comment on rudder

Discussion in 'Boat Design' started by lazeyjack, Sep 16, 2008.

  1. lazeyjack

    lazeyjack Guest

    the balence axis is usually abt 17% of the chord( width) this rudder appears to have no balence. it is what we call A Barn Door, low aspect ratio, \Skegs were essentailly there for boats with weak structural integrity, ply, timber, and some grp, but even those matewrial can be engineered to make them strong enough to get rid of the skeg
    The rudder in this dwg, is in a great position fore and aft, but can not escape the barn door tag
     
  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Lazy, like I said before, the hydrodynamic performance is not the only aspect of rudder design and I guess you know that well.
    When a designer chooses to use a skeg type, low aspect-ratio rudder like this one, he does so because he wants to assure structural integrity in case of contact with the sea/lake/river bottom.
    The fact that the keel is also short and wide and its bottom is in line with the rudder, thus maintaining the same small draft, proves that.
    This boat was designed to be capable of navigating and anchoring in relatively shallow water and its keel and rudder follow that design philosophy.

    Its VMG will not be that of a racer, the forces on the rudder are not balanced, it's leeway angle will be high, but obviously it was not a priority here.
     
  3. lazeyjack

    lazeyjack Guest

    I hear you BUT if the stock and structure is as per Lloyds or ABS, then no fears
    It still does not explain the lack of balence and the huge hole above the top
     
  4. daiquiri
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    daiquiri Engineering and Design

    The hole is not visible in this photo. Do you have a side view to attach?
     
  5. lazeyjack

    lazeyjack Guest

    abt 150mm on cl to cl boat, at 35 degrees would be considerable
     

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  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Yeah, well 150 mm is not a small gap. But are you sure it doesn't become zero at high deflection angles? The transverse section lines look almost horizontal in the area around rudder hinge, so maybe that gap is actually necessary to allow the rudder to reach the max angle.
     
  7. lazeyjack

    lazeyjack Guest

    nah the axis look plumb.
    you know I used to build for one designer, he was very very fussy abt rudders, for cruising boats , most of his were although he designed successful racers,
    Why should a cruising boat have a **** rudder. ? why? same with p struts, faired foil sections, look at this boat? in pic,
     
  8. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Well, just check it out before rushing to the client. The gap seems necessary to me. And also the rudder looks designed for a well-defined purpose. ;)
    Well you could, if you really want to change it, balance it a bit hydrodynamicaly - but I don't know if it is really worth the money, time and efforts.
     
  9. lazeyjack

    lazeyjack Guest

    not my boat not my design
    for the SECOND BEST DESIGNER, in world, well glad I dont pay him
    where in It are you, what is your work?
     
  10. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Well, one need to be pretty bold to call himself the "second best in the world". :rolleyes:
    I'm aerospace engineer, working freelance as engineering consultant in Brescia (northern Italy).
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Lazey, you can't incorporate balance into a skeg mounted rudder, unless the skeg isn't full length (which defeats much of it's usefulness in rudder protection). If the skeg stops short of the bottom of the rudder, then the blade can have some balance incorporated into it. I've never liked this approach, but have seen several designs with a partial skeg.

    This boat appears to have a moderately shoal fin, which necessitates a low aspect rudder plan form. Just one of the many compromises necessary for skinny waters.

    I can't see the gaps very well, but I'm assuming you have issue with the under belly to rudder top. Yep, you're right, the blade would be more efficient with a close fit, but it may well be designed with a close fit. This wouldn't be the first time a builder screwed up some thing of this nature.

    The leading edge gap is also a possible trouble spot. It has to be a very close fit.

    The plan form looks acceptable, considering the sacrifices made for a cruising yacht. The rudder/skeg assembly appear to be 2:1, but the blade alone looks much closer to 3:1 which shouldn't be confused with a barn door.

    If reasonable foil sections were employed and gaps kept to a minimum, then this type of configuration is quite acceptable, understanding it's performance envelope and draft limitations.

    Sure a blade would be more efficient, but they're vulnerable, especially if you need shoal draft.
     
  12. lazeyjack

    lazeyjack Guest

    I know that Paul, unless you put a horn forwards on bottom
    the builder is a good one, I am questioning the designer
     
  13. LyndonJ
    Joined: May 2008
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    LyndonJ Senior Member

    The skeg rudder combo produces more lift from a smaller rudder, you can count the skeg area , also aren't the rudder stock sizes enginneered for the in use loads not the accidental ones.
    I can see a husky skeg supporting the rudder in grounding when a spade rudder would be more vulnerable certainly in a fore and aft direction the skeg will be very strong.

    The gap above the rudder needs to be less than 5 millimeters at all angles (15 degrees either side) for it to be effective, if it's not 5mm it may as well be 50 or 100 and then you can get at the rudder top to antifoul and clean.

    have a look at this thread
    http://www.boatdesign.net/forums/showthread.php?t=22016
     
  14. rwatson
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    rwatson Senior Member

    Hey LJ

    That thread has one of your favourite experts :) talking about this very sort of design

    "I singlehanded a boat like that to New Zealand in my early twenties (Keel Hung Rudder). Downwind she was a disaster to try control. In New Zealand I scrapped the barn door rudder and put a separate rudder on a skeg, six feet further aft. Then I sailed to New Caledonia , Vanuatu and Fiji. The difference in balance going to windward was unoticeable.
    The improvement in downwind control and thus the amount of sail I could carry was huge.
    I made the skeg a keel cooler for the engine.
    I highly recommend anyone with such an underwater profile do the same . People who have were extremely happy with the results.It will be one of the best moves you ever made on your boat.
    They put rudders on airplanes at the back end instead of in the middle for a good reason.
    Brent Swain"
     

  15. LyndonJ
    Joined: May 2008
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    LyndonJ Senior Member

    Oh no .....are you a balanced spade rudder junkie too ???
    My mates father has just paid around 15k Oz money in Italy getting his spade rudder, its shaft and lower bearing housing rebuilt after being blown onto then towed off a nice Italian beach, he is not a fan anymore.

    The rudder on most aircraft is mounted behind a fixed fin or 'skeg' it's still at the back of the beast.

    What about a balanced rudder well aft with the bottom end supported behind a longer keel like the newer Roberts designs now they have abandoned skegs.
    They say it is the best of both worlds.

    On a go-fast boat its different, low wetted surface and high aspect ratios I know, but on a cruiser surely the more robust the better and hey the 'rudderless boat' is the metaphor for every lost cause.



    I was looking at the bit on root gaps at the bottom of that page. Do you have the ship science reports or Gutelle ??
     
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