Effectiveness of tabs on trailing edge of rudder

[andrzej0nl1ne]

I read somewhere that adding tabs at the trailing edge of the rudder adds significantly to the power/efficiency of the rudder. Anyhow, I did this (see photo) but not sure of how well it works. No impact in reverse (maybe making rudder less effective), and uncertain about forward effectiveness. Definitely did not make rudder less effective and maybe improved it somewhat. Curious about the group's input on the engineering logic for this alteration.

Boat is on the hard for winter so not sure if to keep it or remove it.

Thanks in advance for your thoughts.

Ak

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[Bill O]

Hard to say if this would have much help on your rudder behind a full keel. When you were turning in close quarters (forward) did it it help you turn easier/more efficiently?

Most of the time a tab at the end of a rudder that has more shape (NACA, etc.) and looks like a fishtail (Schilling rudder). The tail will provide more lift than a conventional rudder.
Here is a link to a paper describing rudder design.

[Bowdrie]

The more common use for tabs is to balance the rudder against "prop walk", so that the helm can be centered at normal motoring RPMs.
On an outboard rudder such as yours a tab that is independently moveable can also be hooked-up/rigged with a linkage/vane to provide self-steering.

[psk125]

According to the linked paper (Thanks to Bill O) the tabs or "fishtails" or "wedges" work when associated with a NACA profile rudder. Working in tandem with the NACA profile, the fishtails provide added lift, making ships more maneuverable. The paper points out that this added maneuverability comes at a cost, however: drag. On a large ship, the drag is easily overcome by nudging the throttle lever a notch further forward. The added drag may be worth it for them because their maneuverability means they might not need to hire a tug to assist in some operations. On a sailboat, you don't have that luxury or tradeoff option. The drag is going to slow you down. On top of that, because your rudder is not a NACA profile, the fishtail is probably not making you more maneuverable, and is only slowing you down. A hydrodynamicist might have more insight to give you about this.

[Manateeman]

Exactly correct. It’s a sail vessel not a ship. Great research paper and worth the reading. A lot of what this paper contains would help in some design debates such as the catamaran hull extension thread.
Better than calling a boat a pig with lipstick.
Computers are nice but if you build a model, you get reasonable results without a test tunnel, just tow it behind your boat. I think Herreshoff did this but not sure.
Mark

[thinwater]

I assume this would also make the helm much heavier, since it moves the COE aft. On a ship the steering is hydraulic, so they wouldn't notice as long as the system was sized for it and/or the the rudder balanced for it.


On a power boat it makes some sense, because making the rudder bigger is not very helpful; the prop wash only strikes a small area. On a sailboat under sail, fairing and adding area makes more sense.

[hpeer]

Originally Posted by thinwater

I assume this would also make the helm much heavier, since it moves the COE aft. On a ship the steering is hydraulic, so they wouldn't notice as long as the system was sized for it and/or the the rudder balanced for it.


On a power boat it makes some sense, because making the rudder bigger is not very helpful; the prop wash only strikes a small area. On a sailboat under sail, fairing and adding area makes more sense.

This, increase steering effort. It makes it harder for you and your AP to steer.

Unless you are trying to balance out prop walk.

Another possible solution would be to add an “auxiliary rudder” type wind vane steering.

Then when motoring you lock the main rudder to eliminate prop walk, makes it easy on the vane.

When sailing you lock the main rudder to counter weather helm, makes it easy on the vane.

The vane can be driven by an inexpensive tiller AP, makes it easy in the wallet.

Right idea, just need the right solution.

[TrentePieds]

Andrzej:

I am genuinely curious - what had you hoped to achieve by making this modification to so lovely a traditional design by so competent a designer? Was there some particular aspect of her handling that you were not satisfied with?

TrentePieds

[Scorpius]

On Scorpius I have a trim tab, about 4" wide and 5' long down the back of the main, outboard hung, rudder. It has it's own "rudder post" terminating at the top in a small tiller projecting straight out to the side. I use a very small Autohelm 2000 tiller autopilot between that little tiller and the main tiller thus controlling the angle between the trim tab and main rudder. It works beautifully. The boat tracks dead straight and the AP draws negligible power as it is only "steering" the little trim tab. The main tiller is in two sections: a short piece, about a foot long from the top of the rudder post (to which the autopilot connects) and then a long section used for manual steering. The long section can be tipped up out of the way when running with the autopilot (95% of the time), clearing the cockpit. If a wave hits the main rudder it just swings out of the way for a moment (as it is not locked by hydraulic steering or the autopilot) thus drastically reducing forces on the rudder post, pintles, etc. As far as I am concerned it is a wonderful system and I would recommend it to anyone with an outboard hung, tiller steered system. However, you do need a straight edge down the back of your rudder.

[tilostahl]

I confess I haven't read the paper that Bill O. linked to, so I can't speak to that. That said, the only point of the tabs that I can think of is to reduce turbulence off the trailing end of the rudder, to reduce drag. The tabs in the photos pretty clearly do exactly the opposite, so they are counter-productive. And I'm guessing they increase the load on the steering.

This is not a situation like the infamous wings on the keel of a 12 meter yacht, nor like the winglets on the end of an airplane wing. Different purpose and different hydrodynamics.

I suggest you take them off.

[andrzej0nl1ne]

Thanks all for the useful input and especially the design reference paper. The wedge design comments were obviously most applicable and supported what I experienced sailing. Though the experienced changes were so subtle that I needed to be told what to expect to actually realize what I experienced.

With this hindsight my 1st experience was reversing out of a winter slip. I had gained proficiency with turning the boat by propwalk, but on this attempt that skill disappeared. I didn't think much of it, but in hindsight the rudder, still useless for maneuvering, became very effective in negating the prop walk. Provided some excitement at season start. The boat is on a mooring so no further opportunities to validate that experience.

When sailing in stiff winds (my preference) , Ondine is a 12000lb 31' ketch, the rudder seemed to need less angle to balance weather helm. No experience with close quarters maneuvering, other than turning into the mooring ball.

All in all I felt no obvious benefits or negatives, which is one reason I floated this question to the group.

Drag remains a major open question, especially as I recall something from aerodynamics that says the trailing edge should be a smooth transition, which mine is not.

I'll reread the document and see if more insight surfaces. At this point I'm inclined to remove the tabs.

Thanks again for your very helpful guidance.

Ak

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[fxarcher]

You might want to take a look at this.

https://www.youtube.com/watch?v=uRNE64MTefc

[andrzej0nl1ne]

The answer to why I did this has 2 answers. In concept, I tried to reduce the amount of rudder angle I would apply in high winds. In practice, the more likely true reason is that I like to tinker.

The seawind-1 is a beautiful boat, becoming more wonderful the longer I sail her. I will strive to contain my creativity to above the waterline.

Ak

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[Cheechako]

I always thought of it more as a rudder balancing device....

[Scaramanga F25]

Your installation is incomplete. You need a separate control rod attached to the flettner.