Props rotation seen from stern?  
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Props rotation seen from stern? - 7/21/2008 6:25:43 PM   
rva1945



Posts: 144
Joined: 11/8/2005
From: Buenos Aires, ARGENTINA
Status: offline 		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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RE: Props rotation seen from stern? - 7/21/2008 6:42:24 PM   
Dan S


 

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Joined: 8/26/2006
From: Oromocto, NB, CANADA
Status: offline 		B,

Dan.

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RE: Props rotation seen from stern? - 7/21/2008 8:55:21 PM   
rye



Posts: 484
Joined: 1/1/2003
From: chicago, IL, USA
Status: offline 		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

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RE: Props rotation seen from stern? - 7/21/2008 10:41:15 PM   
Dan S


 

Posts: 1458
Joined: 8/26/2006
From: Oromocto, NB, CANADA
Status: offline 		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.

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RE: Props rotation seen from stern? - 7/21/2008 11:32:12 PM   
rva1945



Posts: 144
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From: Buenos Aires, ARGENTINA
Status: offline 		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.


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RE: Props rotation seen from stern? - 7/22/2008 12:00:24 AM   
Justaddwata



Posts: 3479
Joined: 2/24/2005
From: RI-USA, RI, USA
Status: offline 		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.

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I like long walks, especially when they are taken by people who annoy me .....

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RE: Props rotation seen from stern? - 7/22/2008 12:25:36 AM   
Umi_Ryuzuki



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Joined: 12/20/2003
From: PDX, OR, USA
Status: offline 		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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< Message edited by Umi_Ryuzuki -- 7/22/2008 12:26:50 AM >

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RE: Props rotation seen from stern? - 7/22/2008 12:51:37 AM   
Deathwish



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Joined: 7/31/2005
From: St. Louis, MO, USA
Status: offline 		There was a thread about this maybe Umi can find it.

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RE: Props rotation seen from stern? - 7/22/2008 1:27:29 AM   
rye



Posts: 484
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From: chicago, IL, USA
Status: offline 		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=gynvWODhxEM&feature=email

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RE: Props rotation seen from stern? - 7/27/2008 11:13:40 PM   
rye



Posts: 484
Joined: 1/1/2003
From: chicago, IL, USA
Status: offline 		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

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RE: Props rotation seen from stern? - 7/28/2008 1:43:05 AM   
Umi_Ryuzuki



Posts: 1344
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From: PDX, OR, USA
Status: offline 		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.

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RE: Props rotation seen from stern? - 7/28/2008 3:01:31 AM   
rye



Posts: 484
Joined: 1/1/2003
From: chicago, IL, USA
Status: offline 		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

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RE: Props rotation seen from stern? - 7/31/2008 9:19:05 PM   
VFR_RC


 

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From: Little Rock, AR, USA
Status: offline 		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.

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RE: Props rotation seen from stern? - 8/2/2008 2:44:40 AM   
thegoodson


 

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From: harrison township, MI, USA
Status: offline 		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.

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RE: Props rotation seen from stern? - 8/4/2008 1:31:40 AM   
rye



Posts: 484
Joined: 1/1/2003
From: chicago, IL, USA
Status: offline 		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.aspx?ProductCode=m-2139/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 ge