Phones
07 570 3339
0274 919 460
Address
61 Freyberg Street
Tauranga, 3110
NEW ZEALAND
e-mail
dunnrite@actrix.co.nz


Here are some quick answers to common propeller questions

Q: Can my present prop be modified, or do I have to purchase a new prop?

Q: What can I do to increase the pulling power on my boat?

Q: What is the advantage to a cupped prop?

Q: What are the pros and cons to stainless props versus the aluminium version?

Q: Why is my stainless steel propeller rusting?

Q: The splined centre inside my propeller is slipping inside the rubber bush, can this be repaired?

Q: My boat will no longer get up on plane when I apply full throttle. I can idle the motor in gear and the boat goes okay but when I accelerate, the boat actually slows and the motor just over revs. Could this be a propeller problem?

Q: I think my boat uses a lot more fuel than it should. The motor runs sweet so could the propeller be having any affect on fuel consumption?

Q: What information do I need to supply to get a Bronze Propeller Calculation?


Q: Can my present prop be modified, or do I have to purchase a new prop?
A: Most aluminium or stainless steel props can be changed in pitch up to one inch of pitch from the original size, either up or down. Bronze propellers may be changed up to two inches of pitch from the original size. Cupping can be added or taken away. What might help will depend on what a person is trying to gain from their boat.


Q: What can I do to increase the pulling power on my boat?
A: You can always gain pulling power by going down in pitch. Most of the time a drop of 2 inches is common. This will help quite a bit on the low end, and will usually increase top RPM by about 400. If the engine is already running close to red line, you might have to back off the throttle with a light load to keep the RPM's below the red line.


Q: What is the advantage to a cupped prop?
A: A cupped prop can be run higher to the surface, as it holds the water better and slips less than an un-cupped version. A cupped prop is going to run less RPM when wide open as it is pushing more water. It is like running a higher pitched prop. If a boat is running a 21 inch un-cupped prop, a 19 inch cupped prop will usually turn about the same RPM, but will have better power out of the hole, and might even run a little faster as the engine or drive can probably be trimmed up higher.


Q: What are the pros and cons to stainless props versus the aluminium version?
A: The stainless prop, if it is exactly the same dimensions as the aluminium, will usually perform a little better because they can be a little thinner and are more efficient as they don't flex as much under load. A lot of the stainless props are designed for better top end than the aluminium props by having a higher rake angle and better cupping. Of course, the cost is higher, but the durability is a lot better. A concern with a lot of boaters is whether or not a stainless prop will cause drive damage if a strike occurs. If the prop hits a large rock or immovable object, then drive damage might occur. Usually the blades will bend or the hub will slip before this happens, but once in a while a shaft gets bent or a gear tooth does break even with aluminium props. Most boat insurance will cover any of this damage. Remember stainless propellers can be rebuilt to there original strength whereas aluminium propellers cannot.


Q: Why is my stainless steel propeller rusting?
A: Stainless Steel is just as its name implies -- it "stains" less than normal carbon steel. Ironically high quality stainless steel propellers are more prone to this than some low quality stainless steel props. Rust can occur under numerous circumstances, including polluted water and galvanic corrosion. This can be caused by the boat, environmental galvanic activity, (i.e. a marina's electrical system), and oxygen depletion. Since this is a post purchase cosmetic defect and has no bearing on propeller performance, there is no warranty for rust. Polished stainless propellers rust and discolour the least. Dunnrite can remove the rust by polishing the propeller.


Q: The splined centre inside my propeller is slipping inside the rubber bush, can this be repaired?
A: Yes it can. The part you have referred to is called a drive hub and replacement hubs are available for most outboards rated over 8hp. Specialised tools are required to replace conventional drive hubs that have vulcanised rubber. Send your faulty propeller to Dunnrite and we will supply and fit a new drive hub.


Q: My boat will no longer get up on plane when I apply full throttle. I can idle the motor in gear and the boat goes okay but when I accelerate, the boat actually slows and the motor just over revs. Could this be a propeller problem?
A: Yes it can. If you have an outboard or stern drive propeller with a drive hub or drive sleeve it may have failed. Replacement hubs and drive sleeves are available for most of these types of propellers from Dunnrite. Specialised tools are required to replace conventional drive hubs that have vulcanised rubber. Send your faulty propeller to Dunnrite and we will supply and fit a new drive hub.


Q: I think my boat uses a lot more fuel than it should. The motor runs sweet so could the propeller be having any affect on fuel consumption?
A: Most certainly it can. When you say the motor "runs sweet" is it reaching the engine manufacturer's recommended rev range at wide open throttle with your normal load in the boat? The engine manufacturer will have a recommended RPM range of from 500 to 1000 RPM. The nearer you are to the upper limit at wide open throttle the nearer you are to having the correct size propeller and the more fuel efficient your set up will be, regardless of whether you drive slow or fast. A propeller with a bent blade will also affect fuel consumption even though the damage may not noticeably cause any vibration.


Q: What information do I need to supply to get a Bronze Propeller Calculation?
A: Before starting a prop calculation for a Bronze Propeller, Dunnrite requires the following info:

  • What Hull Type - Planing Mono Hull, Displacement Mono Hull or Sail/Fishing Boat with info of deadwood or keel.
    If the hull is a Catamaran, detailed information on tunnel dimensions will be required.
  • Waterline Length of the Hull.
  • Laden Boat Weight.
  • Number of Engines, engine brand and model including the manufacturers rated horsepower with engine RPM.
  • Gearbox Ratio and direction of propshaft rotation when going ahead.
  • Your expected Boat Speed.
  • Propeller Rotation - RH or LH.
  • Maximum Propeller Diameter that can be swung in the propeller space. The General Rule is you need to allow for a Tip Clearance to Hull of 15% of the Propeller's Diameter.
  • Propeller Shaft Diameter with information on the Taper and Key Size for the propeller.
    There will be a surcharge for anything other than a Standard SAE Taper.
 
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