Should twin screws rotate in opposite directions such that the water goes up in the middle or down in the middle?

They should turn inward. If you looking the back they should go clockwise on the left hand side and counterclock wise on thr right. So the answer to your question is down the middle.

lifted off r/c group

fig 1 for backing
fig 2 is the set up you want pushing forward

ok ,i am confused which is the best fig1 or fig 2 my boat is heavy 5 ' long two 12 volt gelcells which would give it the most bang thanks rye

sorry i didnt see the hole bottom but whos right big ford or deathwish

Fig 1 you are looking for. Most Large ships do Fig 1. I belive Fig 2 is for speed boat's which makes them corner better.

Actually Most large ships run with figure 2. We also seem to be finding that many river tugs will run with figure 1.

Here are Titanic's propellers...
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http://www.encyclopedia-titanica.org...volutions.html

Humm i guess i was wrong hehe. But then again that's way the Titanic sunk. LOL

Also noted in the other forum... US and European prop makers are opposite in their LH/RH definitions!
Of course, makes no difference if you're buying a pair...

hi
i guess it all depends on who you ask last year i ran a left and right three blade on my 5'pt boat left conter and right clockwise it ran good not as fast as i would of like it but playing around with battery power i blew the esc now i work on it this winter with two esc and have the props turning in on each other i will find out which way is better in the spring i also send a e-mail to abc props where i got them from and ask him if he e-mails me back i let you guys know thanks rye

It really does not make a difference. IF IT DID. Everyone would be turning the same way.

The wisdom that was passed to me was that turning one way was better for steering, turning the other was better for performance. Cant say tat I've really noted a difference either way. If modeling a real ship, copy that.
The Titanic, like most liners, was intended for running fast and straight - the rudder was more of a trim tab. Serious steering would have been done with differential engine speeds, and changing the speed of a huge great reciprocating engine going flat out was not an instant operation. Even today, 50-odd thousand tons doing 20Kt takes a lot of persuading to go in a new direction.

A lot of fast boats built from the 30s onward UK , had both props turning the same way couple of pictures of ww2 boats running gear.

Peter

Both turning the same way is just plain CHEAP ! There is NO way a twin screw boat can dock in a tide & wind equally well, IF AT ALL, with both screws in the same direction.


CHEAP & DANGEROUS.

Rich

Thats what happens during times of war, and high production counts.
It's a historical precedent. Probably still happens to this day.

Profit talks.

The American bussiness motto.

I think you may find it made them go faster, they where still using this methoud up to the 60s, have you looked at what was used on pt boats ?

Peter

eVERY BODY IS RIGHT

1 OF THE 3 COUNTER ROTATES.

So, while this thread is kicking, is the rotation "design specific"? Tugs go this way, ocean liners go that way or is it individual boat specific? I'm working on a Darby with my father and we were wondering about this topic (a-la the rcg thread and the drawing I did). Our original question was specifically about the Darby tug, and the responses were got were all "fig 2". Is there some hydrodynamic reason some go like in fig 1?

Thanks,
Whitt

Tugs are the most thrust concious bunch around. They would want as much thrust right down the center line. Not spraying it around.

Wow, appreciate all of the input.....but still confused.....let's disect this a bit (then you can set me straight). Tugs may want the stern to be sucked down, that would mean thrusting up in the center? Kinda makes sense....the speed guys may want the rotation down in the center to left the stern.....but does that push the bow down too much?

Well it does not make much difference its just the way in fig.1 kicks up the silt on the botom of ports. (that makes it so they have to dredge more often) so large ships use fig.2. you do get better manuvering with fig.1.

Good question. I am just finishing a Shelly Foss, and I assumed figure 1 would be correct, as it would draw the water from outside and push it down in the center away from the hull instead of pushing it up against the hull. Easy enough to change once I run it to see if it changes performance. It would be interesting to see if it makes a difference on a tug vs. a faster hull as well.

OK, Check out these props on a 47' coast guard lifeboard boat in testing

by the way, that is up in the center if i am seeing those props right