Different sizes, styles can produce other results
Propellers are the final link in transferring all the horsepower from a motor to the water so the boat moves forward. They are sort of like tires: there are big, rugged-tread versions for the off-roaders and smaller slicks, with barely any ground clearance, for the hot rods. Just like tires, there are propeller options to improve performance, but many boaters run boats all their lives with whatever prop came on it. Sometimes that’s right, but often it isn’t.
With the current trend of using smaller motors and trying to get the most performance and economy out of them, the drive for the perfect propeller has risen to the forefront again.
The simplest explanation of the propeller’s job is to grab the water and push the boat. However, just like there are tires that excel in different applications, there are props that do certain things better. Propellers can vary in size, shape, pitch, number of blades, the materials they are made of and more. Understanding the basics is a good starting point to finding the right prop for your boat. Some terms used to describe propellers follow:
Size. This is usually considered to be the diameter of the propeller, how tall the propeller is when standing straight up.
Shape. Shape is a little more complex and involves the shape of the blades and their rake and cup, plus for outboards and some inboard/outboards, whether the exhaust exits through the hub or over it. The shape of the blades tells a lot. In general, large, full, rounded blades are better for carrying heavy loads, and smaller, narrower, blades lean more towards performance. Simply put, if a prop looks aggressive, it probably is.
Rake. Rake is how far the blade leans back towards the rear of the propeller and is important as it influences the lift generated by a propeller. More rake increases holding power in choppy water and creates bow lift, which usually enhances performance, while less rake typically indicates a prop designed to move heavy loads.
Skew. Skew is a term used to describe the angle that the leading edge takes from the hub. This is most important on props that are occasionally above the surface as reduced skew (leaned farther back) reduces the vibration when the blade reenters the water.
Cup. Cup is the term for how much, if any, concave surface is used on the blade of the prop. This is important as it affects other aspects of the propeller. Cup running from the hub to the outer tip of the blade increases rake. Cup running the width of the blade — from the leading edge to the trailing edge — increases pitch. With modern CAD technology used in building propellers, cupping can, and often does, overlap to create specific characteristics in a prop.
Pitch. Pitch is measured in inches and refers to the angle of the blades stated as the distance forward a propeller should move in one full turn. This isn’t absolute, as there is a factor called slippage that reduces this distance a small amount
Number of blades. The number of blades on a propeller makes it easier to tune for specific applications. In the past, two blades were considered to be for absolute speed; three blades were a good all-around compromise and four blades were for power. However, technology over the past 20 years has passed this, and now the number of blades is not an absolute indicator of a prop’s purpose. One thing to remember, though, is that with propellers of the same pitch, the one with more blades spreads the load over a greater area.
Materials. For years, outboard props were made only of aluminum or stainless steel, and larger props — like those on big sportfishermen, yachts, shrimp boats and such, were made of bronze. Carbon composites are now being used for outboard props and do a good job in some applications.
Carbon Composite. These propellers were designed primarily for lower horsepower engines and lighter boats. They do a pretty good job and are usually the least expensive. Some carbon composite props are modular, and individual blades can be purchased to make repairs.
Aluminum. This is the mid-grade prop material, and it does an excellent job. Aluminum props are slightly more expensive than carbon composite but run about a third to half of the cost of stainless steel. Aluminum props work well for lower and mid-range power applications. They will handle higher power applications fairly well, but will flex at high speeds and with heavy loads. Because of their tendency to flex, aluminum props cannot be tuned quite as well as stainless steel props. They are generally considered to be not quite as efficient as their counterparts in stainless steel.
Stainless Steel. These are the top of the line for outboard and inboard/outboard props. There are several aftermarket propeller companies that offer a variety of specific application props, and they are made of stainless steel. Top of the line stainless steel props are state of the art castings that have been custom designed, molded with the tightest of computer generated accuracy and then hand tuned and polished. They seem expensive when buying them, but when one considers the manufacturing process, they are a bargain.
Slip. Slip is the difference, usually expressed in percentage, between the distance a prop could travel with its pitch and what it actually travels in that application. Most companies believe anything less than 10-percent slip is good. Because slip lowers the actual distance traveled by a prop, it also reduces the speed generated by that prop.
There is an equation that reduces engine RPM by gear ratio to determine how many times a prop turns at any RPM. This can be multiplied by the pitch of the prop to get the distance that could be traveled by the prop at that RPM with no slip and that can be converted to speed in MPH. The difference between this potential MPH and the actual speed of the boat with that prop at that RPM is the slip.
Using the formula, I computed the speed of a friend’s boat with a Mercury Optimax 200 and a 19-inch pitch prop to be 59.1 MPH at the maximum RPM of 5,750. By the GPS, this boat actually runs 54.4 MPH at 5,750 RPM. That is a difference of 4.7 MPH and equates to a slip of 7.9 percent
Many outboard props use ventilation holes to help get going. This was developed for 2-stroke engines but has become even more important with some of the 4-stroke engines that don’t have as much power at lower RPM. Ventilation holes are located just behind the leading edges of the prop blades and allow a little exhaust to escape through them at slower speeds and while accelerating. The exhaust going over the prop blades helps them spin a little more easily and boosts acceleration. Once at speed, the exhaust passes freely through the hub and doesn’t need the relief of the ventilation holes.
Mercury Marine’s propeller division refined the ventilation holes to a system they call the Performance Vent System or PVS. PVS props have an oversized, recessed hole cast into them behind each blade, where the ventilation hole would be. PVS inserts with various size holes are used to get just the right-size hole for each application and fine-tune the prop’s hole shot.
The height of a motor on the transom is a factor for props and their performance. When mounted too high, a propeller sometimes breaks free and ventilates, so no momentum is gained. When mounted too low, there is excessive drag. This is also affected with boats that have transom motor brackets, power pockets or any feature that affects how water flows from the bottom of the boat to the prop. You must have enough water for the prop to have adequate grip, but any excess depth on the lower unit creates excess drag, reducing economy and speed.
With modern technology, it is possible to design or alter props to do many things. With elevated running heights and the ability to generate bow or stern lift, there are numerous models to find the best prop for any application. For those willing to go the extra expense, a good prop man can fine tune a prop like a guitar. It really is amazing.
My favorite prop man left us way too early, but I am sure there are others nearly as talented. Emmitt Simms of Mid-Atlantic Propeller in Pollocksville had a way with props you had to experience to believe. One time, I took him two props that had experienced a very rude introduction to a piece of rocky bottom and expected him to laugh at me. He listened to my story and told me to leave them. About a week later, he called me and told me to come by and get them and try them out. I couldn’t believe my eyes when I saw them. They looked great, and he had them tuned almost perfectly.
I went back the next Monday and told him how they performed and what I would like, and he told me to leave them again. When I picked them up the next Friday, they produced exactly the RPM and bow lift I had asked for. I was amazed and sent him numerous customers who called me to relate similar experiences.
Hopefully this helps you understand the multiple variables in propellers and what can be done with them. Using modern technology, most manufacturers build props to handle almost any situation. Whatever they don’t have can usually be reached with a little fine-tuning of an existing model.
If this hasn’t confused you enough, or you would like to learn more about propeller dynamics, all the outboard manufacturers offer some information on their websites. Many aftermarket manufacturers do also. If you would like to get the most performance from your boat, it has to go through the propeller. The more you know, the better the choices you can make. With gas costing in excess of $3 a gallon, wise choices for boating are a good thing.
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