How do you know when your boat propeller is not working hard enough?

A discernible drop in speed could be due to anything from the current to the wind and sea state.

On the other hand, it may be that your boat propeller is corroded or fouled by barnacles and weed.

Or it may be you had the wrong type of prop to begin with and actually, if you replaced it with one of the right diameter and pitch, you could save a chunk of money on fuel and go faster.

If there’s a discrepancy between the manufacturer’s rpm range and your actual rpm, a new prop could be the answer.

David Sheppard is the sales manager at Bruntons Propellers, which designs and manufactures props for commercial and leisure boats.

Over the last 20 years, his customer base has moved away from the individual boat owner to the large boatbuilder looking for the most economical prop for their factory-built design.

However, that might not be the best option for your type of cruising.

“Most production yachts come with a folding prop because it’s the least expensive option,” he says. “If you’re not bothered about fuel saving – say, you’re just going out of the river and back – you’d probably have a folding prop, but if you want to cross the Channel or go further afield to the Med or do an ocean crossing, an Autoprop is a good solution as it can save up to 20% on fuel, increasing your range.”

What does the right boat propeller look like?

With the right boat propeller, you should theoretically achieve the vessel’s maximum speed when the engine is at maximum power.

But prop selection isn’t just about efficiency; it also impacts the health of the engine.

“The prop needs to be sized correctly for the space, maximum power and maximum available rpm,” says David.

“If the prop is too small for this configuration it will be light on engine load and allow the engine to over-rev above its maximum rating. If it’s too big, and you reach maximum hull speed before maximum rpm, you risk overloading the engine and may see black smoke, though that’s not as obvious in a common rail diesel engine compared to a naturally aspirated one.”

For sailing boats, there is another consideration, which is drag.

If your goal is to spend more time sailing than motoring, then reducing drag may be a higher priority than going faster under motor.

What size boat propeller?

So how do you size up a boat propeller? It’s best to consult a propeller expert who’ll carefully consider all the calculations.

Ideally, you’re looking for the largest diameter prop that allows the recommended blade tip clearance.

To calculate this, the technician looks at a range of information. Pitch, diameter, shaft taper and clearance are the basic requirements.

“You need the blade area ratio to be appropriate for the engine’s power and maximum shaft rpm,” says David. “The blade area ratio (BAR) or disc area ratio (DAR) is an important factor to consider. The area ratio is expressed as a percentage of a solid circular disc.

“For instance, if the blade area is too small, the propeller will suffer from cavitation which leads to noise, vibration and loss in performance.”

There are other considerations too. In order for the boat propeller to convert the engine’s torque into thrust, it needs to be matched not only to the power and speed of the engine but the boat’s waterline length and displacement.

What you’re aiming for is the smallest size engine capable of achieving maximum hull speed with low fuel consumption.

A large diameter prop rotating slowly is more efficient than a small diameter prop rotating quickly, but the latter won’t get a sports cruiser on the plane, for example.

Also, you may not have an engine and gearbox combination to match.

“The design scope is much broader with a conventional fixed-pitch propeller,” says David. “For example, you can modify blade area, pitch distribution, skew, blade rake, diameter and so forth, all of which help make the propeller work better.

“Unfortunately, these elements are not as flexible with sailboat propellers and therefore users sometimes have to accept a compromise when fitting a folding or feathering propeller.”

Purpose of the boat

For motorboats, where speed, range and fuel efficiency are the main considerations, a fixed boat propeller is a straightforward choice.

However, for yachts, there are other factors at play.

“One of the things that’s critical, especially on sailboats, is reducing drag but there are trade-offs in terms of performance,” warns David.

“Folding props are not the best in reverse, and while a flat-bladed feathering prop is good for ahead and astern, there is no twist in the blade so there’s some deficiency there. Plus you only have one point on that flat blade where pitch is correct so it can be a lot noisier.”

For a racing yacht, where the only purpose of the engine is to get to and from the mooring, you might select a propeller with the least amount of drag under sail.

A cruiser or motor-sailer might opt for an engine with 30% more power capable of achieving maximum hull speed in harsh conditions, in which case a highly efficient propeller is more important than drag.

Some vessels are deliberately ‘over-propped’ to absorb and transmit the full engine power at a specific engine speed, rather than the maximum rated speed (rpm).

You also get controllable pitch propellers where you can change the angle of the blade while motoring.

Some container ships use these, as well as fishermen, who may need to change the angle of the blades when slowed down by a big catch.

How many blades?

For displacement hull sailing yachts, propellers usually come with two or three blades.

For boats up to around 30hp a two-blade will do the job whereas above this power a three-blade will be better for smoother running.

On a larger vessel above 55ft, when having to power against heavy winds and seas, a three- or four-blade prop will keep your speed up, and the run will be much smoother.

