How do you get a beaver tail on the back of a NACA foil section?

Discussion in 'Sailboats' started by james3232, May 7, 2018.

  1. CT249
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    CT249 Senior Member

    One would think the efficiency of the damping effect would also be strongly related to the hull shape. In an IACC boat, which has very little flare, a very narrow and fairly heavy hull and a huge keel, damping could be an advantage since the immersed shape and area of the hull would not change very much. On the other hand, when I sailed a standard dinghy fitted with hydrofoils on centreboard and rudder, it appeared to be quite dramatically slower upwind through waves than the standard boat. One could feel that the centreboard foil appeared to be forcing the comparatively fat, flared hull to effectively be dragged through the wave crests, resulting in very wide effective entry angles and a major increase in wetted surface area.

    Years ago I sailed an Elliott 7.4, which had a squashed bulb that was sort of like a giant beavertail over its entire length. It felt distinctly off the pace in light and choppy weather; again it seemed that the "foil" was preventing the boat from rising to the waves. One imagines that given the normal shape of most boats, any reduction in drag caused by the boat being held higher in the troughs was significantly smaller than the increase in the troughs, which is perhaps related to the way we try to keep weight out of the ends to reduce motion.

    Having a flatter section lifting up and down through wave action could itself be a cause of higher drag in waves, perhaps. This could be a very delicate series of trade-offs.
     
  2. rwatson
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    rwatson Senior Member

    I don't think they do much for "lift" either, ( that is distance made good by hydrodynamic dynamic lift) , but steerage and particularly damping should be noticeable.

    The fact that flow would occur at points offset at the stern end of the bulb from the direction of travel makes sense, but with a beavertail, the leeward edge would be below the main flow, not in a position to "split" it, due to heeling. I would expect the main flow at the stern to occur over the top of the canted rear beavertail.

    Note also their discussion on the benefits of reducing rudder angle when heeled on a beat.
    "For typical sailing conditions, the numerical computation predicts the induced drag to be reduced by 8% when the rudder side force is reduced from 20% of the total to zero and the keel side force increased accordingly. Figure 16 shows the time gained vs. wind speed when sailing a 34.4 km windward leeward course due to an 8% reduction in induced drag as determined by a VPP. All of these gains, except for a fraction of a second, occur on the 17.2 km windward portion of the course. These time gains correspond to distance gains of about 7 boat lengths in light wind and 4 boat lengths in strong wind. By top level racing standards, it is substantial."

    By reducing the amount of rudder required to hold the boat in the right orientation on a beat, with a bulb appendage, the windward performance gets a boost. At say a 25 degree heel, a beaver tail would provide some resistance to the stern being forced to leeward.
     
  3. rwatson
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    rwatson Senior Member

    I don't think they do much for "lift" either, ( that is distance made good by hydrodynamic dynamic lift) , but steerage and particularly damping would be noticeable.

    The fact that flow would occur at points offset at the stern end of the bulb from the direction of travel makes sense, but with a beavertail, the leeward edge would be below the main flow, not in a position to "split" it, due to heeling. I would expect the main flw at the stern to occur over the top of the canted rear beavertail.
     
  4. jehardiman
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    jehardiman Senior Member

    And it is not just the alpha flow on the body of the bulb, but that also includes the foils TE vortex and bulb/foil horseshoe vortex also. While the ventral flat of the beaver tail may have some effect being in relatively clean water, the dorsal side flow is all caught up in the wake.

    And if everyone will humor me, my personal crusade is to make sure that when doing analysis everyone understands that there is no "inflow" to a keel/rudder/hull/etc. The water is only moving with the current/wave orbitals (if any), and the wake leaves the water deflected in the direction of travel. that is the energy imparted into a wake does not slow the water down, but speeds it up (including adding vorticity). I know Tom know this, but it is just something that twists my nickers having seen too many people mess it up.
     
  5. jehardiman
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    jehardiman Senior Member

    Not only the flatter tail, but the foil/bulb intersection plays a large part also. While a very thin foil to "wall" intersection is preferred for truly zero AoA, as Tom points out that is rarely the case. FWIW, I would question the foil/bulb intersections show above.
     
  6. Dolfiman
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    Dolfiman Senior Member

    As a slight digression to this interesting thread, I have suggested in my Dynamic ballast study that such orientation of a bulb in the leeway direction with heel is possible automatically with a bulb hinged to the tip end of the keel with an ad hoc slight inclination. For example, with a joint inclination of 11°, a 10° heel angle causes a bulb orientation of 1,90° in the leeway direction, a 20 ° heel angle causes 3.74° , a 30° heel angle causes 5,48° (pages 6 and 28 of the attached document to :
    Dynamic ballast https://www.boatdesign.net/threads/dynamic-ballast.59729/ )
    Of course, that needs the complication of an articulation, but on the other hand the eventual advantage to choose the weight of the ballast bulb to the average conditions of the boat for its usual area or use (light winds, strong winds, light crew, heavy crew)
    I wonder if anyone has already tested this solution and/or if there is a serious drawback for that approach.
     
