How bad is a rounded trailing edge on the keel?

Discussion in 'Hydrodynamics and Aerodynamics' started by PDQAltair, Nov 14, 2015.

  1. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    Dingies and race boats go as thin as practical, squared off or mitered to one side. Got that. Basic theory.

    Wandering around the yacht yard I can't help noticing how many boats have really fat, rounded trailing edges on the keel, often 1-2 inches thick. I'm sure they have reasons:
    * Easier to mold.
    * Less chance of cutting the rode with a keel wrap (other ways to avoid that).
    * Reduced/zero chance of damage. But since 1/4" rudder trailing edges are seldom chipped, I think the risk is trivial.

    How much is this costing them in leeway and speed? Are there any hard numbers out there?

    Sharpening up the keel seems like a simple job, spread over 3-4 days. Other minor fairing could be tackled at the same time.
    * Sand down to bare glass and grind a bevel for bonding.
    * Tack in a FRP plate of the correct dimensions, ground down at the edge to the correct thickness (~1/4") and taper. This might be 1/2 FRP, the edge bevel about 4-6 inches long, and the plate would typically have a cord of 6-12 inches, depending of the trailing edge radius and foil section.
    * Tab it in with cloth and biax. Strong as the rest of the foil.
    * Fair as needed. If the edge bevel is right, since there is very little curve in this area, spreading the first coat of filler with a straight edge will be very close.
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,270
    Likes: 584, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Are any of the boats you saw racing competitively?
     
  3. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    Not a chance.
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There are those like you, that know better and keep trailing edges crisp and in good shape. Of course, then there's the ones you saw around the yard.

    I'll assume by FRP plate you mean G-10 or similar. This will work, though you could also simply use fabric too, grinding to shape after it's cured.
     
  5. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    Yes, pre-laminated plate like G-10. It seemed like a simple way to go about this without building forms. The intention is simply to complete the true profile of the foil, since I am assuming that even lower performance boats base the foils upon NACA shapes or something very similar.

    Are there any published figures supporting the common knowledge that thin, crisp edges are better? All of the data I could find assumes taper to a fine edge; quantitative discussion of rounded or severely truncated foils has eluded me.

    How much will a rounded and truncated edge actually affect performance?
     
  6. gonzo
    Joined: Aug 2002
    Posts: 14,270
    Likes: 584, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Since they are not competing, not much. The gains in speed for boats that are sailed leisurely will most likely be negligible.
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There is data and a few books contain some of it, but most production fins, appendages and blades tend to not be as symmetrical as you'd think and this is the place big gains can be made.

    If you have a perfectly rounded over 1/4" wide trailing edge foil, appendage, etc. and compare it's drag to a crisp razor edge, the numbers will suggest a fairly good improvement with the razor, but boat speed differences will be measured in small fractions of a knot. On the other hand if you carefully make the foil, appendage, blade symmetrical, to the section intended and race it against the same production boat with its asymmetric sections, you'll note pointing, handling and speed differences on the course.

    The problem with thin, crisp edges is maintance. It's real easy to ding up these edges, not to mention pop them out of a mold, so more rounded shapes are employed for practical reasons.
     
  8. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    Well, I had made this change a few weeks ago, but had not had a chance for a good workout until today. I measure tacking angles, wind, speed, and VMG, and the results for my boat were staggering. I added about 10" of trailing edge and about 7% area. Almost as importantly, there was certainly some shift in the CLR aft, which since the balance on this boat was poor, reducing rudder angles at the same time.

    I still have more calculating to do, but VMG increased ~ 7% at 10 kt true and the wake is noticeably straighter. This seems like a lot, but both before and after numbers were based on many readings under a variety of conditions. Tacking was more solid and reliable, with less tendency to round up after if the genoa did not come in fast.

    Would the result be the same for a better-balanced boat? I doubt it. In fact more than half of the improvement is probably attributable to improved balance, which I anticipated. The keels on my boat were placed too far forward, good for drying out on a beach, but wrong for sailing. I've been meaning to make this change for years, but haul outs always seemed too rushed.
     
  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    No a better balanced boat wouldn't have as significant an improvement. Moving the CLP aft, likely reduced the rudder incidence angle, reducing drag, plus eased helm pressure and weather helm, from the rig's need for more lateral plane aft. Next time out, figure out what the rudder deflection angle is, sailing close hauled in 10 knots of wind. If it's 4 or 5 degrees and pressure is acceptable, you're where you need to be.
     
  10. Joakim
    Joined: Apr 2004
    Posts: 892
    Likes: 52, Points: 28, Legacy Rep: 422
    Location: Finland

    Joakim Senior Member

    The rounded or thick trailing edge is very bad, but not something a typical cruising sailor would care about.

    This old NASA paper deals with the issue:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930082748.pdf

    NACA 0012 profile is measured with normal 0.25% and three thick (0.68%, 1.4% and 4%) trailing edges. The percentage is related to the chord thus for 1 m chord 2.5-40 mm.

    The profile drag increses close zero lift (reach and run) 10%, 50% and 200% for those three thicknesses and on a beat (Cl=0.4) 10%, 30% and 170%.

    The keel profile drag is about 10% of the total drag of a modern boat at typical speeds, thus the total drag increase is about 1-20% for the three thicknesses. 10-15% more drag will slow the boat about 5%.

    Here is my old presentation about the subject. Unfortunatelly it is in Finnish, but maybe you get something out of it: http://www.avomeripurjehtijat.fi/index.php?option=com_docman&task=doc_download&gid=395&Itemid=250
     
  11. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    ^^

    Thanks. That is the data I was looking for.
     

  12. PDQAltair
    Joined: Apr 2010
    Posts: 12
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Deale, MD

    PDQAltair Junior Member

    ^^ I read through the report, applied the difference in drag, angle of attack, lateral plane area, and change in rudder angle. It turns out that in the case of a boat with too little lateral plane, poor balance and small rudders, the sum can be as much as 5% VMG. As in many things, the worse you are to start with, the more there is to gain!

    When I made the same calculations based upon a moderate design without the above listed failings, the difference is as you suggested, very small.

    Thanks for the link.
     
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.