So for bigger boats, or those that are heavy for their length, the increased blade area of four or more blades is a big advantage.

PBO boatbuilding expert Tony Davies noticed an improvement in thrust and performance when he upgraded his boat to a three-bladed prop.

“I had a three-blade Volvo folding prop on the wing engine of a motor cruiser and this gave remarkable performance compared with the two-blade prop I previously had fitted,” he said.

Which boat propeller design?

In the 1970s the reliability and performance of folding and feathering props was questionable but today’s derivatives are built to withstand all types of conditions and usage.

“Most importantly, they have been rigorously tested and used successfully for many years around the world,” says Bruntons’ David Sheppard.

“The trend is changing. More boat owners are realising the potential of a folding or feathering propeller.”

Fixed or folding?

On a fixed-blade boat propeller, the blades stay open all the time. This is the cheapest option and provides good thrust but also has the greatest resistance when sailing, something David describes as “akin to towing a bucket”.

With a folding boat propeller, available in two-, three- and four-blade options, the blades stay closed, relying on centrifugal force to open them when the engine starts.

Though initially developed for the racing sailor as a means of auxiliary propulsion, modern designs have improved dramatically.

They have little drag when under sail but can be jerky when opening and perform poorly astern as the blades need to open against the water flow.

“Some of the latest designs incorporate small blade overlaps,” says David.

“The rotation of the shaft uses the water pressure as well as the centrifugal force to help open the blades, especially when reversing. Three-blade versions are often seen as a less expensive alternative to the feathering options. They are as good as a fixed prop under power in most cases but lack bite when manoeuvring. The four-blade version is better for low noise and smooth operation.”

Feathering boat propeller

When sailing with a feathering or ‘adjustable pitch’ propeller, the blades can be positioned like a flag relative to the flow of water, minimising resistance.

The pay-off is that the flat blades are marginally less effective under power, but this is offset by reduced drag.

Feathering propellers have gears that allow you to manually adjust the pitch when the boat’s out of the water so that it’s optimal at cruising rpm not maximum rpm, but note you need to take care not to overload the engine.

Another benefit is that the blades present the leading edge in astern as well as ahead, making it more efficient in reverse than a folding prop.

“Flat-bladed feathering props tend to be used on traditional boats, particularly long keel vessels, because they can fit the smaller space available. If there’s an aperture then it’s the perfect solution,” says David.

A folding or feathering boat propeller generally has a low blade area ratio compared to a conventional propeller because the blades have to fold or pass each other when they change direction.

However, the advantage is that when sailing you can increase speed by ¾ knot.

Tony Davies advises: “In my experience, a feathering propeller should give nearly the same performance as your fixed blade prop, and a folding prop won’t be far behind.”

Autoprop

The Autoprop is a British invention manufactured in Essex. It is a self-pitching feathering propeller, which has a higher efficiency over a wider range of rpm settings.

Like with a feathering prop, these blades swing through 180° into reverse, so that the propeller shape and leading edge are the same as in forward, virtually eliminating prop walk.

The difference, though, is that the blades adjust their pitch automatically by balancing opposing forces: centrifugal and hydrodynamic.

“The Autoprop varies the blade angle, giving maximum efficiency within a range of speed, whereas a conventional prop is only efficient at its design point, which is maximum speed at maximum rpm,” says David.

“At cruising speed the efficiency drops but not with an Autoprop because it optimises the pitch for the conditions.”

Bruntons sells a lot of Autoprops to long-distance cruisers and those sailing in the Med, where in the summer there’s very little wind and they need to motor efficiently, yet not increase drag under sail.

There’s also a version for electric engines called the Autoprop Eco Star.

David explains: “On sailing yachts, when speed isn’t an issue but increasing battery energy consumption is, you can allow the prop to turn in the vessel’s wake so that the drive motor becomes a generator and recharges the batteries”.

Why are boats so tricky to reverse?

For boats that do a lot of manoeuvring in tight quarters, such as inland waterways and canals, prop walk and performance in reverse is worth considering.

“Propellers are not designed to go backwards,” says David. “It’s like flying an aeroplane backwards and upside down.”

This is because the blade profiles are designed for ahead performance.

In astern, the trailing edge of the blade has to lead, which is vastly inefficient. Also, many gearboxes use different gear ratios in astern compared to ahead.

The boat propeller is designed for the ahead ratio.

David explains that when you go forward on a boat this ratio might be 2:1 but in reverse it’s 3:1.

Props are designed to work optimally to the power and rpm, so a 16in prop in forwards ought to be 19in one in reverse which, of course, isn’t possible.

What is possible, however, with a feathering prop, is to flip the whole thing around so the performance is the same, and prop walk reduced.

Of course, experienced operators use prop walk to their advantage.

