Fish tail rudder idea!!!

[what_barnacles]

BeeJay,


Pretty interesting that you got some stern lift from the mod. Got any pics?

 

[Seevee]

If your rudder is ss, your aluminum plates will disappear through galvanic corrosion.

Yes that would be and issue, but one could separate them with non metallic washers. But mine's not SS anyway.

 

[Seevee]

Beejay,

Yes, pix would be interesting. I also a bit intrigued with the rudder designs and the shilling, fishtail, etc. mods.

Considered the articulating rudder at one time, but just don't like moving parts down there, plus it's expensive.

All this rudder modification stuff solidifies my decision to get a twin next time.

 

[Xsbank]

The main purpose of the leading edge of the rudder, the bit in front of the post, is to lower the effort needed to turn the rudder. My mod lowered vibration in the rudder, has no effect on cruise speed and improved my low-speed handling immensely. It’s up to you to decide if “immensely” means anything.

 

[MOJO]

I had a piece of 2" angle welded to the trailing edge of MOJO's rudder. No change in speed that I can discern but it does seem to make slow speed handling a little better. I suspect it's a little too small so I may try a larger piece at next haul out. Easy to cut this one off and weld on a 3" or 4" angle.

 

[Nomad Willy]

Our big 45 degree rudder leaves me not in want of anything else.

 

[BeeJay]

Rudder mod pics

 

[MurrayM]

Our big 45 degree rudder leaves me not in want of anything else.

That’s because your boat has a small, pert, slippery rounded arse compared to our large, squat, heavy, water dragging square ones

 

[BeeJay]

That's the kindest thing that somebodies said about my rear end in a long time .....gosh thanks Murray (blush blush)

 

[MurrayM]

That's the kindest thing that somebodies said about my rear end in a long time .....gosh thanks Murray (blush blush)

Sorry Dude... I was talking to Nomad’s Willy

 

[geoleo]

if you want great slow speed turning, the 3 components to success are:

A large rudder.
A significant leading edge (up to 28% of what's behind the pivot point, put in front of the pivot point). Without a leading edge, you lose half of the prop wash.
Being able to pivot the rudder 40 degrees.

This is the rudder on my trawler. It's a foil style, designed for slow speed. There is a significant leading edge and it will pivot 40 degrees. You can push this boat sideways by pivoting the rudder hard over, going forward at idle and pushing the bow with the bow thruster. There is an incredible amount of sideways push with this rudder hard over!

View attachment 80730

View attachment 80731

this is the rudder we built for my bruno & stillman. Large leading edge and 40+ degree pivot. This boat turns on a dime and gives you a nickel back.

View attachment 80732


ted

rudder porn!

 

[BruceK]

When I had a rudder remade the shipwright referred to the section fwd of the post as the "balance". Be that as it may, his reshaped enlarged rudder mods turned the boat way faster than before.

But, he was a very "senior" shipwright. He also used the word "tuck" to describe the transom,uniquely, in my experience.

 

[markpierce]

The million dollar question...

How big of a fishtail to improve turning but small enough to be insignificant drag at cruise?

My thought too.

 

[markpierce]

Our big 45 degree rudder leaves me not in want of anything else.

Ditto. Large rudders rule!

 

[stubones99]

Most machine / metal shops could make this with 4 bends in a brake. You could make it out of stainless as well. I don't know how much drag / turbulence the top and bottom open ends would cause though. Make sure to bolt it with nylock nuts, or you'll likely lose the tail piece from vibration.

 

[CLField]

Dave Gerr Thistle rudder design

I have attached an article that Dave Gerr wrote in Professional Boat Builder back in Aug-Sep 2006. The Thistle rudder has an air foil shape followed by a flared trailing edge. Excellent design for slow speed maneuverability. Also increases angle of stall making the rudder more effective.

 

[garycasey]

I believe the theory behind this is that the boundary layer thickness increases toward the aft end of any surface. Bringing the edge out through this boundary layer increases the sensitivity of steering. How thick? Don't know, but I suspect not nearly to the extent I have seen of the proposed devices. And anything like this will increase drag to some extent. And, exposing a sharp edge into the flow while reversing, I can't imagine any improvement in that direction. As an example, note the rudder on a Cessna Citation - it has a device just like that proposed, and for the same reason. I took sort of the opposite tack - the rudder on my Nordic Tug was just a flat slab of stainless. I made a sleeve that turned it into a streamline shape. I think it increased my speed - not much, but maybe 0.1 knots or a bit more. The velocity of the prop wash? I calculated my propulsion efficiency at about 65%, so the velocity is about 50% higher than boat speed. Any protrusion increases the drag force by a factor of 2.4 compared to a similar shape away from the prop wash, and the power required to drag it through the water by a factor of 3.6. Non-trivial. Last year I hired my bottom redone and I paid extra to have it done in a smoother "sailboat" style. The contractor said, "Why? After all, it's just a tugboat." Drag is drag and the purist in me doesn't like it.

