keel fairing?

Discussion in 'Boat Design' started by kapnD, Nov 8, 2013.

  1. kapnD
    Joined: Jan 2003
    Posts: 546
    Likes: 50, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    I have my 50' Willard (displacement hull) hauled out, and am looking at the deadwood at the end of the keel. It is about 8" wide and is rounded, though not exactly what I would call "fair".
    The keel ends several feet in front of the propeller, so this is probably not as direct an issue as would be it it were right against the prop, but I want to make sure the hull is as slick as possible to squeeze the most speed out of the least rpm's.
    Would there be any measurable gains in adding taper at the back of the keel?
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It's tough to tell without a picture or two, but generally yes, you'll have some net gain if you fair down the aft end of the deadwood. How much gain - well time will tell, once you splash her, but don't expect all that much.

    On your boat, you could do a nice job of "sweetening up" that skeg. I'd put a 2:1 or 3:1 parabolic on the leading edge (full length) then taped the crap out of it. If my understanding of that skeg is correct, it's not as structural as you might think, it's just a skeg for directional course keeping. This would offer a nice clean bit of flow to the prop, easing it's burden. For this boat 2:1 will be fine.

    If this is the 'glass version (MK-4), the laminate in this area is really, (really, really) thick, so you have a good bit of room to do some damage. If not interested in itching to death, a foam nose could be added, with a light layer of fabric to fair it in. As for the back, I'd cut a few "pie wedges" in the laminate and force it into the new, tapered shape. Or you could do it a different way if you like. Make the taper as long as practical.
     

    Attached Files:

  3. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    What price will you pay for a .0001 gain?
     
  4. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 139, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    An 8" thick bluff keel trailing edge? That is a huge drag creator, and it would put a lot of very tubulent water into the prop, making it much less effective as well.

    Taper it down at about 6 to 1, with sharp corners on the TE, just as narrow as you can (like 1/8", the smaller the better, but keep the edges square). it will be noticeable. Anything else you can do to clean up the area around the prop and the rudders should help as well.

    Post some pictures so we can pick it apart and give you some ideas.
     
  5. viking north
    Joined: Dec 2010
    Posts: 1,865
    Likes: 88, Points: 48, Legacy Rep: 1146
    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    If as you say the keel ends several feet in front of the prop you do have room to do some fairing on that 8 in. keel end width, streamlining the flow to the prop and reducing some drag. I have included a couple of photos showing that very idea as done direct from the factory on my keel. Magnify the second photo you can see the tapering (narrowing ) as the keel end extends aft to the cutlass bearing and prop. In your case you could add tapering foam sections to form the shape and simply glass it over as it is not structural. As previously mentioned without photos it is impossible to see your set up but I caution if you have a cutlass bearing do not block the water lubrication intake feeds. I post this only as a reference that one of the designers could use as an aide to guide you along. I think there is also some rule of thumb on the spacing between prop and keel end. As shown here the cutlass end is 6 in. long plus I would allow another 3 in. to install a set of Spur rope cutters between the keel end and the prop. In my case on this conversion build this whole cutlass assembly will be eliminated for one that will be housed inside the tube. Luck would have it i still had photos of the original setup that will give you an idea of a factory layout. Hope it is of some help.

    A yacht is not defined by the vessel but by the care and love of her owner ---
     

    Attached Files:

    Last edited: Nov 8, 2013
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    [​IMG]

    As you can see this is a bit different then most keel/prop interfaces, with lots of room for flow to "reassemble", though some gains can be picked up with a good fairing.

    If it was me, I'd consider removing most of the skeg, with just enough of a well shaped appendage to offer grounding protection. Of course, this is wholesale surgery on this 'glass hull, but doable.
     

  7. kapnD
    Joined: Jan 2003
    Posts: 546
    Likes: 50, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    Thanks for the replies, I will snap some photos next time I'm at the boat.
    As you can tell from Par's attachment (thank you very much) there is plenty of room to add some fairing without having to remove anything.
    I think I will glue on some foam, shape it out and glass over.
    As for cutting the keel off, I would hesitate to do so as I operate the boat in open ocean, frequently rough and windy conditions, and appreciate the directional stability the keel imparts under way, and the rocking it damps while stopped.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.