# Prop School – Part 6: Slip

Response to my Prop School series has been gratifying. It has generated a lot of good discussion (online and off) regarding propeller design, function and application. One of the most common questions is about prop slip. It is the most misunderstood of all propeller terms.

Propeller blades work like wings on an airplane. Wings carry the weight of the plane by providing lift; marine propeller blades provide thrust as they rotate through water. If an airplane wing were symmetrical (air moves across the top and bottom of the wing equally), the pressure from above and below the wing would be equal, resulting in zero lift.   The curvature of a wing reduces static pressure above the wing — the Bernoulli effect — so that the pressure below the wing is greater. The net of these two forces pushes the wing upward. With a positive angle of attack, even higher pressure below the wing creates still more lift.

Similarly, marine propeller blades operating at a zero angle of attack produce nearly equal positive and negative pressures, resulting in zero thrust. Blades operating with an angle of attack create a negative (lower or pulling) pressure on one side and a positive (higher or pushing) pressure on the opposite side.  The pressure difference, like the airplane wing, causes lift at right angles to the blade surface. Lift can be divided into a thrust component in the direction of travel and a torque component in the opposite direction of prop rotation.

Prop Slip

Slip is the difference between actual and theoretical travel through the water. For example, if a 10-inch pitch prop actually advances 8-1/2 inches per revolution through water, it is said to have 15-percent slip (8-1/2 inches is 85% of 10-inches). Similar to the airplane wing, some angle of attack is needed for a propeller blade to create thrust. Our objective to achieve the most efficient angle of attack.  We do this by matching the propeller diameter and blade area to the engine horsepower and propeller shaft RPM. Too much diameter and or blade area will reduce slip, but at a consequence of lower overall efficiency and performance.

Calculating Rotational Speed, Blade Tip Speed and Slip

Our propeller engineers study props at the 7/10 radius (70% of the distance from the center of the prop hub to the blade tip). The 7/10 radius rotational speed in MPH can be calculated as follows:

And can be shown by a vector arrow.

Blade tip speed can be calculated using the following equation:

Forward speed is shown by an arrow in the direction of travel. The length of the arrows reflect speed in MPH for both the measured speed and the theoretical (no slip) forward speed.

Prop Slip Calculator

Back in the day when the Everything You Need to Know About Propellers book was published, the Internet didn’t exist and you had to actually use these cumbersome formulas or rely on the Quicksilver Propeller Slip Calculator.

We Have an App for That

Click here to see some real world examples on how I use the app.

I hope you have found my Prop School blog series both educational and useful. I’ve enjoyed sharing with you.

# Prop School – Part 5: Blade Efficiency

For this, the fifth installment of my Prop School series I will review the various propeller blade designs and how they – along with rotation – affect propeller efficiency and overall boat performance.

Rotation. Propellers come in both right and left-hand rotation. Standard rotation for both outboards and sterndrives is right-hand: the prop spins clockwise when in forward gear.  Left-hand props spin counter clockwise.  Left-hand props are typically used with multi-engine applications. The counter-rotation prop works to balance (or reduce) the torque effects from the right-hand prop. Most twin engine applications are setup with the props “turning in”; the port engine spinning right-hand and the starboard engine spinning counter clockwise.

Hull types and designs respond differently to prop rotations. Some need additional stern lift to reach maximum efficiency and performance. To obtain this, the rotation of both propellers is set up, so they rotate away from each other. We call this turning the props out.  The left-hand rotation prop is on the port side and the right-hand rotation is on the starboard side.

For example, a high-speed catamaran loaded with gear and passengers often runs best with 5-blade cleaver props with 15-degree rake. Turning the props in pulls the stern down, enabling the boat to float over chop. With lighter loads and ideal conditions, the same cat can gain 6 to 8 mph when using 18-degree rake, 5 blade cleavers “turned-out.”

In theory, two blade props are most efficient since they have the least amount of surface dragging through the water.  Two blade props are commonly used on lower horsepower outboards and trolling motors. Three -blade and four-blade props are the most common designs used today. The added blades reduce vibration while maintaining most of the efficiency of a two-blade design at a convenient size and reasonable cost.

Racers and performance boaters raise sterndrive mounting heights (x-dimensions) on ventilated, stepped hulls. The steps create air bubbles, raising the hull off the water on a drag-reducing cushion. This, combined with reduced drag from the higher drive heights, improves hull efficiency. This trend has spawned an evolution of prop designs featuring four, five and even six blades.  The additional blade surface helps offset slip induced by air bubbles flowing from the ventilation steps toward the props.

For efficiency, blades should be as thin as possible to reliably handle a particular power range. A cross section of a typical constant pitch prop blade reveals a flat section on the positive (pressure) side and an arc surface on the negative (suction side) of the blade. Edges are usually 0.06″ to 0.08″ (1.5 mm to 2.0 mm) thick for aluminum props, thinner for stainless steel.

The blade cross section on surfacing props such as our T.E. Cleaver  and Pro Finish CNC Cleavers is wedge shaped. The thick trailing edge adds strength. Surface air ventilates a low-pressure cavitation pockets behind the trailing edge, enhancing efficiency.  The contour or shape of most propeller blade tips (other than cleaver) are round.

