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Common Questions:

The following list is a compilation of the common questions we are asked when talking with prospective customers. Most of this information is based on our own experience, but some is based on feedback from customers or owners of other brands. It is offered in good faith, but we are unable to guarantee the accuracy. If you cannot find the answer to your question below, please do not hesitate to ask us via our contact page.

Please select the category you are interested in:
General Questions
Feathering Propellers
Folding Propellers
Fixed Propellers
Design considerations for feathering propellers


General Questions Back to Top
 
  1. Should I antifoul the propeller and if so what with?
  2. Can I fit my propeller underwater?
  3. Why do SEAHAWK use stainless steel and not bronze like other propeller manufacturers?
  4. Why don't other feathering propeller manufacturers use stainless steel?

1) Should I antifoul the propeller and if so what with?

Unfortunately modern environmentally friendly antifoul paints won't last long on any propeller. If the boat is moored in an area where heavy fouling occurs we suggest mechanically abrading the surface of the blades and applying a suitable primer, followed by several coats of a good quality hard antifoul paint. When applying the paint do not allow a heavy uneven build up of paint to occur at the leading and trailing edges of the propeller blades. Try to keep the total paint thickness reasonably uniform over the blade surface. A heavy build up of paint on the trailing on one side of a blade can cause an uneven water flow over the blade and the blade may not feather true fore and aft which could cause the propeller to rotate under sail if the gearbox was left in neutral. We are constantly monitoring developments in antifoul products and if any significant advances in overcoming the problem of fouling develop we will post them on our website.
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2) Can I fit my propeller underwater?

We strongly recomend against it, particularly with a new instal as it is much more difficult to catch errors in taper etc.

It is less likely to be a potential problem with a reinstal of exisiting propeller for service reasons.

If it must be done we recomend a visit to an engineering supply store and purchasing the new semi solid stick type Loctite to use in place of the original Loctite 262 and/or 242 that is used as standard with Slipstream & Autostream Propellers. This semi solid Loctite will stay in place on the nuts and screws while installing.

Loctite 262 = Loctite 268 Product # 37685
Loctite 242 = Loctite 248 Product # 37615
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3) Why do SEAHAWK use stainless steel and not bronze like other propeller manufacturers?

SEAHAWK have been manufacturing propellers since 1976 and began using stainless steel in 1998. Whilst bronze is an excellent propeller material, it is difficult to cast the accurate and complex shapes, required for a product like a feathering propeller, this results in almost 100% machining of all surfaces. Stainless steel can be cast more accurately and in complex shapes using a casting method known as lost wax or investment casting. This allows greater scope for innovative design: also machining costs are reduced, as only very accurate surfaces need to be machined. The outside surface of the body and the propeller blades are only polished. In most installations the propeller shaft is also stainless steel and being a common material to the propeller there is less likelihood of electrolysis occurring.
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4) Why don't other feathering propeller manufacturers use stainless steel?

The tooling cost to set up for investment casting is much more expensive than for casting in bronze. Other manufacturers have a large range, which would be prohibitively expensive to tool up for in stainless steel. We only make one hub size for 15" up to 20" diameter, so we have a low tooling cost per propeller. And we have now reached economies of scale that offsets the high tooling cost. We also like to think we are more innovative, after all, would you buy a bronze shackle or a bronze propeller shaft in preference to stainless steel?
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Feathering Propellers Back to Top
 
  1. I have a fixed blade propeller, what is the advantage of fitting an Autostream self-feathering propeller?
  2. What is the range of pitch adjustment for the Autostream Propellers?
  3. Why is the SLIPSTREAM made from stainless steel and not bronze like other propellers?
  4. Is fouling a problem, will barnacles and other marine growth stop the propeller from opening and closing correctly?
  5. I have a folding propeller, what is the advantage of fitting an Autostream self-feathering propeller?
  6. I have a Max Propeller, what is the advantage of fitting an Autostream self-feathering propeller?
  7. I have an Autoprop, what is the advantage of fitting an Autostream self-feathering propeller?
  8. Will it fit my existing propeller shaft?
  9. I have a hydraulic gearbox; do I need a shaft lock?
  10. Can I run a shaft alternator?
  11. There is a zinc anode on the Autostream propeller: do I still need a zinc anode on the propeller shaft?
  12. Won't barnacles and other marine growth stop the propeller from feathering correctly?
  13. I'm going on an extended cruise, what spare parts should I take, and should I take a spare propeller?
  14. My boat is a fast Multihull; will the propeller stay feathered at high speed?
  15. The propeller looks complex with bearings and seals, am I going to be constantly replacing these parts?
  16. How fast do I have to be sailing for the propeller to feather?
  17. I have a 'Saildrive' leg, does the Autostream saildrive model have a cushion hub?
  18. Can I use an AUTOSTREAM self-feathering propeller with an electric drive such as the that made by Solomon Technologies?

