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

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

  1. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi Folks,

    Some of you may have seen my aluminum Double Eagle build, under the metal boatbuilding section.
    I have a used Cummins 4BT 150 hp that I will used with a traditional inboard / shaft drive on this Glen-L Double Eagle design. It is a hull that is supposed to go fairly smoothly from displacement speeds, through semi-displacement to planing with to speed around 20 - 24 kts

    Plans show the aluminum full keel version with a cutaway in the full keel/ skeg ahead of the prop. Yet other pics in plans (likely borrowed from the wood version plans) show a solid full keel (no cutaway). I believe at least one Glen-L member's pics show his wood model with this solid full keel, with no cutaway.
    I am considering not having a cutaway in my aluminum version as I could use one piece of approx. 2" (ID) schedule 80 aluminum pipe for a continuous shaft log and cutlass bearing holder all in one pipe - rather than a separate shaft log and then a strut with the cutlass bearing aft of the cutaway. I think it would be easier to align one long pipe, and build a box keel around it (tapered). I also think the stress or loads on the hull would be less if the stress is distributed along the entire ( but mostly aft end)length of the box keel where it is welded to the hull (a few inches on either side of the centerline) rather than the prop / bearing having it's force put soley on the V strut where both legs meet or pass through the hull to be welded to the engine girders.
    I have increased the hull thickness to 3/16" from plans of 1/8" and the plans had called for 1/4" hull inserts where the struts pass through. I am no engineer, but feel that my 3/16" bottom is likely nearly as strong as having a 1/8" bottom with 1/4" inserts welded into it, as the welds reduce strength in the HAZ zone. I could add stringers in this area above the prop, in either case, to further increase strength as well.

    Really looking for thoughts on how the cutaway affects performance/ flow to prop, etc and as well, on other pros and cons of the two methods.
    If I go with no cutaway, and a one-piece pipe for the shaft log and cutlass bearing holder, I would likely divert a bit of seawater to feed into the tube to keep the stuffing box and cutlass bearing well lubricated. I have no experience with this, but have read about members here doing this with good results on other posts.
     
  2. Northeaster
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    Northeaster Senior Member

    Hi folks,

    Should this be moved / reposted under Design, or Prop headings?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I'm not familiar with this design/hull. Do you have any images of with and without, so we can see the difference, and thus be more informed to comment?
     
  4. Northeaster
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    Northeaster Senior Member

    Hi Adhoc,

    I have a few pics of my plans, but was unsure how much I could or should post without putting up more than the designer would want posted. Here are a couple of plan drawings, which I don't think would be too detailed to post.
    You will see different lines for full keel ( with cutaway per aluminum plans) as well as the smaller keel line for the outboard or I/O version. Finally a full pic shows the boat with no cutaway, as usually built in wood.
    Thanks for taking the time to look!
     

    Attached Files:

  5. Ad Hoc
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    Ad Hoc Naval Architect

    The skeg performs several functions.

    1) Directional stability. With vessels that, in general, have a a shallow draft at the transom, and a deep forefoot far fwd, the directional stability can be suspect. The deep forefoot and acts "like" you have a rudder up fwd and therefore when you engage the main real rudder aft, it does not "bite" so much and you struggle to steer. Also in following seas the skeg helps to prevent broaching, same reason but in reverse.

    2) Structural. The skeg can add longitudinal strength to your hull

    3) Damage control. The skeg protects your prop and also the prop when grounding and also the hull when grounding. Also if the skeg is full, having extra stiffness at the prop boss rather than some distance from it, reduces any bending moment athwartships much easier, again reducing the loading on the structure from possible fatigue. An "open" or without as you call it, requires thicker plating/sections at the aft end to be as equivalent stiffness.

    Full skeg is easier to fabricate than having the cutout. Less labour.

    In either case, the most important aspect it to make sure that the training edge of the skeg is knife edge to promote good flow into the prop.

    Each method has its prop's and con's but, if done correctly should not influence the vessel too much other than your own build and maintenance. Since having a fully supported shaft and enclosed, can prevent issues that occur with a more exposed shaft.

    Finally, yes, add some additional long.ts iwo of the prop on the hull. You get a lot of pressure pulses from the prop so you need to limit vibration. Adding weight/mass to the structure locally increases the natural freq of the structure and thus helps to minimise any damage; prevention rather than cure. I would also increase the plate thickness iwo of the prop too, circa 1.5 times the surrounding shell plate thickness.

    Any of this help you to make up your mind?
     
  6. PAR
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    PAR Yacht Designer/Builder

    In a perfect world, this is the approach, though more practically, a knife like trailing edge is imposable to maintain. The better (more practical) trailing edge, is as fine as you can make it, but with enough meat left so you can square it off, with crisp edges being the goal. Picture a knife edge that's been filed/ground down, so it's about a 1/4" (6 mm) flat along it's length. The corners of this flat are crisp, not rounded, for good flow separation. Maintaining this type of trailing edge is much easier.
     
  7. FAST FRED
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    FAST FRED Senior Member

    Cruising boats run into "stuff" as well as run aground.

    The full keel would handle cruising better.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Not necessarily.

