  Boat Design Review
The type of hull determines what kind of propeller is required to move the boat through the water. There are two basic hull types: displacement and planing. Displacement boats are comparatively slow. They are supported by the weight of the water they displace (static water pressure). They are generally designed with a somewhat pointed or rounded stern as well as a pointed or rounded bow. They ride low in the water like large ocean-going ships. The propellers on these boats run totally submerged and will generally be of lower pitch, when powered by an outboard or stern drive.
  Planing boats, which include most recreational boats, operate like a displacement hull at low speed (below 10 MPH). With sufficient power, these boats rise to the water's surface and operate in a skimming or planing mode, supported primarily by the water's velocity pressure. Planing boats are generally faster and more efficient than displacement boats. Their design is distinguished from a displacement design by having a broad transom that meets the bottom with a basically sharp corner. The propellers on these boats often are not fully submerged and thus need to provide holding ability as well as higher pitch and rake because of the higher top-end speeds.
Following is a description of typical hull designs for outboard and stern drive boats.   Flat Bottom
When a boat's bottom has no "v" but is simply flat from side to side, it is generally referred to as a flat bottom. Where the bottom meets the side is called the "chine." It may have squarish corners (called square, sharp, or hard chines) (Figure 7-7) or rounded corners (called round or soft chines) (Figure 7-8). Vee Bottom This is presently the most common bottom design (Figure 7-9), offering good speed with a softer ride that depends on the angle of the vee (called "deadrise," see Figure 7-10), the radius or shape of the   keel line, and the use of strakes (Figure 7-11). To increase top speed with only a little loss of softness in the ride, some boats are made with a small flat at the very bottom, called a "pad" (Figure 7-12). Each manufacturer chooses how many and how far back (toward the transom) to bring each strake in order to achieve the desired performance characteristics. A greater use of strakes tends to make the boat run higher and flatter, but a little harder. Tri-Hull or Cathedral Hull These hulls usually are vee bottoms with some degree of added outside hull, often most predominant near the bow   (Figure 7-13). The benefit is a boat that is more stable, particularly at rest. The penalty is a rough ride in choppy water. Tunnel Bottom This design is most popular in racing circles. It differs from the older catamaran bottom (Figure 7-14) in that the inside corners (between the bottom and the tunnel) are quite sharp (Figure 7-15). This allows incredibly sharp high-speed turns and a very soft ride. Some of these hulls have experienced handling problems at low speeds.  Tunnel Vee
This combines a shallow vee bottom with twin tunnels (Figure 7-16), one on either side of a center pad. Top-end performance is usually superior to a true vee bottom, but this generally comes with a rougher ride in choppy water and does not carry a load as well as a conventional vee. Pontoon A pontoon is basically a flat surface floating on two or three pontoons (round or squarish), usually made of aluminum. This design maximizes a boat's usable space. Pontoons can easily be run at planing speeds, but the front ends must be properly shaped for planing (Figure 7-17). Condition of the Boat Bottom  For maximum speed, a boat bottom should be as flat as possible in a fore-aft direction (longitudinally) for approximately the last five feet (1.5 m) (Figure 7-18). For best speed and minimum spray, the corner between the bottom and the transom should be sharp.
The bottom is referred to as having a "hook" if it is concave in the fore-and-aft direction (Figure 7-19). A hook causes more lift on the bottom near the transom and forces the bow to drop. This increases wetted surface and reduces boat speed, but it helps planing and reduces any porpoising (rhythmical bouncing) tendency. A slight hook is often built in by the manufacturer. A hook can also be caused by not trailering or storing the boat with support directly under the transom. A "rocker" is the reverse of a hook (Figure 7-20). The bottom is convex or bulged in the fore-and-aft direction. It can cause the boat to porpoise. Any hook, rocker, or surface roughness on the bottom, particularly in the all-important center-aft portion (critical bottom area) (Figure 7-21), will have a negative effect on speed, often costing several miles per hour on a fast boat.  Single- and Multiple-Engine Application
In single-engine installations, right-hand rotation propellers are almost always used. This is merely the result of tradition, but is perpetuated by nearly all recreational boat manufacturers placing their operator's position on the right-hand side to compensate for the boat roll resulting from a right-hand rotation propeller. When a boat is equipped with dual engines, counter-rotating propellers are preferred (Figure 7-22). This balances any steering pull when the outboards or drives are trimmed evenly. Most experienced boaters prefer to rotate the propellers "out." That is, right-hand rotation on the right side, left-hand rotation on the left side, believing that overall handling is a little better. This can also afford more balanced steering when forced to operate with only one (off-center) engine. Many twin engine boats use propellers of the same rotation. The disadvantages are that steering torque is greater when trimmed well in or well out (assuming no power steering) and, in very rough water, if a hull goes airborne, a pair of right-hand propellers (for example) can walk the stern to the right a little. Choosing a Propeller Propellers are described in numbers: the first number is the diameter, and the second number is the pitch. So, a 14 x 19 propeller has a diameter of fourteen inches, and a pitch of nineteen inches. In conversation, most people would describe this propeller as a 19-inch pitch, or simply a 19. What is the most important thing when choosing a propeller? Pitch. No matter what size engine or what kind of boat, if the propeller isn’t the