There is perhaps no sight more graceful than a sailboat gliding across the water. Did you know that in addition to sails, sailboats also have engines? A sailboat’s engine and propeller enable it to maneuver in and out of a marina or harbor, and also to sail under power when the wind is uncooperative or blowing from the wrong direction (i.e., the same direction that the sailboat is headed).
But how to choose a sailboat propeller? There are several important considerations to keep in mind starting with your budget and needs. Fixed blade propellers are the most economical option but produce a lot of drag when under sail, while folding and feathering propellers produce less drag but are more expensive.
With the right propeller, a sailboat’s performance under sail can be optimized as far as speed and minimal engine wear, while being efficient and highly maneuverable moving forward and in reverse under motor power.
The Different Types of Propellers
Ideally, you want to achieve maximum power, or thrust, and optimal engine efficiency when the sailboat is moving under power, and zero drag while under sail. Drag is essentially a negative force that acts against the forward movement of the vessel, hindering speed, reducing sail and motor efficiency, and increasing wear on engine and propeller components.
Drag in the context of sailing is created by the propeller while the sailboat is moving under sail. Because the propeller is an immovable object attached to the stern (rear) of the sailboat, it creates a substantial amount of resistance in the water. Sailors liken the effect of propeller drag to towing a large bucket under water, or driving a car downhill with the brakes engaged.
There are three primary categories of sailboat propellers, (1) fixed blade propellers, (2) folding propellers and (3) feathering propellers. Each type has its benefits and limitations, and the price range vary widely from one to another.
Fixed Blade Propellers
This is the most common type of sailboat propeller and is typically the type of propeller that is pre-installed by manufacturers. As the name suggests, fixed blade propellers have two or three (sometimes four) blades that are arranged in a fixed, unadjustable position. Because they are the simplest in design and the easiest to maintain, fixed blade propellers are typically the most economical option for sailors and deliver the most bang for your buck.
As far as forward thrust goes, fixed blade propellers perform well, but when operated in reverse (such as maneuvering into a boat slip), they perform poorly, losing as much as 50% of the thrust. This is due primarily to the fact that when the propeller is run in reverse with blades that are fixed, the trailing edge of each blade must act as the leading edge, which is highly inefficient.
The main drawback of fixed blade propellers is the amount of drag they create while under sail. The negative effect that this type of propeller has on a sailboat’s performance can be put in several different ways:
- The amount of drag created by a fixed blade propeller can reduce a sailboat’s speed under sail by as much as 1 to 1 ½ knots
- Up to 20% of a sailboat’s speed can be reduced by dragging a fixed blade propeller underwater
- A fixed blade propeller can generate upwards of 70 pounds of pull in the opposite direction as the sailboat’s heading
- A fixed blade propeller’s drag can add one hour to a sailboat’s 20-mile daysail
Certain sailboat engines allow the sailor to disengage the transmission gear, thereby allowing the fixed blade propeller to spin freely while under sail. While this does reduce the amount of drag at certain speeds, the resulting wear on bearings and other transmission parts may cause more harm than good. For longer trips, it may be advisable to place the sailboat engine in gear or operate a shaft brake.
Folding propellers usually come in two or three blade configurations, although newer sailboats are outfitted with four blade propellers. Folding propellers have the lowest drag characteristics of all types of propellers and are a popular choice among sailors willing to pay substantially more than they would for traditional fixed blade propellers. (Although still less than feathering propellers.)
Another benefit of folding propellers, aside from their low drag quotient, is that their streamlined profile, when closed, allows the propeller to be less prone to getting entangled in kelp, flotsam, jetsam, or rope. Such items do pose problems for other types of propellers, and experienced sailors carry sharp knives to free their sailboats from entanglements, often involving fishing nets and crab pot lines in commercial waters.
When under sail and with the engine disengaged, the flow of water past the propeller forces the blades to fold inward with the tips of the blades pointing aft (toward the rear of the sailboat). When the sailboat engine is engaged and revved, the centrifugal force of the shaft spinning will cause the blades to open and propel the sailboat forward or in reverse.