I will discuss propeller slip more thoroughly in Prop School – Part 6.

# Hi-Performance Boat Operation – Part 1: Introduction

It’s boat show season – the perfect time of year to check out the latest performance boats and Mercury Racing propulsion. For those of you who are about to purchase your first performance hull, congratulations!

With Winter in full swing, now is the time to review the basics of high-performance boat operation to ensure you and your passengers have safe experiences out on the water.  We include a Guide to Hi-Performance Boat Operation with every engine we ship. We encourage new and current owners to review the book and then take in-boat driving lessons from your local high-performance dealer or boat builder.

Our operation guide is packed with general performance boating information, including  propellers, hull types and overall boat performance. Let’s first review the various performance-boat hull configurations.

Vee-Bottom

The traditional vee-bottom is the most common hull design. It offers good speed and a softer ride, especially in rough water. The softness of the ride depends on the angle of the “V” (called deadrise), radius of the keel line and the use of strakes.

If your boating is mostly in larger bodies of water such as the Great Lakes or open seas, you may want to consider a boat with this hull type.

The most recent change in this design over the past decade has been the incorporation of strategically placed notches or steps in the hull. The steps create air bubbles, raising the hull off the water on a drag-reducing cushion.

Some vee-bottom hulls feature a small flat area toward the rear of the keel called a pad. Similar to steps, the pad reduces the wetted surface area the hull runs on, increasing top speed with minimal effect on the ride quality. Mercury Racing offers a full array of outboard and sterndrive propulsion options for the vee-bottom boater.

Tunnel

Outboard tunnel boats are the fastest-turning race vehicles on earth. The sharp, 90-degree transfer where the tunnel sides meet the bottom of the sponsons helps the boat settle in the water as it enters a turn.

The submerged sponsons make the boat turn as if it were on rails. It is common for drivers to experience 4.5 to 5Gs as they enter a turn at 120 mph and come out at 90+ mph. Obviously, only experienced racers should consider this type of hull.

Catamaran

I like to refer to catamarans (or cats as they are often called) as tunnel boats on steroids. The design principal is similar. The boat rides on two sponsons or hulls separated by a tunnel. Air entering the tunnel generates lift as speed increases. The wetted surfaces and hull drag are reduced, for enhanced speed and ride quality. This design is not for the novice operator.

The air entrapment hull is sensitive to engine trim, wind, and water conditions. In general, they produce a smoother and faster ride over a vee-bottom in calm to mild chop. The vee bottom is king in rough water.

The fastest recreational cats on the water today are powered by the venerable dual calibration 1550 sterndrive. The V8 4.6L 300R FourStroke is a popular choice for those who choose outboard power for entry into the cat experience.

In Hi-Performance Boat Operation – Part 2: Rigging Fit & Function, I will review the important things to consider when preparing your new Mercury Racing outboard – or sterndrive-powered boat for the upcoming season.

# Prop School – Part 4: Blade Cup

Continuing from Prop School…Part 3. Here I will explain everything you need to know about Blade Cup.

Cup is a curl formed or cast into the trailing edge of a propeller blade.  When done correctly, the face of a cupped prop blade is completely concave.

The first three-blade aluminum props for MerCruiser powered boats featured flat blades, with 15-degree rake. The heavy, deep-vee hull ran best with the drive trimmed up (raising the bow, reducing the wetted surface, and increasing hull efficiency). We got our first experience with cupped, 3-blade aluminum props in the mid ’70s. We immediately realized greater top-end speeds. We also noticed the engine didn’t work as hard. The cupped props were more efficient. Our measurement? The paint was still on the blades at the end the season. Cavitation burns, mostly from abusive teenage kids over trimming dad’s boat, would burn away the paint. The cupped prop definitely made  a difference.

Location. Location. Location.

Originally, cupping was done to gain similar benefits as you get from progressive pitch or higher blade rake. In fact, cupping reduces full-throttle engine speed 150-300 RPM below the same pitch prop with no cup. The location of cup on the blade determines the affect it has on performance. When the cupped area intersects pitch lines, pitch increases. Cupping in this area will reduce engine RPM.  Cupping can also prevent prop cavitation or blow out.   Blade rake can be increased when the cup intersects the rake lines. Slip is a measurement of propeller efficiency as it turns through the water, the normal range is 10-15%. Most racing and performance boats slip can be as low as 5-7% where as performance vee and step vee bottom boats with high X dimension (outboard engines or sterndrives mounted high) can see slip as high as 20-22% at WOT

Cleaver Cup.

Adjusting cup on cleaver-style propellers is more difficult. The trailing edge is very thick and runs straight out on the rake line. Pitch can be altered some by grinding away some of the cup. Rake may also be altered slightly.  The rake can be reduced by decreasing the cup near the tip of the blade. Rake can be increased by reducing the cup near the prop hub. Remember that any change in cup affects engine RPM. The Bravo I propeller family is a good example of how cup changes RPM and the attitude of the boat I will discuss blade configurations and factors that effect propeller efficiency in Prop School – Part 5.

