Ventilation results from excessively tight turns, a motor that is mounted too high on the transom, or an engine that is over trimmed. Ventilation can also occur from prop designs that are not matched to the application, poorly designed props, props with little or no cup, or props that are worn or have damaged edges or cup profiles.
Causes of cavitation include incorrect engine height outboards , dings or sharp corners in the leading edge, poor polishing, too much cup or crummy blade design. Cavitation can also occur from thru-hulls, sensors, or other turbulence-producing protrusions under the boat forward of the prop.
We stock propellers to fit most outboards and sterndrives. Choices include propellers and hubs made by Mercury Quicksilver and Turning Point. Regardless of the brand you prefer, you will need to gather as much information as possible see below to make the best selection. After processing this information, you can make a better-informed decision.
Hubs are purchased separately from props. You will find an explanation of modular hub systems at the end of this article. Mercury also continues to offer one-piece, traditional props. Propeller size is expressed with two numbers, diameter and pitch, with diameter always stated first. Diameter is two times the distance from the center of the hub to the tip of any blade. Smaller prop diameters generally go with smaller engines, or with fast high performing boats. Pitch is the theoretical forward distance, in inches, that a propeller travels during one revolution.
Turning Point Propellers use a squeeze casting process to create thinner blades. They boast stainless performance at aluminum prices. Rake is the degree that the blades slant forward or backward in relation to the hub. Rake can affect how water flows through the propeller, which can make a difference regarding boat performance. The blade tips of these new propellers may strike the older style trim tabs on your engine.
Cupping of the trailing edge of the propeller blade is common on many propellers. A cupped prop may allow the engine to be trimmed with the prop closer to the surface, and will also decrease rpm by to 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. The lower the ratio, the more pulling power it has from a standstill. This is the same physics that is applied to a boat propeller. A boat with a lower pitch propeller could accelerate much faster from a standstill position than one with a higher pitch propeller. Unfortunately, this lower pitch would make your engine reach maximum rpm at much slower speeds, albeit faster acceleration.
On the other hand, a higher pitch propeller will give you better top speeds but much slower acceleration.
While the boat will achieve higher speeds, it will do so for a short while before getting bogged down. Pro tip: Remember that each inch of pitch is equivalent to about rpm. So if you are going to be changing the propeller on your recreational boat and decide to go from a 21 pitch prop to a 19 pitch prop, you will be essentially increasing the engine rpm by around revolutions. As is the case with pretty much everything else, the answer to this question is highly subjective; it depends on what it is you need in particular.
As such, here are some tips that will help you determine which one between 21 pitch prop vs 19 pitch prop you need. As soon as you define the problem, you will be in a much better position to find the perfect solution. Under revving or over-revving, your engine will damage it faster than you would like. If you find that your boat engine is either under revving or over-revving in contradiction to the specs, the simplest solution is to buy a propeller with the appropriate pitch for your specific boat, which leads us to the next point.
Our Manager of Technical Services Bob Adriance would like to remind you that the wrong prop can wreck an engine. 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 nave 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.
The size of a propeller is defined with two sets of numbers, diameter and pitch, with pitch always following the 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, 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.
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. Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. When this happens, boat speed is lost and engine r. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.
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.
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