How much clearance to leave between hull and the propeller blade tip?
The question of available space and tip clearance is a bit complex, and requires some additional information, and ultimately a judgment call.
Minimal tip clearance has little effect on performance of a propeller.
The real reason for tip clearance is to reduce noise and vibration created when the vortices coming off the blade tip reach the hull.
Due to the shape & efficiency of Flexofold blades, we have lower tip vortices than any other low drag propeller to begin with.
There are 3 factors that come into play in deciding how much tip clearance to leave:
1. What is the hull shape? A flat hull above the prop will create more resonance (like a drum skin), while a V-shape or rounded hull form will reflect more of the tip vortices
2. What is the hull construction? A modern, light laminate will resonate more than an older, heavier laminate. A cored hull may increase or decrease resonance, depending on the core material.
3. How tolerant are the boat owners of noise and vibration under power? If you’re used to a relatively vibration prone 1 or 2 cylinder diesel, you may not notice the noise created by low tip clearance.
Can I install a Flexofold under water?
Yes, but you have to use locking screws with pre-applied, underwater Loctite (supplied with all Flexofold propellers). The normal Loctite, which can be purchased in a hardware store, will not cure underwater. And, once a screw with the pre-applied Loctite has been used, it will not work a second time underwater. New screws, with the pre-applied underwater Loctite, are available from Flexofold.
What about prop walk?
Prop walk, which is produced by all propellers, is exacerbated by extreme down shaft angles and small tip clearance. Many people don’t realize there is prop walk in both forward and reverse. However, in forward the prop is pushing a moving column of water over the rudder’s surface, and you have steering control right away to counter the walk. In reverse, you can control walk only by getting the boat moving backward, thereby creating water flow over the rudder. The Flexofold’s superior reverse performance should overcome most all prop walk concerns.
Can I change the blades on a Flexofold Propeller?
(ie. by installment of a new engine)
Yes. New blades can be bought separately and easily be mounted on the existing hub (the existing hub must be with shock absorbers – feel free to send us a photo if any doubt.
Can I change the pitch and size of a Flexofold Propeller?
Yes. Changing the pitch of a Flexofold Propeller is as simple as changing the blades. This can be done either in or out of the water. You do not have to change the hub.
Can I use a bronze propeller on a stainless steel shaft?
Yes. Bronze and stainless steel are very close in the galvanic series and will corrode only very slowly in the absence of an anode. With a properly installed anode on the shaft, you should be protected against any slight galvanic corrosion that might exist as well as against stray current.
Where can I buy a Flexofold Propeller?
You can buy a Flexofold direct from our manufacturing plant in Denmark, our local sales office in the USA or through a sailboat manufacture / -dealer.