# Future Techs

I’ve led many tours of Mercury Racing over the past 30 years. People are constantly amazed to see our skilled labor handcrafting outboards, sterndrives, propellers and accessories. Some of the more common questions asked are, “where does your labor comes from and how do they learn their skills?”

Our employees come with a strong skill set and work ethic in place. The only training needed are for things that may be specific to the job at hand. I attribute their strong  work ethic to the Midwest culture. In Fond du Lac, it most likely also stems from the rich German ethnic mix and heavy farming influence. I truly believe farming brings with it an inherent mechanical aptitude that has been ingrained within Mercury since the late Carl Kiekhaefer founded the company in 1939.

The education and interests of today’s generation has changed. Millennials grow up using technology – aspiring to play video games and becoming “device” experts from an early age. They are used to instant gratification.

I recently came across an article in Forbes magazine entitled, “Why We Desperately Need To Bring Back Vocational Training in Schools.”  The timing of the article couldn’t have been better.

Industries such as ours are beginning to feel the pinch in finding skilled labor with a strong work ethic and passion to build and service the products we manufacture. Many say Millennials don’t want to get their hands dirty or have the desire to actually learn skills to build or repair products. I personally believe they are as interested and as capable as ever. We just need to provide them the education and tools they need to succeed.

Millennial Diamond

Marty Signorelli – owner of Diamond Marine – a Mercury Racing dealer located in Ft. Lauderdale – made me aware of one school that is making a difference when it comes to filling the void in skilled labor. His nephew Michael attends Coral Shores High School in Key Largo, Florida. The school has a dedicated marine vocational program. Students who attend the 4-year program learn skills to service marine engines.  Several current and former students work in marine  work in marine shops or related businesses on the water.  I spoke with instructor Chris Catlett regarding the program. Chris has been teaching  for 13 years. He is a 20-year Coast Guard Veteran with over 30 years of marine experience.  Eighty students are currently attending the program.

“Mercury Marine helped launch the Marine Service School program. We have 60 Mercury outboards made up of a mix of 2-stroke and 4-stroke models. The kids learn  everything; from  rebuilding powerheads and gearcases to diagnosing and repairing hydraulic and electrical systems. We are one of five marine mechanic trade schools in the nation which provide students an alternative to a formal four year college education,” said Chris.

For the past several years, Chris has taken the students to Key West for the annual Super Boat International Offshore World Championships. Twelve students got to work with race teams this year.

“The kids see the boats go past the school on their way down to Key West for the races. I feel it is important for them to see the engines in use – be it the recreational outboards they work on day in and day out or the exotic – high powered race motors they see competing in the extreme race environment. They get to see cutting edge technology in their own backyard, ” Chris said.

We encourage poker run and race promoters and participants to invite tech school students to their events. Get them involved. It lights a fire in the students for sure.

2-Stroke DNA

Michael Signorelli has mechanical aptitude built into his DNA. His uncle Marty and Joe are legendary in their ability to maximize the performance of  our legacy 2.5 EFI 2-stroke competition outboards. His father Frank is a private boat captain. Michael started the Marine Service School program in 2015. His first project was rebuilding a 2-stroke 9.9 h.p. Mercury. His current project is a tear down and rebuild of a 75 h.p. OptiMax.

We are thankful for instructors such as Chris Catlett and the various vocational programs around the country. I am confident program graduates will  provide tech support for Mercury and Mercury Racing products well into the future.

# Prop School – Part 3: Blade Rake

Continuing from Prop School….Part 2 .   Here, I will explain the science behind blade rake.

Rake is the angle of a propeller blade face relative to its hub. If the blade face is perpendicular to the hub, the prop has zero-degree rake. As a blade face slants back toward the rear of the prop, blade rake increases. Rake is either flat (straight) or curved (progressive). Most lower horsepower (“lower” by Mercury Racing’s reckoning)  propellers, like Black Max aluminum and Vengeance, have 15-degree rake and are designed to operate fully submerged to push a boat across the water. Typically, higher horsepower outboard and sterndrive propellers have a higher flat or progressive rake.

A greater rake angle generally improves the ability of the propeller to operate in a ventilating situation. Ventilation occurs when blades break and re-enter the water’s surface — such as occurs with 1) a Bravo sterndrive (XR, XR Sport Master or XR Sport) installed with a high “X” dimension, 2) a surfacing drive (M6 or M8) or 3) an outboard installed or jacked high on a transom. In surfacing operation, higher rake can hold the water better as it’s being thrown into the air — deflecting it aft and creating more thrust.

On lighter, faster boats with a high prop shaft, increased rake often will improve performance by holding the bow higher. This results in higher speeds due to less hydrodynamic hull drag. However, on some very light boats, more rake can cause too much bow lift. That will often make a boat less stable. Then, a lower rake propeller (or a cleaver style for outboard) is a better choice.

Looking at examples:

• A runabout with Alpha sterndrive usually performs best with a lower rake Black Max or Vengeance pushing the boat. The aim is broad capability and utility for many recreational activities.
• A lighter weight runabout with Alpha drive may increase performance with higher rake Enertia propellers lifting the bow offering less wet running surface (lower drag).
• Bass boats can vary widely because of the design differences among hulls in the market. Mercury offers high rake propellers such as the Tempest Plus and Fury for these applications. Mercury Racing specialty props for the bass market include the Lightning E.T., Bravo I FS, Bravo I XS and Pro Max.
• The Bravo XR drive, used with higher horsepower multi-length and weight applications, typically use props with high rake and large blade area — such as the Bravo I and Maximus.