1) I have a fixed blade propeller, what is the advantage of fitting an Autostream self-feathering propeller?

Compared to a conventional fixed 3 blade propeller the Autostream will have around a knot less drag under sail, similar performance under power and improved reversing with "prop walk" virtually eliminated in most cases. The turbulence created by a fixed blade propeller, being dragged through the water, can affect the laminar flow over the rudder blade resulting in vague feel at the tiller. Depending on the proximity of the propeller to the rudder this can be quite dramatic. Fitting an Autostream propeller will allow the sailboat to comfortably sail a course closer to the wind. Similarly compared to a fixed 2-blade propeller the drag would be around 0.5 of a knot less under sail.
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2) What is the range of pitch adjustment for the Autostream Propellers?

A standard Autostream prop will have an adjustment range of between 3” - 21" in Forward and 1” – 14” in reverse.
By using alternate pitch screws, or changing the main gear orientation, pitches outside of this range can be achieved for high torque / low speed applications (such as electric or steam drives)
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3) Why is the SLIPSTREAM made from stainless steel and not bronze like other propellers?

Whilst the correct grade of bronze is an excellent propeller material, it is difficult to cast the accurate and complex shapes, required for a product like a folding propeller, this results in almost 100% machining of all surfaces. Stainless steel can be cast more accurately and in complex shapes using a casting method known as ‘lost wax’ or ‘investment casting’. This allows greater scope for innovative design, while reducing machining costs, as only very accurate surfaces need to be machined. The outside surface of the body and the propeller blades only require polishing, while the gears are used as cast, with only the taper, pivot pins and locking screws requiring machining.

It is worth noting that in most installations, the propeller shaft or saildrive leg internal components are also stainless steel. Being a common material to the propeller there is less likelihood of electrolysis occurring. This is a key point in aluminium boats.

As far as life expectancy of your propeller goes, we have found in our repair experience, that folding bronze props often suffer from electrolysis. This usually renders them unrepairable, as it is not possible to weld them. Stainless steel is much more resistant to electrolysis and we have found the few that have suffered from minor problems, can usually be welded back up.
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4) Is fouling a problem, will barnacles and other marine growth stop the propeller from opening and closing correctly?

No, the centrifugal force of the blades being swung open and the leverage the blades have against the gears, means even the worst marine growth is going to be crushed. There is one technically possible occasion where growth may interfere with operation, although in our experience we have never seen it. If the propeller is left in the open position for an extended period of time, there could be a large amount of hard barnacle growth on the gears, this may prevent the prop from closing properly for a period of time. Once the propeller has gone from forward to reverse a few times then it is unlikely there would be any growth remaining. Should you be in a situation where you feel this is possible, simply close the propeller up and place a thin rubber band over the tips of the blades to keep them closed. Next time you start using the propeller the rubber band will break and you can be sure of correct operation of your propeller.
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5) I have a folding propeller, what is the advantage of fitting an Autostream self-feathering propeller?

The drag would be slightly more than a two blade folding and about the same as a three blade- folding propeller. Stopping and reversing power would be significantly improved. Ahead performance would be similar and in most cases improved due to the ability of the Autostream Propellers pitch to be easily adjusted to tune the propeller to suit the engine and boat. The life of an Autostream Propeller will be at least three times greater than a folding propeller due to the moving parts being permanently lubricated and protected from the elements. The fact that the Autostream is made of stainless steel, the same material as most propeller shafts, plus a significant zinc anode right on the propeller reduces the possibility of damage due to electrolysis, the main cause of propeller failure, especially in folding propellers.
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6) I have a Max Propeller, what is the advantage of fitting an Autostream self-feathering propeller?

