Tips on choosing the right outboard propellers
A boat that has the right boat props will function properly for many years and provide one with many hours of fun and enjoyment. Boat props, more commonly known as boat propellers, help to steer the boat and give it sufficient power to run properly. The type of propeller chosen will depend on what type of boat one has and which activities the boat is used for.
Choosing the Right Material
A good propeller will be made from either sand cast aluminum or stainless steel. Stainless steel boat propellers are in fact the best, but they are quite expensive. Aluminum propellers are well made and will last for many years, even though they are not as high quality as stainless steel propellers.
What Should be Considered When Choosing a Propeller?
Before buying a propeller, check the owner’s manual to see which type of propeller is recommended. The owner’s manual will detail what the diameter and pitch of the propeller should be. It will also note whether the propeller should turn to the right or the left and whether a three or four blade propeller should be used.
The weight of the boat will determine how high or low the pitch should be. “Pitch” is the measurement of how much a boat moves when the propeller turns one full time. The lighter the boat is, the higher the pitch should be. It should be noted that the heavier the boat is, the stronger the propeller will need to be. Most heavy recreational boats use stainless steel propellers, although aluminum propellers are by far more widely used.
One should also check the engine shaft before buying a new propeller and see what shape it is made in. The propeller hub may be cylindrical, conical, radius, or barreled in shape and it is important that the hub fits properly onto the engine shaft. The propeller should also turn in the same direction as the shaft.
There are so many boat props on the market that it can be hard to know which one to purchase. First of all, one will want to make sure the propeller is the right size and make for the boat. One will then want to find a prop brand name and model that is known to be high quality.
Process of selecting the correct propellers
When you are shopping for high performance outboard boat propellers it is important to do your homework so you choose the right replacement for your old propeller. If you fail to choose the right propeller you will not improve your boat’s performance on the water. You will need to consider size, style, and material when you are comparing all of the different top name brands on the market today. Understand how to determine which size and style is best for your boat and make the best investment of your hard-earned money.
Prop Size and What to Consider
You may already know that not all boat propellers are the same. Understanding how they differ in size in important. Prop sizes refer to the diameter or the propeller and its pitch. Boat propellers with smaller diameters are meant for smaller board with smaller engines. As you might assume, larger propellers are designed for maneuvering larger boats. Pitch means how much the boat will move in one revolution. If you purchase a propeller with a higher pitch your boat will move faster as long as the engine is powerful enough to keep the RPMs in the right range.
Number of Blades
If you currently have a propeller with 3 blades you may be able to use a 4 blade prop interchangeably. You can also go from a 4 blade to a 3 blade propeller without a drastic change in performance. The diameter may require an adjustment if you choose a different number of blades for your replacement.
Which Material is Best
Boat propellers are made of composite, aluminum, or stainless steel. You should understand the pros and cons of each material before you make a purchase. Composite propellers are inexpensive and durable. Aluminum propellers are easy to repair and are most common today. Stainless steel propellers offer the best durability and performance and are the most expensive.
Choosing boat propellers is not an easy task. Make sure you ask propeller experts for advice if you are ever overwhelmed. By considering size, styling and material, you can improve the performance of your boat on a budget.
Stainless or Aluminum outboard propellers?
When you are shopping for a replacement prop for your boat you need to do your homework. With so many different styles and sizes, choosing the right propeller can be a daunting task. If you know you want a boat propeller with the same diameter and the same style of blades you need to move on to choosing a material. There has been a lot of debate on whether stainless steel or aluminum boat propellers are better. To make the best investment, you should understand the pros and cons of each. Review the benefits and challenges of each and then make your decision.
Benefits of Aluminum Boat Propellers
Aluminum propellers are the most common because they are very affordable and offer good performance under most circumstances. One reason why many boaters choose aluminum propellers over stainless steel applications is because aluminum weighs about half as much. These propellers are also very easy to repair when damaged.
While aluminum is a great choice it can flex under extremely high rpms and this will cause a drop in pitch. The blades often tend to be thicker than stainless steel blades so they will cause more drag. They are not ideal for sandy conditions so you must consider use.
The Benefits of Stainless Steel Propellers
Stainless steel propellers are designed for high performance and they are the most durable choice on the market today. They have thinner blades which will reduce dragging and will not flex when you are traveling at high speeds. They are also designed for abrasive environments and can handle adventures in sandy and shallow waters.