    We adopt the same approach as we do for the stems on the bow. Most of our designs are is multihulls, and the hulls are extremely fine up in the bow. The side plates butt into the thicker vertical stem bar and the stem bar is ground down to a knife edge creating a near perfect knife edge to the side plate. (That is a straight line from the stem bar to the weld of the side plates to the stem bar to the faired line of the side plate).

    knife edge.jpg

    We do the same approach for the trailing edge of skegs, when we require them. But you need to make sure the skeg is sufficiently stiff for the job to begin with.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Agreed, there are ways to get a tough edge, but in 'glass and wooden construction, a much more difficult edge to maintain. Alloys (like this boat) have an advantage. I use metal on wooden trailing edges of appendages, for this reason.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    That's also what we do on our composite vessels, for the bow. We have a metal insert (iwo of the DWL) which we grind down to a knife edge.
     
  11. Northeaster
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    Northeaster Senior Member

    Thanks to all for the responses!

    Adhoc - yes, this does make me lean away from trying to make a cutaway area in the full skeg - as you mentioend, I see it being substantially more difficult to build (incorporating a nice trailing edge into the cutaway area, which would not be a straight vertical line).

    One other option I was considering was to build the smaller skeg, perhaps just slightly larger than the outboard version of the skeg, and then build the separate prop strut (using two pieces of flatbar to connect to the hull in a vee per plans, as opposed to only one point). On the bottom of the strut I could have another single leg, below the prop and connect this ahead to the aft bottom of the skeg, using a heavy piece of flatbar (perhaps with a small tee for less flex). The idea here would to essentially use the smaller skeg and strut to reduce surface drag similar to having a large cutaway area, but it would seam to me to be easier to fabricate.
    Would there be any noticable performance / efficiency gains due to the reduced drag (having roughly 25 kt top speed).
    Or, are there major flaws in this approach or lack of gains over a full keel and I should just stick to the full keel?

    re: hull thickess above the prop - I have already increased the hull thickness 50% in the entire hull (I know, not normally recommended, but I know a couple of other builders of same design who did similar or greater increases and were ok with performance). Plans called for 1/8" hull with 1/4" inserts at prop strut. I went with 3/16" hull and was thinking that if I used extra stringers it would be sufficient. Do you think I should still insert 1/4" or greater sections in this area?
     

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  12. Ad Hoc
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    Ad Hoc Naval Architect

    Since the skeg would be at least the same, if nor larger than the outboard version, you should not have any directional stability issues. If you deviated too much from the original plans, that would be different. But in this case you are not considering anything detrimental, since the inboard skeg size is just to protect the prop anyway.

    Make sure your FBs are very thick and stiff. You need to make sure the seatings/foundation for them are very high quality and have sufficient stiffness behind them inside the hull. I would also shape the FBs to be stream lined as much as possible, not leave them square.

    Try it without. Since if your FBs are stiff enough, well 'root' to the hull and do not create turbulence by being out of line of the natural water flow, the prop should be sufficiently supported.

    A Tee section btw, would not assist much more than a FB. Since the principal mode of strength/vibration is athwartships. Its out of plane of the web of the tee.

    Golden rule of thumb, 50% greater plate thickness than surrounding plate.
     
  13. Northeaster
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    Northeaster Senior Member

    My boatbuilding will now slow drastically for about 6 month, as I primarily work on it in the winter months.
    Spring has finally arrived to Eastern Canada, and I have to get the cottage ready, docks in, sailboat in water, mast on , and then enjoy the summer.

    I appreciate all of the advice this far and hope to have as much interest and help next winter, with the goal of having it in the water next summer (perhaps not completely finished, but able to test out and enjoy a bit - with the ability to put it back in the shop in following winters and add / improve as needed.

    Thanks again!
     
  14. FAST FRED
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    FAST FRED Senior Member

    "with the goal of having it in the water next summer (perhaps not completely finished,) "

    No boat is "finished" till it sinks in deep water or is burned for the metal.
     

  15. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    hi Folks,

    Back at the (amateur) aluminum boatbuilding - likely should have asked for advice earlier, but here it is...

    I mocked up a full keel vs smaller skeg, both with a bar extending aft to lower strut leg, to protect prop and then extend further aft to become lower rudder support. I really did not like the look of the smaller skeg, with a then steeper extension to the lower strut leg.
    So, I decided on a full keel, and have "modelled" a cutout / cutaway in the keel, similar to what I have seen in images from the Shamrock keel drive boats.

    I plan on tapering the trailing edges of keel, strut, etc to as fine a point as practical. But, I wan't sure if that was required on the flat bottom of the keel.
    Right now, the keel is about 5" wide at the widest section, and I had planned on just cutting and welding in a piece of flat sheet.

    I did use a few clamps and easily pulled the open bottom keel sides in an inch or more, before the clamps started sliding off. To pull the sides closer, I could tack on a piece of angle, on each side near the edge, to prevent the clamps from slipping. But, I think that If I try to pull it much closer, the sides may start to bend/ buckle, as I would think they would be trying to move in two planes at once.

    Perhaps If I can get the opening to close to about 2 -3 " at the widest, without buckling the sides too bad, I could then weld it up and use either epoxy and filler to fair a knife's edge on top...or more likely weld a sharpened piece of heavy flatbar or a piece of angle on with the mid point making the "point" of the keel. Then fair where necessary.

    But, this would then add a couple of inches of depth to the keel, to go from what's there now to a knife's edge... So, what's worse, not having a knifes edge or adding a couple more square feet of wetted surface area to get to a knife's edge?????

    Looking for advice on best way to finish what I have started....relating to the keel bottom...i.e. flat, rounded, ground to a point....
     

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