Ahead performance and drag under sail would be similar. Reverse performance in most cases would be better with the Autostream due to the ability to adjust the reverse pitch to a fine setting totally independent to the ahead pitch. The ahead and astern pitch is simply adjusted with two adjustment screws set into the side of the propeller body, the pitch can be adjusted in the water, there is no dismantling required. Greasing via a permanently fitted grease (zerk) fitting. The Autostream propeller has replaceable bearings and "O" ring seals to keep the grease inside the propeller, both greatly improve the life of the propeller.
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7) I have an Autoprop, what is the advantage of fitting an Autostream self-feathering propeller?

The drag under sail would be less with the Autostream, also more responsive stopping and reversing at lower engine RPM results in more confident manoeuvrability when docking. Turbulence off the propeller under sail is significantly less with the Autostream, providing a cleaner flow of water to the rudder blade, resulting in a more sensitive feel at the tiller, critical when trying to sail a course close to the wind.
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8) Will it fit my existing propeller shaft?

Autostream propellers are taper bored and keyed the same as conventional propellers and are stocked in sizes 1"(25mm) to 1.5"(38mm) diameter in 16:1 SAE and 10:1 metric tapers. A standard SAE taper shaft has a plain section on the end where the cotter pin passes through. The special nut we supply with the Autostream propeller will not accept more than 1 1/8"(29mm) of thread, which means the plain section, may have to be cut off to fit the nut. We supply a bottle of loctite compound and a locking pin to secure the nut. The hub can be custom bored to suit almost any shaft up to 1.5"(38mm) diameter.
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9) I have a hydraulic gearbox; do I need a shaft lock?

No. When sailing, for the propeller to feather from the ahead driving position a slight resistance to the propeller shaft turning is required for a few seconds. This is achieved with both hydraulic and mechanical gearboxes by stopping the engine whilst in forward gear. In some cases, such as gearboxes with cone clutches, it may be necessary to engage reverse gear for a few seconds after the engine has stopped. This will stop the propeller shaft from turning and the propeller will feather. The propeller will not come out of feather until it is driven by the engine. Once feathered the gear lever can be placed in the most convenient position, forward, neutral or reverse.
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10) Can I run a shaft alternator?

Yes. If reverse gear is engaged with the engine running then shifted to neutral, the propeller will not feather and the propeller shaft will rotate being driven by the propeller. To feather the propeller from this reverse configuration, drive the boat forward under engine power at a speed greater than the speed being sailed and follow the procedure as outlined in the answer to Question 8.
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11) There is a zinc anode on the Autostream propeller: do I still need a zinc anode on the propeller shaft?

Yes. The propeller has moving parts and non-conductive bearings, which result in a poor electrical connection of the various parts of the propeller to existing shaft anodes so we put an anode on the propeller to protect the propeller body and blades. The shaft anode is required to protect the propeller shaft and the propeller hub. Most problems associated with electrolysis in under water gear occur due to faulty electrical equipment on or around your boat.
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12) Won't barnacles and other marine growth stop the propeller from feathering correctly?

No. The seals on the blades and the main gear hub keep the grease inside the propeller body maintaining an unsuitable environment for marine growth. When the engine is put in gear the shaft torque rotates the blades on their axis with substantial force and any marine growth in the way is instantly wiped off by the edge of the blade.
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13) I'm going on an extended cruise, what spare parts should I take, and should I take a spare propeller?

Autostream propellers are extremely robust and are only likely to get damaged by grounding or hitting a large heavy object. Anything that is likely to damage the propeller would also most likely damage the shaft and strut so having just a spare propeller would not get you going again. If a blade should get damaged beyond repair we can supply a replacement off the shelf that will be the exact weight to maintain perfect balance. Just quote the serial number of the propeller and the number stamped on the blade, we have all the details recorded. We supply spare seals, anode and locking wire with each propeller. A seal would only need to be replaced if fishing line damaged it or similar, they are standard "O" rings readily available in most parts of the world. Running the propeller without a seal in place would only result in the gradual loss of grease and thus slightly increase the wear rate over a long period of time. Our suggestion for an extended cruise would be to carry an additional set of seals and lockwire, plus two spare anodes per year.
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14) My boat is a fast Multihull; will the propeller stay feathered at high speed?

