# Water velocity and propellers

## Main Question or Discussion Point

hi to all!!
Say i have a uniform flow of water of 1m/s.. if i put a propeller with known diameter and blade angle...what would be its RPM???...i want to design a flow meter...pls help me!!..i need to have basic and approximate formulas on the blade parameters and its RPM..thanks!

Regards
dioross

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rcgldr
Homework Helper
Normally blades are rated with a pitch. That is the distance they would travel through jello with each revolution if there was no compression of the jello. The prop blades are normrally twisted so the pitch (advance rate) is about the same along the radius.

If you know the prop pitch, say 15 inches, and it's speed, 1000 rpm, then it advances 15,000 inches or 1250 feet every minute, or 14.2 mph. This is asumming the prop is free wheeling, (no drag or thrust).

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thanks for the reply..it really helped me a lot...do you have some basis or references about the calculations?...does the analysis of the propeller in air same as that when it is in water?...does the density of water affect the rpm of the prop?...

@Gabe21: the angle of prop in ur example is with respect to the plane of rotation?..why is it that it is divided by 90?..

you divide by 90 because u have to get your blade angle into a percent. if your blade angle was 5 degrees you would divide it by 90 and get .0556. this means 5.56% of the water flow speed is put into rotational speed of the propeller. assuming no friction or drag of course. think about it on a x,y grid. if you travel from the origin(90 degrees is vertical and 0 degrees is horizontal) at a 5 degree angle moving at a constant speed, then 5.56% of that speed is put into verticle motion and the other 94.44% is put into horizontal motion. the same concept applies to the propeller angle. however, the closer the propeller angle gets to a 90 degrees the less efficient it becomes due to friction.

and the air calculations would be the same, but i have to say if u r trying to build an airflow meter it would be alot cheaper to just buy one. because air is so much less dense than water making your own meter would be very impractical. the one i use is very accurate and only set me back 120 bucks, and that was 3 years ago. so im sure they are cheaper now.