the boatbuilding.community

What makes the propeller work? How do we choose the best propeller? And just how can we get the most performance from our propeller design? This is a multi-part article on the engineering basics of what makes the propeller work. In “History & Design of Propellers, Part 2”, we'll look at the parts of a propeller and. advanced propeller design and propping techniques.

Let’s first understand the different parts of a propeller, so that our terminology is consistent.

1.4 Propeller parts

Figure 6 Propeller Parts

1. Blade Tip - The full-length dimension of the blade from the center of the propeller hub. The tip is exactly between the leading edge and the trailing edge.
2. Leading Edge – Thin edge of the blade closest to the boat; is the first part of the propeller to cut through the water; the edge goes from hub to tip.
3. Trailing Edge – The edge of the blade farthest away from the boat; the water leaves the blade at the trailing edge, after being ‘processed’ by the propeller. The edge goes from the tip to the hub.
4. Cup - A small lip on the trailing edge of the blade; permits the propeller to hold water better and has the effect of adding about a ½" of pitch.
5. Blade Face - The side of the blade facing away from the boat; this side creates positive pressure.
6. Blade Back - The side of the blade facing toward the boat; this side creates a negative pressure (relative to the blade face).
7. Blade Root - The point in which the blade attaches to the hub; opposite end of the blade to the tip.
8. Inner Hub – Mechanical component that contains the shock-absorbing hub. The fore end is the metal surface that transmits the propeller forces through the forward thrust hub, through the propeller shaft and thus, to the boat.
9. Outer Hub – (Through-hub exhaust propellers) The exterior hub surface in contact with the water; blades are attached it, and its inner surface is in contact with the exhaust gas passage. It has mechanical ribs that reinforce the outer hub to the inner hub.
10. Ribs – (through-hub exhaust propellers) Connections between the inner and outer hub.
11. Shock-Absorbing Rubber Hub - Rubber molded to an inner splined hub to protect the propeller drive system from damage of impact and when shifting the engine. (First introduced by Carl Kiekhaefer at Mercury outboards).
12. Diffuser Ring - Reduces exhaust backpressure and prevents exhaust gas from feeding back into propeller blades.
13. Exhaust Passage – (Through-hub exhaust propellers) The hollow area between the inner hub and the outer hub through which engine exhaust gases are discharged into the water.

Figure 7 Propeller parts

So where does this all leave us today? Let's look at propellers as they are designed and used today. To make it as organized as possible, we'll look at the major contributing features to propeller performance and selection.

Figure 8 - Modern day 5-blade propeller

Changing the pitch of your propeller can be the key to fine-tuning boat performance. Pitch is the theoretical distance the prop would travel through water during one complete revolution. We can think of it as similar to the distance a wood screw would travel in one revolution while penetrating a piece of wood. However, a 19-inch-pitch prop never actually travels 19 inches in one revolution. This is because slippage occurs in the water. Pitch is the primary measurement of prop performance. Other design features of the propeller such as cup, rake, diameter and number of blades can modify what pitch delivers. The best way to use pitch to your advantage is to apply the rule of thumb – "one inch of pitch equals 100-200 rpm". (Move up an inch of pitch and you will reduce rpm. Move down an inch and you will increase rpm).

The diameter of a prop is the overall width of the rotating blades, or twice the radius (distance from the center of the prop to the outermost blade tip). Prop diameter is limited by clearance between the prop shaft and the cavitation plate on outboards and stern drives. For inboards, the limiting factor is clearance between the prop shaft and the hull.

Figure 9 Modern 4-blade propeller design

By convention, props are identified by diameter and pitch. Typically, this information is stamped on the hub. For example a 12X19 propeller is 12” diameter and 19” pitch. So, when fine-tuning your prop, which is better to experiment with, diameter or pitch? The usual answer is pitch. While it is possible to alter rpm and performance by changing diameter, professionals start with pitch. This is because diameter is limited by fixed components, and since manufacturers make more options for pitch than for diameter.

There is a wide range of propellers from which to choose: three-blade, four-blade, five-blade; cupped, double-cupped, vented, blueprinted, stainless steel, aluminium, and even plastic materials. These options give you the ability to improve overall performance, and also to fine-tune the precise kind of performance you want to improve. Here are some of the most common options for advanced propping:

Three-blade propellers are the most commonly found props because they offer the best potential for all-around efficiency, performance and pricing.

