Feathering propellers and folding propellers

Darglow Ltd

Based on the south coast of England, we've been specialising in feathering propellers and folding propellers for the sailboat market for over 30 years.

We are the sole U.K.agents for the Italian Max-prop and have been since the propeller was first patented in 1976. In addition, we are agents for Flex-O-Fold propellers.

We have also been manufacturing and selling our own aperture feathering propeller for over 15 years. We have a large stock level and can usually supply within a few days.

Darglow is a relatively small, family-run business and we pride ourselves on giving each customer the individual attention that they need for their specific installation.

Thirty years of experience, a large database of previous installations and the most up-to-date propeller sizing software combine to guarantee optimum results whatever the installation.

Max-Prop feathering propellers

The Max-Prop is the original feathering propeller and is now widely recognised as the industry standard. Its legendary reputation for performance and reliability have ensured that it is by a very wide margin, the market leader.

For shaft or sail drive transmission, Max-Prop feathering propellers require no operating system and fit directly to the existing shaft. The appropriate forward or reverse drive is automatically achieved when the engine is run ahead or astern. When the yacht is sailing the Max-Prop automatically feathers to give minimum drag. The shaft remains stationary and there is no need for a shaft brake.

Flex-O-Fold Propellers

Originally, folding propellers were designed to minimise drag under sail, usually at the expense of powering performance. For many racers, this compromise was acceptable.

Flex-O-Fold, on the other hand, took another approach, by designing blades that were efficient under power, and incorporating those somewhat larger blades into a folding configuration. The result is a no-compromise propeller that delivers powering performance comparable to fixed bade props. At the same time, the drag is among the lowest. Darglow, who manufactured their own folding propellers in the 1970’s and 80’s, is now pleased to be offering this marked improvement in propeller design.

Featherstream propeller

Many traditional style yachts have apertures between the keel and the rudder. These restricted spaces often make it impossible to fit a folding or trailing propeller, or even a Maxprop.

DOES THIS APPLY TO YOU? DO YOU STILL WANT:

Maximum sailing speed through almost eliminating propeller drag?

Maximum motoring performance with 3 efficient blades?

Astounding reverse thrust with less "prop walk" due to equal thrust in ahead and astern?

THEN THE Featherstream propeller IS A SOLUTION

Simple Installation

Can be used with most rope-cutters

Simple annual maintenance with no need to dismantle the propeller.

Fixed Propellers

Darglow can supply a full range of 2 and 3 bladed fixed propellers.

We stock most sizes from 12" to 24" in diameter for both shaft and saildrive transmissions. Most sizes can be supplied within 1 week of ordering. Larger propellers can be made specifically to order.

PROPELLERS ON SAILING YACHTS

Ever since inboard auxiliary engines were first fitted into sailing yachts, it has been

appreciated that, although the benefits of having an engine are considerable, the sailing

performance is adversely affected by propeller drag. On early heavily built yachts with

relatively low hull speeds this was of no consequence. Also the power available was low and

hence propeller sizes were small, so drag was minimal. As hulls developed and became

sleeker and faster under sail, the adverse effect of propeller drag became increasingly

obvious. Also engines improved, giving more power from smaller physical sizes, so more

power has become the norm. This requires larger propellers in order to use the power

efficiently, hence greater drag under sail.

Today, modern yachts have enormous resources spent on them to improve hull, keel, rig and

sails, all in order to increase potential sailing performance. Then the builder sticks a huge

propeller onto an exposed shaft, effectively cancelling the benefit of all that development.

The result is like tying a bucket onto the stern and dragging it along when the yacht is sailing.

Not only does it reduce sailing speed, but also causes turbulence from the propeller resulting

in unpredictable water flow over the rudder surfaces making steering problematic in many

instances.

The drag of a propeller is not easy to quantify but some generalisations are in order such as:

1 The drag effect increases with speed – actually it increases with the square of the

speed, so faster hulls are penalised much more than slower ones.

2 The average sailing speed is reduced by about 15 - 20% by the drag of a 3 bladed

propeller of a size suited to propel a particular hull.

3 The drag of an exposed fixed three bladed propeller can account for as much as 35%

of the total resistance of the hull through the water.

Two bladed propellers with thin narrow blades were the first response. These did help a bit, but they also compromised the motoring performance, particularly in adverse weather conditions. The propellers simply did not have enough surface area to efficiently transfer the power available into the water to create thrust. Also they were pretty hopeless in reverse so manoeuvring became difficult if not impossible.

