skeg/keel cutaway vs solid full keel on inboard Double Eagl

Discussion in 'Inboards' started by Northeaster, Apr 24, 2015.

  1. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    I would leave it flat and very smooth even polished.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Given the aspect ratio, you don't need to make any special attempts at foil shaping this skeg. Just round over the leading edge (3:1 ellipse) and taper the trailing some, but don't go nuts, because it's not going to do much for you. Roll over the corners on the bottom of the skeg, leaving enough of a flat, to install a sacrificial rub strip of some sort. In fact, I'll bet the very sharp leading edge on the skeg will "hunt" around a lot, as flow switches which side it wants flow around. On a bow it's one thing, but below the waterline, you do want to blunt the leading edges, so this edges aren't stalling every other second underway.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    The skeg is best made from welding two plates, via plug welding to a sectional shape, such as an I or a box. What you have is two plates, but I cannot see any internal stiffening? Thus It appears you will close off the top (or rather bottom edge) with a plate/FB?

    The way you have it, it will require correct sequencing to prevent distortion which may well lead to buckling. And you must weld both sides with the same amount of heat input. That's the reason for having a shape and plug welding the plate sides to it. It stiffening the skeg athwartships and also provide you with an easy way of welding (plug welding to the face flat) and thus you can also control the heat input better too.

    The connection of the skeg to the bkt holding the boss for the prop is very weak and in addition you have a poor looking welded joint at the connection. It would have been far better to have shaped that bkt to have an extension fwd, like an L shape. So the welded joint is away from the right angle connection.

    Also, since you are not breaking world records..the bkt supporting the prop/boss would have been better as an A shape, that is two tangs coming from the hull to the support. This stiffens up the joint significantly - currently its too weak. And those tangs should also be shaped at their base, with curves in both direction (fore/aft) and then welded to a thick doubler which is stiffened directly above it, internally, to transfer the loads. Since as you have it, as it vibrates, as it will, it is weak athwartships and so it will also work on the base joint. It will crack there. Having a doubler as the base prevents cracking of the hull plating, just the doubler. In addition, if with the curves rad ends fore/aft with doublers and 2 off (A-frame style), it prevents all the above.
     
  4. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Thanks for all of the detailed replies!

    Adhoc - yes, I do plan to add internal brackets/ stiffeners to the keel, to creaet a stronger box. I may not even need to plug weld most of it, as I can get my mig gun in the wider areas, and weld from inside, to join sides to brackets. And then, yes, I was planning on just cutting a piece of 1/4" plate to fit the open shape and weld it shut.

    re: the joint of the aft end of keel to the lower strut leg - There will be a piece of heavy flatbar welded underneath of this joint, which could be tapered and extend forward some distance...which will extend aft to become the lower rudder support bracket (actually will be 2 pieces, overlapping about 1 foot, bolted together to allow for rudder removal). Would extending this flatbar forward and tapering / welding into keel bottom add a significant or acceptable amount of strength?

    re: the weaker single strut leg - I know.... plans called for a two legged strut as you recommend....plans also called for a single 3/8" plate keel....I went with a single strut leg as I had planned on wrapping a full keel (with no cutout) completely around the strut and cutlass area, but later decided against that.
    The single strut leg does pass through the hull about 8 inches and is welded to a main frame on forward edge and there will be another large support bracket welded on aft edge, so that it forms a large H.

    I acknowledge the fact that it is not as strong as it could be, or should be (if it were to have 2 legs). But, where this is a recreational boat, to be used maybe 50 -100 hours per year... when would I start to potentially see any cracks / failures? I am not trying to be a smart@ss.... I really appreciate your help! I just would like to know if it could start cracking within a few uses or after hundreds or thousands of hours (excluding any hard groundings, abuse, etc)
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    That is wholly dependent upon your quality control.

    Does the joint has sharp edges on it, or small saw cuts, i.e not smooth?? If the weld is not a good full-pen weld and/or has porosity, do you grin back your stops before you do the start etc, if the alignment of joints/plating is off by more than 10% of plate thickness etc etc...and of course your shaft line up, is it perfectly 100% in-line?? All these things degrade the life of the joint.
     

  6. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Well, my QA would be no where near what would be expected in your professional environnent... In general though, I do follow practices mentioned like die grinding stops, cleaning / degreasing well, using router cut or cleaned up edges, etc.
    As always, than for your input!

    Out of curiousity, how much do you think performance would be affected by now adding two strut legs, in a V, tied up into large engine girders, as should have been done originally, but also leaving in the one that is already there?
     
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