How much should a skeg flex on a Roberts 44?

Discussion in 'Sailboats' started by Sailor 45, Nov 15, 2007.

  1. Sailor 45
    Joined: Nov 2007
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    Location: Appleton, WI

    Sailor 45 Junior Member

    I have a Bruce Roberts Offshore 44 fiberglass Sailboat with a skeg hung rudder. We had considerable movement at the skeg area which also protruded into the hull. Before adding several layers of glass over the area I had almost a 1/2" of movement from side to side. Now I have about 1/8" of movement from side to side and no movement protruding into the hull. At this time the only force I have applied to it is with a 3 x 3. I can only guess about 300lbs or so. I know at times rudder loads can be over 1000lbs. I am considering adding a few more layers but I am concerned about getting too stiff if that is possible. The prop shaft exits the boat through an aperture in the skeg. Should there be any flex at the skeg?
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I'd add more material if you're concerned. Bruce Roberts are home built boats. Unless you know the builder and can contact him with some questions about the laminate schedule, you're better off having additional reinforcement.

    I would be wise to spread out the stresses imposed on the hull by the skeg. As you place additional fabric, take it well out in every direction, to transmit these loads over a wide area of hull shell.
     
  3. Sailor 45
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    Sailor 45 Junior Member

    Thanks, I am definitely leaning towards adding a few more layers. I don't think I can ever become too stiff. I would like to have no movement at all. I can't think of any reason that would cause a catastrophic failure from being too stiff. I can only imagine more would be better in any situation. As far as spreading out the load over the hull, I did consider that when I layed up the first set of biax and chopped matt. I added 3 layers of chopped matt and 3 layers of biax over the whole area and I alternated 5 layers of biax at varing widths starting with an 8" width and working up to 14" down the inside radius. When finished I had about 1/2" on the inside radius and 1/4" on the rest of the area along with about 15 gallons of vinylester resin. When I started I could feel the hull flexing when the skeg was pushed to one side or the other and now the hull does not seem to be moving with the loads that I am able to apply. However if I build up the lower part of the skeg where it is still flexing a little then the fact that the skeg is stiffer might cause the flex to articulate back up into the hull. The hull was professionally layed up by Fortuna Yachts in Cape Town S.A. They are no longer in business.
     
  4. Sailor 45
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    Sailor 45 Junior Member

    Here are a couple photos of my project.
     

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  5. MMNet SEA
    Joined: Feb 2007
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    Location: Thailand

    MMNet SEA Junior Member

    Skeg Roll Bar Installed

    Hi Sailor45.

    If you look at the posts " Skid "roll-bar" Sked to Keel
    and Skeg Roll bar installed
    06 26 >>> 06 30 -- 2006

    You will see what we did to protect the prop. but also you will see what was done to strengthen a Roberts 44 skeg - by installing a wide plate between the
    aft end of the keel and the bottom of the skeg.
     
  6. lazeyjack

    lazeyjack Guest

    i dunno what your rudder is like, but the one on the boat next door is the worse rudder I have EVER seen, and I've seen some Boy!! would stall out so fast look at how far away is from the hull I'm so pissed off with that rudder I would build him one for free, and it would make a different boat!! How are you paul?
     
  7. PAR
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    PAR Yacht Designer/Builder

    Sailor 45 seems to have it well on the way. Good to see it's taken well out onto the belly of the yacht. This will transmit loads nicely.

    I'm doing quite well big guy . . .
     
  8. Sailor 45
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    Sailor 45 Junior Member

    Hello MMNet Sea,

    I checked out your modification. Looks like it will keep most everything off your prop but I can't see how it will reduce the side to side movement that I am experiencing. Do you feel any movement in your hull around the skeg while underway. While on the hard can you deflect the skeg at all? I had two bulkheads broken loose from the hull around the skeg area. I also have a poorly mounted rudder post which was leaking when I first purchased this boat. Now that the hull is not moving I intend to fix the bulkheads and add support to the rudder post on the the inside of the boat.

