Trim tab keel, why not?

Discussion in 'Sailboats' started by wetass, Aug 27, 2008.

  1. wetass
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    wetass Junior Member

    Hello all,

    Have browsed the posts here about daggerboards and keels a couple of times. Have browsed the net for articles on the topic and probably read them all. I think that I understand the theory around trim tab foils vs conventional foils. These are supposed to be superior due to their smaller size (wetted surface), adjustable lift and angle of attack. The drawback being increased minimum drag and a more complicated design.

    My interest is not academic. I am building a small (7 metres, 600 kg) sportsboat. I am at the crossroads of choosing a keelfoil. I am not worried about the mechanics of a lifting trim tab keel. I am however confused about the lack of proof of the benefits of trim tabs on modern light displacement yachts.

    The Metre-boats, IACC-boats and similar lead-mines benefit from trim-tabs. Is this due to the steady motion? Low aspect ratio keels (not valid for IACC though)?

    My choise is either going for a naca 63010 foil with 400 mm chord (no tab) or say the esotec PCK310 foil with 250 mm chord, 25% flap, deflection < 10 degrees. The keel blade is 1300 mm and ends with a bulb.

    Does anyone have any insight?
    Anyone less confused?

    Thank You for Your interest,

  2. tspeer
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    tspeer Senior Member

    I'm not sure the PCK310 has any advantages over the NACA 63010.

    I've uploaded the XFOIL-computed polars for the PCK310 with its 25% flap deflected 10 degrees, the NACA 63010a scaled 160% to represent the 400 mm chord vs the 250 mm chord, and the NACA 63010a with a 25% flap deflected 10 degrees.

    The predicted maximum lift is the same for both of the tabbed sections. The NACA section has lower drag inside its drag bucket. Neither of the tabbed sections match the larger chord's maximum lift at the low Reynolds number I've selected. The minimum drag of all three is quite similar.

    I'm not sure what sort of maximum lift coefficient you want, or what thickness you need for strength and stiffness. The 400 mm chord @ 10% thickness has 60% more physical thickness than the 250 mm chord. I would compare different chords and sections on the basis of equal physical thickness, requiring the smaller chords to have a higher thickness ratio - in this case, 16% for the smaller chord.

    A better choice may be something in between, say, 333 mm chord, 12% thickness, with a tab.

    Attached Files:

  3. wetass
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    wetass Junior Member

    Thank You for Your interest, again.

    To be perfectly honest, I am out on thin ice here.

    The PCK310 is something I found at the esotec - site. It was used there as an example of a contemporary, trim tab keel foil. This is why I thought that it would be particularly good, but apparently I am out in the dark. The PCK310 does, however have one advantage over the n63012 as it is thick enough in the aft sections for a proper "rudder post". This could be solved with a bigger tab-portion on the naca (29% tab on the n63012, 25% tab on the PCK310 for same chord).

    Is the naca 63-series a good way to go for a trim tab foil? Any airfoils specially designed to have a smooth suction side when the tab is deflected? Isn´t this one of the critical design issues? Since airplanes have tabs, are the naca-foils already designed with tabs in mind? Thick aft sections (at tab joint) is a plus...

    The 63010 foil with 400 mm chord creates enough lift for the boat. I can make the foil strong enough with a 35 mm thickness.

    Going back to the original problem - why isn´t anyone using trim tabs on light-displacement boats?
  4. wet feet
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    wet feet Senior Member

    Some suggestions about the absence of trim tabs:Could be a source of leaks and would require the manufacture of another foil and the hardware to locate and control it.In addition,to get maximum benefit,the trim tab will need to be operated by somebody.Unless the boat is to be dry sailed there is a possibility that the accuracy of the slot gap could be compromised by growth or foreign bodies.It would seem to be a simpler and less expensive solution to use a slightly larger foil in a less critical operating condition with less hardware and fewer demands on the crew.
  5. Doug Lord

    Doug Lord Guest

    trim tab/light displacement RC 44

    Did a short "google" on"trim tabs on light displacement boats" and came up with this:
    "The trim tab on the keel was an example of that thinking, to reduce keel area and yet still achieve good upwind performance and maneuverability. It adds to the complexity of the boat, but the racing enthusiast will enjoy exploring the different tab angles and the resulting benefits in performance.”
    another boatdesign thread on trim tabs on the keel:
    here's a mini design using a keel trim tab:
  6. wetass
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    wetass Junior Member

    Thank You for Your interest,

    Yes, the boat gets more complex and there is one more lever to turn every tack, but I would imagine, that since a lot is done on some racers to find the last speed potential, trim tabs would be exploited regardless of the small hassle involved.