Yes. The geometry and uniform accurate cross section of the blades result in stability in the feathered position at high speed. Autostream propellers are fitted to many Multihulls that regularly exceed 25 knots under sail.
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15) The propeller looks complex with bearings and seals, am I going to be constantly replacing these parts?

No. The seals will not need replacing unless damaged by fishing line or similar, and the extra large bearings ensure minimal wear. It is difficult to predict the service life before reconditioning will be required due to the multitude of applications, but based on 20 years experience manufacturing feathering propellers we believe most will get more than 5000 hours before requiring refurbishment, in 7 years we have not sold any parts due to 'wear and tear'; to suit the current range.

When fitting the Autostream propeller, it does not need to be fully disassembled like other brands. The two main sections of the propeller are seperated and the drive gear is fitted to the shaft. Then the blade assemly is refitted and the job is done.
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16) How fast do I have to be sailing for the propeller to feather?

The faster the boat is sailing the quicker it will feather. The surface area of the blades result in considerable force to feather the propeller. Even at very slow speeds, say 2 knots the propeller will feather in a few hundred yards or metres.
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17) I have a 'Saildrive' leg, does the Autostream saildrive model have a cushion hub?

Yes. The Autostream saildrive model has a unique 'fail-safe' cushion hub, even if all of the rubber were to fail in the hub, full drive would still be available at the propeller. We also use our own unique nut and locking screw to positively secure your propeller.
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18) Can I use an AUTOSTREAM self-feathering propeller with an electric drive such as the that made by Solomon Technologies?

Yes, an AUTOSTREAM propeller can be used as normal with an electric drive system. There is the added advantage that it can also be used to provide battery charging if the drive system is capable of regeneration, that is, it can operate as a generator as well. Simply engage reverse then release the power, the propeller will not feather from reverse so will provide drive for power generation. Once the batteries are fully charged, simply drive forward briefly and feather the propeller as normal.
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Folding Propellers Back to Top
 
  1. Will a folding propeller reverse?
  2. Will a folding propeller vibrate more than a fixed propeller?
  3. Will a folding propeller power my boat as well as a 2-bl fixed propeller?
  4. Will a folding propeller fit onto my existing propeller shaft?
  5. Should I antifoul a folding propeller and if so what with?
  6. I’m going on an extended cruise, what spare parts should I take, and should I take a spare propeller?
  7. I have a Saildrive leg; does the SLIPSTREAM saildrive model have a cushion hub?
  8. How fast do I have to be sailing for the propeller to fold back?
  9. Must I secure the propeller shaft in a particular position for it to fold back fully at low speed?
  10. Don't folding propellers make a big clunk when they open?

1) Will a folding propeller reverse?

Yes. Folding props will reverse, within the realms of physics. The blades rely on the centrifugal force of the spinning propeller to open. However, as the blades try to thrust the boat back, in effect trying to close the blades back up, they counteract this centrifugal force to a degree. In operation, the blades find a balance point, where the centrifugal forces balance out with the thrust of the blades. The reverse performance varies from good on higher revving propellers, with low pitch, to poor for slower revving, high pitch propellers. This is explained in more detail in the “Useful Information” “Folding Propellers” section of this website. Reverse is not as effective as a fixed, or feathering propeller, but it is usually adequate. Usually more engine speed is required for the same thrust, as a fixed prop and substantially more than a feathering prop. Our unique blade geometry is designed to maximise the reverse performance and ensure the blades always open, even while still moving ahead.
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2) Will a folding propeller vibrate more than a fixed propeller?

No, a well-designed and balanced folding propeller will be as smooth as a well-designed and balanced fixed propeller.
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3) Will a folding propeller power my boat as well as a 2-bl fixed propeller?

Yes, in fact a folding propeller usually performs better than a typical fixed ‘sailing’ propeller, as it will have a larger blade area than the ‘slimmed-down’ fixed ‘sailing’ propeller blades.
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4) Will a folding propeller fit onto my existing propeller shaft?