    Hello Lazeyjack,

    The rudder I have is mounted to the boat with a cup bolted to the skeg on the bottom and coupled to the rudder shaft on the top. It is approxmately 12.5 sq.ft and turns to about 35 degrees. Sorry I don't have any photos from before the rudder was removed. I have found through a google search an article from cruising world that is titled "Rudders Loads On The Modern Cruiser". (www.cruisingworld.com/capable-cruising/ systems/rudder-loads-on-the-modern-cruiser-37603.html - 36k) This article brought to light the amount of potential force applied to my rudder at specific angles and speeds. I found at 6 knots and 10 degree rudder angle there will be 222lbs and at 35 degree rudder and 6 knots the force is 734lbs and if I end up surfing down a wave face and accellerating to 9 knots it goes up to 500lbs and 1651lbs respectively. I would never imagined that much force before reading this article. Thanks again for all the feedback.
     
  9. MMNet SEA
    Joined: Feb 2007
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    MMNet SEA Junior Member

    Hi , you are quite right , my roll bar probably would not stop movement in your skeg - the addition of the flat bar on the Roberts does help ( however I did not add that both boats shown are steel ) Elsewhere on this board there is a discussion covering the reinforcement and bracing that was carried out in the inside - again steel .
    What you have done to freeze movement looks good !
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I think you'll find the bulkhead repairs will stiffen up the skeg quite a bit. The skeg can exert quite a bit of force on the hull shell in a reasonably localized area. Once the bulkheads get tabbed back in, this local deformation will be limited and the skeg much stiffer as a result.
     
  11. lazeyjack

    lazeyjack Guest

    dear boy forget all that math cos you can not marry it to what you a re doing, show us a pic of your rudder eh
     
  12. Sailor 45
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    Sailor 45 Junior Member

    Hello Par,

    Thanks for all your input. I am at a point now where the hull does not seem to flex at all when I apply pressure to the skeg.(Yes I still need to reglass the bulkheads as well) All my flex seems to be coming from the skeg itself. From below where the skeg attaches to the hull. Like I said though the skeg is moving about 1/8" at the lowest point of the skeg while applying pressure. At this point I only have a couple concerns.
    1. Is it possible to build the skeg up so much that in the event of a major impact on the skeg it could snap off versus bend or flex?
    2. Is some flex normal?

    Hello Lazeyjack

    Thanks for all your input as well. Here is a pic of my rudder and a drawing of about how it looks on the boat.
     

    Attached Files:

  13. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Without having a laminate schedule in hand, it's difficult to speculate about what Roberts intended with the skeg.

    It sounds like your laminate on the skeg itself may be thin, though some flexing isn't unusual. How much pressure do you have to exert to get the 1/8" deflection?

    Lazy, I see what you mean about the rudder hanging on the boat to his port side. The huge aperture is maybe to permit the flow to reattach, but there's no excuse for that giant gap along it's top. It must make some interesting noises.
     
  14. lazeyjack

    lazeyjack Guest

    rudder aspect ratio is good, cant see any balence, but that rudder wll steer the boat
     

  15. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Sailor 45, sounds like you've been given the "if you make it too stong you risk taking a big chunk of boat with it in impact or grounding" line, this is the standard response from Bruce, I beleive. Consider the size of the stock incorporated into your rudder- thats a very strong & reliable lever, so in my estimation if you let the skeg bust off easily you may also allow the rudder stock to "lever out a sizable chunk of boat" as the skeg gets busted off. My opinion is that- that skeg can't be too strong, reliability of steering is vital & also considering that your skeg also supports your stern bearing & that without a rudder in circumtances of severe grounding all you may have is fwd & reverse when pointed in the "right" direction you are doing exactly the right thing in reinforceing the skeg. All the best from Jeff.
     
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