 

[funangler]

Type thistle rudder on YouTube and look for the green boat. I can travel at full speed turn the rudder over and make 180 turn in 30 ft circle with maybe 5 ft of forward progress.

 

[MurrayM]

Type thistle rudder on YouTube and look for the green boat. I can travel at full speed turn the rudder over and make 180 turn in 30 ft circle with maybe 5 ft of forward progress.

Yup. Pretty dang impressive!

 

[markpierce]

Yup. Pretty dang impressive!

Irresponsible joy-riding within a marina. Don't want that guy anywhere near my berth.

 

[dhays]

Irresponsible joy-riding within a marina. Don't want that guy anywhere near my berth.

Yeah, I have to admit feeling the same way. Impressed by the turn radius, but not his choice of where he chose to demonstrate it.

 

[Xsbank]

What a load of old poops you are!

 

[MurrayM]

What a load of old poops you are!

Enviuos/jealous

 

[Sailor of Fortune]

I think Mike Thompson (former TF member who since passed) made a Thistle rudder for his Jensen/Defever 40. Mike was a very talented guy and his work was documented in a thread on here somewhere.

 

[garycasey]

Interesting design. However, you imply that you can put the helm hard over at full speed AND turn with less than 5 feet of forward travel. I think you meant that you meant to replace the AND with OR. If I might summarize (correct me if I'm wrong) there are at least 3 features that are important: 1. The leading edge is fairly blunt. This might cause a little more drag, but at trawler speeds, I doubt it. It mostly increases the stall angle so that at higher turn angles the flow tends to stay attached to the "back" side, increasing the rudder power during aggressive maneuvers. 2. The leading edge extends further than normal ahead of the axis. While this might make the steering effort too light, or even negative (making it unstable), it does have the advantage of capturing more of the propwash, increasing the effectiveness at high angles. 3. The endplates increase the effective length of the rudder by reducing the "spillage" of water around the ends. However, a longer rudder, though not usually practical, would be a more efficient way to do this. Further, the concave section toward the rear might make the steering more effective at small angles.

I like the overall idea. Worthy of implementing, although the forward overhang might be a little excessive.

 

[nwboater]

The main purpose of the leading edge of the rudder, the bit in front of the post, is to lower the effort needed to turn the rudder. My mod lowered vibration in the rudder, has no effect on cruise speed and improved my low-speed handling immensely. It’s up to you to decide if “immensely” means anything.

Did this mod add more torque to your steering (wheel feels "heavy"...hard to turn over) when traveling at cruising speeds (7-8 knots)?

 

[Xsbank]

No. Post 14

 

[garycasey]

I would bet that the reduce "vibration" is due to the better (although I'm not sure what it was like to begin with) airfoil shape and the smooth rounded leading edge that is bothered less by the pulsations in the propwash. Going by airfoil characteristics, the center of pressure is usually in the 25 to 35% of chord. If the center of pressure is ahead of the rudder post the load on the rudder goes the "wrong" way - turn the rudder a little and it wants to turn more. It just looks like the pivot is even further back. How far back is it? Another question - how did you construct the shape? On mine I used a .060 stainless steel sheet that I bent around the leading edge of the original 3/8 flat stainless steel plate.

 

[kapnd]

Makes sense to me in terms of forward travel, but I’m not getting how the flared trailing edge has a beneficial effect when reversing.
There is no prop wash to direct, and the rudder has no effect at all until some rearward motion is initiated, so wouldn’t the flare need to be on the forward edge to influence a backing turn?
On my vessel, (huge plate rudder) the immediate effect of reversing is walking to port, no matter where the rudder is pointed. Only after it is moving backwards and shift is placed in neutral does the rudder have any effect.

 

[Xsbank]

In that video, he is using way more power than most of us would use while maneuvering. I see that as a very educational video!