Yes, SLIPSTREAM propellers are taper bored and keyed the same as conventional propellers and are stocked in sizes 1”(25mm) to 1.5”(38mm) diameter in 16:1 SAE and 10:1 metric tapers. A standard SAE taper shaft has a plain section on the end where the cotter pin passes through. The special nut we supply with the SLIPSTREAM propeller will not accept more than 1 1/8”(29mm) of thread, which means the plain section may have to be shortened to fit within the nut. We supply Loctite locking compound and locking screws to secure the nut. The hub can be custom bored to suit almost any shaft up to 1.5”(38mm) diameter.
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5) Should I antifoul a folding propeller and if so what with?

Unfortunately modern environmentally friendly antifoul paints wont last long on any propeller. If the boat is moored in an area where heavy fouling occurs we suggest mechanically abrading the surface of the blades and applying a suitable primer, followed by several coats of a good quality antifoul paint. When applying the paint do not allow a heavy uneven build up of paint to occur on the propeller blades. Try to keep the total paint thickness reasonably uniform over the blade surface. Do not apply anti-foul to the gears, as this will hinder the operation of your propeller. We are constantly monitoring developments in antifoul products, and if any significant advances in overcoming the problem of fouling develop we will post them on our website. We have been hearing nothing but good news about the new generation of silicon based anti-foul paints such as 'Pellerclean' and Prop-Speed 780. These silicon based anti-fouls are usually applied over an epoxy primer and remain soft to the touch. It seems, the idea is that nothing can get a grip in this anti-foul and simply falls off when the prop is spun up.
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6) I’m going on an extended cruise, what spare parts should I take, and should I take a spare propeller?

There is a user service kit available for the SLIPSTREAM propeller. This kit has all the service parts that you will require once your propeller has had considerable use and needs refurbishing. There are two occasions where a spare propeller may be useful…

A – If the propeller or a part of it falls off – SLIPSTREAM propellers have been designed to stay securely assembled and on the shaft until the owner decides it is time to disassemble, AS LONG AS THE PROPELLER IS FITTED CORRECTLY. Follow the instructions carefully using all locking screws and correct loctite grades, as per the instructions and you should never lose a part, or the entire prop from the shaft. The only foreseeable exception to this is if the propeller is used extensively in a badly worn state.

B – If the propeller is damaged by a foreign object or collision – The SLIPSTEREAM propeller is one of the strongest propellers on the market, any force strong enough to damage the propeller will probably also result in additional damage to the strut &/or shaft. Use your own judgement when deciding what spares you may need to carry.
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7) I have a Saildrive leg; does the SLIPSTREAM saildrive model have a cushion hub?

Yes. The SLIPSTREAM saildrive models have a unique ‘fail safe’ cushion hub, plus our own unique nut and locking screw to positively secure your propeller. These designs have been proven over the years on the Autostream range of propellers.
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8) How fast do I have to be sailing for the propeller to fold back?

Not much, just a knot or two.
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9) Must I secure the propeller shaft in a particular position for it to fold back fully at low speed?

With the geared blade design, it does not matter which position the propeller is in, it will fold back as soon as you start to sail.
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10) Don't folding propellers make a big clunk when they open?

Folding propellers often have a noticeable ‘clunk’ when closed and then driven forward, as the blades swing open and hit the stops. We try to minimise this ‘clunk’ by fitting urethane ‘bump stops’ to cushion the blades as they hit the stops.
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Fixed Propellers Back to Top
 
  1. I have a fixed blade propeller, what is the advantage of fitting SLIPSTREAM folding propeller?

1) I have a fixed blade propeller, what is the advantage of fitting SLIPSTREAM folding propeller?

Compared to a conventional fixed 3 blade propeller the SLIPSTREAM 3 blade propeller will have around a knot less drag under sail and ½ to ¾ for the two blade comparison. Usually performance under power is improved over the small blade area ‘sailing’ style fixed blade propeller. The feel of the rudder is also often noticeably improved, as the turbulence created by a fixed blade propeller can affect the laminar flow over the rudder blade, resulting in a vague feeling. Depending on the proximity of the propeller to the rudder this can be quite dramatic. Fitting a SLIPSTREAM propeller will allow the sailboat to comfortably sail a course closer to the wind.
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Design considerations for feathering propellers Back to Top
 