Our Pro Finish 5-blade CNC Cleaver prop is available with 15, 18, or 21-degree rake.

Performance applications using Mercury Racing’s CNC Pro Finished Cleavers with  M6 or M8 drives have three rake choices: 15, 18 or 21 degree. Most “V” and step “V” bottom boats utilize a 15 degree rake — unless the center of gravity is forward of the helm; then, 18 degree rake works best. The higher rake helps lift the bow — positioning the boat to ride appropriately on the steps. Air entrapment hulls (catamarans and tunnel hulls) pack air and lift during forward motion; they typically use props with 15 to 18 degree rake — since air pressure does most of the lifting.

The 15-degree and 18-degree rake Pro Finish CNC Outboard Cleaver is being used in a variety of applications including bass boats, performance center consoles and catamarans.

Your head probably hurts by now, so I will discuss blade cup in Prop School – Part 4.

# Prop School – Part 2: Terminology

Continuing from Prop School….Part 1 .   Here, I will explain basic propeller terminology and fitment.

Rotation

Propellers are available in both right-hand and left-hand rotation. Most single engine outboard and sterndrive powered boats use right-hand rotation propellers. A right-hand rotation propeller will spin clockwise when pushing the boat forward, while a left-hand propeller will spin counter-clockwise.

The most popular propellers used for recreational boating have three or four blades. Three-blade props are efficient and do a good job of minimizing vibration. Four blade props are popular for suppressing vibrations even further while improving acceleration by putting more blades in the water.

Diameter

In “prop speak,” diameter is the distance across a circle made by the blade tips as a propeller rotates. The proper diameter is determined by the power that is delivered to it and the resulting propeller rpm.

Type of application is also a factor. The amount of propeller in the water (partially surfaced vs fully submerged) plays a role in determining diameter. The more a propeller is surfacing above the water, the larger the diameter needs to be (so what’s left under water can still push). On rare occasions, diameter may be physically limited by drive type or in close, staggered engine installations where tips can touch.

Within a specific propeller style, diameter is usually larger on slower boats and smaller on faster boats. Similarly, for engines with a lower maximum engine speed (or with more gear reduction), diameter will tend to be larger. Also, diameter typically decreases as propeller blade surface areas increase (for the same engine power and rpm). A four bladed prop replacing a three blade of the same pitch will typically be smaller in diameter.

Physical limits

Mercury Racing engines fitted with the Bravo One XR or Bravo Three XR drives are designed for props up to 16-inches in diameter. Bravo One XR drives fitted with the short Sport Master gearcase accepts props up to 15-1/4 inch in diameter. Sterndrive engines with surface piercing M6 or M8 sterndrives run cleaver props up to 18-inches in diameter. Our 4.6L V-8 250R and 300R FourStroke outboards as well as the 400R Verado accept props up to 16-inches in diameter.

Pitch

Pitch is the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw in wood. When we list an outboard four-blade Pro Max prop as a 14-1/2 X 32, we are saying it is 14-1/2 inches in diameter with 32-inches of pitch.

Pitch is measured across the face of a propeller blade. Actual pitch can vary from the pitch number stamped on the prop. Modifications made by propeller shops may alter the pitch. Undetected damage from a submerged object may result with a bent blade, altering the pitch as well.

There are two common types of pitch; constant and progressive. Constant pitch means the blade pitch is the same – from the leading edge to trailing edge. Progressive pitch, referred to as blade camber, starts low at the leading edge and progressively increases toward the trailing edge. The pitch number, “32” in the Pro Max example, is the average pitch over the entire blade.

Pitch is like another set of gears. Since an engine needs to run within its recommended maximum rpm range, proper pitch selection achieves that rpm. The lower the pitch, the higher the engine rpm. Mercury Racing propellers are designed so that a one-inch change in pitch results in a 150 rpm change in engine speed.

A lower pitch propeller may provide greater acceleration for water sports activities, but your top speed and fuel efficiency may suffer. If you run at full throttle with a prop selected for acceleration and not top-end speed, your engine rpm may be too high, placing an undesirable stress on the engine. If you select too high of a pitch, your engine may lug at a lower rpm – which can also cause damage.  Acceleration will be slower as well. It will be reduced further with a full load of fuel and maximum capacity of people on board.

Proper pitch selection allows the engine to operate near the top of its recommended rpm range at light load (1/2 fuel tank and two people). Using this pitch selection method, the engine usually operates near the low end of the recommended engine operating range when the boat is fully loaded (full fuel tank, boating gear, full live wells, and maximum capacity). Full load engine speed is usually reduced 200 to 300 rpm. The power output of naturally aspirated engines can be affected by high heat and humidity which is another factor that can reduce engine speed by 200 to 300 rpm.

The Mercury Racing Prop Slip Calculator App is very helpful in determining the proper pitch for your application.

