When all is said and done, it is the propeller that turns engine horsepower into speed. Despite this critical point, many myths exist in the world of propellers sometimes creating an aura of mysterious black magic. To the contrary, propeller selection and overall performance is really black and white. A boat can only go so fast based on its shape, horsepower and ambient conditions. It is all about physics. So, no matter what anyone promises you, a new propeller will not turn your 15-knot cruiser into a 35-knot power racer unless it is on a trailer being towed down the interstate. When the goal is to maximize performance and efficiency avoid the dockside spin-doctors and seek out a qualified prop shop with the experience and equipment to tune your propellers.

Here are some commonly pitched ideas about propellers.

Adding pitch will make my boat go faster. Maybe, but the real question is how long do you want to go fast? In sizing propellers, the diameter is listed followed by pitch. Thus a 26 x 28 prop has a diameter of 26" and a pitch of 28". (An exception is twin props turning simultaneously on a single shaft, which are called propsets). Pitch is the theoretical distance the prop travels in one revolution. However, a prop turning in water will slip because of the medium, so the actual distance covered will be less. Nonetheless, the propeller���s pitch is what delivers the speed.

The most important variable when determining the correct propeller pitch is that the engine must reach full rpm with diesels, and attain the recommended operating range in inboard and outboard gasoline engines. If the engine cannot meet these specs, it will labor, guzzle fuel like party hearty frat boys, smoke more and develop higher thermal stress, which will wear out engine parts prematurely. Too much pitch will quickly drive your engine to the scrap pile.

Many factors, including air and water temperature, load, bottom slime and wind to name a few, can affect speed and rpm. But if your boat cannot consistently reach the recommended rpm as described in your engine operation manual, a problem exists and it must be corrected. However, before you start cutting or adding inches of pitch to the propellers (adding one inch of pitch can result in a reduction of 50 to 100 rpm and vice versa), be sure you have the information needed to make such a decision. Collect as much data as you can so you and your prop technician are not guessing at a solution. For example, a dirty bottom or a small amount of scale build up on the prop is enough to cost you 100-200 rpm. Even a small bend in one blade can throw the propeller out of balance and chip away at the rpm, not to mention introducing vibration. A dirty fuel or air filter, or fouled spark plugs or blown injector tip also can cause power issues. The bottom line is that if the boat has reached its rated operating parameters with the current propellers in the past, it is unlikely the problem is the props, unless you have run aground or done something to change the shape of the blades. The best thing you can do at the end of everyday on the water is open the throttles and see how the engines perform.

Vibrations are normal. If they occur just adjust the throttle to eliminate it. There is vibration and then there is VIBRATION. Some vibration is normal, like the sensation you feel in the cockpit as the water rushes past the hull bottom. But, if the bow rail shakes or the helm seat feels like it is motorized, adjusting the throttle is the wrong answer.You need to haul out or get a diver to remove the props and have them dynamically and statically balanced (See ���Getting Scanned, page 36) and reconditioned as required. Propellers are designed to tear up water, not slice through sand bars. Once the vibration starts it can lead to metal fatigue if allowed to continue. Ignore the problem and you may lose a propeller blade one day without warning. If this does happen, hopefully the prop blade will sink when it breaks and not come through the bottom of the boat.

If three blades are good, four must be better, and five unbelievable. Unbelievable perhaps, and sometimes true, but not always. Three-bladed props give good all-around performance. But if you notice a slight chatter and it bothers you even though the prop shop insists the blades are true and balanced, a four-bladed prop will probably smooth things. Likewise, a fivebladed prop is smoother still. Additional blades also provide better acceleration. The downside? More blades cost more money and the additional metal may cause a slight downturn in top speed if you have to decrease the pitch in order to reach full rpm.

Stainless steel is the best material for propellers. Stainless is a popular choice for outboards and stern drives. It is stronger than aluminum and the blades can be thinner, which reduces water friction and results in a few more MPH. However, aluminum is a better choice in rocky waters because if you hit an obstruction at a high rate of speed, the prop will take the impact by bending or breaking and save the lower unit from gear damage. Aluminum props generally cost half as much as a stainless steel one, which means you can easily afford to carry a spare. They also are less expensive to repair. Inboard propellers are generally made of bronze or Nibral, which is a combination of nickel, brass and aluminum. Bronze is a good choice for modest speed, while Nibral has additional strength and is better suited for higher speeds.

Also See Getting Scanned : Diagnostic technology can enhance propeller performance