Although engineering and design breakthroughs have greatly enhanced their performance, folding propellers are still shedding a bad reputation for being problematic, particularly with regard to:
- At low speeds while under sail, the pressure of water flowing past the propeller is not strong enough to push the blades into a folded position
- Blades can act independently of each other (e.g., one open and one closed), particularly two-bladed propellers that are in a vertical alignment where the top blade is closed, but the bottom blade is open due to the force of gravity
- The amount of centrifugal force needed to re-open the blades requires revving the engine to higher RPMs
- Very poor thrust when moving in reverse
Many of these problems have been resolved by innovations such as geared blades that are completely synchronized with one another and, therefore, open and close in unison. Recent blade designs have also made forward thrust even more efficient while improving mobility moving in reverse. Folding propellers are now featured standard equipment on many new sailboats from reputable manufacturers.
Folding propeller blades are not attached to the shaft in the same manner as fixed blade propellers and because the blades fold in and out depending on the sailboat’s speed, you can expect higher maintenance and repair costs associated with this type of propeller.
It should be noted that folding propellers may not be an option for larger sailboats with longer keels because these vessels often have an opening (called an aperture) between the rudder and the keel where the propeller sits. Because a folding propeller has blades that fold when under sail, the length of the folded blades may extend into the rudder.
When under sail, the blades of a feathering propeller pivot in relation to the shaft so that the leading edges cut straight into the water with minimal drag. This type of propeller is by far the most costly option as far as sailboat propellers go, but depending on the sailor’s needs, it may be the best one as well.
When the engine is engaged, the torque of the propeller shaft causes the blades to pivot to a certain pitch to provide a highly efficient forward thrust. And unlike fixed blade or folding propellers, because of the unique pivoting action of the blades feathering propellers are nearly as efficient moving in reverse. This feature is particularly appealing to sailors who must navigate in and out of congested marinas or tight corners in cramped harbors.
Another feature offered by feathering propellers is the ability to adjust the pitch, or the blades’ angle of attack to the water, to best suit the engine to which it is attached. This will result in optimized thrust moving forward and backward and yield improved fuel efficiency.
In higher end models, this pitch adjustment is done automatically. As the engine revs, the pitch of the blades will adjust. In other versions, the adjustment must be performed manually with the propeller out of the water. But in either case, feathering propellers afford the sailboat owner the ability to pair the propeller’s performance with the sailboat’s engine capabilities, thereby producing a custom fit.
On the downside, aside from their cost, feathering propellers also have more moving parts than a fixed blade propeller. Although they are considered more robust than folding propellers because the blades do not fold in and out, the blades on a feathering propeller are mechanically attached to the shaft and rely upon internal mechanisms to pivot the blades straight under sail and in forward or reverse pitch.
Therefore, on top of the expense of purchasing one, you can expect that regular maintenance costs will exceed those associated with a fixed blade propeller.
Choosing the Right Propeller for Your Sailboat
Ultimately selecting the right propeller for your sailboat depends on your motoring needs as well as your budget. If maneuverability under power is an absolute must, then strong consideration should be given to a feathering propeller. If your main concerns relate to minimal drag under sail, then either a folding or feathering propeller may be options for you, with cost being the deciding factor.
If, however, you are looking for basic sailboat motoring and you occasionally go out for a leisurely sail around local waters, then perhaps a fixed blade propeller is the right choice for you.
It should be noted that in order to make a truly informed decision, you should look beyond the three main categories of sailboat propellers and consider specific characteristics of propellers such as diameter and pitch, which will provide you with the true performance parameters you can expect out of a propeller.
Will a Bigger Propeller Make Your Boat Faster?
In simple terms, when a propeller turns, its blades create displacement of water that generates thrust and propels the sailboat forward or backward depending on the direction of the propeller’s rotation. On most sailboat engines, forward motion correlates to right-handed rotation (the propeller turns in a clockwise direction when viewed from the rear of the vessel).
The hydrodynamics of a propeller’s blades propel the sailboat in much the same way that the shape of an airplane’s wing creates lift. The sailboat’s engine turns the propeller at varying speeds, which correlate to revolutions per minute (RPM). This rotation is converted by the propeller into thrust.
So, will a bigger propeller make your sailboat faster? Not necessarily. While a bigger propeller will have larger blades that, in theory, should produce greater thrust, the size and length of the vessel and the power and capacity of the sailboat’s engine must be considered. An underpowered engine would struggle to turn an oversized propeller and could become damaged in the process.
