Frequently asked questions

The following are questions we are often asked by customers, we hope the answers are of some help to you.

When does a propeller need repair?

Apart from the obvious blade damage or drive hub slippage there are other tell tale signs you should watch for.

  1. Loss of power - poor pullout etc.
  2. Loss of speed
  3. Vibration - not only uncomfortable but can lead to destruction of your shaft bearings and seals.
  4. Poor fuel economy
  5. Slippage ( the propeller feels as if the bush is slipping, like a slipping clutch.)

Other items to check on through prop. exhaust propellers are:-

  1. The condition or even existence of an exhaust diffuser ring.
  2. The condition of the exhaust sealing ring on the front of the prop.

Damage in either of these places can lead to exhaust gases mixing in with the blades and causing ventilation and lack of power.

Competition ski boat propellers i.e. O.J.Johnson, Dymex, Legend, Mastercraft, Federal, etc. are particularly susceptible to vibration after the slightest knock, one you may not even have noticed, Castle marine provide an annual checking service for these props. Which is FREE OF CHARGE between 1st November and 1st March . The only cost is carriage and packing if applicable. (If any repairs are required and your consent is given they are chargeable at our normal rates.)

How is a propeller Measured?

A propeller is measured by its Diameter and its Pitch.

The diameter is the diameter of the circle the propeller subscribes when it is rotating or double the distance from the hub center to the tip of one blade.

The pitch is the distance a propeller would in theory move forward in the water in one revolution if there was no slippage i.e. a 21" pitch propeller would in theory move forward 21" in one revolution. In practice most propellers do slip the amount being dependent on the boat and as a general guide slippage would be between 8% for a race boat with a planing hull to as much as 55% for a full displacement craft. A 10" pitch propeller on a displacement craft with a 50% slip ration would only move the boat forward 5" per revolution of the propeller. The pitch is the second number in the description i.e a 13 3/4 x 21 propeller is 13 3/4 diameter and 21 pitch.

What is D.A.R.?

D.A.R. stands for Disc Area Ratio and is the developed area of all the blades as a percentage of the total area of the circle described by the propeller.

What is cupping?

Cupping is a small curved lip on the trailing edge of the propeller and has the effect of increased pitch it can increase speed and help prevent cavitation. It is normally most effective at speeds in excess of about 35 knots. It can also be carried on through the tip of the blade having the effect of increasing the rake giving better bite and improve ventilation handling.

What is ventilation?

Ventilation is when air from the surface or exhaust gasses are taken into the blades and is often confused with cavitation. This causes loss of thrust, vibration, a reduction in speed and an increase in engine revs. This situation is encountered when making tight turns or mounting the engine to high on the transom. In special racing set-ups supper ventilating propellers are used, these propellers are designed to handle the situation and often take advantage of it. Chopper and Cleaver style propellers are examples of these. Using a high rake propeller can often help in this situation. The best way to avoid ventilation is to get the propeller further under the surface.

What is cavitation?

This is bubbles of partial vacuum caused by excessive propeller speed or loading. This is often caused by incorrect pitch, damaged blades, or bad water flow to the propeller. The tell tail sign of this is usually pitting or marking along the face of the blade. Cavitating propellers can still give lots of thrust but the small bubbles can implode causing vibration and pitting of the blade surface. A perfectly balanced but incorrectly matched propeller can under these circumstances give the impression of bad balance. The bubbles form and implode erratically causing uneven pressure on the blades and this is what causes the vibration. The cure for this is a correctly matched propeller with as clean a view of the water as possible.

What is the Rake of a propeller?

The rake is the angle at which the blades slope relative to the hub. This can be either backwards or forwards (positive or negative rake). Using a higher rake propeller can often help in a ventilating situation such as on tight turns or rough water operation. Aft rake helps to trim the bow of the boat upwards which often results in less hull surface area on the water and therefore higher speeds. Forward rake propellers help to hold the bow down and is often more suitable for workboat applications.

Which is most important in propeller selection Pitch or Diameter?

The answer to this is both. Under normal circumstances and taking into account hull type, weight and engine horse power there is normally only one ideal propeller for a given rig. Unfortunately boats where the weight changes dramatically in operation such as dive boats or working boats, i.e. lobster fishing, the prop often has to be a compromise between the boat running full and empty. Calculating and assessing this is normally a specialist job.

Can a propeller pitch be changed?

Yes. Normally Up or Down by 1" on stainless and 2" on bronze or aluminium.

Will I get more speed from a stainless propeller?

Probably yes. Stainless steel propellers normally have thinner blades because of the greater strength over aluminium and are therefore more efficient, they can give increased speeds of 2-3 mph. They may also improve fuel economy.

Can high performance competition ski boat propellers be repaired?

Yes. These props. are very sensitive to correct blade configuration and balance. At Castle Marine we have specialist equipment to ensure that they perform as new after repair. We can also repitch these up or down by as much as 2".

Competition ski boat propellers i.e. O.J.Johnson, Dymex, Legend, Mastercraft, Federal, etc. are particularly susceptible to vibration after the slightest knock, one you may not even have noticed, Castle marine provide an annual checking service for these props. Which is FREE OF CHARGE between 1st November and 1st March . The only cost is carriage and packing if applicable. (If any repairs are required they are chargeable at our normal rates.)

Can Duo Props be repaired?

Yes. No problem. We may ask for both props of the set to ensure correct performance after repair. After damaging one prop. it is quite possible that the other prop although looking OK may need resetting. We can also replace the centre hubs in these props. even if they are glue in type.

Can glue in hubs be replaced?

Yes. Glue in hubs are carried in stock. In some instances we can replace the glue in type with an equivalent press in type.

