Troubleshooting Tips for Boat Engine Propellers

Log on to any online boating forum and ask a couple of simple questions about your boat’s propeller. Chances are you’ll get dozens of answers – and we’ll wager that most of the responses are different.

Yamaha gets a lot of calls about the various aspects of boat operation and performance, but questions about propellers are right there near the top of the list. Perhaps it’s because the subject of propellers is shrouded in mystery.

Not to worry – we’re here to straighten things out and present suggestions based in reality– the reality of a lot of seat time testing boats, engines, and especially propellers. We’ll address a fistful of the most common generic prop-related questions.

Q. At full throttle, my engine rpm is too low (according to the owner’s manual). Would a different propeller bring the rpm up to where they should be?

A. Try a propeller with less pitch. Every inch of pitch usually changes the engine’s speed 150 – 200 rpm, so full throttle rpm should increase when using a lower pitch prop (of the same style). For example, if your outboard is rated for 6000 rpm at full throttle and it only gets to 5200 rpm, dropping two to four inches in pitch should allow the engine to operate near its maximum rpm. In addition to gaining rpm, you may notice improved acceleration on the low end, and snappier throttle response.

Q. My outboard exceeds the engine manufacturer’s recommended rpm at full throttle (or the engine hits the rev limiter). Do you think the prop has anything to do with this?

A. A propeller with more pitch could help. Engine rpm should decrease about 150 – 200 rpm per inch of pitch with a higher pitch prop (of the same style). A propeller with more pitch should bring the outboard’s rpm down to where they should be, improving the engine’s mid-range cruise economy, and if you’re lucky, you could gain a little more top speed – maybe.

Q. Running a three-blade propeller, my boat has good top end speed, and the engine’s rpm is where it should be at full throttle. I’d like the boat to get on plane quicker, but I don’t want to give up any top speed. Do you think a four-blade wheel would help?

A. Choosing a propeller is usually a compromise. Seldom do we find a prop that pops a boat on plane and delivers blistering top speeds – but there are exceptions, and it doesn’t hurt to try. Bolt on a four-blade propeller an inch less pitch (to keep the outboard’s rpm up) than your three-blade. Your boat’s planing times ought to be better; however, in most applications, a four-blade prop may not be quite as fast on the big end as a three-blade propeller.

Having said that, we’ve seen a few boats (mostly really fast bass boats) actually pick up speed with a four-blade wheel, because the four-blade reduced the boat’s tendency to chine-walk, allowing the driver to push the boat closer to the ragged edge of its performance envelope. However, these bass-racers are the exception, not the rule.

Q. During a sharp turn, the outboard revs up, the boat loses speed and falls off plane. It seems like my three-blade propeller isn’t gripping like it should. What’s going on and how do I fix it?

A. It sounds like your propeller is ventilating – ingesting air from the water’s surface when the boat leans over in a turn. The propeller doesn’t work well in this aerated water, and as you suspected, the prop loses its grip – simply spinning rather than pushing the boat.

Try trimming the engine down when you turn, to keep the prop deeper in the water for better ‘bite’. If the three-blade still ventilates, you could try a different style of three-blade prop with a larger diameter, more cup on the blade tips, or perhaps a four-blade wheel to stay hooked up better in the corners.

Q. I’m trying to increase my boat’s top speed, but the outboard’s rpm is at the upper end of the manufacturer’s specified rpm range with the boat loaded, and trimmed for maximum speed. Will changing the propeller make my boat go faster?

A. The short answer is not likely – it appears that the boat is doing all it’s going to do. The long answer is that pitch=speed. The more pitch the engine can turn (while staying close to the max rpm), the faster the boat will go. If you put a higher-pitch prop on your boat (without changing anything else), you’ll lose rpm (and probably drop some top speed), throttle response and acceleration are going to be lackluster, and the boat may be slow to get on plane.

Without getting into engine height, boat setup, or driver ability issues, you have to realize that a boat’s power-to-weight ratio is one of the biggest keys to performance.