  1. BASIC OPERATION
  2. Driveline:
  3. Transfer of drive to blades:
  4. Pitch adjustment:
  5. Independence of Pitch Adjustment
  6. Method of Pitch Adjustment:
  7. BLADE DESIGN
  8. Blade drive gear ratio:
  9. Fitment:
  10. Sacrificial Anodes or Zincs:
  11. Servicing:
  12. Saildrive Propellers:

1) BASIC OPERATION

The basic principles behind all of these more common self-feathering propellers are similar…
The propeller shaft is permitted to rotate between stops within the propeller hub, this rotation is transferred via a drive gear to a gear on each propeller blade. If the propeller shaft is driven forward the propeller blades are orientated to present the leading edge to the forward drive position. Conversely when the propeller shaft is driven in reverse, the blades are rotated approximately 180 degrees on their axis. This orientates the leading blade edge in reverse providing higher efficiency than a fixed or folding propeller working backwards.
When no drive is applied via the propeller shaft, and the vessel is under sail, water flow past the propeller blades forces the propeller hub to rotate around the stationary shaft until the blades align with the water flow and cease rotating the hub. The blades stay in this feathered position until engine power is provided to the shaft, forcing the blades to rotate back into pitch and provide drive once more.
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2) Driveline:

Most current self-feathering propellers have a gear connected to the propeller shaft. The gear and shaft are free to rotate within the hub of the propeller, but are limited in the extent of their travel by some form of limit stops. The limit stops are often adjustable in some way to allow adjustment of the pitch. The actual design in this area varies considerably. Primarily in the method employed to transfer the drive torque to the propeller blades once in position and the how the shaft/blade rotation is limited to adjust pitch. The design and materials used for the drive/feathering mechanism also vary considerably.
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3) Transfer of drive to blades:

AUTOSTREAM, amongst others, use the gear mechanism purely to rotate the blades into the correct position. There are limit stops that restrict the propeller shaft movement placed between the shaft and propeller hub. This means that once the propeller shaft and blades have rotated to the correct position and up to the limit stops, all engine torque used to physically drive the propeller is transferred directly from the shaft to the propeller hub and blades, not via the gear mechanism. The AUTOSTREAM propeller uses heavy limit stops that are capable of withstanding full power ahead to full power astern without threat of failure. It is not necessary to pause when changing direction, although along with most manufacturers we do not recommend repetitive hard direction changes, this can increase wear on the propeller mechanism.

There are propellers manufactured with the limit stop mechanism placed on the opposite end of the propeller. The propeller shaft and drive gear drive the propeller blade gears, which in turn drive another gear that is in some way limited in it’s travel, the same end result is achieved but ALL drive torque must pass through the relatively small propeller blade gears. At best this increases the load and wears the gears, at worst there is the potential for a complete lack of drive situation should the gears fail, for example in a hard emergency stop situation.
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4) Pitch adjustment:

By adjusting the limits within the hub that the propeller shaft / gear can turn, it is possible to adjust the pitch of the blades. Again there are considerable differences between manufacturers in this area.
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5) Independence of Pitch Adjustment

Some manufacturers such as AUTOSTREAM allow independent adjusting of both the forward pitch and reverse pitch. The reasoning behind this is, it is often preferable to have less pitch in reverse for better control and less sideways "prop walk" when pulling away from a standstill. Some manufacturers have a single adjustment that increases, or decreases pitch for both forward and reverse simultaneously. This could cause problems if you like a high forward pitch, possibly for motor sailing, but still wish to have a smaller reverse pitch to limit "prop walk" and have the best reversing characteristics.
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6) Method of Pitch Adjustment:

While it is possible to calculate the required diameter and pitch of a propeller to suit a given boat, the engine, transmission, shaft angle, keel design, skin fittings, hull shape, weight and even personal preference, all have a subtle influence that make exact calculations near impossible. Almost always, a benefit is to be gained from making small adjustments in pitch to suit the owner and actual boat the propeller is fitted to.

There are many differences between how manufacturers adjust the pitch on their propellers. Some require the return of the propeller to the factory for adjustment, and cannot be self-adjusted by the owner. This does not allow for economical fine-tuning during sea trials to find the best pitch for the boat in question.
Other manufacturers require disassembly of the propeller, to reposition a gear or other component, this method is often impractical unless the boat is out of the water and does not make it an easy task to fine tune the prop during sea trials.