One of the biggest setbacks has to do with price. Stainless steel is simply a more expensive material and it is more difficult to repair when damaged. These propellers weight twice as much as aluminum which means you need a more powerful engine.
Ultimately, the decision depends on use. If you are not looking for power, the aluminum prop may be the right choice. If performance is your primary concern, stainless steel props will better meet your needs. Consider durability, pricing, and overall use when buying your boat propellers.
Propeller Terms: Understanding Ventilation and Track
Many non-boaters think of a propeller as simply a spinning assembly of three blades that provides propulsion for a watercraft. The truth, however, is that outboard boat propellers are highly complex pieces of equipment that are designed according to exacting specifications so as to maximize issues such as fuel efficiency and minimize others such as the drag due friction. As might be expected, therefore, there is a specialized vocabulary used to describe the many aspects of outboard boat props.
Common terms such as diameter, pitch, and blade number might be familiar even to casual boaters, while other terms are much less widely known and understood. These include such terms as ventilation, pitch reference line, cylindrical section, and track.
Ventilation refers to a circumstance in which either exhaust gases or air from the surface is pulled into the spinning blades of the propeller. This is not a good situation as it causes boats to lose speed. It also strains the engine by increasing the level of RPM the motor must put out. Ventilation has several possible causes. It may occur when boaters turn corners much too tightly or when a motor is mounted excessively high along the transom. A boater who over-trims his or her engine may also contribute to increased ventilation.
Track indicates the maximum difference between the height of one blade on outboard props and the height of another. Propeller blades, of course may not have exactly equal heights. Because track measures the difference between the highest and the least high, it is always a positive number.
Lesser-Known Propeller Terms
There are a large number of different manufacturers of outboard props, and each manufacturer makes a great many different models. The huge variety of propellers that results from this can seem overwhelming to a boating enthusiast, particularly to a novice. Thankfully, all manufacturers of propellers do use standardized terminology to describe each of their models. This allows for easy comparisons from one brand of propeller to the next, or from one model of propeller to the next inside the same brand.
Blade Thickness Fraction
Usually abbreviated as BTF, this describes the maximum thickness of the blades on outboard propellers as they extend to the center line of the propeller, also known as the diameter. Blade thickness is an important characteristic because blades that are manufactured to be too thin will not prove strong enough under load; they may crack or break. On the other hand, a blade that is thicker than necessary will be demonstrably less efficient, in part because of the excess weight created by the additional thickness.
This refers to the total area of the circle that is “swept out” by the motion of the blades on outboard boat props. If the disc area of a propeller is not given, it can be easily derived from other values that are given. Simply take the diameter figure and halve it, then square the resulting number. Then multiply the result by pi, which is generally given as 3.14. For additional accuracy, use a more exact value of pi; this will involve multiplying by a number with more decimal places.
Outboard Propellers and the Issue of Vibration
When it comes to having a look at the various outboard boat props on sale today, boating enthusiasts will find that they have several major categories from which to choose. Outboard propellers may consist of Apollo Stainless Steel Propellers, Ballistic Stainless Steel Propellers, Vortex Aluminum Propellers, or Michigan Match Propellers. Among these various kinds of outboard propellers, boaters can find virtually any combination needed of pitch and diameter in both right handed and left handed rotations.
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.
Outboard Propellers 101
Once you have the outboard motor for your boat it is important that you choose the right outboard propeller as well. The propeller should be suitable for the type of boating that you plan to do, different props are available for shallow draft boating, for water skiing, trolling and for other types of boating. You also need to make certain that the prop will allow the engine to reach the manufacturers recommended RPM’s when at full throttle or you risk damaging your propeller and engine. Choosing the right propellers is critical for the performance of your boat and for your boating enjoyment.
Boat propellers are often described by their diameter and pitch. The larger your engine is in horsepower, the larger diameter prop you will generally want. The pitch describes how far the propeller advances with each revolution. A lower pitch gives better pulling power, however if you use a propeller with a pitch that is too low you won’t be using all the power of your engine. A higher pitch allows the boat to go faster, but this is only true if you have enough horsepower to keep the RPM’s at the optimum range. A pitch that is too large makes the propeller heavy and it demands more power than the engine can provide.
After determining the proper diameter and pitch you need to consider how many blades to have on your prop. Outboard propellers typically come with either 3 or 4 blades. Unless you are running a high horsepower motor or using the boat for bass fishing, you will most likely want to choose a 3 blade propeller. 4 blade propellers are designed more for high speed usage with less steering torque and vibration at high speeds. Water skiers can also benefit from using a 4 blade propeller.