Smart, pressure charged engines like the supercharged 400R outboard and our turbocharged QC4 sterndrives will auto-regulate power output for heat and humidity. Adaptive Speed Control, a standard feature on our 250R and 300R outboards, is another factor to consider when dialing in your boat for maximum power and top-end speed.

In my next Prop School post, I will discuss blade rake.

# Prop School – Part 1: Introduction

Working in performance boating is exciting: It’s fast-paced. Propulsion systems and hull designs are in continual evolution. Our customers are generally astute, technically oriented and often quite colorful characters. We’re all performance freaks! We’re all continually learning. That’s what makes my job so much fun!

If you are like me, your first boating experiences were in lower horsepower boats used primarily for family recreation, fishing, skiing, wakeboarding, or general cruising. And like me, your boating experiences and knowledge have evolved over time.

When working with high-end performance boats and experienced customers, one tends to assume people have basic product knowledge. However, a propeller is complicated. Because our backgrounds vary widely, our levels of understanding vary widely, too. So, we’ll revisit the basics and then dive deeper on propeller form, fit and function. Read more

# Don’t Knock It!

So, what exactly is engine knock?  Well, put on your engineering hats for a moment, as we’re going to get a little technical here.  Simply put, engine knock (aka “detonation”) is an undesirable phenomenon that occurs when a “left over” pocket of air-fuel mixture in the combustion chamber ignites after the spark plug has already fired. When this happens, cylinder pressure jumps as high as 25 times that of normal combustion, and in doing so creates a sharp metallic noise audible to the human ear.  This noise is referred to as “knock”, and left unchecked, it can lead to engine damage ranging from relatively mild to complete engine failure. The extent of engine damage that can occur from knock is highly dependent on the specific output of the engine (horsepower/cubic inches or liters) . . . the higher the specific output, the more extensive the knock damage may be.  In addition, external factors influence an engine’s propensity to knock.  For example, higher air and/or water temperatures make it harder to cool the engine, and therefore create an easier environment for knock to occur, while higher humidity helps reduce the chances for knock.  By now you’re probably wondering “how can I protect my engine from the weather?”

The good news is using high quality fuel with an octane rating that complies with your engine’s requirement is your single best defense against engine knock.  This is why certain Mercury Racing consumer outboards and sterndrives require a minimum of 91 octane (98 RON) pump fuel.  The higher-octane fuel allows your Mercury Racing engine to safely produce maximum power while protecting against engine knock.

Octane, in fact, is a measure of gasoline’s antiknock performance. There are two test methods used to measure gasoline octane rating. One method results in the Research Octane Number (RON); the other produces the Motor Octane Number (MON). Octane ratings at the pump are typically determined by the following equation:

(RON + MON)/2; commonly written as (R + M)/2. This is called the antiknock index (AKI).  In general, a higher-octane fuel, such as 91, provides greater protection against engine knock than a lower octane fuel, such as 87 or 89.

400R and Automatic Knock Protection

All Mercury Racing outboards and sterndrives require a certain minimum fuel octane to protect against knock while maximizing performance. The Verado 400R outboard takes it one step farther with an advanced computer controlled knock protection system. The 400R produces its advertised horsepower at 7,000 RPM on 91-octane (98 RON) pump fuel; however, the engine control unit will automatically adjust spark timing on individual cylinders should it start to detect engine knock. The amount of spark removed and subsequent power reduction is highly dependent on ambient conditions (water and air temperature, humidity) and other factors.

The great thing about the 400R’s knock protection system is it is designed to always give you as much power as possible under all conditions while still protecting the engine from knock damage.  Running your 400Rs with the recommended 91 octane fuel will help ensure you always have the full 400 horsepower at your fingertips, but sometimes on the water, 89 octane or Rec 90 is the best you can find.  Don’t sweat it . . . your 400Rs will run safely and reliably on this fuel as well, you just may not see the same top speed you will with the premium fuel.  This knock protection system provides the ultimate flexibility by allowing you to maximize performance on 91 octane without having to compromise where you run your boat based on available fuel grades at the gas dock.

Currently the 400R is the only Mercury Racing engine with a built-in knock protection system.  This means with other Mercury Racing outboards and sterndrives, it is absolutely necessary to comply with specified fuel requirements for each engine. Using a fuel with an octane number lower than an engine’s specified rating will likely result in engine failure, which is not covered by Mercury Racing’s limited warranty. While most fuel grades are readily available for boats that are trailered, it is important to understand what is sold at the gas docks for boats that are kept in the water or run for long distances.

Fortunately, all but three of Mercury Racing’s outboard and sterndrive products are designed to run on 87 or 89 octane (see chart below for octane requirements by engine model).  Since most gas docks carry 89 or Rec 90, the vast majority of Racing product may be operated virtually anywhere.  The consumer Race product which requires 91 octane includes the 300XS outboard and the QC4 1350 sterndrive.  Finally, the dual cal 1350/1550 QC4 consumer model requires 91 octane in 1350 mode and 112 octane race fuel in 1550 mode.  Typically race fuel (112+ octane) is not readily available and must be ordered in advance for speed runs or competition race activities.  Recommended race fuels per the chart below are Sunoco Supreme 112 AKI or Sunoco 117 MON, VP C16 or equivalents.

