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Props rotation seen from stern?

Old 07-21-2008, 12:25 PM
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rva1945
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Default Props rotation seen from stern?

As seen from stern, how (which direction) do twin counter-rotating props rotate, no kort nozzles?

A or B (see image)?
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Old 07-21-2008, 12:42 PM
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Dan S
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Default RE: Props rotation seen from stern?

B,

Dan.
Old 07-21-2008, 02:55 PM
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rye
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Default RE: Props rotation seen from stern?

why b, i have my boat turning (a) and i think it runs great ,arent they suppot to put the bottom of the hull down to get the bow out of the water? rye
Old 07-21-2008, 04:41 PM
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Dan S
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Default RE: Props rotation seen from stern?

if I would have answered "A", someone else would have said why A???

one thing that would help is to know what boat are we looking at?

if you are using the props for steering go with "B", if its a twin with a rudder system go with "A".

Having counter rotating props eliminates prop walk, prop/strut angle will bring your bow up or down.

Dan.
Old 07-21-2008, 05:32 PM
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rva1945
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Default RE: Props rotation seen from stern?

I'm using 3 bladed, 35mm left and right handed props from Robbe, on a 22in long lifeboat. I guess the boat steers bad in reverse if I install them the "A" way.

Old 07-21-2008, 06:00 PM
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Justaddwata
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Default RE: Props rotation seen from stern?

Both A and B

I asked several of the large offshore race teams why they ran one direction against the other. Each said it was found to run best with their particular boat.
Old 07-21-2008, 06:25 PM
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Default RE: Props rotation seen from stern?

What we seem to see most on scale ships, is that "B" is most common on large ships.
And "A" is often used on large push boats on the rivers.

However, the image below comes from an old Chris Craft owners manual. [&:]
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Old 07-21-2008, 06:51 PM
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Default RE: Props rotation seen from stern?

There was a thread about this maybe Umi can find it.
Old 07-21-2008, 07:27 PM
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rye
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Default RE: Props rotation seen from stern?

here the thread http://www.rcuniverse.com/forum/m_7125577/tm.htm my 5' pt boat runs in (a ) but i am going to try (b) to see if there is a diffenets thanks rye here a youth tube of the pt boat running in a click on quilty veiw for better picture http://www.youtube.com/watch?v=gynvW...&feature=email
Old 07-27-2008, 05:13 PM
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rye
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Default RE: Props rotation seen from stern?

hi umi
ok i had them running in a and try b this weekend and yes i got more speed and bow came up out of the water more but i lost a lot of rudder ie little turning and if i turn the rudders all the way boat just about stop in the water or side stepped but at slow speed it turns but not as good as (a) do you think i need bigger rudders ,i notes that the rudders were kind of up to hide about 3/4 of it was in front of the props or the props were lower then the rudder and 1/2 shorter from the back of the boat ,this is a 5' build pt boat and i us the dummas blades for the dummas pt boat at 40 " thansk rye
Old 07-27-2008, 07:43 PM
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Default RE: Props rotation seen from stern?

Really it should be what you prefer.
If "A" works better use A, if "B" accomplishes what you want use B.

When I was taught to build model boats, I was told to over size the rudders by a fourth, or
enlarge the surface area by 125%. We used to draw the rudders on to graph paper and
count the squares, then add an appropriate number of squares to get the rudders 25% larger.

If you are up to building some new rudders, then by all means give it a try.
But don't throw out the old one,.. You might want to switch back.

How many rudders does the PT boat use?
Are they behind the propellers, or between them. That might make a difference.
More than 35 degrees of rudder usually acts like a speed brake.
Old 07-27-2008, 09:01 PM
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rye
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Default RE: Props rotation seen from stern?

hi
i am going to try that and the rudders are right in line with the props and there are two of them thanks for your help rye ps ill let you know next sunday for the boat is out at are summer place and we dont get there until friday
Old 07-31-2008, 03:19 PM
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Default RE: Props rotation seen from stern?

from what i can see, if you have one rudder down the center of the boat, with A you would be pushing water down where the rudder is and thus reducing effectiveness. With B, you push water up and through more of the rudder. Now regarding speed, with A you are following the current created by the hull while with B, the props have a more difficult because of the direction the hull is pushing the water. Anybody please correct me if I am wrong.
Old 08-01-2008, 08:44 PM
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Default RE: Props rotation seen from stern?

differences in counter-rotation in fullsize hulls has a lot to do with how 'wet' the hull is designed to run. using rotation where the right hand prop turns counter-clockwise usually gives stern-lift and is supposed to be more stable in turns. rotating the right prop clockwise gives bow lift and helps in higher speeds, but is more prone to blowing out in turns. cat hulls usually benefit from the props turning against each other, and vee hulls away. of course, different deadrise vees and weight distribution are also factors.
Old 08-03-2008, 07:31 PM
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rye
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Default RE: Props rotation seen from stern?

hi
ok big weekend, i put on bigger rudders and got the boat to turn a lot better thats with the right turning clockwise and left turning counter clockwise then i try it the other way and i see more speed the frist way and higher bow on the second way ,i also check on land to see how many amps each motors where pulling with a amp meter that gose around the power wire and got 00.2 one one and 00.5 on the other do you think one motor is having trouble they are http://206.206.85.209/xtdoc/Details....w&store=mhzusa dose grauper make motors that will match or are better then these had trouble with these from day one even with water coolers they get hot and stall then at the end of the day on sunday i hit a log or something under the water and bent a rod and prop i was thing of going from a 1.75 3 blade prop to a 2" 3 blade 2 pitch to get better running thanks rye
Old 08-03-2008, 07:35 PM
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rye
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Default RE: Props rotation seen from stern?

fogot the pictures rye
Old 08-03-2008, 07:39 PM
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rye
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Default RE: Props rotation seen from stern?

and again sorry
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Old 08-04-2008, 11:40 PM
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rye
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Default RE: Props rotation seen from stern?

hi
talk to hobby lobby today they said on motor is drawing to much not the right kind anyway they are for raceing and said to use 2 graupner 700 bb 12 volt and i think i am going with the 2" three blade props 2 pitch even that it will raise the amps ill let you know when i get them and put them in ,for now i am going to put the 2" props with the new rods this weekend so i can still run we see what happens with the new props going from 1.75 to 2"
Old 08-11-2008, 10:21 AM
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rye
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Default RE: Props rotation seen from stern?

hi
ok i found the trouble of the motor stalling it would only happen as the boat just came of of the turn and then coming at me so the antenna was faceing back, the trouble is that the esc were to close to the radio rec so i sheild the wires from all the power wires and made a pop can sheild for the radio and bingo it works and then i put the more powerful motor to run in rever an now they are close in speed so i got it right? no i nick a water cooler hose and got water in the boat not bad clean up but after all this time building and ajusting for almost a year i had to laugh a small rubber hose got me in the end so i think this boat like my attention almost like a small child lol next week replacing all the hoses about 12 ' in all, let see what she throw at me next weekend ,i name her {christine} and as for the 700 graupner 9.6 i am going to use then in my large ww-11 lcm tie toghter into one esc and a 12 volt gelcell thanks rye

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