The next thing to consider is the construction material of the propeller. Outboard propellers can be made from aluminum, stainless steel, or composites. Aluminum is the most common choice for the manufacturer’s stock factory equipment. They are inexpensive and easy to repair and they work well in a wide range of applications. Stainless steel propellers are more expensive, but they are also more durable and less likely to suffer damage. Composite propellers are typically only utilized on very small horsepower applications, and are not nearly as popular as aluminum and stainless propellers.
Your main goal when choosing a propeller is to ensure that it allows the engine to perform optimally at wide open throttle. Boat dealers and Prop Shops can provide you with formulas to determine the proper pitch, diameter and number of blades based on your engines power or horsepower.
Cavitation is an issue that many users of outboard propellers do not fully understand. They may be familiar with the effects of cavitation, of course, without being cognizant of the nature of this phenomenon or the variety of factors that can contribute to it.
What is Cavitation?
Cavitation happens when water vaporizes due to an area of reduced pressure along the back side of the blade of a propeller. All outboard boat props are subject to cavitation to one degree or another. Small amounts of cavitation, in fact, are perfectly normal during the operation of outboard boat props.
A large amount of cavitation, however, is not normal. When a propeller cavitates too much, it can cause actual damage to the surface of the propeller blades. This damage happens because microscopic air bubbles are collapsing against the surface of the blade. Too much activity of this sort is not good for the physical condition of a propeller blade.
Causes of Excessive Cavitation
There are several reasons why excessive cavitation may occur. One common cause of the problem is a mismatch between the style of propeller installed and the intended use of the watercraft. If the blade edges of a propeller are already damaged, this can lead to the cavitation levels that are much higher than normal. A third common cause is a propeller with the wrong pitch installed on a motor boat.
To avoid cavitation, be sure to install an outboard boat propeller that is appropriate for the boat and that matches all relevant specifications for the motor.
Understanding Pitch and Slip
Pitch is becoming an increasingly familiar term to boating enthusiasts interested in purchasing aftermarket outboard propellers for their watercraft. Usually defined as the distance a propeller will move through a soft medium such a balsa wood after one full turn of the blades, pitch is an idealized figure that is rarely seen in actual use of outboard boat props.
Boaters, therefore, might question the purpose of a pitch specification for a given propeller. If the prop is advertised as offering 13 inches of pitch, but this is a theoretical number, what can a boater expect in terms of real world performance?
Fortunately, there is another term that helps to clear up this conundrum: slip. When it comes to outboard boat propellers, the word “slip” gives the difference between the theoretical and the actual. That is, slip describes the degree to which a given propeller usually achieves its stated pitch. A propeller that is properly installed and appropriate for both motor and boat can usually achieve as much as 90 percent of “perfect pitch,” though in some cases a figure closer to 80 percent is more realistic.
Why Not Just Scale Down Pitch Numbers?
The concept of slip usually raises a simple question: why do propeller manufacturers use idealized pitch numbers in the first place? Why not just multiply ideal pitch by 80 or 90% and use that figure as the pitch? The answer to this lies in the complexity of watercraft usage. Boats will achieve closer to their ideal pitch when conditions are optimal. Slip, therefore, depends on context whereas pitch values remain constant.
Propeller Terms: The Intricacies of Exhaust
Even an absolute newcomer to the sport of pleasure boating begins with a basic understanding of engine exhaust, but it may come as a surprise to some that outboard boat props actually have several ways of releasing or venting the exhaust produced by the motor. One of the most common arrangements is through “hub exhaust.” These propellers have blades attached to a hollow barrel through which exhaust can pass. This allows exhaust to avoid contacting the blades directly, which improves the way water flows onto the blades.
Another arrangement is “non-through hub exhaust.” As one might expect, in this system there are not any holes for exhaust near the shaft of the propeller. These systems are most commonly found on stern drive motors with low horsepower and on some low horsepower outboard motors.
Still other outboard propellers use a system known as “over the hub exhaust.” In this arrangement, blades are directly affixed to a small tube that runs near the propeller shaft. This eliminates a large exhaust tube. Over the hub exhaust systems are often employed when boating enthusiasts want to reach the maximum possible top speed of their motor.
The final system commonly employed is termed “over/through hub exhaust.” This is a combination arrangement that uses features from both the over-hub and through-hub systems.
Since outboard props can use a variety of exhaust systems, the boater determined to get superior performance should carefully consider which system would best suit his watercraft and his needs.