Several manufacturers offer solutions that are adjustable while the propeller is fitted. AUTOSTREAM use independent forward and reverse adjusters of a good size with large locknuts. These adjusters make it relatively easy to adjust either forward or reverse pitch, while free or scuba diving, using just a wrench and hex key, both of which are supplied with the propeller. There are no lock-screws to drop or small fiddly fittings that are hard to work with, or clog up with marine growth.
The actual techniques used by different manufacturers vary substantially and are worth consideration when appraising a propeller.
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7) BLADE DESIGN

Blade design is a complex area, with many books available on the topic. It is also another area where approaches and claims vary widely between manufacturers.

The one area where most manufacturers of self-feathering propellers seem to agree, is that a flat blade is the way to go. While a flat blade is not the optimum shape for efficiency under power, it does allow the minimum of drag when feathered. There is at least one self-feathering propeller available today that does have a more efficient twisted blade, but the downside is greater drag under sail.

Without delving too deeply into the depths of propeller theory, it is accepted that most of the actual work done by any propeller is achieved by the outer half of the blade. The inner half is generally ineffective and mostly just along for the ride. In the case of fixed blade propellers, extra twist is applied at the root (base) of the blade to increase efficiency. With the flat blades used by most self-feathering propellers, adding extra twist in this area is not desirable, as it would add extra drag, so there is a trade-off for the lower drag.
Another point of consideration is the thickness of the actual blade. It must be remembered that we are not dealing with the near perfect helix shape of a twisted blade on a fixed propeller, that gains a benefit from increased efficiency on the low-pressure side of the blade if a slight wing shape is utilised. In effect this wing shape can help pull the blade along in addition to the push from the high-pressure side of the blade. We have learnt by experience with AUTOSTREAM propellers that by making a flat blade as thin as possible the efficiency improves substantially. We have also proven that there is no benefit to be gained by shaping a flat blade. In fact the thicker and more shaped a flat feathering propeller blade becomes the more it is inclined to twist in the water flow, this starts the propeller turning and then the propeller tries to come out of the feathered position and drive the shaft.

We know of several cases of this happening on high-speed sailboats. In one case the propeller engaged itself in the opposite pitch, this spun the engine over backwards until it sucked water up the exhaust, the ensuing hydraulic action destroyed an engine. Not the sort of thing you want to happen in the middle of a high speed run.

We have run AUTOSTREAM propellers on high-speed sailing catamarans at over 25 knots with no problems. A shaft brake is NOT required with an AUTOSTREAM propeller.

AUTOSTREAM‘s approach to blade design is to minimise the area and thickness at the inefficient root of the blade, while also making the working area of the blade as thin as possible. The use of the very strong stainless steel alloy 2507 allows us to achieve thinner and smaller areas than previously obtainable with any form of bronze material. The thinner the blade the less resistance it creates, regardless if it is being dragged through the water while feathered, or if cutting the water while under power (try cutting a block of butter with a thick knife, versus a thin knife).

Cavitation is possibly the most common source of complaints about noisy operation from a self-feathering propeller. It is a well-known phenomenon that the boiling point of water decreases as pressure drops. Cavitation is where the water on the low-pressure side of a propeller (backside) actually boils due to the extremely low pressures generated in this area. It can occur either at the tip of the blade, when the tip speed becomes high due to larger diameters or higher rpm, or at the root of the propeller due to the inefficiency inherent in this area. The first can be minimised by careful propeller selection and the second by making the inefficient part of the blade as small as possible.

