Which Level Of Prop Reconditioning Is Right For Your Boat?
By Capt. Harald Dett
The old adage that prop shops perform a lost “black art” goes back a long way. It refers to the quality of craftsmanship that a “propman” puts into his work and the tricks he has learned over the years to perfect his trade. While the tricks may still be “black art”, there is a lot of modern technology involved as well. “propman” has evolved into “propeller technician”.
Today’s propeller technicians are highly trained and require computer knowledge. The work is still done manually and the mechanical skills required are very much like a modern day blacksmith. Visualize a man heating a damaged propeller with a torch and then striking the red hot propeller with an assortment of hammers against a variety of anvils in order to repair it to its original condition. Additionally, hydraulic presses and rams may be used. Some props come in ripped and torn and require heli-arc welding. Balancing is another labor-intensive effort requiring repeated removal of material from a heavy blade and then re-measuring until it is in proper balance. Finally, to complete the job and make the surface smooth, a variety of grinders and polishers are used.
A modern shop can combine their experience and craftsmanship with a computerized propeller scanning system such as the MRI and a dynamic balancing machine to ensure the highest levels of vibration-free performance.
After a propeller is sandblasted to remove sea growth, it can be scanned with an MRI to determine the pitch, rake, track, spacing, geometry, and camber. Armed with this information, the prop technician knows exactly where adjustments need to be made on the propeller in order to bring it into a specified tolerance. After the first adjustments are made, the prop is re-measured, marked again for further adjustments, and the process is repeated until the target tolerance is met.
Most shops with computerized measuring equipment offer 3 tolerance levels of reconditioning; Standard or Class II, Class I, and Class S. Standard is described as medium accuracy, Class I as High Accuracy, and Class S as Very High Accuracy. Remember, time is money, and to bring a propeller to higher accuracy levels takes time. The primary benefits of bringing your propeller to a higher tolerance are increased performance and reduced vibration. Aside from a more pleasant, smooth ride, when you reduce vibration you will reduce wear and tear on your running gear as well as the whole vessel. Increasing performance means going faster and burning less fuel. This is especially beneficial for those of you who travel extensively.
A Standard recondition is adequate for most slow moving vessels like a trawler or the “Sea King”; a 75’ head boat I used to captain down in Marathon. Most Sport fishing yachts choose Class I because it assures no vibration and provides excellent performance. The highest tolerance is Class S which is ordered by the most demanding owners and captains for the ultimate in performance, smoothness and fuel economy. Most custom wheels today like Federal CX series and ZF Faster arrive Class S from the factory.
Now comes balancing; Static or Dynamic? What’s the difference? To statically balance a propeller, an arbor is placed through the hub of the prop and then rested on a stand with bearings on both sides so the propeller can rotate freely. The heaviest blade will always fall to the bottom. Material is then removed from the back of this blade until the prop will remain still, or static, no matter where it is turned on the stand. It is now said to be in static balance. For many applications, this will be sufficient. For medium to fast boats and especially sportfishing yachts, you should take it to the next and highest level, and dynamically balance the wheel. Now the prop is placed on a machine that will rotate it at high RPMs and a computer will measure exactly where the imbalance is at speed. After the technician removes material the same way as he did before, the propeller will now be dynamically balanced and vibration free.
I like the way ITB uses surveys and polls to address the issues you want covered and to elicit your expert opinions on our profession. We can cover a variety of topics such as progressive, constant, and regressive pitch propellers, cavitation burns, electrolysis damage, variable pitch distribution, propeller terminology, or any topic you like. Send Dale an email at email@example.com and let him know. You can email me at Prop Tec in Jupiter Harald@PropTec.us
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