Based on what you’ve told us, you’re going to have to add a substantially more powerful outboard to turn a prop with more pitch (more pitch equals more speed) to see any significant increase in top end speed.

Q. A guy at the marina told me that stainless steel props are faster than aluminum propellers. Is this true?

A. If you swap your aluminum prop to a stainless steel propeller (same style and pitch), you’re not likely to experience a noticeable change in your boat’s performance.

However, because aluminum is a rather soft material, the blades on an aluminum prop tend to be thick (thick blades create additional resistance, called drag), and the metallurgical properties of aluminum limit the creativity of the propeller engineers when creating performance-oriented blade styles.

Stainless steel is much stronger than aluminum; therefore the blades on a stainless prop are thinner (and offered in a wider variety of blade shapes and styles) than is possible with aluminum. Thinner blades and advanced blade shapes (geometries) can result in improved acceleration and speed.

If your boat’s hull design and engine have untapped performance potential, a stainless steel prop might be faster than your standard aluminum wheel. But if you have a 15’ jon boat with a 25 hp outboard on the transom, a stainless prop might not be worth the investment.

Caveat

Keep in mind that every boat is different, and the variables that affect a boat’s performance can change by the moment; therefore, your results may not be quite the same as our suggestions. We’ve given you our best shot – the finest recommendations available, based on our cumulative years of experience on the water.

For more propeller information, check out www.yamaha-outboards.com or www.turbo-props.com. You owe it to yourself to become an educated and knowledgeable boater to be able to separate prop fact from prop fiction.

The propeller industry has its own unique language, which can add to the confusion factor when prop shopping. Here are some common propeller terms, including definitions, and how these factor into how a propeller works – important information in the propeller selection process. If nothing else, at least you’ll have a clue what the prop folks are talking about.

Size

We use numbers to describe propellers: the first number is the diameter, and the second number is the pitch.

Diameter

If you could draw a circle around a propeller’s blades, diameter would be the distance, (in inches) across this circle. Large diameter propellers push a great volume of water and extend deeper into the water under the boat. You’ll find large diameter propellers on big, heavy boats – or on outboards mounted high on the transom.

Lightweight boats with the prop deep in the water, or a high-powered boat with a lot of lift built into the hull design, often use smaller diameter propellers.

Pitch

The distance, in inches, that a propeller would theoretically travel forward through a solid during one complete revolution. (Theoretical distance, because the propeller operates in a liquid medium – water – it slips a little bit).

The correct propeller will allow your boat’s engine to reach its maximum recommended rpm (revolutions per minute) at full throttle, trimmed up for best speed, with a typical load in the boat.

An inch of pitch should change the engine’s speed about 200 rpm. Increasing pitch lowers engine rpm; decreasing pitch raises engine rpm.

Ventilation

Ventilation occurs when excessive air enters the water surrounding the propeller, causing the blades to lose their bite in the aerated water. Rpm increases, but speed typically drops.

Many pleasure boats experience ventilation during a sharp turn, especially if the engine is trimmed up for maximum straight-line speed. Trim the engine down a bit when turning to reduce ventilation and maintain better boat control.

Some engines require a predetermined amount of controlled ventilation, letting the prop spin for a moment until the engine can turn up sufficient rpm to create the horsepower necessary for extreme acceleration. This is why some propellers have small slots or holes in the sides of the hub – to allow engine exhaust gases to aerate (ventilate) the water around the propeller during takeoff.

Number of Blades

Three blade propellers typically offer good overall performance and speed for most boats; however, a three blade prop may tend to lose its grip (ventilate) in hard turns, and may not be the best handling propeller at top speed.

Four blade propellers can get a boat on plane faster, can keep a boat on plane at a lower speed, are less likely to lose traction in turns, and change a boat’s handling characteristics by lifting the bow or the stern. However, a four-blade propeller may not be quite as fast (1-2 mph) at top speed as a three blade prop.

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