  7. revintage
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    revintage Junior Member

    James, seems you didn´t get your specific question answered . This is in a paper from Chalmers University of Technology:

    "NACA 65017 is used for the side profile. This is a NACA section of series 6, with pressure minimum at 50% chord length and with a section thickness 17% of the chord length. This profile was then scaled to a larger size and cut off to create a beaver-tail on the bulb. Beaver-tails are often seen in existing designs, and the main purpose of these is to make the flow follow the shape in draft direction and possibly to push the flow down. The pressure minimum for this scaled profile occurs where the trailing edge of the fin is connected to the bulb."

    http://publications.lib.chalmers.se/records/fulltext/148387.pdf

    I used these guide lines using Freecad for a very small 50kg bulb with a 0,85m top/bottom 65012 profile together with same 65012 profile scaled to 1,0m for the sides. Image is taken from the CNC-router program.
    [​IMG]
     
    Last edited: May 12, 2018
    Doug Lord likes this.
  8. DCockey
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    DCockey Senior Member

    The pressure distribution around the 3D, extremely low aspect ratio bulb, will differ significantly from the 2D airfoil pressure distribution.
     
  9. CT249
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    CT249 Senior Member

    I'm not sure if it's what you mean, but the keel on Greg Elliott's 5.9 metre sportsboat had a fairly narrow steel girder at the front of the section, and an L bulb. The ide was to allow the girder, and therefore the keel, to twist under the weight of the aft part of the bulb when the boat was heeled, therefore giving the keel a higher angle of attack than the hull. The idea didn't seem to go anywhere.
     
  10. Dolfiman
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    Dolfiman Senior Member

    As I understand this should lead to a larger mean angle of incidence of the fin keel so to have a larger side anti-drift force. My suggestion is very different : the fin keel orientation is unchanged, only the bulb hinged to its tip end, which orients automatically in the leeway direction (= inflow) when the hinges axis is not choosen horizontal but with a slight rising inclination (say 11°) : when the boat heeled (say 20°), the bulb is orientation is then 3,74° (/ boat axis in the horizontal plane) similar to a typical leeway angle, so the bulb is the inflow and drag is then supposes minimal. I hope that the image in page 6 and the computation of this effect in page 28 (a combination of sinus) of the thread Dynamic ballast document can give you all clarification.
    The remaining question is the drag of the hinged axis itself, to maintain low so to have a global reward from the concept.
     
  11. jehardiman
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    jehardiman Senior Member

    I think the most important takeaway for the cited paper is that all bulb keels investigated have more drag and less lift than the fin keel for the same span. And the fin keel is not that well shaped either, being low aspect and with too much positive sweep and insufficient taper. Unless there is a) an overwhelming need to restrict the draft, b) you have a beam restriction that prevents adding form stability while maintaining a moderate aspect ratio keel, or c) you intend to start playing with winglets, there is no need for a bulb. A bulb is only used to address a problem with the overall design requirements, never something you should willingly choose.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ======================
    Really? It is interesting that almost every single racing monohull keelboat has a bulb + fin keel. In other words, designers of racing mono's consider a bulb keel to be fast including canting keels. But maybe you were referring to a limited type of bulb keel?

    6 Conclusions
    The Aim of this Master’s Thesis was to identify which of the chosen basic keel designs that has the best performance upwind, both evaluated alone in a wind tunnel domain using CFD and wind tunnel tests and together with a hull using VPP. The results as analyzed throughout chapter 3 to 5 has consistently shown that for both these cases the T-bulbed keel performs best of the bulbed keels, and also better than the fin keel in higher wind speeds. Another aim was to determine whether a proper design of one of the basic design types could change the outcome of the comparison. When the T-bulbed keel was completely redesigned after its poor performance from the Bachelor’s Thesis [2], and this new design proved to be the best performing one this aim was definitely answered. Finally the results from the wind tunnel were found to be of a high order of accuracy, and as stated in the Aim and Objectives will be open for further validation of the keels using other CFD software and theories.
     
    Last edited: May 15, 2018

  13. jehardiman
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    jehardiman Senior Member

    That assumption is critically flawed based upon the authors own assumptions and text.
    See Para 1.3
    Yet in the analysis the authors say in para 3.3.2
    So really, by using the effective draft and based upon that a heeled Cd & Cl which they did not calculate, they " manipulated a bit " the VPP to show a less than 1% advantage to keel 4 in Table 8, when in Fig 2 they stated that the induced drag was 8% of the total drag and heel was 5%. Furthermore in Section 5 Discussion, the authors acknowledge this issue in a backhanded way:
    Really, with no bias, I can say after doing this type of analysis for 30 years that if even half the advantages of having an end-plated foil could be found, all the military submarines and warships would have them. They don't
     
    Last edited: May 15, 2018
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