Prop walk or ’kick’ is caused by the way water flows over the blades. As the prop rotates, it creates a flow of water which exerts a sideways force on the stern.

A single right-handed fixed propeller will tend to push the stern of a vessel to starboard when going forward and to port when going in reverse.

Although it can be challenging to control, when you’ve figured out your boat’s prop walk, you can use it to make tight turns, pivot on the spot and reverse into a berth.

Troubleshooting your boat propeller

There are many reasons why an engine can lose performance.

Loss of drive from a prop fouled with barnacles can be surprisingly dramatic, folding props can fail to open and the blades of the feathering prop can become restricted or seized solid, though these days this is uncommon.

On larger yachts with high-powered engines, particularly over 100hp, cavitation can be an issue, causing noise and vibration: ‘an uncomfortable idiosyncrasy in the owner’s stateroom on a luxury yacht!’ says David.

If the prop becomes seriously fouled by flotsam, this can cause damage to the propeller and shaft line and lead to excessive vibration and rattling.

It might damage a blade or bend the shaft and this will put things out of balance.

Losing a blade on a feathering or folding prop is also very rare and usually only occurs to a prop with moving parts that is badly maintained.

Boat propeller maintenance

Hard antifoulings are best for propellers and outdrives and most brands have a specialist option different to their regular antifoul such as International’s Prop-O-Drev, Hempel’s Ecopower Prop Spray or Seajet’s Propeller Triple Pack.

Be sure to note the materials they’re suitable for, so you get no reactions, and always use a primer when applying to bare metal.

Check your prop each time the boat is lifted, for wear, corrosion and movement.

When it comes to the mechanics of the gears, a feathering boat propeller requires the most maintenance.

Folding and feathering props can be stripped, cleaned and lubricated (with waterproof grease) annually, while all you can do for a fixed prop is protect it from galvanic corrosion.

Propellers that use geared blades and hubs will wear out eventually, this includes folding and feathering propellers.

The Autoprop is an exception: the wearing parts, bearings and seals are replaceable with a service kit.

Corrosion

Most leisure boat props are made of manganese bronze – which is more susceptible to corrosion, and tends to be used for fixed propellers.

Nickel aluminium bronze is used for folding and feathering propellers and is the best, most resistant material for sailboat props.

Some props will be made of stainless steel; particularly feathering props where there are lots of moving parts.

“Most corrosion happens in vessels at the dock, which are plugged in and susceptible to stray currents,” says David.

“Anodes should be bonded to the shaft and the propeller, and the hull anode should be bonded onto the shaft. Most sailboat props come with an anode-type fitting, certainly all the ones we sell do. A lot of people are moving towards aluminium anodes now as it’s better for the environment.”

As well as being susceptible to electrolytic corrosion (where the current is caused by an external source such as the battery or shore supply) props can suffer galvanic corrosion.

This occurs when dissimilar metals are connected and immersed in salt water. In other words, your boat is like a giant battery.

For example, you might have a stainless steel boat propeller (cathode) connected to an aluminium outdrive (anode) in saltwater (the electrolyte).

Because of the dissimilar metals, the anodic metal gets used up in the process of generating electricity.

To prevent the used-up metal from being the expensive boat propeller or outdrive, you need to fit a sacrificial anode to both.

The idea is to install enough anode mass to last the season.

No anode corrosion means you have a problem with bonding, but if it’s more than 50% the anode isn’t big enough so you need a larger one.

Dezincification

Marine surveyor Ben Sutcliffe-Davies commonly encounters manganese bronze propellers damaged by dezincification.

These contain up to 40% zinc, which is consumed by galvanic action, leaving behind copper residue.

The first signs are pink patches on the blade, and crumbling where the metal is thinnest, such as the edge of the blades.

Poor – or no bonding – of the anode means that the prop itself corrodes when in contact with another metal, or its own alloy makeup.

Ben cites a five-year-old powerboat he surveyed with twin engines and propellers.

One prop was correctly bonded to the anode, which had all but disappeared, but on the other side the anode had been painted over with antifoul so wasn’t doing its job.

Ben used his ball-pein hammer on both props. The correctly bonded propeller blade had a nice ring to it – it sounded high-pitched and tuneful and looked to be in good shape. The one that wasn’t properly bonded was, in Ben’s words, “As dead as dead can be.”

The hammer sounded dull and hollow and the tips of the blade had even broken away where he hit them.

Make do and mend

If your prop is looking worse for wear but you’re satisfied with performance it may be possible to get it reconditioned.

For example, T. Norris Marine in Chichester can reweld and, if required, polish, balance and re-track propellers made of bronze alloys and aluminium.

Bronze props can also be repitched. Note though the company advises that some props can be bent beyond the point of no return, and those with severe electrolysis can’t be repaired.