    There are some intresting boats in the field, the RC44, one minitransat form 1999, some Ker-designs, and so on. There is also a lot on the drawing board, as the mini Doug found.
    There aren´t too many success stories though. The mini in ´99 did okay against canters aso, the RC44 is fast but given the parameters it should be, the ker 50? "Magic glove" is doing ok, the Ker 37 "Jump Juice" was fast with her keel but has now been castrated. (Quote on the Ker37 "Jump Juice" - "During the winter of 2006, we changed keels from an experimental trim tab keel to a normal fin keel and the results were exceptional..." This might of course have to do with the severe rating punishment of the tab.)

    I want a tab but not convinced yet.
  7. Cheesy
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    Cheesy Senior Member

    Sounds like a cool project, are you designing from scratch?
  8. bntii
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    bntii Senior Member

    One of the glass sloops I thought of buying years back had a keel trim tab. The gear was rigged with the linkage leading to a smaller wheel mounted in front of the main wheel at the helm. I don't remember the build but she was a late 60's or so racer of 37' on deck. Might be there is some info about on the net about how this all went.

    But these may be different horses eh?
  9. Steam Flyer
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    Steam Flyer Junior Member

    I dunno about foil sections, but...

    If you're talking about a sportboat with a higher level of performance than a conventional "fast" sailboat, then you may want to look at structural issues.

    The loading on the keel will be really intense at high planing speeds. Stronger definitely= better, and this is not a time to distract the crew on a small boat by fooling around with trim tabs.

    The range of Reynolds numbers for a sportboat keel will also be higher than a conventional keel, and one of the desirable characteristics is to get a thin foil that is strong & rigid enough, that does not start blowing off big turbulence at 20+ knots. This would make fitting a trim tab even more of a challenge.

    Years ago (decades in fact) I fooled around with a trim tab on a Force 5 daggerboard. This is a small one-design singlehander very similar to a Laser. The other people I was racing against knew about and were interested in the experiment... they also enjoyed passing me while I was distracted by trying to get the adjustment 'just right' on the trim tab, which I only used in light-to-moderate conditions.

    After about a year of sailing with it, and experimenting with ways to get a very fine & repeatable control system, I found that in about 4~8 knots of breeze close-hauled, I could gain about a boat-length to windward in 200~300 yards of straight-line sailing. Not a big gain for the complexity & distraction. I had visions of climbing over everybody right off the starting line, and that wasn't happening.

    FB- Doug
  10. wetass
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    wetass Junior Member

    Steam Flyer: The leadmines (Metre-boats) are truly climbing over their non-tab competitors right off the starting line.
    I agree with You that it is a challenge to get the tab "in tune". It adds to the complexity of the boat and is of course a compromise between ease of use and performance. Rigidity of the foil shouldn´t be a problem...
    I am happy to hear of Your experiments. So, in light winds, You were faster upwind than the competition (ok, just a little). You probably tested the tab in heavy weather? How bad was it? Problems when planning downwind?
    Failure in Your experiments strengthens my doubts about the possible benefits of a tabbed keel on a light boat. I still want a tab...

    Cheesy: No and yes, scratch-built by myself and some friends who come mostly for the beers, designed with the help of people much wiser than me.


  11. Steam Flyer
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    Steam Flyer Junior Member

    I want one too, tabs are COOL

    A trim tab would be really cool, I'd love to have one myself.... but at this point I don't believe it's worth the trouble on a light fast boat. The reason I did that experiment with the Force 5 was because I was young and overloaded with theory ;) ...and also because I had a spare daggerboard and experimenting with foil sections wasn't dramatic enough....

    The problem with testing the Force 5 trim tab in heavy weather was that I consistently underestimated the force on the tab lock. It kept stretching or breaking outright, and letting the tab flop at random which made the boat difficult to sail. Eventually I quit using the trim-tabbed DB any time the wind was over about 12k.

    In retrospect, I think the performance difference in hiking, constant sheet adjustment, and helming, would make a much bigger difference than the trim tab ever would. In a boat with enough crew to have one man dedicated to working the tab, this wouldn't be a problem.

    But it does seem to me that a trim tab would benefit a slow "tactical" boat more than a zippy sportboat.

    Did you think about a mechanism to duplicate the action of a gybing board... being able to put the keel foil at a positive angle of attack... maybe even one that was adjustable. That would be easier than a trim tab and probably carry more benefit for a go-fast boat.

    FB- Doug
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