Selecting the right propeller

Apart from keeping your boat’s hull clean and your outboard or engine tuned, selecting the right propeller is one of the easiest actions you can take to optimize boat performance.

Depending on how you use your boat and its current performance, you might be wondering if you should “pitch up”, “pitch down”, switch from aluminum to stainless steel or move from three blades to four. Your prop might be damaged and need replacement. Or maybe it is in good shape but you’re looking to improve your boat’s performance.

The proper propeller size for your boat and engine combination is based in part on the wide open throttle (WOT) operating range for your particular engine. You can find this in your operator’s manual, expressed in terms of a certain horsepower at a certain r.p.m.

The goal in propeller selection is to determine what style and size will maximize your boat’s performance, while allowing your engine to operate in the recommended r.p.m. range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum r.p.m. where the maximum horsepower is produced, with ideal engine loading.

Using your existing propeller, determine your maximum obtainable r.p.m.. If during this test, you begin to exceed the maximum rated r.p.m. of the engine, reduce the throttle setting. If the engine over-revs beyond the maximum recommended r.p.m., you may need to increase the pitch of the propeller.  Increasing the pitch increment by 2″ will result in approximately a 200-400 r.p.m. drop. Also, switching from an uncupped to a cupped propeller will reduce your r.p.m (see “Cupping” paragraph below). The cupped propeller of the same pitch and diameter will typically reduce your r.p.m. by approximately 200. If you cannot reach maximum r.p.m., then pitch should be decreased. These recommendations apply to single engine installations only. For most twin engine installations it is necessary to increase pitch by 4″.

Once your WOT r.p.m. falls within the recommended range of the engine manufacturer, you have a propeller that is suited to your boat with respect to r.p.m. However, you may not be satisfied with your boat’s skiing performance or trolling speed. It may be advisable in these circumstances to have multiple propellers, each to accommodate different boating activities. In all likelihood, more than one propeller will be suitable for your boat and motor combination, depending on your usage. Ski boats need more top end speed, and should choose a prop with a higher pitch. Cruisers and houseboats need more performance at displacement speeds, and should use a prop with a lower pitch to acheive low-end power. It is imperative, however, that the WOT r.p.m. fall within the range specified by your engine manufacturer. If your engine is not able to reach this r.p.m. range, it’s operating under an extremely loaded condition and premature failure is highly likely. Our Senior Sales Engineer Giovanni Sorrentino (@sultanofwind)  would like to remind you that the wrong prop can wreck an engine. “After 15 year spent to test and optimize new prototypes, the selection of the propeller is always something that many boatbuilders don’t care about, thinking that is not their job, but only after so many test on the boat you can find the right propellers for the different situations, and this is a job that every builder should do for his customers.In Riviera boat   for example we are used to do many days of sea trial to optimize the selection of the propellers, with teams of engineers and experts dedicated to this issue. Additionally our experience in the maintenance in Riviera Yard,  advice us that using the wrong prop is the single greatest cause of premature engine failure” says Giovanni.

While most of these comments are geared to outboard engines, some also pertain to inboards.  There are other factors that can adversely affect the performance of your propeller. One of these is dings on one or more blades. Another would be having the blades out of alignment, as would occur if you hit something. Either of these could cause vibration or undue stress and ultimately damage your transmission, cutlass bearing and other components. Usually the best way to deal with a propeller issue with an outboard is to get a new prop if it’s damaged or, if you’re not sure you have the correct prop, try on different ones, with the recommendation of a qualified dealer, until you’ve got it right. But with inboards typically you’ll need to have the propeller(s) pulled and sent to a good prop shop so that they can work their “magic” on your existing prop to repair it or determine that you need another. This includes things such as “swinging it” to determine balance, checking for alignment of the blades, and actually working on blades, such as adding or removing cup and many other adjustments. You would give them all the information about your boat that they’d request and describe fully all the issues you’re experiencing.   

Propeller Size

The size of a propeller is defined with two sets of numbers, diameter and pitch, with pitch always following the diameter.

Diameter

The diameter is two times the distance from the center of the hub to the tip of the blade. It can also be looked at as the distance across the circle that the propeller would make when rotating.

Pitch

Pitch, the second number listed in the propeller description, is defined as the theoretical forward movement of a propeller during one revolution. Since there is almost always a small amount of “slip” between the propeller and the water, the actual distance travelled is slightly less.

Cupping

Many of today’s propellers incorporate a cup at the trailing edge of the propeller blade. This curved lip on the propeller allows it to get a better “bite” on the water, resulting in reduced ventilation and slipping, and allows for quicker acceleration, or “hole shot,” in many cases. A cupped propeller also works well in applications where the motor can be trimmed so that the propeller is near the surface of the water. The cup will also typically result in a higher top end speed.

Some Problems to Avoid

Ventilation

Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. When this happens, boat speed is lost and engine r.p.m. climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.

Cavitation

Cavitation (which is often confused with ventilation) is a phenomenon of water vaporizing or “boiling” due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller’s blade surface due to the collapse of microscopic bubbles on the blade. There may be many causes of cavitation, such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, water flow obstruction caused by parts of the boat’s hull or running gear too close to  the propeller and others.

What material is best?

Most outboards and IOs are originally sold with aluminum props, which are inexpensive and repairable. Inboards use three- and four-bladed props of bronze, or a nickel-bronze-aluminum alloy. Replacement props for IO or outboard boats are available in aluminum or stainless steel. These materials compare as follows:

Aluminum is the most common, less expensive material. Suitable for most outboard and sterndrive applications.

Stainless steel offers a performance advantage over aluminum due to stiffer, thinner blades and more advanced designs. Best choice at speeds over 50mph, or if your boat is running over oyster beds or sandbars regularly. Stainless costs more but is five times more durable than aluminum. Stainless props can be repaired, at a higher cost, to like-new condition, while repaired aluminum will suffer from metal fatigue and a loss of strength.

Should you choose a four-blade prop?

Three or four blades work well in either sterndrive or outboard applications. Three-blade designs give you all-around performance with an advantage on top end speed. Four-blade designs work well with boats that are difficult to get on plane, underpowered or used in watersports where top-end speed is not critical.

The composite cores of modular hubs are designed to break away upon significant prop strikes, helping to protect the prop body and engine drive train from damage.

Four blades in many cases will drop your rpm by 50 to 150rpm with identical pitch. Three-blade props are generally best for recreational boats with three-, four- and six-cylinder outboards and sterndrives, giving good hole shot and top-end performance.

The blades on three-blade props fill up about 50 to 55% of the available area inside the circle formed by the prop’s diameter (referred to as the Diameter Area Ratio). Adding a fourth blade increases the DAR to between 60 and 65%, so you can expect more thrust to keep your boat planing at lower rpm, a potential boost in fuel economy, but also a reduction of 50-100 rpms at WOT.


If you are convinced that your propeller is not the right one send us a message we will help you to select the right propeller for your boat.

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