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05-26-2017, 09:51 AM
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#1
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Guru
City: Italy
Vessel Name: Didi Mau
Vessel Model: Currently looking for next boat
Join Date: Jul 2015
Posts: 1,081
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rudder angles
I noticed yesterday for the first time that my rudders are not exactly parallel. When the rudder connected to the rudder sensor is parallel to the prop shaft, the other rudder is out by a few degrees. I would have thought both would be parallel, but this is my first two engine-boat.
A friend told me that if both are parallel, I will get vibration from the rudder, because of disturbed water going by the rudders. Does not seem intuitive to me.
Thanks in advance.
Gordon
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05-26-2017, 10:14 AM
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#2
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Guru
City: Pender Harbour, BC
Vessel Name: Gwaii Haanas
Vessel Model: Custom Aluminum 52
Join Date: Sep 2013
Posts: 3,791
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Rudders are frequently toed in a bit to compensate for torque.
I find it interesting that if you look carefully at a twin engine prop airplane, the vertical tail is mounted with the vertical tail offset, it's not oriented "square." Same principle.
__________________
Don't believe everything that you think.
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05-26-2017, 10:18 AM
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#3
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Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,775
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Quote:
Originally Posted by Xsbank
Rudders are frequently toed in a bit to compensate for torque.
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And to put a little water pressure on them to avoid the prop wash vibration.
David
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05-26-2017, 10:20 AM
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#4
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Xsbank,
That would not be best. In a way.
But in reality it will come to pass that one rudder will compensate for the propwalk (not torque) and together they will keep the boat going straight.
Interesting part pf this is that the single screw vessel will not require nearly as much rudder deflection to compensate for the propwash .. the rudder being so much larger. A detail that would make the single more efficient. But there are other factors. Fly stuff of course.
__________________
Eric
North Western Washington State USA
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05-26-2017, 11:07 AM
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#5
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Moderator Emeritus
City: Au Gres, MI
Vessel Name: Black Dog
Vessel Model: Formula 41PC
Join Date: Jul 2015
Posts: 21,131
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If they are truly parallel you may get some flutter in the rudders. With some toe in, there is some pressure to hold the rudders steady.
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05-26-2017, 02:57 PM
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#6
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Comodave,
Why dosn't my single screw ever flutter?
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Eric
North Western Washington State USA
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05-26-2017, 03:19 PM
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#7
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TF Site Team
City: Jacksonville
Join Date: Nov 2008
Posts: 11,670
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Quote:
Originally Posted by Nomad Willy
Comodave,
Why dosn't my single screw ever flutter?
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Maybe the alignment of your rudder or your bearings are extremely tight? I have no idea. I get a flutter occasionally at certain RPMs/sea conditions but its so infrequent I don't worry about it. I have to be by the rudder post to hear/feel it.
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05-26-2017, 05:01 PM
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#8
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Larry,
Rudder not that tight at the shoe.
With my small prop (18") and small power (40hp) the turbulence is probably low. Also the rudder is large so LE to TE distance probably drops some turbulence too.
__________________
Eric
North Western Washington State USA
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05-26-2017, 05:51 PM
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#9
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Moderator Emeritus
City: Au Gres, MI
Vessel Name: Black Dog
Vessel Model: Formula 41PC
Join Date: Jul 2015
Posts: 21,131
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I am not sure why a single doesn't flutter, maybe less connections and no tie rod?
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05-26-2017, 06:20 PM
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#10
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Guru
City: Ft Pierce
Vessel Name: Sold
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Posts: 28,119
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My single flutters under certain conditions...maybe a worn bearing in the keel shoe, but cant duplicate by hand every hauout.
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05-26-2017, 07:09 PM
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#11
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.
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Kar-KEEN-ez Koot
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05-26-2017, 07:36 PM
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#12
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Guru
City: Pender Harbour, BC
Vessel Name: Gwaii Haanas
Vessel Model: Custom Aluminum 52
Join Date: Sep 2013
Posts: 3,791
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If your rudder flutters, it's more likely to be the linkage. Mine started vibrating and I needed a new gudgeon pin. The new Wagner pins have grease nipples.
I bet dollars to donuts the rudders on a twin are no more likely to flutter unless something is worn. It makes no sense.
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Don't believe everything that you think.
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05-26-2017, 07:45 PM
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#13
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Guru
City: Gulf Shores, Ala.
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Quote:
Originally Posted by markpierce
No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.
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Please educate me on that port rudder/ prop walk. Is that not a Rt. hand screw? Would that not create a prop walk in the same direction rather than an opposed direction ?
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05-26-2017, 07:58 PM
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#14
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Guru
City: St Augustine,Fl
Join Date: Sep 2009
Posts: 3,789
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That looks like a left hand wheel to me. Counter clockwise rotation, will back to starboard.
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05-26-2017, 08:11 PM
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#15
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.
__________________
Kar-KEEN-ez Koot
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05-26-2017, 09:28 PM
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#16
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Same w my boat Mark.
Left hand wheel, back to stbd and pull to stud going fwd.
LarryM just the mass of my big rudder probably keeps it from fluttering.
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Eric
North Western Washington State USA
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05-26-2017, 09:39 PM
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#17
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Guru
City: Boston Area
Join Date: Feb 2017
Posts: 2,605
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could the difference be that single screw rudders are often attached top and bottom, while twin screw rudders are often only attached at the top ?
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05-26-2017, 09:40 PM
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#18
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Guru
City: St Augustine,Fl
Join Date: Sep 2009
Posts: 3,789
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Quote:
Originally Posted by markpierce
Correct, it backs to starboard and turns to starboard going forward under power without a three-degree port rudder.
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Eric, maybe no play in the rudder tube, or tight bushing in the drag link would be why you have no play or "chatter".
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05-26-2017, 10:42 PM
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#19
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
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Well it dosn't chatter and I'm not upset about it.
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Eric
North Western Washington State USA
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05-26-2017, 11:59 PM
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#20
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Dauntless Award
City: Wrangell, Alaska
Vessel Name: Dauntless
Vessel Model: Kadey Krogen 42 - 148
Join Date: Jan 2013
Posts: 2,820
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Quote:
Originally Posted by markpierce
No flutter here. A three-degree port rudder, however, is needed to counteract prop walk while underway.
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Me too, but it's 3 degrees to stbd
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