Can’t I Just Add Octane Boost?

As previously mentioned, most marinas today are carrying Rec 90; a cross between 91 and 89 octane pump fuel. In other areas, the highest available fuel rating is often 89 but sometimes 87-octane. This is a huge gamble if your engine requires 91-octane fuel and is not equipped with knock control, one that often leads to serious engine damage.

As a result, it comes as no surprise that many of our customers turn to aftermarket products (fuel additives) promising to raise the octane in their boats to acceptable levels which comply with our specified fuel requirements.  Internet forums are full of discussion threads arguing whether these products actually work.  Here at Mercury Racing, we are frequently asked for our opinions on these additives. To date, we have not been able to validate the effectiveness of any aftermarket octane boosting products on any Mercury Racing product.  Thus, we do not recommend or support using them as a substitute for using the specified minimum octane fuels our products require.

Many of these products advertise that they boost the octane by a certain percentage or factor when mixed with the fuel at a certain ratio.  It is unclear how much of the product is truly required to boost the fuel octane in the tank by a full point.  Higher volume fuel tanks found on most powerboats or center consoles would likely require a substantial amount of octane booster, and even at that, there is risk the final octane in the tank may still not be enough to meet the specified requirement. This is a major risk to take with your high dollar investment, and one that would not be covered by Mercury Racing’s limited warranty.

# Prop Slip? We Have an App for That!

The Mercury Racing Prop Slip Calculator App, available for both Apple or Android devices, provides all of the functionality of the calculator featured on our website. The app enhances the versatility of the tool – making prop testing much easier and a ton more accurate.

I use the slip calculator daily when assisting customers with propeller questions. It is a great tool that provides much more information than just slip. Results for each of the five fields can be derived by populating the other four. Following are a few examples of what this powerful app can do.

Outboard Example

Let’s look at a single engine OptiMax 250 ProXS outboard powered hull.  When communicating with customers, I always first ask for baseline information such as the pitch of prop they are running,  gear ratio and engine RPM at wide open throttle. Using the app, I can plug information I’m provided (pitch, gear ratio, engine rpm and speed) to first determine slip.

Now – after getting some additional information, the customer informs me the rig is under a heavy load and thus he is looking to increase RPM for enhanced hole shot and mid range acceleration to carry the load. Let’s see what happens when we drop the propeller pitch by two inches. Typically, one inch change in pitch affects engine speed by 150 RPM. In this case – dropping two inches of pitch will increase engine speed by 300 RPM. I plug in the new RPM and pitch size into the app – leaving everything else the same.  The engine is running at the upper end of it operating range. This will provide the added thrust needed for the enhanced hole shot and mid range performance the customer desires. Note the top end speed drops by 2 mph.

The customer sometimes runs under lighter loads and he is concerned with it being so close to the rev limit using the smaller wheel. Lets see what happens when we go back up one inch in pitch. I go to the app and change the pitch to 24 and decrease the engine speed by 150 RPM. I again select Actual Speed and find a gain of one mph in top end speed.

Let’s see what happens when we go down one-half inch in pitch. Knowing the engine speed will increase 75 RPM, I change the RPM  to 5925 and pitch to 23.5 and select the Actual Speed button.

Top speed is comparable to what is achieved with using the 24″ pitch prop. The higher engine RPM will enhance the hole shot and mid range performance. There is enough of a gap between the actual engine speed to the upper engine operating range to allow for occasional light load applications without worrying about hitting the rev limit. This is the prop I would recommend for this application.

Sterndrive Examples

M6 Application

Here I wanted to share a couple of examples. The first is regarding a boat originally equipped with twin 800 h.p. engines coupled to our dry-sump M6 drives.  The customer has supplied us with gear ratio, engine RPM,  pitch and speed. By inputting our known data, we find the slip to 12.83%.

The customer then decides to have his engines rebuilt and updated to 1,000 H.P. He would like to know the pitch size he could run with the updated engines.

Running a conservative calculation we can expect a 10% increase in speed (1.10 x 103 mph) for a total of 113 mph.

The new engines need to rev to 5800 rpm. I plug 5800 into the App and change mph to 113. Keeping everything else the same and select pitch, which changes to 35.42″ pitch.  Here, I would suggest he start out by running his existing 36-inch pitch cleavers. He can break the motors in and then check wide open throttle to see if he can get to the recommended 5800 RPM.  If he gets there and finds he has more throttle – he can consider higher pitch props.

Using the old general rule of thumb, it takes 10 additional hp for a gain of 1 mph – for both single and multiple engine applications. Remember – this is the old rule of thumb. In this case – we bumped the power from 800 to 1,000 h.p. or 200 h.p./10 h.p. = 20 mph.

Let’s see what the results are with that. When I change the speed to 123 mph in the slip calculator the pitch comes in at 38.55-inch. Now the customer needs to make a decision to go up to either 38-inch or 39-inch pitch props.

