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Wayne's Prop Shop

1101 Salem Road

Mt. Vernon, IL 62864

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618-242-PROP

618-242-7767

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Understanding Props

Propeller Selection
The propeller that came with your new engine isn't necessarily the one that is best suited to your needs. This is because many original equipment props, understandably, are made to meet a range of average conditions for many kinds of boats, hull styles, and loads. This can limit their performance. What you need is a propeller specifically selected for your kind of boat, and your style of boating.

The size of a propeller is described using two sets of numbers. These correspond to the diameter and pitch. The pitch always follows the diameter when describing a propeller.

Diameter: Diameter is two times the distance from the center of the hub to the tip of the blade. It also can be looked at as the distance across the circle that the propeller would make when rotating. It is the first number listed when describing a propeller.

Pitch: Pitch is defined as the theoretical forward movement of a propeller during one revolution -- assuming there is no "slippage" between the propeller blade and the water.
For most boats, there is slippage and therefore the distance advanced is less than the design pitch. The amount of slippage varies from boat to boat. Pitch is the second number listed in the propeller description.

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. This results in reduced ventilation, slipping, and allows for a better hole shot in many cases. A cupped propeller also works very well where the motor can be trimmed so that the propeller is near the surface of the water. The cup will typically result in higher top end speed on one of these applications.

Rake: Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller, and has implications with respect to boat performance.

Aft rake helps to trim the bow of the boat upwards, which often results in less wetted surface area and therefore higher top end speed. Aft rake propellers also typically "bite" better on ventilating type applications.

Forward, or negative rake, helps hold the bow of the boat down. This is more common in workboat type applications.

Selecting the right propeller for your boat and engine combination: The proper propeller size for your boat and engine combination is based on the wide open throttle operating range for your particular engine. This can be located in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM.

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

Using your existing propeller, determine what is the maximum RPM you are able to obtain. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.

If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 2" will result in approximately a 300-400 RPM drop. If your testing shows, however, that you are only able to obtain an RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM.

Switching form an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.

Once your wide open throttle RPM falls within the recommended range of the engine manufacturer, you have a propeller that is suited correctly for your boat with respect to RPM. You may, however, not be satisfied with respect to skiing performance or trolling speed. It is best in circumstances like this to have two propellers. One to accommodate on set of circumstances and the other to perform best under the different load. It could, in fact, be that more that one propeller will be suitable for your boat and motor combination depending on your usage. It is imperative, however, that the wide open throttle RPM fall within the range specified by your engine manufacturer.

Propeller Styles

"THRU-HUB EXHAUST"
PROPELLERS

The thru-hub exhaust propeller design utilizes a round barrel to which the blades are attached. The exhaust gases pass through the lower unit torpedo and exit through this round barrel hub. Advantages of this type of propeller include good planing performance ("hole shot" in propeller language), especially on boats where exhaust induced ventilation needs to be controlled or eliminated. Additionally, all props with this type or similar types of underwater exhaust create an effective exhaust noise reduction.

NON THRU-HUB EXHAUST PROPELLERS

Typical applications for this design would be for:

A) Inboard boats using shaft driven propellers.

B) Inboard-outdrives using thru-transom exhaust

C) Some outboards using above propeller exhaust. (shown)

OVER-HUB EXHAUST PROPELLERS

Over-hub propellers eliminate the larger outer barrel, instead attaching the blades directly to the small inner cavity surrounding the propeller shaft. This results in massive exhaust flooding the prop during takeoff, with characteristically high rpm's for the first few seconds of planing. Typically used on applications designed for maximum speed.

OVER/THRU-HUB EXHAUST PROPELLERS

This type of design is normally used when the desired degree of exhaust flooding at takeoff is more than is readily obtainable with thru-hub exhaust, but less than that inherent in the true over-hub design. Additionally, the superior strength of the thru-hub design (shorter blade length mean less leverage force on impacts) is retained.

What's New

Wayne's Props is a proud sponsor of many of the regional fishing tournaments and boat races.

Wayne's Props was recently featured in the Oct/Nov issue of Sportsmans Magazine.

Bring your boat in and I'll remove your prop for you and even give you a FREE out-drive inspection.

Large parking lot to accommodate boats up to 50 feet in length.

This is the busiest time of the year, and we want to make sure you can get back on the water with a properly tuned prop for your boat as quickly as possible.

Sommer Consulting & Design