“As a good guide, if a propeller is held at the boss and a blade tapped with a metal object, it should ‘ring’. If a dull thud results it is most likely beyond repair.”

If you do need a new boat propeller, don’t just replace new for old without exploring the options.

Designs have changed, and there may be a better solution out there than the one fitted to your boat.

“So few people understand the world of propellers, and that includes shipwrights and engineers!” says David.

“It’s a dark science. Seek advice from a propeller expert and if you can, talk to an existing user of the brand before committing.”

Boat propeller case studies

Autoprop for a Bavaria 34

Jan and Marianne Stenqvist installed an Autoprop on their Bavaria 34. They’ve since travelled over 1,700 miles, allowing them to test it in varying conditions from the Dutch canals to the North Sea

“We, as well as our dog Trinda, are very pleased. We are cruising at 6-7 knots. If the wind isn’t good enough we add speed by having the motor running with 1,000 to 1,500rpm. Before we had the Autoprop the average with the motor running was 2,700 rpm. Now we make the same speed through the water with the motor running at 2,000 rpm. We are enjoying less noise and reduced fuel consumption.”

Folding prop for a Leopard 46

Karl Marits swapped his fixed propellers for Varifold folding props on his Leopard 46 Powercat.

“I had sailed the Caribbean, and crossed the Atlantic back to the Mediterranean, along the northern coast as far as Turkey before heading west to Tunisia and Malta. I was not happy with our fixed propellers. The performance was fine, but because we did a lot of multi-day passages, and it wasn’t recommended to lock our propellers, the spinning of the props underway was maddening.

“Since installing the Varifold props in Spring 2023, we sailed from Malta to England, approximately 2,500 miles, and noticed no significant changes in performance under power, other than having to apply a bit more power to get the props to bite in reverse. However, after a few dockings, it became second nature once again. I have full confidence in their performance in tight manoeuvres around a marina.

“There is no question that our performance under sail has improved and saved us hours on shorter passages and will save us days on our longer ones, but for me, the sound of silence is golden when under sail, especially on night passages.”

Folding prop for an Oyster 66

Martin Dent replaced his feathering boat propeller with a folding one on his Oyster 66.

“The old prop served the yacht well for six years and 90,000 miles but it was always very noisy in the aft section of the boat and particularly in the owner’s cabin. Since we were about to embark on a six-month stint living aboard and crossing the Pacific I thought I’d treat myself to a few good nights’ sleep.

“The difference [with the Varifold] was amazing. The reduction in noise is massive. Not only is it a great deal quieter but we are achieving much improved fuel efficiency when motoring; I’d estimate this at 20%. The supposed negative would be a reduction in responsiveness when engaging reverse and in handling in tight spots.

“This concern turned out to be a non-issue and it was just a matter of getting used to it. The other big advantage is in removal and installation which is a great deal more straightforward.”

Boat propeller dimensions

The shape of your boat’s hull imposes space limitations on the diameter of the propeller, but you can compensate for this by adjusting the pitch, that is, the angle at which the blades are set.

If you follow this angle through one revolution and measure the distance between start and finish, this is the pitch measurement.

The minimum clearance between the propeller and the hull should be 10% or greater than the propeller’s overall diameter.

All propellers can be described by diameter x pitch. Hence, a propeller with an 18in diameter and 12in pitch is an 18 x 12.

With outdrives, PBO’s Tony Davies points out that adjusting the pitch of the prop is often the only option, as the maximum diameter propeller is usually already installed.

Increasing the pitch will decrease the revs and vice versa.

If the engine revs seem OK with the current prop, but you want better acceleration, it’s often possible to reduce the diameter and increase the pitch by roughly the same amount to give added acceleration without losing the top-end speed.

You can find a boat propeller calculator at vicprop.com.

Boat propeller types: pros and cons

Folding

Pros: Low drag (esp. 2-blade); Good ahead; Simple; Low maintenance
Cons: Average astern; Poor emergency stop; Lifespan 5-6 seasons; Worn out folders can lose blade
Brands: Varifold, Flexofold, Volvo Penta, Slipstream

Feathering

Pros: Low drag; Adjustable pitch; Good stopping and reverse; Long lasting
Cons: Flat blades less efficient; Can be noisy; Prop walk; High cost; Cavitation
Brands: Max Prop, Variprop, Featherstream, Kiwiprop, Autostream

Autoprops

Pros: Automatic pitch; Good motoring & motorsailing; Fuel saving; Less prop walk; Good stopping and reverse; Long lasting; Low maintenance
Cons: High cost; Needs more rpm initially
Brands: Autoprop

Fixed

Pros: Best under motor; Fuel saving; Less prop walk; Good stopping and reverse; Long lasting; Low maintenance
Cons: Increased drag under sail
Brands: Solé, Vetus

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