Bravo Application

For this example I wanted to share an example of an air entrapment vee-bottom hull and the how it affects propeller performance. The subject boat is a 42-foot Fountain equipped with twin 525 EFI sterndrives. The customer provides me with the baseline information. He tells me his is running 34-inch pitch Lab Finished Bravo I props with a 1.50:1 gear ratio. The 525’s turn 5200 RPM at wide open throttle. His slip is high at 22% – resulting with a top speed of 87.05 MPH.

Stepped hulls such as that featured on the Fountain aerate the water just forward of the propeller blades. This creates slip as the propeller is not large enough overall  – or in blade area specifically – to grab clean water. Here I would suggest the customer switch from the four blade Lab Finished Bravo I to the five blade Lab Finished Maximus. Let’s go to the app and see if what we find out.

I know from experience the pitch will go down two inches when switching from the 4 blade Bravo I to the  larger 5 blade Maximus. I also know the the slip will go down to approximately 12%. I plug in the constants; 5200 RPM, 1.50:1 gear ratio, and revise the pitch to 32-inches and plug in the 12% slip. The resulting top speed calc is impressive and representative of what the Fountain with 525s is capable of producing. The customer is pleased to learn he will gain five MPH when switching to the Maximus. He’ll also gain that speed in the mid-range offering a great cruising speed.

I hope these examples provide a sense of the various calculations you can perform with our new app. Remember, the calculator is a tool to help us better understand a number of variables. It provides a base from which to test the results for your particular application.

# Cat Nip!

The Verado 400R has fueled the resurgence of the outboard performance boat market. Multiple outboard (two or more) installations capable of speeds in excess of 85 mph require the use of an external rear tie bar assembly to keep the motors parallel and equalize loading.

Factory Installed Tie Bar Kit

400Rs destined for go fast duty on catamaran hulls are equipped with a custom Mercury Racing designed rear tie bar kit. The factory installed kit includes custom rear engine mounts and a heavy duty, stainless steel tie bar wing plate.  The engine mounts feature an addendum for the mounting of the wing plate. A tie bar is not included.

The custom rear mounts are critical for safe and secure installation of the wing plate. They serve a function and –  at the same time – maintain the integrity, form and function of the 400Rs’ Advanced MidSection. Most people mount aftermarket rear tie bar wing plates via the powerhead studs. This places a great amount of stress on the studs, engine mounts and the powerhead which could lead to cylinder distortion and possible engine failure.

Verado 400Rs with factory installed tie bar kits are backed with a full warranty.  The Mercury product warranty does not cover any damaged related to the use of tie bar kits or other accessories not manufactured by Mercury Marine.

Sport Master

One of the features which differentiates the Verado 400R from all other four stroke outboards is the availability of the Sport Master gearcase. Designed  for boats capable of speeds in excess of 85 mph, Sport Master 400Rs deliver fresh adrenaline pumping excitement to the go fast cat world.

We are excited to see our high performance boat builders embrace the 400R. The response thus far has been phenomenal. The biggest kick I get is people seem to be as awestruck by the pure power and torque of the engine as they are its  drivability and and overall  quietness. People are as excited to be able to carry on a conversation at 80 mph as they are going for the big number. These are exciting times for sure.

# Hush the Magic 520!

Our new 520 sterndrive has been a resounding success since it was introduced one year ago at  the LOTO (Lake of the Ozarks) Shootout. It’s become even more popular since the release of a  Joystick Piloting for Sterndrives – Axius option for selected models fitted with the Bravo Three XR sterndrive. Formula boats have been early adopters of the engine package. They are also the first OEM boat builder to install the potent engines with joystick control. A Formula 400FX used to demonstrate joystick 520 maneuvers at the Miami Boat Show had standard through transom exhaust fitted with aftermarket mufflers. For our traditional sport boat crowd – they had a nice exhaust note. For those looking for enhanced performance without all the rumble – it might have been a bit much.

We took note, no pun intended, and went to work to create an X-haust Noise Reduction system designed specifically for the 520.  The system will appease our friends at Formula and a variety of our OEM boat builder partners, dealers and consumers looking to take advantage of the 520’s performance value.  It will also be adopted in European Union countries where the engine is certified in meeting the stringent RCD (Recreational Craft Directive) exhaust emissions standards. Read more

# Vital Fluids

We spend a lot of time validating our products. This is because we are responsible for entire propulsion systems – not just independent components. Everything (engines, transmissions, drives and propellers) must work together and be tolerant of each other. This includes oils and lubes. They are the system’s lifeblood.

We validate our engines using specific oil types and weights. Same goes for the drives and  lubes. Over the years, our two-stroke outboards have evolved from carbs to electronic fuel injection to OptiMax low-emissions direct fuel injected technology. Similarly, our higher horsepower sterndrives have evolved from traditional 2-valve, push rod engines to a quad cam, four valve engine of our own design. Oil and lube requirements have evolved along with the products. Read more

# Bravo for Go Fast Charters

It’s been a year since we introduced the Bravo I FS outboard propeller. It was originally developed for single engine four stroke outboard applications. We’ll, it didn’t take long for the word to spread regarding the prop’s performance. Folks running multiple four stroke outboard rigs started asking for right and left-hand rotation sets of the popular prop.

Being the conservative person I am, I opted to first work with a handful of people running various hull types to prove the concept before releasing the counter rotation Bravo I FS models. It took longer than I thought to get feedback from the field.

