I’m referring to a Skerry rudder, but this could apply to ANY rudder: Has anyone considered/tried a rudder trim tab (servo tab) like on an airplane, whereas instead of deflecting the rudder, you deflect the trim tab, which in turn deflects the rudder? Would that be too much drag on a boat? Alternatively, moving the rudder the traditional way, but movement aided by trim tab in opposite direction. I’m looking at making a new rudder blade with a NACA 0012 or 0009 profile and figured while I am at it, I may as well try a tab. Finally, I think I recall reading that the trailing edge of the foil is better blunt (square off to like 1/8’) than the actual pointy profile (not just for structural integrity, but for laminar flow); I seem to recall reading a pointy trailing edge induced turbulence and a “fuzzy feel.” I’m fully aware that a Skerry is hardly a racing machine and the speeds are relatively low. It is easy to put too much rudder deflection and have the rudder acts like a brake/spoiler; I’m hoping a profiled rudder gives me more authority within a smaller blade angle, and I would put a physical stop to deflection beyond that. While I am able to manually deflect the rudder just enough without overdoing it, I still feel the Nordic style tiller is a pain (despite using it for a long time) and especially given the slanted pivot axis of the rudder. This is not a reflection on the design, but strictly user preference.
The point of using trim tabs (aircraft or boats) to move control surfaces is to allow the use of control surfaces that are too big and heavy to move by hand. Or to allow smaller and less expensive autopilot actuators to be used. Trim tabs use some of the power in the fluid moving past the control surface to operate the surface. You’re trading a small bit of extra drag against having to provide a power source (your arm or a servo) to operate the surface. You are also opting for a more complex mechanical linkage and extra weight.
The Skerry is much too small and lightweight to need trim tab rudder actuation and much too small and lightweight to justify the weight of the setup. If you want to do it for fun and education, by all means go ahead, but there is no practical reason to do so.
The other reason to use trim tabs on aircraft is to maintain a constant altitude, velocity or attitude without pilot input. Sailboats let you accomplish the same thing by setting and trimming the sails and moving backward or forward to change the center of gravity. No trim tabs needed for that, either.
You are correct - the right NACA profile will make your boat’s appendages more efficient and a blunt trailing edge will perform better than a sharp one.
Finally, keep in mind that there are times you want to swing the rudder hard and wide. If you need to slow down quickly is one case. Sculling back to shore when the wind dies by flapping the rudder is another. Sculling the stern around to get through irons is a third reason not to physically limit the rudder excursion.
Laszlo
Thank you, Laszlo!
Yes, I am aware of the complexity and reasons for tabs, on the aviation side; well articulated; thank you! Part of me is looking to tinker with something that probably doesn’t need tinkering with. Definitely no hydraulically actuated tabs, but simply a linkage and horn on the tab, with one end articulated and attached to the boat side (say starboard). When the rudder is deflected, say to starboard, the rod pushes the tab to port, which further moves the rudder to starboard.
You make another excellent point about sculling the boat back by flapping the rudder and more importantly, helping the bow through a tack, which given the little momentum of the Skerry in little winds, becomes helpful. So, definitely won’t go with a mechanical stop.
Thank you for confirming the blunt trailing edge of the blade. I will likely, while I’m at it, make two blades and experiment with the tab (for the fun of it, and maybe I’m nicely surprised); If it is less than resourceful outcome, I will always have the second blade to swap back in.
Cheers,
Eric
Agree with Laszlo’s comments above except for those dealing with the trailing edge. All other things equal, an airfoil with a very sharp trailing edge will have less drag than one with a blunt trailing edge. That is why the trailing edges of jet aircraft, keels/rudders of high performance sailboats and hydrofoils all have sharp trailing edges. Another example is racing kayak rudders like Stellar 4” weedless in these pictures. This rudder has a max thickness of 10mm but the trailing edge is too thin to measure without a micrometer. It is something less than 1mm.
For a boat like the Skerry, making a sharp trailing edge for the rudder would technically reduce drag but the difference would be minute, and it would be very damage prone. A better approach would be to get the trailing edge down to a thickness 1/4-1/8” then plane it square. Whatever you do, do not round off the trailing edge as this will create more drag.
If you are intent on making a better rudder for your Skerry, a cambered foil is a good first step. You should also consider increasing the aspect ratio of the rudder by making in longer (depth) and more narrow (front to back). Lastly, I’d suggest trying to come up with a way to mount the pintles so that they are more vertical. This would decrease the amount of down force the rudder creates at high deflection angles.
I guess I need to expand that a little. A blunt trailing edge on a boat like the Skerry will perform better. By blunt, I meant “not sharp” rather than not rounded.
The type of boat matters because the physical thickness of the foil is not the same as its hydrodynamic thickness. The latter, what the water flowing past the foil reacts to (“feels”), is a combination of the physical thickness and the thickness of the boundary layer, that is, the thin layer of fluid flowing past right next to the foil. Since water is incompressible, this layer acts as if it was part of the foil and makes the foil appear thicker. It also has the beneficial affect of forming a nearly frictionless layer that reduces the drag of the foil through the water.
For a given set of conditions, the boundary layer is thicker at low speeds and gets thinner as the boat speeds up. The result is that for a slow-ish boat like the Skerry, the boundary layer is thick enough that there’s no real point to having a sharp trailing edge.
What’s more important is that the trailing edge shape match the foil profile at the normal operating speed. That lets the boundary layer flow smoothly off the foil. If it flows off turbulently, it will lose its frictionless property and cause drag. If the flow becomes turbulent enough, it will actually cause vibrations that will be felt or heard in the boat. If you’ve ever heard your centerboard or daggerboard buzzing in its case, that was the likely cause. For slow speeds like the Skerry’s, the ideal trailing edge shape is that square that Mark mentions. A rounded edge will cause turbulence as the 2 streams curve toward each other and collide.
Note also that the cases that Mark mentions for using sharp trailing edges are all high speed, or at least substantially higher speed than a Skerry. He’s absolutely correct in that. All those examples have thin boundary layers and the thickness of the trailing edge makes a significant contribution. And for his weedless rudder, you can see that it has a long chord which allows the boundary layer to flow smoothly off the foil, the same way the weeds do. So that’s why the apparent difference. We’re just talking about the same effect in different speed regimes.
Hope that clears things up,
Laszlo
Thank you, Mark and Laszlo! It does make sense and yes, I’ve never intended to round off the trailing edge, but rather square it … My concern was the fragility of a pointy leading edge. Ditto of the speed regime … I’m trying to improve the performance of the rudder but am in no way looking for improvements too tiny to even notice on a Skerry.
Pintles closer to vertical would be my preference, but it comes at an aesthetics price as I would have to modify the stern with a sled extension of sort. Probably not going to do that,
I’ve toyed with the idea of a higher aspect ratio rudder blade (slightly narrower chord and longer blade with a bit of a raked tip. Not a big fan of the current “half moon” shape.
Finally, I was wondering whether terminating the rudder blade with a winglet style (airbus winglet style) perpendicular extension might increase the aspect ratio effect without extending the rudder blade length too much and introduce flex. Perhaps not a sharp 90 degree angle, but a faired transition. Details, details … I intend to carve it out of hardwood staves glued, together, route a groove and insert a length of carbon fiber rod, and sheath the whole lot with fiberglass/epoxy.
Anyway, I’m rambling at this point. Thank you, both very much!