All propellers share the same primary characteristics, whether they are powering a sailboat, a luxury yacht or even a cruise ship. Some of these characteristics directly affect the thrust characteristics and overall efficiency of the propeller in specific relation to the sailboat engine on which it is installed.
Hub and Shaft
The hub is the central core of the propeller and is where the propeller blades attach. The hub, in turn, is attached to the propeller shaft, which is part of the sailboat engine’s drivetrain and rotates the propeller.
Propeller Blade Anatomy
Although today’s sailboat propeller blades come in a wide variety of shapes, their basic anatomy and how they function remain the same. Understanding the basic terminology of the various blade parts is instrumental in making an informed decision when selecting a propeller for your sailboat.
- The root is the innermost portion of the blade that attaches to the propeller hub. Since it is closest to the shaft, it travels the shortest distance per revolution. On the outermost edge of the blade is the tip which is the furthest away from the shaft and travels the longest distance per revolution and therefore undergoes the greatest amount of stress when the sailboat is moving under power.
- An important concept to understand with regard to propellers and blades is that the blade root and blade tip are different points on the same blade. Even though they are different distances from the hub, they still complete a revolution at the same time. This explains why the tip moves faster than the root, and this is particularly relevant to propeller pitch, which is discussed later.
- The blade face is the side of the propeller blade that faces aft (rear of the sailboat). Also referred to as the pressure side, the blade face pushes the water away from the propeller as the sailboat moves in a forward direction. In other words, water is forced away from the propeller, which is mounted at the rear of the sailboat, thereby propelling the vessel in the opposite direction.
- The blade back is the opposite side of the propeller blade that faces forward (front of the sailboat). This side of the propeller blade is often referred to as the suction, or negative pressure, side. As the propeller rotates, the blade back is the first surface of the propeller to come into contact with the water, and it creates a pressure differential which in conjunction with the blade face, creates thrust.
- The leading edge of the propeller blade is the forward portion that leads or cuts into the water first. As the propeller rotates, it generates the flow of water across the blade.
- The trailing edge is on the rear portion of the propeller blade adjacent to the aft side of the hub and is the edge of the propeller blade that exits the water flow. The leading edge and trailing edge must work together to produce clean, efficient thrust.
This is considered one of the more vital dimensions to differentiate one propeller from another (the other being pitch) and provide a quantifiable means of predicting performance. If you imagine a circle formed by the blade tips rotating, the diameter would be the length of the line (usually in inches) that passes from one edge of the circle, through the center of the circle and continuing to the opposite edge.
Although propellers with larger diameters are able to convert power into more thrust through the water, they must be properly sized to the sailboat’s engine and drivetrain due to the greater amount of power needed to drive the blades through the water. Larger diameter propellers run at lower revolutions per minute versus smaller diameter propellers, which typically run at higher RPM.
Another important consideration with larger diameter propellers is the increased amount of drag that they create under sail, particularly fixed blade propellers.
This refers to the distance that the propeller would travel in one full revolution if it were rotating in a solid medium with zero slippage. Pitch is usually indicated in inches, so if a propeller has a 20 inch pitch, then it would move 20 inches when turning one revolution. The pitch of a propeller can be likened to the turning of a screw into wood.
A propeller rotating in the water will not achieve perfect or full pitch because as the sailboat moves in the water, it experiences drag against its hull and the propeller. In other words, there is a loss of efficiency, and it can vary anywhere from 5% to 40%. The better the propeller is matched to the vessel and its engine, the lower the propeller slip value, and the greater the efficiency at which the propeller performs.
As with propeller diameter, the higher the pitch the greater the potential amount of thrust that can be generated by the propeller’s blades. The caveat, however, remains the same. A propeller’s pitch must be properly matched to the capacity of the sailboat’s engine and drivetrain. A mismatched pitch can result in very poor performance under power, or worse yet, serious damage to the propulsion system itself.
It should be noted that pitch is a broad and general term, or characteristic, with respect to sailboat propeller blades. There are specific types of pitch available, and each is worth considering.
- Constant Pitch – When the propeller blades have the same angle of attach toward the water from the root to the tip, they are said to have constant pitch. Constant pitch propellers tend to be more affordable because they are cheaper to manufacture.
