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.
Replacement props for IO or outboard boats are available in aluminum or stainless steel. These materials compare as follows:. Aluminum is the most common, least expensive material.
Suitable for most outboard and sterndrive applications. Stainless steel offers a performance advantage over aluminum due to stiffer, thinner blades and more advanced designs. Best choice at speeds over 50mph, or if your boat is running over oyster beds or sandbars regularly.
Stainless costs more but is five times more durable than aluminum. Stainless props can be repaired, at a higher cost, to like-new condition, while repaired aluminum will suffer from metal fatigue and a loss of strength.
Three or four blades work well in either sterndrive or outboard applications. Three-blade designs give you all-around performance with an advantage on top end speed.
Four-blade designs work well with boats that are difficult to get on plane, underpowered or used in watersports where top-end speed is not critical. The composite cores of modular hubs are designed to break away upon significant prop strikes, helping to protect the prop body and engine drive train from damage. Four blades in many cases will drop your rpm by 50 to rpm with identical pitch.
Three-blade props are generally best for recreational boats with three-, four- and six-cylinder outboards and sterndrives, giving good hole shot and top-end performance.
Adding a fourth blade increases the DAR to between 60 and 65 percent, so you can expect more thrust to keep your boat planing at lower rpm, a potential boost in fuel economy, but also a reduction of 50—rpm at WOT. Modular hub systems consist of a hub that fits on to the splined shaft of the engine and the prop body which fits on to the hub.
These systems offer two main advantages. First, in the event of a significant prop strike, the composite hub breaks away, which reduces the chance of damage to the engine, engine drive or propeller.
Also, if the prop is bent but the hub is intact, replacing just the prop is also less expensive. Recreational boat propellers are usually offered in two-inch pitch increments within a prop model line, but some high-performance props are offered in one-inch increments to allow for fine-tuning boat performance.
Propeller diameter is simply the diameter of a circle scribed by the blade tips of the prop. Propeller pitch is the distance the prop would move forward in one rotation if it were moving through a soft solid—think of a screw being turned into wood.
The blades on a propeller are analogous to the threads on a screw. Some propellers have a constant pitch, meaning the pitch is the same at all points from the leading edge to the trailing edge of the prop blades.
Progressive pitch starts lower at the leading edge and increases to the trailing edge. The pitch number assigned to a progressive-pitch prop is the average across the entire blade. 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. The magazine provides boating skills, DIY maintenance, safety, news and more from top experts.
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