right pitch, the boat isn’t going to live up to its potential. Pitch is the theoretical distance, in inches, a propeller moves forward every time it turns (i.e. a 19” prop should move forward 19” with each revolution; in reality, it’s a bit less, because the propeller isn’t 100% efficient). Here’s the skinny: You want to be able to run the engine at, or near, the manufacturer’s recommended maximum RPM at full throttle, trimmed up for speed, with a typical load in the boat – and if you can, your propeller is the right pitch. Too much pitch (over-propped) - the engine won’t reach its max RPM, is sluggish getting on plane, and has poor throttle response. Not enough pitch (under-propped) - the boat pops on plane and accelerates with confidence, easily exceeding the recommended full throttle RPM. Neither condition is good for engine longevity or fuel economy. The cure? Since every inch of pitch is worth about 150-200 RPM, decreasing pitch should provide a proportionate increase in RPM – drop pitch an inch, gain a couple of hundred RPM. Conversely, increasing pitch usually results in a RPM decrease – go up an inch, lose a couple of hundred RPM. After you have the pitch part of the equation figured out, then the propeller selection process can continue. How Do I know What Pitch is Right for Me? Consult the engine owner’s manual to find the recommended wide-open-throttle (WOT) range for your engine. If the current propeller is at WOT RPM within the specified RPM range, select a replacement or upgrade propeller with the same pitch as the current propeller. If the current propeller isn’t at WOT RPM within the recommended RPM range, select a replacement or upgrade propeller with a larger or smaller pitch using the following rules: If you’re upgrading from a three blade to a four blade propeller, remember that a four blade propeller generally turns 50 to 100 RPM less than a three blade prop with the same pitch. Pitch Points Generically, every inch of pitch is worth about 150-200 RPM. Increase pitch an inch and the full throttle RPM should drop around 150-200 RPM. Decrease the pitch an inch and the RPM ought to go up roughly 150-200 RPM at full throttle. Too few RPM at full throttle? You might need a prop with less pitch. Too many RPM at full throttle? A propeller with more pitch could help. Stainless Steel vs Aluminum It’s a question of strength and performance. Aluminum propellers usually cost quite a bit less, however, stainless steel is over five times more durable than aluminum. If you’re looking for quicker acceleration, superior top speed, or better overall performance, you should consider a stainless steel propeller. Aluminum propellers aren’t expensive, they work well, and are relatively easy to repair, but aluminum isn’t particularly durable, making aluminum props susceptible to wear and damage. Stainless steel propellers cost more up front; however, because stainless steel is stronger than aluminum, a stainless propeller’s blades are usually thinner and can be made in a wider variety of styles and shapes than an aluminum prop, thus the potential for better performance and increased service life. Remember, it’s stain-less steel, not stain-proof - a stainless propeller can still rust, so you need to keep it clean, just like the rest of your boat.   Three Blades vs Four Blades
Four blade propellers usually: Plane the boat faster than 3-blade propellers. Keep the boat on plane at a lower speed. Give improved mid-range speed at the same RPM as a 3-blade propeller. Provide quicker acceleration than most 3-blade propellers. Run smoother than 3-blade propellers. Have better holding power in rough conditions. Are less likely to ventilate in sharp turns. Provide better low speed handling. Are not quite as fast on the top end as a comparable 3-blade propeller. In broad terms, three blade propellers offer good overall performance; however a three blade prop may lose its grip in turns and may not be the best handling propeller on high-horsepower rigs. Four blade propellers can get a boat on plane faster, they’re less likely to lose traction in turns, and help many boats handle better, although a four blade propeller is often a bit slower (1-2 MPH) at top speed than a three blade propeller. What are Some Common Signs it is Time for a New Propeller? Your boat takes longer to get on plane than it used to. It uses more fuel. It doesn’t seem to run as fast as it did when it was new. If your boat feels sluggish, one of the first things you should check is your propeller. Is it showing signs of wear and tear? Does it have more than its share of nicks, dings and missing paint? These symptoms can accumulate and significantly impair the performance of a perfectly good engine. How Should I Maintain and Service My Propeller?  Essential to good propeller maintenance is periodic inspection to detect even small dings, which can lead to blade failure if not dressed or repaired. A damaged propeller, even one that only appears slightly damaged by running through silt and sand, can significantly reduce performance efficiency and fuel economy, and can more severely damage itself through cavitation erosion emanating from the blades' irregular leading edges (Figure 8-42). In one test with a damaged propeller, top speed fell more than 13%. Acceleration was off over 37%. Optimum cruise miles slowed 21 %. Worse yet, damage usually is not done to each blade uniformly and, therefore, the damage can set up imbalance vibrations that can cause fatigue damage to other parts of the engine or drive. If you boat in shallow or rocky waters, you will want to check your propeller more frequently for possible damage.
Up to a point, dealers can have a propeller restored to like-new condition; however, extreme damage can be more expensive to repair than the cost of a replacement. Minor damage corrected early on can prevent much more serious and costly repair later on to both the propeller and other parts of the engine. Propeller repair, as well as customizing, can include adjustments to pitch, rake, cup, and blade thickness. To aid in the future removal of the propeller, liberally coat the propeller shaft spline with Anti-Corrosion Grease. To assure that the propeller remains secure on the shaft during the season, periodically check the self-locking prop nut for tightness. |
