Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: boatmadman on March 08, 2008, 11:54:59 am
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Now, we all know that cavitation on a prop is undesirable, but, does anyone know of a way to figure out at what revs a scale prop is likely to start cavitation?
Ian
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Unfortunately there are so many variables that have an effect on cavitation that the only sure way to determine when it will happen is experimentation.
People tend to think with models boats that cavitation is the point at which the propeller "breaks away" and looses it's grip on the water but in actual fact cavitation can occur at any time and can cause erosion of your propeller. Cavitation is actually the creation of gas bubbles on the low pressure side of the propeller blade, which can then impinge on the blade surface and slowly erode it away. Exactly the same effect can be seen in centrifugal pumps where cavitation can definately erode the impeller away.
Although the root cause is the undesirable low pressure areas when these actually occur can be determined by the propeller configuration, as in pitch, diameter, blade profile etc., as well as the speed it is turning at, the draught it is at, the hull form, the distance the propeller is from the hull etc..etc..
For model boats cavitation manifests itself when it appears to "stall" as the prop is spinning so fast and creating so much of a low pressure area that the following blade is also passing through the same low pressure area and is therefore not capable of creating lift. Your options when this happens are obviously slow down the prop or fit a prop with a smaller pitch or possibly a smaller diameter.
Unfortunately experimentation is the only way forward to ensure you get the best performance and one of the issues I discuss regarding real ships quite regularly is the fact that it doesn't matter how much power you can generate if you can't get it into the water it's no use to you!! Exactly the same applies to model boats.
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Cavitation at scale-models depends on the combination of Motor/prop. As we don`t need to have "racing-drifters" a slow-turning motor should be first choice. We had some tries in my old club, by measuring the pull on standard props parallel to the Rpm of the motor. And all the Standadr-props you can buy in model-shops will give up between 5700 and 6800 rpm. The pull becomes lower, the higher the rotation was. So it was the obvious sign that the cavitation starts to effect the pull and therefor effectivity.
The props itself in their effectivity depend on the shape and the angle of the blade.
To find out what matches best to your model, just have a look (if available) on the prop-shape of the origin. This will give you a clue about the style of the prop on your model. The rest is now up to the motor.
Hope that was out of any help.
Jörg
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I have found that cavitation can also occur when a model is sitting in the water, and FULL neck is given at a standing start. sometimes its better to feed in the power, winding it up to full power, rather than simply dumping the power.
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Cavitaion or Aeration??