Careful attention to ensuring that you have sufficient clearance between the propeller and the hull will assist with making sure any sounds emitted by any cavitating are minimised. Cavitation on a typical sailboat self-feathering propeller is more a nuisance than anything else, unless it is becomes excessive and then it can create ‘cavitation burn’ and begin to erode the propeller.
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8) Blade drive gear ratio:

While it is easier, due to space limitations within the hub, to manufacture a gear assembly with small gears on the propeller blades and a larger gear on the propeller shaft drive gear, care is required in selecting the correct ratio. As the blade gears are reduced in size, versus the shaft drive gear, the effort required by the propeller shaft to turn the blades is increased and vice versa. If the blade gears are too small in relation to the shaft drive gear, there is the possibility when changing direction from forward to reverse at speed, that insufficient power is transferred to the blades to fully rotate them to the reverse position.
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9) Fitment:

One area where propellers vary considerably is the design of the propeller hub and the process of physically mounting the propeller. By their nature, self-feathering propellers generally need to be at least partially disassembled to fit them to the shaft, some completely.
The hub on the current AUTOSTREAM range is split in such a fashion as to give two main assemblies, one being the blades and nose, the other the hub and drive-gear. Firstly the hub and drive-gear assembly is easily mounted onto the shaft, and then the blade assembly is reattached. This allows the propeller to be fitted without the need for complete disassembly and reassembly.
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10) Sacrificial Anodes or Zincs:

The application of sacrificial zinc anodes varies considerably. SEAHAWK and most other manufacturers have determined that one cannot rely purely on shaft anodes for protection of a self-feathering propeller. This is because there are a number of components involved that although they are in contact with each other, electrical connection is often poor. Addition of an anode or zinc directly on the propeller greatly increases its resistance to electrolysis, this is the biggest killer of propellers on sailboats.

At SEAHAWK we have always manufactured our zinc anodes with a stainless steel core, securing the zinc anode via a bolt through this core. By making them this way the zinc anode is assured of doing its job until the very end.

Using screws to secure a zinc anode directly through the zinc material, particularly if positioned in a thinner part of the body can create problems. What often happens, is the first part of the zinc to be eaten away by electrolysis is the area around the screws. This results in a loss of the ability to retain the zinc anode tightly causing the electrical connection and protection to deteriorate. As the material is eaten away from the screws over the course of time, they can often become loose and some even fall off completely.
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11) Servicing:

There are two aspects of servicing a self-feathering propeller to consider.

1 – Regular maintenance. In the case of AUTOSTREAM propellers the following is recommended to keep them in top condition…
A)Greasing of the propeller via the permanently installed grease nipple/s, one on shaft drives models and two on saildrive models. As we use O-ring seals on the hub and blades grease is retained more effectively and greasing is usually required only once a year. B)Regular inspection of the zinc anode, replacing it if less than 25% remains, cleaning and inspecting for damage and replacing the blade o-ring seals if damaged.
A competent free (snorkel) or scuba diver can do all of the above with the propeller still installed and underwater.

2 – Overhaul. In the case of AUTOSTREAM propellers, as long as the propeller has not suffered any physical damage or electrolysis, the owner, without special tools can complete all overhaul work. All bearings and seals are easily replaceable, even the cushion hub on saildrives is easily removed for replacement.
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12) Saildrive Propellers:

Many manufacturers provide propellers to suit saildrives, although some are simply adapted shaft drive models. All Saildrive manufacturers recommend a cushion drive hub to protect the small gears in the leg from the shock of gear changing or impact with foreign objects. Most will not honour a warranty if a non-cushion hub propeller is used.

SEAHAWK manufacture a propeller specifically for saildrive applications with several unique features.

1 – An owner serviceable cushion hub, although we have yet to replace one in 9 years.

2 – The cushion hub is designed to be fail-safe in such a way that even if ALL the rubber were removed from the cushion, full drive would still be available.

3 - Many people have already learnt the hard way that saildrives are renowned for dropping propellers. There are two common reasons for this. The main one being that the cushion hub allows the hub of the propeller to rotate slightly around the shaft spline, part of this rotational force is often transferred to the propeller nut via the washer, this force slowly but surely works the nut loose. The other problem is the slight differences between manufacturers in the way the splines on the propeller shaft are made. This leads to a situation where most propellers are not a perfect fit on the splines and tend to move slightly on the shaft. This again works on the propeller nut to loosen it. SEAHAWK have modified the female spline in the propeller with a subtle difference that substantially limits the ability of a propeller to move on the shaft splines. We have also developed our own propeller nut/lock-screw arrangement that is steadfast in its ability to stay tight. Due to demand we are now starting to supply this nut to other propeller manufacturers. You can be assured that if you follow our instructions that you will never lose an AUTOSTREAM saildrive propeller to the depths due to your propeller nut coming undone.
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