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Old 06-21-2012, 05:14 PM   #1
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Rudder Angle Maximum for GB 32

I have a 1974 fiberglass Grand Banks 32. The rudder stops are set at 16 degrees from dead center. Want to improve forward turning efficiency when low speeds manuevering. Can I get better performance to increasing maximum angle stop and if so, to what angle? Does this create over issues, or concerns? I have read that generally a rudder can be deflected 35 degrees before it starts to stall. Does anyone have specific experience this issue with the GB 32 or other full keel semi deplacement trawler which they are willing to share?
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Old 06-21-2012, 06:37 PM   #2
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Several Monk 36 owners have modified their rudders by welding a "fishtail" vertically on the trailing edge usually just a piece of 2" SS angle the piont of the angle welded to the edge of the rudder. They are very happy with the results claim much better handling especially at slow speeds. I have not done it but may in the future.
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Old 06-21-2012, 08:06 PM   #3
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Several Monk 36 owners have modified their rudders by welding a "fishtail" vertically on the trailing edge usually just a piece of 2" SS angle the piont of the angle welded to the edge of the rudder. They are very happy with the results claim much better handling especially at slow speeds. I have not done it but may in the future.
Steve W
The PO of our Ennos Sapphire 32 did exactly that to the rudder, after experiencing poor low speed handling. I can't say what it was like before, but it seems okay now.

He apparently was advised by Bill Garden, who suggested that the angle of the fishtail be about 17 degrees.
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Old 06-21-2012, 08:21 PM   #4
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Conrad do you have any photos of this?
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Old 06-21-2012, 09:24 PM   #5
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Quote:
Originally Posted by Zambezi View Post
I have a 1974 fiberglass Grand Banks 32. The rudder stops are set at 16 degrees from dead center. Want to improve forward turning efficiency when low speeds manuevering. Can I get better performance to increasing maximum angle stop and if so, to what angle? Does this create over issues, or concerns? I have read that generally a rudder can be deflected 35 degrees before it starts to stall. Does anyone have specific experience this issue with the GB 32 or other full keel semi deplacement trawler which they are willing to share?
I suggest you post your question to Bob Lowe on The GB Owners Board on Grand Banks Owner's Resources.
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Old 06-22-2012, 12:44 AM   #6
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The rudder on my 30' Willard is stopped by the slave cylinder at 45 degrees. Works great. Turns really sharp at slow speeds and even has fairly good steerage in neutral gear. The Willard is a bit slower than a GB and has a smaller rudder so suspect 45 degrees would not be as effective as w the Willard but I'd try 35. I'd think 16 degrees would be WAY too little deflection.
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Old 06-22-2012, 12:48 AM   #7
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Conrad do you have any photos of this?
I was hoping nobody would ask, because the only one I have was taken just after the boat had been hauled, and the bottom looks pretty grotty!
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Old 06-22-2012, 01:04 AM   #8
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Conrad, I seen worse.
Thanks I have to research this so more, it sounds interesting.
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Old 06-22-2012, 06:54 AM   #9
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The problem with just arbitrarily changing the stops is not knowing the why behind where they are now.

Displacement boats can have large rudder angles but planing/semi-planing usually have much smaller. Not sure what your GB32 is...is it powered high enough to be semi-planing?

If not...going up to 30-45 isn't out of the question but I would probably stick to the 35 guideline. If you are running 15 knots or higher sometimes....then I would definitely research some more.
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Old 06-22-2012, 06:56 AM   #10
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I was hoping nobody would ask, because the only one I have was taken just after the boat had been hauled, and the bottom looks pretty grotty!
I'm guessing not much...but any "more definite" clues as to how much drag the fishtail adds?
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Old 06-22-2012, 09:15 AM   #11
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The picture posted by Conrad is basically what some Monk owners have done, some have also added a plate about 3" wide welded on the top and bottom of the rudder to guide the prop thrust over the "fishtail".
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Old 06-22-2012, 09:48 AM   #12
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Slower boats have the leading edge toward the prop called the balance it catches the thrust and diverts it to add control at slower speeds and angles. Also they are more of a square blocky looking Rudder, Go Fast boats have less rudder and no balance forward of the rudder shaft, This is for on plane turning with less drag if you had a displacment rudder on a semi planning boat and you turned the wheel it would be an agressive turn and spill your drink after the microwave bounced off the salon windows. Online i learned alot about this just google Rudder balance it explains loads and reasons and cause and effect !
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Old 06-22-2012, 09:54 AM   #13
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Slower boats have the leading edge toward the prop called the balance it catches the thrust and diverts it to add control at slower speeds and angles. Also they are more of a square blocky looking Rudder, Go Fast boats have less rudder and no balance forward of the rudder shaft, This is for on plane turning with less drag if you had a displacment rudder on a semi planning boat and you turned the wheel it would be an agressive turn and spill your drink after the microwave bounced off the salon windows. Online i learned alot about this just google Rudder balance it explains loads and reasons and cause and effect !
the original reason for a balanced rudder game long before hydraulics and power steering...it was all about the ease of holding that tiller.