Many yachts are still like this – it is after all the cheapest possible way to propel the hull and reduce propeller drag!A fixed propeller when working in reverse has the water pressure on the wrong face of the blades. This surface is all the wrong shape and the result is that the propeller only produces about half the thrust in reverse compared to forward mode. Thus it is necessary to spin the propeller faster in order to get it to function in reverse, with consequently greater sideways prop walk.

As long ago as 1920s folding propellers were developed with complicated operating

mechanisms. When these worked, they were effective in eliminating drag, but they still suffered from limited blade area and poor propulsion. Also they were expensive and only produced in small numbers as a result. The first serious improvement came in the 1970’s with the development of reasonably efficient folding propellers with geared blades.

Within limits these now had acceptable performances and are still the most cost effective way to eliminate propeller drag. However they are not a complete answer as there are inbuilt limitations to the concept, the most serious of which is the problem of opening (and staying open) in reverse.

The water pressure on the blades tries to shut the propeller, and the only force available to hold them open is centrifugal force. To make them operate in reverse at all, it is necessary to increase the weight of the blades particularly towards the tips. Some older designs have thick heavy blades, which absorb a significant proportion of the available power simply cutting their way through the water. So even after decades of development, most inexpensive folding propellers are still a bundle of compromises.

Lately, 3 bladed, and even 4 bladed folding propellers have arrived on the market. These improve the efficiency of the folding propeller concept by increasing blade area and allow greater horsepower to be transferred into the water but they still suffer the penalty of heavy blades that are needed to operate in reverse.

Probably the most advanced state of the art folding propeller available is the Danish

Flex-O-Fold. Although this propeller works in the same way as older folding propeller designs, it offers thrust comparable to fixed propellers in forward and powerful thrust in reverse. This is due to its greatly improved blade design having wider blade area, more efficient shape and optimal weight distribution. Many boat owners want to sail faster than the boat next to them, but don’t want to have to give up powering performance in forward or reverse. The Flex-O-Fold offers a solution.

Another innovative propeller, unique in its own right, is the “self pitching” propeller. It offers blades of a peculiar shape, each independently hinged to a hub with ball -bearings. These are designed to adopt the optimum blade angle for all hull and shaft speeds, though the resulting benefits are not easily defined.

The blades are also heavy as weight is a critical part of the self pitching concept, with the same effect on efficiency as that of most folding propellers. Under sail, the blades are free to adopt a trailing position in the water, thus reducing drag although some part of the blade is necessarily exposed to the flow due to the influence of gravity. At higher hull speeds this can cause the propeller (and shaft) to rotate which results in the blades flying out from the trailed position, partly exposing the propeller to effectively drag once more. The propeller is very sensitive to balance and can experience vibration problems when marine growth forms on some areas of the blades.

Back in the 70s self feathering Maxprop propellers were developed, and patented, initially 2 bladed, only for the racing yachts, which thereby gained a small rating advantage. When a 3 bladed version was produced, the world of yachting finally had an answer to the problem of propeller drag, without compromise. At last a yacht owner could have the best of both worlds.

Minimum drag under sail with each blade lined up for – and – aft offering virtually the same miniscule resistance as a folding propeller, whilst still giving the full sized blade area and diameter required to allow the engine power to be fully utilised. Many yacht builders fit undersized, over-pitched fixed propellers in an attempt to slightly reduce drag. The Maxprop having virtually no drag, can be of the larger, correct size for the engine. Motoring performance can be expected to therefore exceed that of a fixed 3 bladed propeller in these circumstances.

A fixed propeller is efficient in forward, with the concave blade shape giving best drive and the aerofoil section of the blade helping by adding hydrodynamic “lift” in the forward direction.

In reverse this all falls to bits! The “lift” is still in the forward direction thus working against the screwing effect of the propeller, which is now trying to work with the load on the back (convex) surface of the blade- in itself a very ineffective shape. Hence fixed propellers are ineffective providing only about 50% thrust in reverse. This all adds up to problems of stopping and manoeuvring in reverse with excess engine revs, paddle-wheeling of propellers and general mayhem in crowded marinas. A Maxprop propeller overcomes this disadvantage as it remains equally efficient in both directions.

By eliminating propeller drag, a yacht’s sailing potential is greatly enhanced. In a good blow, there is enough power in the rig to overcome the drag and achieve the potential hull speed.

In calmer conditions, where most of us prefer to enjoy our sailing, the effect is more

pronounced. With a fixed 3 blade propeller the yacht may be reduced to 3 knots or so,

whereas without the propeller it may still be doing 4 or 5 knots. Then, if you need to be at your destination quickly, you can still switch on your powerful diesel auxiliary and punch your way back to the marina against wind and tide using your efficient full sized Maxprop feathering or Flex-O-Fold folding propeller to effectively deliver that power into the water.