Does polishing a bronze or Stainless prop. make a difference to performance?

Very little on speed but possibly up to 5% saving on fuel economy.

Why is my stainless steel propeller rusting?

This can be caused by polluted water, galvanic corrosion ( can be caused by the boat or the environment it is in), or oxygen depletion (can be caused by wrapping the prop in a plastic bag. The effect is not detrimental to the propellers performance.

Why has my stainless propeller turned white?

This is caused by a high lime content or calcium in the water. Any limescale remover available from most super markets will remove the coating. Occasional polishing with a chrome polish will help prevent this.

What about plastic propellers?

Nothing to date has been developed that has all the qualities of propellers made of metals. A good propeller must be durable, repairable and above all, it must perform well. So far the available plastics fail in these important requirements.

Can one propeller give top speed plus lots of power?

Generally no. To gain high speeds involves sizes or pitch ratios unsuited to load carrying or working conditions. Decide what is most important and select the prop accordingly, or change props with boating conditions.

What is the purpose of the rubber cushion hub in an outboard prop?

The rubber drive hub is NOT intended to prevent blade damage as is sometimes assumed, it does give some protection to the lower unit parts by cushioning the shock of propeller impact. Its prime purpose is to prevent excessive shock to the gear train which is experienced in the process of shifting gears.

The rubber hub in my propeller seems to be slipping. Is this possible?

It is a definite possibility, but it does not occur too frequently. Take a look at the propeller. If the blades are visibly bent or distorted, you are very likely experiencing cavitation-and cavitation is often mistaken for a slipping bushing. Have it checked by Castle Marine . The bushing can be replaced if it needs it or the blades can be restored to proper accuracy to eliminate the cavitation. On propellers with through the hub exhaust check the diffuser ring for damage and the fit of the front end of the hub to the gearbox, a fault in either of these can give the same effect as a slipping hub.

Why do I break so many shear pins without apparently hitting anything?

The engine may be turning too fast in the shifting range. The shear pin hole or slot may be sloppy or oversized. The shear pin may be of improper strength for the amount of power involved, or the cushion hub may be frozen tight.

How can I work out the maximum speed of my sailing boat under power?

There are many factors involved in the speed calculation. Go to our pitch calculation page and download our FREE propeller calculator. The maximum speed of the vessel is included in the calculation report.

What causes "Bogging Out" or "Squatting" in a power boat?

Many things. A slipping or cavitating prop. digs a hole which the stern settles into. Correct prop size will normally cure this. Anything on the hull which restricts the free flow of water can also cause cavitation and subsequently bogging out.

How can I tell if I require a right or left hand propeller?

Stand at the back of the boat. If the props rotates in a clockwise direction in forward gear then you need a right hand prop.

What clearance should there be between the tip of my propeller blade and the hull ?

The clearance should be at least 10% of the diameter of the propeller. i.e. a 20" diam. prop. should have at least 2" clearance from the hull. Any closer and the shock wave from the blade as it passes the hull could cause a drumming noise in the hull and even cut into a fiberglass hull.

What is Electrolysis?

Electolysis is a flow of electricity caused by the difference in potential of two dissimilar metals immersed in an electrolyte (in our case water). It acts like a plating process and one metal is eroded and deposited on the other. To help prevent this sacrificial zinc anodes can be fitted close to the part being protected. These must be connected to the part being protected so as to form a circuit when a test meter is applied, they should also be replaced from time to time as they show deterioration.

Why is my bronze propeller turning pink?

The pink colouring is caused by galvanic corrosion / electrolisys which causes loss of zinc in the material. The pink colour is copper. The problem with the propeller is not repairable and as it becomes more advanced pieces will start to break of the blades, it should therefore be replaced.

You can minimise the risk and effect of this corrosion by doing the following:-

  1. Check that all your boat electrics are of insulated return and not simply returned through the engine or running gear. This includes items on the engine such as oil pressure senders etc.
  2. Provide and use an isolating switch in the main battery circuit.
  3. Fittings such as sockets, bilge detection devices etc. must all be protected from water ingress.
  4. Avoid overloading any one cable by looping several items back to it. This causes overheating and insulation breakdown.
  5. Make sure cables are fixed correctly to avoid sagging and possible insulation fracture.


That's the first part of the problem. Dissimilar metals in salt water create their own current so you must fit a zinc anode close to the propeller which is connected internally through the running gear to the propeller shaft in such a way that you can get a circuit between the anode and the prop. The corrosion will then take place on the anode rather than the propeller. The anode should be changed annually.

All the above is the best you can do without going to the expense of electronic cathodic protection which is another story.

On a sailboat under sail should a prop be locked or unlocked?

There is less drag when a prop is rotating.

What Causes Prop Walk (Crawl)?

Prop walk or crawl is produced by all propellers to some extent and is the effect the propeller has on your steering. It is exacerbated by too little tip clearance and a high downward angle on the propeller shaft. From the propeller point of view it can be made more evident by having too high a pitch or diameter on the propeller. There is normally no prop walk or very little effect if the propeller shaft is is parallel to the water surface. For a more information download this report courtesy of The inland Waterways Cruising School.

Can you compensate for lack of diameter by increasing pitch?

You can to a point. The rule of thumb would be that "One inch on diameter would absorb the torque of 2-3 inches of pitch" or vice versa. This is only a quide and is no more than that. You could not use this rule for proper propeller selection.

How can I measure the taper in my propeller?

Do the following:-

  1. Measure the diameter of the small end of the taper
  2. Measure the Large end of the taper
  3. Measure the length of the taper
  4. Subtract the small bore diameter from the large bore diameter
  5. Divide the hub length by the result in (4)

Here is a diagram