On a twin shafted ship, is a balanced rudder or an unbalanced rudder better for turning? I have seen a lot of unbalanced rudder ships turn fast, but those rudders are mounted either tandem, or in front of a single prop. Is it better for an unbalanced rudder when thrown letting the diverted thrust from one prop, and the other props wash being unimpeded??, or, having a more balanced rudder, that places the rudder in front of both props when turned? ( rudder with the rudder post in the very front of the rudder, vs rudder with post in the middle of the rudder) So far I have only built rudders ruffly scale shape/location, or used what was commercially available, Second question, is it better to devote the area equally in length and height? (roughly square) or with a greater degree devoted to either height or width (rectangular?)
I can't offer much in the balanced vs unbalanced cause I haven't tried the unbalanced type yet. I do like having a balanced rudder that will deflect the propwash from the entire prop disk instead of half of it. I bet it would vary from ship to ship and prop/rudder configurations As for size, I tend to go with just enough length to put the bottom of the rudder around a 1/4" below the prop and add the rest to length to try to catch all the prop wash possible ... again with a balanced rudder.
My two-cents: I prefer a semi/balanced rudder in all applications, though I can't conclusively say it turns any better than an unbalanced one in a dual prop scenario, I would assume so since you are vectoring more thrust if you cover a prop and a half than if you only cover one. I prefer my rudders to be long enough to cover the prop + a little more depending on distance from the prop when hard-over, and the rest can go to height. My Derf's rudder violates this a bit by being much longer than need be because I did not want to add extra height and cause the rudder to be hanging lower than the bottom of the hull.
I assume this is a twin rudder ship as well. I'm of the opinion that thrust/wash fans out a marginal amount from the rudder. As far as balanced designs go, I believe having your rudders positioned close to the props minimzed its ability to avoid contact with the rudder. Then having axis of the props intersect the rudder axis perpendicular to the vertical center of the rudder should be ideal. I agree with RichelieuBB with vertical height/ proportions of rudder. Fluid in motion tends to entrain the surrounding fluid. Therefore if you ensure no wash is "escaping" below your rudder then a greater amount will be present to act on it's surroundings. Shoot two stream of relatively equal strength water in the same direction on a converging course. Shoot one straight then hit it with the other at a 45deg angle. They both deflect to an overall shallower angle. If the angled one is more powerful then the overall deflection will favor it. The "angled" wash (in relation to a ships centerline) should deflect the "straight" wash therefore acting like an extension in length to the rudder in theory. The question lies in what your personal conviction is as far as effectiveness. Which is better catching all the wash vertically but potentially not deflecting it as strongly? Or, missing some of the wash vertically but ensuring what is deflected is deflected strongly? I am in favor of the vertical bias. 1) Because of my fluid entrainment theory 2) Because what flow escapes should be relatively undisturbed/ laminar and will contribute to speed in the turn. With an unbalaned design your props could be slightly inside your rudders thereby giving more wash over the inside rudder and helping you maintain speed in a turn. This could potentially create a similar effect as steering differential. Also don't forget about steering differential in reference to rudder placement. This is where through linkage the inside rudder has more throw than the outside rudder. Thus allowing more thrust to pass on the outside helping maintain speed in a turn. I apologize for any redundancy... Das Bütts
Unbalanced rudder, hang it all over the one prop. I use this design on both my Arizona and my Saratoga. All designs have been extensively tested and this is what I found works best for me.
I re-read your post a couple times. It appears you are talking single rudder. In which case I heartily agree with iceman! Das Bütts
I ran some CFD a while back on this question while working on rudder shapes... the answer seemed to be, can you keep the leading edge of the balanced rudder in the propwash at all times? If yes, it will turn with the rudder post in the leading edge rudder. If no, the leading edge post rudder turned better, Also, shape of the rudder can have significant impact on the forces exerted at high angles of attack. Some shapes I tried had a large range of angles of attack with similar side thrust before stalling and having the thrust drop off dramatically, others had larger peak thrust but the difference between peak thrust and stall in angle of attack was very small.... There are a lot of things that affect rudders (aspect ratio relative to props, airfoil shape, where the LE of the rudder is at all angles of attack, max angle before stall, severety of stall, etc). Relatively speaking a well engineered rudder can be significantly better performing than a flat plate rudder...
I am curious can you explain the method you used to simulate the rudder system Greg? I could probably benefit from analyzing it myself. For building simplicity on my dual shaft dual rudder ships I have been using a balanced rudder with a flat profile and 170 degrees throw, but I usually want to completely kill my forward speed and pivot the ship in place as quickly as possible when maneuvering during battle. If I want to maintain forward speed and turn shallower to follow another ship's movements I use less throw on the stick. Ron Hunt
I basically had a part of a hull surface defined (a baden in my case) along with a volume around the hull of water. no bulk flow (although that would be easy to add and look at things). The prop was simply a cylindrical region where the mass flow was specified at the inlest side and mass flow plus an estimated swirl at the outlet. I played with prop location and geometry from there and just checked for lift forces normal to the long axis of the hull at a multitude of angles of attack. It has been awhile, that is about all I recall. It would be very interesting to look at it experimentally as well
Looking at it experimentally would be pretty time consuming, especially when you consider multiple rudder shaft locations, flat vs airfoil rudders, toeing in the props, toeing out the rudders, clearance between the props and rudders, varying thrust from the prop, etc. Quickly becomes too many scenarios to reasonably test with a physical model. Best guess modeling on a computer would seem to be the way to go here. What software did you use to model it? It would be interesting to see how the simulation's predictions change if you define a different sized prop as well. Ron Hunt
I used it as training at my previous position for solidworks simulation. All I have access to now is the output from those runs I saved into excel, although I have been looking at getting it set up with OpenFOAM for flow simulation, although there is a steep learning curve on that one.
Greg's input on my rudders for Baden made a huge difference in the turning versus a flat rudder. The 2 Badens in Georgia can do donuts in my pool.
