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Old 06-20-2017, 09:42 AM   #1
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Thistle rudder again

I'd like to revisit the thistle (schilling or fishtail rudder). Looking at some of the old threads a lot of the links don't work any more.

Seems like quite a few folks have modified theres with great success for low speed handling.

I've got Boat Mechanical Systems Handbook, by Dave Geer on order, which has been mentioned several times as a good reference.

I like the idea of a bolt on addition, easy to install and remove if necessary.

Right now, I'm thinking of a two section rudder mod, one for the nose and one for the tail.

What I don't know is the dimensions that would work, but reading from others I'm guessing the following: Hopefully Dave's book will give some guidance.

X = 3 in
Y = 2 in.
Z = 3 or so, to fit shaft

As for the tail design, some leave it open on the end and some box it in. Why not a trailing diamond or curved shape, like in C and D below. The reasoning is to reduce turbulence in cruise, and give "some" effect in reverse, which would be better than a flat or open end.

Also, I'd figure out how to add a top and bottom plate.

[/URL][/IMG]

Thoughts?
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Old 06-20-2017, 10:27 AM   #2
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Our New Boat ...

Scroll down about 10 photos to see our rudder modification ..... extremely pleased.
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Old 06-20-2017, 12:57 PM   #3
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Seavee

Isn't this a new vessel to you? Maybe all is OK but more close in maneuvering time and experience is needed.
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Old 06-20-2017, 01:25 PM   #4
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Seavee

Isn't this a new vessel to you? Maybe all is OK but more close in maneuvering time and experience and is needed.
+1

Get some runtime under your belt and learn your boat before modifying.
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Old 06-20-2017, 08:09 PM   #5
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+1

Get some runtime under your belt and learn your boat before modifying.
I've got about 60 hours on it, how much more do I need? Always can use more but with the reports on rudder modifying, I have yet to find one that has been dissatisfied. It turns fine, but just want more, as a lot of other owners have done.

I can turn it around in a 50 ft circle in no wind... BUT it takes time and with a wind I don't have the time. I do that every time I launch from my home port which I been doing 4 or 5 times a week. With wind I can't turn fast enough without blowing back into the pier unless I back further downstream.

Just looking for better options. Yes, I'll modify the rudder, just want good info before I do it. Thanks.
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Old 06-20-2017, 08:11 PM   #6
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Our New Boat ...

Scroll down about 10 photos to see our rudder modification ..... extremely pleased.
Boat Poker,

Very interesting mod. You have water flow between the mod and the rudder. That might mitigate the turbulence behind the fish tail. What do you think?
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Old 06-20-2017, 08:18 PM   #7
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Boat Poker,

Very interesting mod. You have water flow between the mod and the rudder. That might mitigate the turbulence behind the fish tail. What do you think?
I believe it does. I've also watched it with an underwater camera under hard load backing up with no visible deflection of the plates and I believe that gap is also responsible for that. The design was a wild ass guess that cost about $40.00 and 20 minutes to install. Could also be removed in even less time if it didn't work. Neither is there any noticeable drag in forward.
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Old 06-21-2017, 08:00 PM   #8
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'Thistle' rudder is a great tool especially for guys like me that may be a bit underqualified as skipper for a 57 ton 6 foot draft vessel like Libra.
This aggressively shaped rudder pictured below really helps when making little way in close quarters.
I think it is shaped a lot like Seevees first drawing.
For perspective, the wheel is 38" and the flat trailing edge of this rudder is over 6'wide. Really kicks the stern over with a little thrust and when combined with the large wheeled hydraulic thruster on the bow almost allows for a walk perpendicular to the boats axis to port. A little tougher to starboard but still good.
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Old 06-23-2017, 08:44 AM   #9
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Got a few good ideas here, but where are the rest of you that have modified a rudder?
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Old 06-23-2017, 09:00 AM   #10
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I think there are very few with modified rudders- hence the low response volume.
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Old 06-23-2017, 09:19 AM   #11
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I think Peter is right. Not mant people mod mechanical things. Not a natural thing for conservative boaters and this group (as a whole) is very conservative.

You say "Why not a trailing diamond or curved shape, like in C and D below. The reasoning is to reduce turbulence in cruise,"
IMO there's lots of turbulence in this type of rudder so it matters little what variation you choose.

