Rudder Spacing From Hull ??

Discussion in 'Hydrodynamics and Aerodynamics' started by Extra300, May 25, 2015.

  1. Extra300
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    Extra300 Junior Member

    Are there any advantage or disadvantage to keeping the rudder as close to the bottom of the hull as possible. Or is it better for there to be a gap on top of the rudder?

    Looking for the least drag resistance at higher speeds.

    Example in the picture.
     

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  2. gonzo
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    gonzo Senior Member

    You need enough clearance for the rudder to be able to swing. Of course, unless it is a flat bottom boat, then it can almost touch.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Ideally, you want as little if any at all gap, between the hull and rudder top. Of course as Gonzo mentions, hull shape can play a big role in this, but a bustle or pad can be employed to permit the rudder to sweep the bottom at the same distance.

    If you want the least resistance at high speed, then toss that rudder in the trash, as it's not remotely close, for a high speed rudder. There's a few trains of thought in this regard, but a recent race boat I designed, has showed a marked improvement with a new wedge style of rudder, I drew up, as a replacement for the more traditional foil section.
     
  4. Anatol
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    Anatol Senior Member

    " a new wedge style of rudder"

    do tell...
     
  5. PAR
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    PAR Yacht Designer/Builder

    The wedge rudder is fairly common on high speed craft and nothing new, except to the design I updated after it's first racing season.

    "The performance of High-speed rudders in a cavitating environment" by D.L. Gregory & G.F. Dobay, SNAME 1973 is a good reference to get a handle on this.
     
  6. whitepointer23

    whitepointer23 Previous Member

    Have a look at the shoe under the rudder. That is not a hi speed hull. Its either displacement or semi displacement.
     
  7. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante


    Higher speeds ?
    Your example is not for higher speeds.

    The main problem on rudders for high speed is cavitation.
    About 60 years ago a Rudder with Göttingen Profile under a high speed vessel designed by Vertens suffered during test trials for the first time with a desastous result under cavitation.
    The cavitation occured of course on the thickest part of the profile.
    The rudder therefore became abrubt overbalanced --100%---and the boat described immediatly a curve and run up the shore.
    At first sight one put the blame on the helmsman, but , after repair a few days later it happened again.

    The trick was to use another profile , a profile on which cavitation behind the thickest part can be accepted....the rudder with delta profile was born.
    Further developments lead to asymmetric deltaprofiles , counteracting the propwash 's helice.
    Normally all these high speed rudders are balanced spaderudders with an endplate on top.

    Cavitating foil
    http://www.heliciel.com/en/images/cavitation 2 profil.jpg

    A good explanation of cavitation
    (scroll down For good pictures of cavitating rudders )
    http://www.heliciel.com/en/aerodynamique-hydrodynamique/cavitation- helices-hydrofoils.htm

    An advanced rudder w. deltaprofile ( Boesch Beulenruder):
    http://www.boesch-boats.ch/image_rendered/uploaded/7/300x250_ratio.jpg

    pogo
     
  8. Extra300
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    Extra300 Junior Member

    -The rudder will NOT be like the one in the original picture. It will be smaller and have a slightly different design. (pic below)

    -Speed of this boat is projected to run close to 50 Mph, and we are looking for everything we can get from it.

    -The rudder will be made from 1/2" Plate and about the first 5" (leading edge back 5") will be tapered. The remaining part of the rudder is to be 1/2" with a square edge on the backside.

    If anyone has any suggestions, we are open to hearing them.

    Thanks
     

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  9. pogo
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    pogo ingenious dilletante


    Also a plate is a profile,
    a profile with many disadvantages,
    especially at higher speeds.

    the geometrie of your rudder (" will be made") is, apart from the balance, not really important.

    pogo
     
  10. PAR
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    PAR Yacht Designer/Builder

    If that's the hull you're trying to make go 50 MPH, you got a lot more to worry about than the miniscule amounts of drag the rudder brings to the table. You have a huge deadwood assembly or built down hull, either of which will generate tremendous amounts of drag, compared to the rudder.

    What are the basic dimensions and weight of this boat? How much HP available?
     
  11. JSL
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    JSL Senior Member

    The angle fairing over the stock (& pintle) will reduce the rudder's effectiveness by about 50% at low rudder angles.
     
  12. Extra300
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    Extra300 Junior Member

    It is a builtdown hull. I understand the skepticism, but there are the same and other builtdown hulls producing similar results.

    This boat is 38' long; being built light as possible (roughly 18,000lbs) and will have 1200 hp.


    My question is concerning the rudder, and opinions on how to make the most efficient rudder design possible.
     
  13. tom28571
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    tom28571 Senior Member

    Now some really pertinent facts show up. I guess the OP is planing on entering the lobsterboat races along with the other wild and wooly crazies. 1200hp can cure a lot of drag issues and control becomes the main determining factor of success. An effective rudder is near the top of the list of necessaries for these events. Those hulls were never meant to go that fast and handling is probably a bit hairy.

    What the flow looks like behind that keel at 50mph is an interesting thing to consider. A rudder with some foil shape will probably be better than a flat plate even with disturbed flow. PAR suggests a wedge which may be good since it would be more effective than a nice foil at larger angles of steering and probably better in disturbed flow. There are several forms of a wedge with the simplest being a bit of of fan out on the trailing edge which gives more positive control at small steering angles, or that is what I have read.

    I'm assuming some disturbed flow at that speed although that is just a guess.
     
  14. gonzo
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    gonzo Senior Member

    An efficient rudder is one that will steer at all speeds. Faired blades work well at low speed but cavitate and lose control at higher speeds. That is why high speed boats have wedge shaped rudders. Drag is of secondary importance. Fairing the keel is of more importance as far as resistance is concerned.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Agreed, the rudder's efficiency, behind that built down hull is marginal at best, regardless of plan form and to a lesser extent it's section. At 50 MPH, the wedge will start to rear it's head, but with these flow numbers, you'll still need a bit more speed to possibly justify the wedge. Simply put, a good foil section on the same plan form will likely be on the edge of cavitating, while the wedge will just be coming effective.

    Considering the presumed lobsterboat races (that was my guess too Tom), the wedge will likely prove ever so slightly superior, mostly because of it's "bite" at these higher speeds. Everyone I know that designs these puppies, has a specific set of angles they'll employ, based on several factors. For a "good" one, it'll be like pulling teeth getting much more than some generalities and certainly no firm shapes and angles, without some serious study on your project (read lots of questions from a designer).

    The likelyhood you'll "wing it" and get a good one is remote, but if you want to give it shot, a 4 degree sweep on the wedge is the usual starting point, on a square tipped delta plan.

    All this said, your real speed (I played with these some years back) will come from the fair and smoothness of the hull and the most possible wetted surface reduction, you can get away with. Most of these puppies are on the verge of longitudinal stability at these speeds, so control is pretty import, but there are many other factors that can make a slightly slower boat win consistently, simply because it gets up quicker, turns more precisely and accelerates faster. This is the key to winning these types of races. It's true that the boats that turn the fastest, typically win, but there's usually more to it than this observation.
     
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