Four-blade props have become increasingly common in recent years. With more blade area in the water, these props run at slightly lower rpm, and deliver slightly slower top speed. They make up for it by improved handling in rough conditions, increasing hole-shot and reduced blow-out in turns.

In certain applications, four blade props actually can improve top end performance, but these are limited. Such applications include stepped hulls, tunnel hulls and boats equipped with jacking plates or high trim angles that raise the tips of the blades near the surface of the water in operation.

Four-blade props can work well on such hulls because the flow of water to the prop is disturbed (filled with air bubbles) as it separates from the running surfaces (hull bottoms). Disturbed water is less dense than solid water, and the 4-blade propeller helps in two ways:

• the aerated water allows the blades to spin easier, enabling the engine to run at greater rpm
• the four-blade grips more than a three-blade because it has more blade area.

Surfacing propellers (whose tips run close to, or break the surface of the water) are used on boats with jack plates and high trim angles. These blade tips can draw in air as they turn, making the water less dense. In these conditions, the greater blade area of the four-blade prop enables the prop to maintain its bite (less slip).

Most props today are cupped. This means that a curved lip is added to either the leading edge or the tip. Some props add a curved lip on both locations to create a “double-cupped” design.

Adding cup to the prop's trailing edge has the effect of increased pitch. Adding cup along the tip of the blade has the effect of adding rake. (Rake is the angle that the blades tilt toward or away from the hub).

Cupping can increase the prop's grip on the water (less slip) and decrease blow-out (a condition caused by excessive aeration). Cupping can thus, allow the boat to run with higher trim angles and transom mountings.

The three most common prop materials are composite plastic, aluminium and stainless steel. Each offers differences in price and performance. (Bronze props, another option, are usually found on slower speed yachts.) Prop materials in many ways determine their suitability when matched to engines of varying horsepower. Plastic props are used on engines of less than 50 hp (and I prefer less than 20ph). Aluminium props are suited use on engines of up to 150 hp. Stainless props are expensive, but justify it because they make the best use of any amount of horsepower.

Many boaters familiar with composite plastic props use them as spares rather than everyday equipment. Although economically priced, their efficiency is limited by blade flex and inability to accept cupping, because. of the relative weakness of the thin plastic blades. Most plastic props cannot be repaired, although some offer replacement blades.

Aluminium props are slightly more expensive than composite, but deliver more efficient performance through reduced blade flex and the ability to accept some amount of cupping, Aluminium blades can sometimes be repaired after being damaged.

The most versatile and expensive props are stainless steel. Steel is extremely durable, enabling the blades to be as thin as possible to reduce resistance in the water and virtually eliminate flex. Despite being thin, the blades are strong enough to accept significant cupping. Other benefits include the ability to accept repair and the toughness to withstand punishment, such as hitting submerged objects.

An increasingly popular trend on high-performance boats these days are vented props - holes drilled in the outer hub.

Traditionally, props vent exhaust through the center of the hub to ensure undisturbed water delivery to the blades. Vented props allow a limited amount of exhaust to pass into the blade area through holes drilled in the outer hub. This aerates the water and reduces drag resistance on the blades. As the boat accelerates, the vented prop turns easier, allowing engine rpm to rise more rapidly for quicker hole shots. This means that vented props are hole-shot specialists. Once a boat is planing, water passes over the vent holes so fast that the holes effectively become ineffective and only minimal exhaust slips through. With the blades operating in solid, non-aerated water, there is no effect on top-end speed.

Too much aeration through vent holes, will cause a prop to slip. To keep this from happening, Mercury's new Performance Vent System (PVS) allows you to regulate flow by installing plugs into a series of holes with graded sizes. Each PVS prop comes with a set of plugs, which allowing you to find one that delivers the best performance for your set-up.

Another new prop trend is a process called blueprinting. To get a prop ‘labbed’ a specialized shop will check it against the original supplier design - blade distortion, pitch, rake, cup and other engineered elements. The prop is ground, heated and formed until it meets its design specification as precisely as possible.

Tests have shown that blueprinting can enhance top-end performance by 3-5 mph. While the cost can be several hundred dollars, some boaters believe it is the easiest way to add speed. There’s always a downside, though, and ‘labbing’ usually thins blades and consequently reduces prop reliability after some time.

OK, that’s enough for now. Next, in “History & Design of Propellers, Part 3”, we'll look gear case design, nosecones and causes of blow-out.