boat with 4 propellers

Hi,

I would like to know why each of the propellers on a battleship with 4 propellers turns against the other. Clockwise, anticlockwise, Clockwise, anticlockwise.

Is it because of the moment on the ship ! Does it have the same effect on model boats ? or is it negligible?

thanks,
ronald

 

I've tested such boat configuration and the best propulsive coefficients have been obtained with SB screws turning anticlockwise and PB screws turning clockwise...

Only If the model propellers have the same geometrical characteristics of the real.

 

"why" the props counter rotate is becouse props have a wheel effect, they tend to walk away sidewards

 

What is interesting to note is that you reach the best results (propulsive coefficients) if the SB props are turning anticlockwise and the PB clockwise.
If the SB props are turning anticlockwise and PB clockwise the global efficiency il lower...
Perhaps because in the first case the propellers are working in a better homogeneous fluid.
The difference btw. the 2 installations increases with speed.

 

For dual and quad props, the inboard wheels should turn so that the rotary flow is "down" with relation to xz centerline plane (i.e. "inward turning"). This is to prevent increasing the "up" following flow that occurs due to hull shape along the CL that reduces apparent AOA. The outboard props should then turn "outward" in the oppsite direction to prevent loss of apparent AOA due to sympethic rotation in the inflow. See Saunders, Hydrodynamics in Ship Design, v1,sec33.6.

Edit....For three wheels, you are on you own....

 

so does this effect occurs on a model of 1.8 meters ?

thanks
ronald

 

Yes, but it also depends on the props and what you want out of the model. Most non-engineering (i.e. towtank) models use much larger props/shafts than would be to scale and run at a much lower Renyolds Number so getting it exactly right is not really an issue. There is some scale effects on towtank models and this is corrected for for self propulsion tests akter wake surveys are conducted

 

if 3 props... wouldent it decide rotation from whether the center prop was lined up,aft or forward of the outboard drives to avoid the harmonic circulation effect noted above, however there is a exelent example of 3 props in a built fast boat PT boats had 3 props i think given they did 50+ knots id say thats a good place to start.....however im almost tempetd to say all same direction and treat it as 1 prop because of the odd number ....or you want to counter rotate the port shaft to offset torq and maby slightly diffrent props....1, 2 and 4 shafts is a lot easier to plan for and set up though probably

 

Yes, PT boats had 3 props, and yes they all turned in the same direction (an interchangability decision, not a performance one). But they were also very flat aft, with no deadwood, the wheels were all in the same plane, and seperation between the wheels was greater than 1 diameter. They also had a horrible/useful strenwalk problem which on one occasion got 2 men killed and made another president.

Now the wheels on the KMS ADMIRAL HIPPER class are a different story.

 

carbon prop

Hi

There is a New Zeland outfit called Q-SPD they manufacture carbon prop system to the best of my knoweledge

 

When props turn, obviously the blade on the bottom is deeper in the water than the ones on the top, so it has a better bite (actually it has to do with pressure differentials but we aren't hydrodynamicists ) So the stern of the boat moves in the opposite direction the blade is moving. This can be a pain at low speeds, and it can be very useful if you know how to use it.

If you have two props, one turning clockwise and the other counterclockwise, they cancel each other out and the stern doesn't go left or right. Same with 4 props. Anytime you have an odd number of props you have this phenomenon.

 

I heard that there where diferent benefits with the props turning clockwise or anti clockwise on multi prop systems. I was efficensy, but I forgot what the other one was.