Very little increase in drag if you just lengthen the rudder plate. I have a very large rudder on my 30' boat, 45 degrees swing each way and my low speed handling is great. The fishtail trailing edges will enhance dead slow turning abilities and those that have employed them say there's no noticeable increase in drag. I disagree. The rudder is in the propwash, is high velocity and will definitely produce more drag.

For you I'd just add several inches to the TE and a little to the LE according to design standards. Easy to do and easy to take back. And very little drag increase. Also as a rudder to deal w following seas the straight blade surface is probably better. I'd bet on it but can't show proof. You could even make the TE at a little angle to compensate for propwalk.

Here's my large rudder that swings 90 degrees.
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Old 06-23-2017, 09:48 AM   #12
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....or, add a stern thruster...
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Old 06-23-2017, 10:22 AM   #13
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I think Peter is right. Not mant people mod mechanical things. Not a natural thing for conservative boaters and this group (as a whole) is very conservative.

You say "Why not a trailing diamond or curved shape, like in C and D below. The reasoning is to reduce turbulence in cruise,"
IMO there's lots of turbulence in this type of rudder so it matters little what variation you choose.

Very little increase in drag if you just lengthen the rudder plate. I have a very large rudder on my 30' boat, 45 degrees swing each way and my low speed handling is great. The fishtail trailing edges will enhance dead slow turning abilities and those that have employed them say there's no noticeable increase in drag. I disagree. The rudder is in the propwash, is high velocity and will definitely produce more drag.

For you I'd just add several inches to the TE and a little to the LE according to design standards. Easy to do and easy to take back. And very little drag increase. Also as a rudder to deal w following seas the straight blade surface is probably better. I'd bet on it but can't show proof. You could even make the TE at a little angle to compensate for propwalk.

Here's my large rudder that swings 90 degrees.
Eric,

Looks good, and good info.
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Old 06-23-2017, 10:31 AM   #14
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....or, add a stern thruster...
Pau,

I have a stern thruster (and a bow thruster which is much more effective), and it helps. The thrusters can't overcome much wind, however, and I'd like more control in my bag.

I'm not overly hell bent to make a mod, but of those that have seems like no down side. The virtually all report significant improvement. I'm just collecting more data and info to make an intelligent design.

I live in an unusual place where tides have a significant effect if I can get out and have about a 50ft wide area to turn the boat around without hitting a dock or going aground. In no wind, it's fairly easy, just takes a bit of time. With wind it's a bitch and easy to get sideways where it's hard to finish the turn. I also have to turn against the prop walk.

At times I wished I had twins which would be easy, but I don't.
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Old 06-23-2017, 10:49 AM   #15
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Adding a wedge shape extension to a rudder is very common here in SE Alaska, especially in the single screw commercial fishing fleet. Those that I have spoken with about this design all agree it is a great improvement to slow speed handling and no noticeable down side. Is there increased drag? Probably there is but if the increased drag for a slow boat is not noticed then it is not significant. I have experience operating numerous single screw slow boats and there a few that could use a wedge extension, especially in windy/tidal conditions. I would not hesitate to do this modification if I thought it would improve performance.
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Old 06-23-2017, 12:08 PM   #16
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Klee Wyck, I notice your rudder zinc is nearly done - when I did mine last, I moved the zinc to the bottom edge of the rudder out of the wash and I found a significant reduction in the amount of vibration through the steering system, even went below and stared at the rudder while I was running and there was a remarkable improvement. I was sensitive to vibration as I had recently replaced a worn-out Wagner king pin with a newer piece that had a grease fitting on it, a huge improvement in servicing too.

My trouble is the bow, with little or no keel up there and very little weight, without a way on it can move pretty fast in the wind or if I am backing and do not stop before changing direction, inertia keeps it moving in whatever direction I may have inadvertently chosen. Enough rudder, not enough control of the bow.
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Old 06-23-2017, 01:45 PM   #17
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Pau Hana (Peter) is right again .. thrusters.

I wouldn't go Thistle just because of the drag and consider then pretty much a substitute for a bigger rudder. However I have seen demonstrations of what the Thistle Rudder can do. At zero speed the Thistle rudder may be more effective that a bigger rudder. But I'm sure while cruising at normal speed the bigger rudder would be more effective. If you're at the beginning of a broach w full throttle and max rudder deflection I'd bet on the bigger rudder. Either way you should make sure your rudder system is up to the increased forces.