but I can see where the emphasis is now on putting more rudder in front of the propwash.
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Old 06-22-2012, 11:27 AM   #14
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I'm guessing not much...but any "more definite" clues as to how much drag the fishtail adds?
Excellent question, but unfortunately I don't know, since the mod was done by the PO. When I'm back at the boat next month I'll go back through the log book and see if anything was mentioned. The PO did a lot of tuning - prop etc. - and kept good records, so there may be something there.
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Old 06-22-2012, 12:59 PM   #15
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Increasing deflection increases drag not at all and increasing area increases drag very slightly compared to the wedge. No studies to back that up but I feel quite confident.
I have had an idea for a long time that rudder performance could be greatly increased by putting holes along the leading edge or possibly just behind the rudder shaft. Or both perhaps. The idea being that the hydraulics on the back side would benefit much the same way as slotted leading edges of aircraft wings .... Permits normal functioning at much higher angles of attack. Can't imagine an aircraft w big wedges on their trailing edges. I have a hunch too that many rudders on boats aren't very well thought out or researched.
Personally I would prefer a big rudder w power hydraulic steering but if one went in that direction one would/should consider all aspects of structural integrity before going down to the sea. A couple more inches TE on Willy's rudder w power steering would be GREAT.
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Old 06-22-2012, 01:41 PM   #16
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Increasing deflection increases drag not at all and increasing area increases drag very slightly compared to the wedge. No studies to back that up but I feel quite confident.
I have had an idea for a long time that rudder performance could be greatly increased by putting holes along the leading edge or possibly just behind the rudder shaft. Or both perhaps. The idea being that the hydraulics on the back side would benefit much the same way as slotted leading edges of aircraft wings .... Permits normal functioning at much higher angles of attack. Can't imagine an aircraft w big wedges on their trailing edges. I have a hunch too that many rudders on boats aren't very well thought out or researched.
Personally I would prefer a big rudder w power hydraulic steering but if one went in that direction one would/should consider all aspects of structural integrity before going down to the sea. A couple more inches TE on Willy's rudder w power steering would be GREAT.
At some speed...not sure what...it will definitely increase drag because the fishtail will stall and create turbulence. Again like I first said...not much but I'm curious. (It should be acting just like a spoiler on a jet wing..same shape)
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Old 06-22-2012, 10:03 PM   #17
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Here's another take on the fishtail/thistle rudder:

Thistel Sandwich-Hold the Mayo.... | SailAngle.com

Low speed handling greatly improved and minimal cost (<$200)

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Old 06-27-2012, 02:02 AM   #18
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Thanks everyone for your posts about the Thistle/fishtail rudders. I increased my maximum rudder angle from 15 degrees to 26 degrees which is about the maximum I can given the chain and cable steering system on my 1974 GB 32. This has shorten my low speed turning radius by half and I can almost do a 180 degree turn within a boat length. When I haul out later this year I plan to add a fishtail rudder to my flat-plate rudder. Does anyone has suggestions on the angle of the fishtail. Boat Mechanical Systems Handbook by Dave Gerr specifies 45 degrees.
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Old 06-27-2012, 02:55 AM   #19
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The rudder on my 30' Willard is stopped by the slave cylinder at 45 degrees. Works great. Turns really sharp at slow speeds and even has fairly good steerage in neutral gear. The Willard is a bit slower than a GB and has a smaller rudder so suspect 45 degrees would not be as effective as w the Willard but I'd try 35. I'd think 16 degrees would be WAY too little deflection.
Yeah. Forty-five degrees works well with the slow Coot. Sixteen degrees might work with a speedboat, but not for a trawler.

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Old 06-27-2012, 12:22 PM   #20
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Suggest that you order down Dave Gerr's article on rudders from Professional Boatbuilding. Go here: Professional BoatBuilder - #102 PBB Aug/Sept 2006

and you can get a PDF via email for under six bucks. He gives all sorts of good info, including plotting the fishtail based on rudder size.

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