- Variable Pitch – A variable pitch propeller blade has a higher pitch at the root of the blade and lower pitch at the tip producing a twisted shape to the blade. This variable pitch design is needed to produce an even thrust throughout the blade because the tip travels at a faster speed than the root and would, therefore, produce more thrust.
Of all the various characteristics of propellers, diameter, and pitch are the most important when it comes to efficiently creating thrust, and therefore, these are the measurements that should be carefully considered when choosing a sailboat propeller. Sailboat propellers are often stamped with two sets of numbers, the first being the diameter and the second being the pitch.
- Example: A propeller marked 20 x 16 would indicate a 20-inch diameter with a 16-inch pitch
Fuel Consumption Chart
Under ideal conditions, a sailboat would travel completely under sail with minimal time spent motoring under power. Unfortunately, sailors are at the mercy of winds, currents, and waves. As such, a sailor may be forced to cruise under power and sometimes for long or unknown stretches. It is, therefore, useful to know how much fuel a sailboat consumes when running under power.
There are various methods for estimating fuel consumption, which would also enable projecting range and engine running hours. One method is to estimate burn rate and then extrapolate estimated running hours based on the sailboat’s fuel tank capacity.
- Example: A common fuel burn rate that seems to apply to a broad range of engine sizes is .75 gallons per hour (GPH). If a sailboat’s fuel tank has a maximum capacity of 50 gallons, then dividing 50 by .75 equals 67 hours of sailing under power. However, it is important to note that this .75 GPH burn rate is under ideal weather conditions (no current, calm waters).
Unlike automobiles, it is virtually impossible to convey fuel efficiency for sailboats in terms of average distance per gallon of fuel (e.g., miles per gallon/MPG). This is primarily due to the many variables that a sailboat running under power would encounter, namely ocean currents, large waves, and head winds, any one of which could easily double the amount of fuel that a sailboat consumes.
The most common expression of a sailboat’s fuel consumption is stated in terms of gallons of fuel burned per hour. Sailboat engines have varying rates of fuel consumption depending on their size, typically indicated by number of horsepower (HP). As a general rule, the higher the horsepower, the greater the fuel consumption.
Here is a simple chart showing approximate fuel consumption rates (gallons per hour) for sailboats with different engine sizes (horsepower):
|ENGINE SIZE||FUEL CONSUMPTION|
|10 HP||0.5 GPH|
|20 HP||0.9 GPH|
|30 HP||1.4 GPH|
|40 HP||1.8 GPH|
|50 HP||2.3 GPH|
The above figures reflect estimated fuel consumption under benign weather conditions. In adverse sailing conditions, the fuel consumption figures above would increase by a minimum of 40%.
There are other factors that directly affect a sailboat’s fuel consumption and fuel efficiency, including:
- Sailboat Hull Size and Shape – The size, shape, and length of the sailboat’s hull will have a direct impact on fuel consumption and fuel efficiency. As a general rule, the more streamlined the shape, the less water that has to be pushed out of the way, and the less fuel that has to be consumed in the process. Water displacement is one way to quantify this.
- Engine Size – Engine size is another factor that directly contributes to a sailboat’s fuel consumption. The larger or more powerful the engine, the greater its fuel consumption. However, this is not to be confused with fuel efficiency. A larger engine may consume more fuel but yield more nautical miles traveled per gallon than a smaller engine that burns less fuel but runs at higher RPMs and therefore produces less range.
- Propeller Type – Certain propellers, as we have seen, produce more thrust and deliver power more efficiently than others. For example, a variable pitch blade will consume less fuel than a constant pitch blade.
- Hull Condition – A clean hull cuts through the water more efficiently than a dirty one and will consume less fuel.
- Weight – Heavy cargo can weigh down a sailboat and dramatically increase its fuel consumption rates.
- Adverse Weather Conditions – Weather conditions can have the greatest effect on a sailboat’s fuel consumption. Sailing under power into a strong current or headwind can double your fuel consumption, as can riding through strong waves.
- Over-revving the Engine – Revving the engine too high can unnecessarily burn fuel, not to mention increase wear on the drivetrain. Most sailboat engines achieve optimal running efficiency at 70-85% of maximum RPMs.
Now that you have learned everything you need to know to make a wise choice when it comes to selecting a sailboat propeller, now all you have to do is go shopping for one!