All About Propellers

[rickshaw]

The 'Turbinia', the world's first turbine powered ship, originally had multiple turn screws arranged in triplicate along it's three shafts!

Even when developed they still kept the three props in line ahead on each of the three shafts. They certainly wanted to absorb the power available.

However, they didn't understand about cavitation and that is the major difference between aircraft and marine propellers.  The faster you turn the aircraft propeller the only thing you have to worry about is tip speed.  Even moderately fast turning marine propellers have to worry about cavitation.

[kitnut617]

However, they didn't understand about cavitation and that is the major difference between aircraft and marine propellers.  The faster you turn the aircraft propeller the only thing you have to worry about is tip speed.  Even moderately fast turning marine propellers have to worry about cavitation.

Hydro-electric plants have the same problem.  I was working on a project quite a few years ago where the engineers were puzzled over the indentations in the turbine blades.  At first they thought that too much garbage was going into the feeder pipe (called penstock) which was orientated vertically down and straight onto the turbine blades and they came up with all sorts of weird trash racks to correct the problem (which I had to build) but to no avail.  In the end they discovered that the culprit was nothing more than 'air-bubbles' in the water and the solution was to shorten the feeder pipe by as much as half.

[rickshaw]

AIUI, cavitation is a funny condition that can occur at moderate speeds on poorly shaped prop blades in water.  I've seen pictures and films taken of cavitation experiments which the RN carried out in the 1950-60s and its quite remarkable to watch in slow motion how quickly the bubbles form.   Lots and lots of work been done since then on trying to fix the problem so that underwater noise is reduced.   CNC milling has allowed a lot of very fine shaping in propeller design.  The Japanese got in big trouble about 20 years ago for selling large scale CNC machines to the then Soviets who immediately used it to improve their submarine propellers.  Caused a bit of a scandal at the time at the height of the Cold War.   Washington spanked them quite badly over it IIRC.

[PR19_Kit]

Cavitation's not just an issue in water either, any fluid that has a vapour phase at attainable temperatures can develop cavitation.

It's quite common in the hydraulics world, where poorly designed pumps and valves can be totally destoyed by the fluid cavitating in very short order. As rickshaw says, it takes almost no time at all for the bubbles to appear under some conditions. We managed to destroy a big 7 piston radial pump in less than 5 mins once while testing some 'new and improved' inlet guide ports.

Needless to say they did not get approved!