its optimal speed in a fuel-efficient manner.  It also means that the throttle and controls will be able to respond quickly to changing demands on the engine.  In addition, excellent propeller design does allow some reserve power to be applied as needed.

In contrast, a propeller that is not properly sized will cause the engine to work a great deal harder than it should have to.  This leads to several negative outcomes, including high costs for maintenance and repair, a shorter engine life, and poor fuel efficiency.

When to Replace an Outboard Propeller: The Vibration Issue

Many boaters are not completely clear on the issue of when they need to replace a propeller.  In some situations, of course, the need to purchase a replacement propeller is beyond doubt.  If the propeller shows visible signs of wear such as cracking, chipping, or warping it must be replaced.  To continue with the propeller in that state is inadvisable from a performance and fuel economy standpoint, and from a safety perspective.

What if, however, the outboard motor seems to be vibrating a great deal when this has not typically been true of your system?  In this case, if the propeller looks as good as new, or very nearly so, boaters may conclude that the problem must not lie with the propeller.  This, however, could be a misjudgment.  In many cases, damage to the blades of a propeller is not visible to the naked eye.  Blades may be bent slightly out of true, in which case the propeller should be replaced.

Also featuring .060 cupping is the Acme 1209, which has the same diameter as both of the other Acme boat propellers but features 12 full inches of pitch.  It should also be noted that the 1218 and 1604 have right-handed rotation while the 1209 has left-handed rotation, a distinction that can be important, particularly when watercraft feature two propellers.

Acme propellers are manufactured by the Acme Marine Company, which uses a CNC process to reproduce accurately the exact diameter, cup, and pitch desired for each unit.  With a CNC system, incremental changes are made possible, which explains the small gradations seen from one propeller to the next.  This type of system is essential if boating enthusiasts are going to have a propeller that truly fits all the design specifications that will allow them to make the most out of the boating experience.

Acme propellers enjoy a very low return rate, which indicates high customer satisfaction with the finished product.  Their operation is considered smooth, and perhaps best of all for many American consumers, the propellers are wholly made and manufactured inside the United States.

Ballistic Yamaha boat propellers, in contrast, are available in stainless steel instead of aluminum and offer boating enthusiasts the choice of either 3 or 4 blades – though in the Ballistic line, there are many more 3-bladed than 4-bladed options.  In addition, the Ballistic line does offer an occasional propeller with left-handed rotation; the majority of options have right-handed rotation.

Vortex aluminum Yamaha props all offer right-handed rotation.  Again, most are 3-bladed but some 4-bladed options are available.  Diameters in this line range from 10″ up to 15 ¼” and a wide variety of pitches are available, resulting in many different permutations so that boaters can choose the propeller most likely to yield the performance results they desire.

For example, the Vortex 992003 propeller has a diameter of 15″, very nearly the maximum in this line, and offers a pitch value of 17″.  It has three blades and uses right-handed rotation.  This propeller will work with a large variety of engines, from a 200 HP Thru Hub Exhaust first produced in 1976 to a 300 HP E-Tec engine still in production today.

With such a diverse collection of propellers that will work with Yamaha outboard engines, boaters can truly customize their watercraft to make their boating experiences all the more enjoyable.