I  grew frustrated because I wasn’t getting any details regarding performance results. All I would get was, “They’re great! Thanks. ” or more often than not – no news at all. It was like pulling teeth. I found out over time the props worked so well that they wanted to keep their performance secret to themselves. Eventually, I got the detailed information I was looking for and I am happy to share it with you here. Read more

# Bravo on the Bayou

I’m fortunate to annually represent Mercury Racing at the Bass Master Classic in the Mercury booth. This year, a fisherman named Rick asked me if there was a Mercury prop that would work for him. He had recently purchased a 2012 Triton 19XS powered by an OptiMax Pro XS 200. Rick was frustrated with the performance. The sharp turns and switchbacks on the Bayou where he runs were causing his propeller to break loose. This forced him to back off the throttle, causing the boat to lose speed and drop off plane. Rick had contacted his Triton representative regarding the issue. Although they discussed various options, the rep suggested Rick continue using a three blade prop.

The performance facts that I gathered in our conversation pointed me to a Bravo I XS. I told Rick the prop is designed specifically for low-emissions 2-stroke OptiMax outboards. Rick responded, “Isn’t Bravo I a sterndrive prop?” Read more

# Hi-Performance Boat Operation – Part 3: Prep & Drive

In my previous post (Part 2) regarding high performance boat operation, I reviewed basic information on rigging fit and function. Now its time to head to the ramp.

While the boat is still on the trailer, walk around for a visual inspection of the hull.  Next, climb aboard for a visual inspection of the interior and engine compartment (motor well for outboards): ensure everything is in place and secure. Don’t forget the drain plug(s)!  Check your other safety accessories: aboard? In secure locations?

Once your boat is launched,  review the helm to familiarize yourself with the location and function of all instruments and controls. Make sure the steering wheel, throttle and shift controls are well within your reach and that you are comfortable with their operation.

If your boat is fitted with K-Plane trim tabs, be comfortable with the location and operation of the tab trim switches. The driver needs to know the location and function of accessory switches such as bilge blower, bilge pump, running lights, horn, courtesy lights and related fuses, or circuit breakers. Read more

# Hi-Performance Boat Operation – Part 2: Rigging Fit & Function

Spring is a great time for newbie and veteran performance boaters alike to get familiar with their craft. For starters, you should review your owners manuals — really, you should — and review the key components of your new boat.

Performance boats vary widely in propulsion and size. Outboards come in 20, 25 and 30-inch drive shaft lengths to accommodate a variety of applications. Mercury (and other brand) outboards are fitted with a standard gearcase for most applications. Hulls that can take advantage of the high power-to-weight ratio of a 300XS may benefit from its wide range of gearcase options. Similarly, Mercury Racing offers a variety of sterndrives for differing power capacities and hull types.

Mechanical control: High performance outboards are usually rigged with with dual steering cables, a shift cable, throttle cable and fuel line. With performance sterndrives, throttle and shift are accomplished with cables, but steering is hydraulic. These include 600 SCi  and 700 SCi Mercury Racing packages.

Digital control: On Digital Throttle & Shift compatible outboards, such as the 400R and sterndrives including the 520, 540, 565, 860, 11001350 and 1550 mechanical throttle and shift cables are gone — replaced with a single electronic cable. Steering is either electric (Verado) or hydraulic (MerCruiser). Read more

# On Their Own Plane

I was going through my literature archives the other day and came across a copy of the original Kiekhaefer Aeromarine, Inc., K-Plane Trim Tabs sales brochure. I’ve always respected the quality and functionality of Kiekhaefer’s literature.  I  thought a blog post regarding the history of K-Plane trim tabs would be of interest. More importantly, it will serve as a refresher regarding the fit, form and function of the world’s most durable trim tabs.

Kiekhaefer Aeromarine Motors first introduced K-Plane Trim Tabs in 1970.  They were designed to keep the fastest, hardest running racing boats on an even keel in just about any water condition. US (APBA) and World Offshore (UIM) champions, Doc Magoon and Carlo Bonomi ran nothing else. In the mid 70s, Fred Kiekhaefer upgraded the product for recreational use. Read more

# Walleye Prop

You may wonder how we go about testing props. We have a number of our own outboard and sterndrive boats that we use for initial testing. Nevertheless, I’m a firm believer of getting feedback from those who use the product everyday in the real world. Recently, I wanted feedback on performance differences between our Lab Finished Bravo I and Pro Finish Bravo I XS outboard props. My target applications were Walleye and bass anglers.

# Refreshed & Rejuvenated

Is your propulsion system in good shape and ready for another season? Now is the time to check over your equipment. If your engines have reached a maximum of 150 hours, now is the time for a refresh to insure a hassle-free 2011 boating season.

We introduced the Factory Fresh engine refresh program in 2006 as a service for owners of our big block sterndrive engines (850 SCi, 1025 SCi, 1075 SCi and 1200 SCi). We’ve learned how our customers from around the world use the product, how various applications relate to engine wear and the affects maintenance (or lack of) has on engine life. More importantly, we have built valuable relationships with our consumers, OEM boat builders and dealers.