Keel Section

Discussion in 'Hydrodynamics and Aerodynamics' started by motorbike, Feb 21, 2015.

  1. DCockey
    Joined: Oct 2009
    Posts: 4,622
    Likes: 281, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    How accurately will the new keel follow the designed section, particularly near the leading edge?

    Is "total drag" in the statement above the total drag of the hull and keel? If so how was the estimate of the hull drag including wave making obtained?
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Again, if this is a shoal, built down section, old CCA style of yacht, you can play with the numbers all you want, with zero noticeable gain on the water. This assumes the current appendages are clean, symmetrical, smooth and fair. Simply put, the hull form and appendage configuration have to be able to take advantage of these minute gains. Which hull (make model and year) are we talking about, as generalities concerning any potential, is mostly dependent on what design (especially if shoal) you're working with? ORC successes are typically two fold; a favorable rate and crew skill/decision making.
     
  3. Karsten
    Joined: Jun 2004
    Posts: 184
    Likes: 8, Points: 0, Legacy Rep: 33
    Location: Sydney

    Karsten Senior Member

    I would be careful using very thick keel sections. The viscous drag might be not much different to a thin section but it is possible that the wave drag with a thick keel is larger. I'm not sure how much research has been done in this area. I found this thesis which shows in Section 9.2.2 that including the appendages into the model for the hull does make differences to the pressure distribution on the hull. Might be an interesting topic for a thesis to look at the difference in wave drag depending on foil thickness.

    http://dspace.mit.edu/handle/1721.1/50364
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I've done some rudimentary testing with thicker sections, looking at pressure wave reactions on hulls and Karsten has a valid concern about the thicker section. If this is the type of boat I'm thinking it is, the section is probably fairly fat anyway.
     
  5. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    Who uses 4-digit sections for keels? I don't know the cruiser market that well, but I haven't seen a 4-digit section on any C/R or racing boat from 80's or later. 63-series is likely the most common one and lately there are more and more 64-66-series or other sections similar to them. Quite a few rudders still use rather thick 4-digit sections due to better performance at higher AoA.

    Some C/R boats have gone to very big surface are almost rectangular keels, which seem to work very well with a suitable section. E.g. racing keel for First 35.
    [​IMG]
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Without knowing what design you're attempting to modify, all this is just playing around and guessing. Since you don't seem to want to divulge the make, model and year, how about this question; does this yacht have the potential to utilize a 64 series appendage?

    This suggests it hasn't this potential, within it's performance envelop.

    Lastly Joakim, you do know who forum member Tspeer is, right?
     
  7. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    Yes I do and I was quite surprised to read that statement from him. I know that the low drag bucket of 6-series may very easily be compromised, but still those and similar are the ones all keel modifications and new designs go for.
     
  8. Canracer
    Joined: Aug 2009
    Posts: 621
    Likes: 9, Points: 18, Legacy Rep: 47
    Location: Florida

    Canracer Senior Member

    That planform is very similar to that on this Newport 30.
    [​IMG]

    [​IMG]
     
    Last edited: Apr 27, 2015
  9. DCockey
    Joined: Oct 2009
    Posts: 4,622
    Likes: 281, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    What is the marketing value of a 6- series section vs a 4 digit section on performance oriented sailboat?
     
  10. Ja guar
    Joined: Jan 2013
    Posts: 22
    Likes: 1, Points: 1, Legacy Rep: 10
    Location: English Bay

    Ja guar Junior Member

    Yes. Recall the Mickey Mouse eared keels in the 80's IOR boats. Part of the reason for them was to get an elliptical load distribution, but you don't need an elliptical planform to accomplish that. The other reason was the reduction in keel volume at the hull/keel joint which in theory reduced interference drag.

    With regards to sections, here is a link to an article written by David Vacanti in which he seems to mainly recommend a 63 or 64 for high AR keels and 4 digits for rudders and low AR keels http://www.sponbergyachtdesign.com/Keel and Rudder Design.pdf

    I've had 3 boat over the years and what they've had in common are that they are all lightweight fractionals circa 1981 with thin 64A series keels.

