Counter Rotating Props !

[Pibber]

:eek: I fully understand the principal behind counter rotating props, at least I think I do. What I want to know is, is there a differrence in the direction that they turn for basicforward thrust? If I was looking at the stern should the props be turning in to the center, up and out at the top or in to the center, down and out at the bottom ??? I can't see a big difference but I bet there really is one! :confused:

[CaptLee]

Pibber Over the years have tried both ways and this is my take on it. Center moving down tends to dig a hole and the boat squats more. Center up tends to hold the stern higher out of the water. Military tends toward center down, tug boats tend towards center up. Freighters tend to be all over the place on direction. But center down seems to raise the bow for ease of turning on a long skinny boat. Hope this helps a bit, I have been known to reverse the directions after a testing session for a better look in operation. It is a bit of a pain in the posterior but that is the nature of modelling.

[towboatjoe]

In my world of river towboats, I've noticed open wheels have the clockwise rotation on the starboard shaft and with wheels in kort nozzles the clockwise rotation is on the port shaft. The reason they have the kort wheels turning this way is to draw more water from the bottom of the hull to avoid cavitating when going hard down pushing a tow.

I think almost all blue water vessels have the clockwise rotation on the starboard shaft.

The reason for counter rotating wheels is to enable backing in a straight line and no steering adjustment from the wheel thrust pulling the hull to one side. With towboats you can leave the steering rudders straight, go forward on one engine and back on the other. By turning the backing (flanking) rudders the direction you want to go, you can walk a tow sideways into a fleet.

[mxz70017]

I run my Donzi as well as my DeepVee.As Capt.Lee explained center down.The one thing I have heard.The most Important thing to take Into mind when running counter rotateing props.Is the motors!!!Ok make sure you have zero timed motor's.What this does Is to make sure both motors.Are running at the same rpms.If you have a motor that the timing's not the same.One will run at a different speed when that motors In reverse.. So zero timed motor's run the same rpms.No matter If they are In forward or reverse.

[rye 332]

hi
captlee is right ,i build a 5' pt boat and try both ways prop turning out and down had speed but no turning props turning in and down bow comes up turns on a dime ,one thing i learn from Umi_Ryuzuki is bigger rudders make a big differents , rye

[nick_75au]

We run top swinging outboard on all our full sized boats, offshore open props, It helps with manoeuvring as the paddle wheel effect works with you when trying to dock. As for providing basic forward thrust top swinging inboard has a slight efficiency advantage, I would say that's why the kort boats run that way, there's no paddle wheel effect from a kort nozzle to help (or hinder) manoeuvring. Pretty much the same as Towboatjoe has said.
Nick

[Ghost 2501]

i tried running both directions on Nord Icelandia, though have settled to inward turning props to try and push the outside edge of the boat upwards during turns

[Jay Hargest]

nick 75au has it summed up prety well. But note that a right hand prop in the US is a left hand prop in Europe.

So forget left and right hand. The top of the prop swings outboard going forward for the greatest manoeuverability at low speeds. Docking of a vessel is acomplished by the use of the gears without touching the wheel.

[Pibber]

WOW !!! Thanks to everyone for the great info. I will certainly put it to good use and even try chnging it up on my 26 footer and see which I like better. Truth be told, I don't even know which drirection they swing in.

[Ghost 2501]

Pibber, if looking at the props on my model is anything to go by, the curved face of the propeller is the leading edge, so when going forward, the flat side of the blade will be trailing through the water

[ScottOram]

Try this document from the Coast Guard. Its answers a lot about how the directions of props affects navigation...