Judging from your avatar it appears your problem os probably mostly windage. Do you really need that big penthouse up there? Why is the pic so smal? Taken w a cell phone?

Xsbank,
I moved my zincs up out of the propwash just to reduce drag. The only thing in the propwash now is the perfectly straight and flat rudder plate.
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Old 06-23-2017, 03:53 PM   #18
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Pau Hana (Peter) is right again .. thrusters.

I wouldn't go Thistle just because of the drag and consider then pretty much a substitute for a bigger rudder. However I have seen demonstrations of what the Thistle Rudder can do. At zero speed the Thistle rudder may be more effective that a bigger rudder. But I'm sure while cruising at normal speed the bigger rudder would be more effective. If you're at the beginning of a broach w full throttle and max rudder deflection I'd bet on the bigger rudder. Either way you should make sure your rudder system is up to the increased forces.

Judging from your avatar it appears your problem os probably mostly windage. Do you really need that big penthouse up there? Why is the pic so smal? Taken w a cell phone?

Xsbank,
I moved my zincs up out of the propwash just to reduce drag. The only thing in the propwash now is the perfectly straight and flat rudder plate.

Norman Willy,
The thistle has a different shape that won't stall as quickly as a straight rudder, that's why folks use them for tighter turning. The straight rudder may help, but different animals.

How much the Thistle will slow up a cruise can be figured out, or found out by trial and error, but but in the trawler world, it's not much at all.

I'm pretty much set on the thistle, just getting more info as to size, etc. As far as reshaping the front, not sure how much the effort would be worth, but I'm sure there would be some gain. Also a top and bottom plate would help too.....
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Old 06-23-2017, 06:23 PM   #19
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SeeVee,
As to top and bottom plates it depends largely on the aspect ratio of the rudder. Tall narrow rudders will gain little to very little. Low aspect rudders like a square one almost needs end plates and then they are very beneficial.

As to the drag issue you could extend the rudder's TE and put a Thistle "Y" not very wide. Compromise. A good rule of thumb is to measure performance 1st w/o mods. Then do a mod and measure. Repeat w more mods if desired.

I would humbly suggest to 1st check your rudder swing or deflection. If it's a fairly small rudder and the boat goes 15+ knots at times don't increase swing beyond 30 degrees. If it's a med to large rudder increase the swing to 36 to 38 degrees. If it's a big rudder go for 40 - 43 degrees. If it's as big as mine .. 45 degrees. And make sure your mechanical system is strong enough.

Then if you're in the mood to increase the blade area lengthen LE & TE in appropriate proportions. OR/and put a Shilling "Y" on the TE.

If you can only be bothered w one mod pick bigger rudder or Shilling and go for it. Would be nice if you were to measure your rpm/speed before and after if your measuring devices are up to it. Then of course report back.

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Old 06-23-2017, 06:44 PM   #20
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SeeVee,
As to top and bottom plates it depends largely on the aspect ratio of the rudder. Tall narrow rudders will gain little to very little. Low aspect rudders like a square one almost needs end plates and then they are very beneficial.

As to the drag issue you could extend the rudder's TE and put a Thistle "Y" not very wide. Compromise. A good rule of thumb is to measure performance 1st w/o mods. Then do a mod and measure. Repeat w more mods if desired.

I would humbly suggest to 1st check your rudder swing or deflection. If it's a fairly small rudder and the boat goes 15+ knots at times don't increase swing beyond 30 degrees. If it's a med to large rudder increase the swing to 36 to 38 degrees. If it's a big rudder go for 40 - 43 degrees. If it's as big as mine .. 45 degrees. And make sure your mechanical system is strong enough.

Then if you're in the mood to increase the blade area lengthen LE & TE in appropriate proportions. OR/and put a Shilling "Y" on the TE.

If you can only be bothered w one mod pick bigger rudder or Shilling and go for it. Would be nice if you were to measure your rpm/speed before and after if your measuring devices are up to it. Then of course report back.

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Eric.
Good info, thx. Makes a lot of sense.

Except for the rudder swing. Why would more swing work better for a bigger rudder?
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