    My current boat is a Laurie Davidson design. I've taken a fair number of keel measurements and from what I can ascertain has a 64008 at the root transitioning uniformly to a 64012 at the tip. At 8% it is quite thin at the root, supported by a single row of quite large keel bolts (for obvious reasons)

    Having said all this, my observation from going through boatyards is that most fin keeled boats have a 4 digit keel. You can usually tell by feeling the leading edge.
     
  11. DCockey
    Joined: Oct 2009
    Posts: 4,622
    Likes: 281, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Another reason I've heard for a narrow chord at the root of the keel is to reduce the wave generation and drag from the keel, particularly if the boat frequently sails heeled far enough that the root of the keel is near the surface.
     
  12. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    Seems to be zero, since I have never heard any marketing about 6-series or any other section. Elliptical profile was marketed in the 80's, later came bulbs and T-keels. X-Yachts markets now accurate NACA profile, but doesn't mention which one. When I did keel profiling for my boat, I couldn't find any information which profile it had. Not even from its designer.
     
  13. Ja guar
    Joined: Jan 2013
    Posts: 22
    Likes: 1, Points: 1, Legacy Rep: 10
    Location: English Bay

    Ja guar Junior Member

    That is interference drag.

    A thin short chord at the root is one way of minimizing it. Vacanti advocates a lip near the leading edge root. I'm not sure if this has proven out, I suspect the reasoning is that if you spread out the keel area you get a smoother transition at the "keel bump".

    Note I just made up the term "keel bump". Some designers just use the hull in coming up with the sectional area curve, while others advocate superimposing the keel area over the hull area. If you do the latter, there will be a "bump" in the area curve beginning at the leading edge and ending at the trailing edge.

    Similarly, I suspect the worst case scenario for interference dag is if the keel footprint coincides with the point of maximum hull sectional area (i.e. the shoulder of the curve). In such a case, another way of reducing interference drag may be to shift the keel forward or even aft to smooth out the combined curve and more importantly, reduce the size of the combined shoulder. Keep in mind this is just speculation on my part and I may be completely out to lunch here.
     
  14. myszek
    Joined: Jan 2013
    Posts: 56
    Likes: 6, Points: 8, Legacy Rep: 22
    Location: Lodz, Poland

    myszek Junior Member

    Using the technology the owner is going to apply - a steel frame, lead core, GRP skin, putty, and paint - the inaccuracy could be no more than 0.5mm. In compare to 800mm chord it's 0.06%.
    I tested, how the inaccuracy about 0.1-0.33% affects the foil performance. In the worst case it reduces the drag bucket from 4 to 3deg.

    Of course, when in use, the shape will become worse and worse. And the surface roughness will grow, which worries me more.

    The "total drag" is taken without the wave making drag - only the foil drag, the total induced drag, and the hull skin friction. This was easy to estimate with XFLR5, taking the hull as a root section of the foil.

    In order to evaluate the wave making drag, I need the exact hull offsets. Haven't got them yet. They are necessary, anyway - some small changes in the hull may be needed to reduce the interference with the new keel.

    The present version of my foils look like this

    regards

    krzys
     

    Attached Files:


  15. DCockey
    Joined: Oct 2009
    Posts: 4,622
    Likes: 281, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    0.5mm accuracy to the designed section for a 600mm chord keel is very impressive. How will the keel be shaped - precision CNC machining of a plug to be used when molding the GRP skin? Has that level of accuracy been verified with previous keel builds?
    How do you calculate wave making drag if you have the offsets - CFD or a more empirical approach such as used by the ORC rule or DelftShip?
     
Loading...
Similar Threads
  1. Chaos
    Replies:
    3
    Views:
    117
  2. Howlandwoodworks
    Replies:
    17
    Views:
    2,348
  3. RumnCoke
    Replies:
    21
    Views:
    2,432
  4. PDQAltair
    Replies:
    11
    Views:
    3,306
  5. dale.frahm
    Replies:
    6
    Views:
    3,200
  6. Remmlinger
    Replies:
    9
    Views:
    2,446
  7. Mick@itc
    Replies:
    0
    Views:
    1,404
  8. Dabrownone
    Replies:
    42
    Views:
    13,154
  9. valefrisbee
    Replies:
    10
    Views:
    2,826
  10. Joakim
    Replies:
    23
    Views:
    10,270
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.