Deck sweeping sails and effective aspect ratio

Discussion in 'Hydrodynamics and Aerodynamics' started by Will Fraser, Mar 31, 2015.

  1. Will Fraser
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    Will Fraser Senior Member

    There is no mystery in the fact that closing the gap between the deck and the foot of the sail has aerodynamic benefits. For the record, I am not just referring to headsails, but any sail in general.

    What baffles me is the fact that the concept receives so little attention from designers - especially those that get to start with a clean sheet of paper. Land- and ice yachts in pursuit of speed records seem to appreciate the benefits, as do some of the latest AC72 multis.

    But the benefit is not just available at the upper end of the performance spectrum. In lieu of additional speed, the concept can also be used to point significantly higher, reduce rig hight, sail area and righting moment - or various combinations of the aforesaid depending on the application.

    Sure, considerations such as forward visibility, available deck space for crew movement etc. makes it an impractical proposition for a lot of boating applications. But surely not all. For these remaining candidate applications I would imagine it becomes a trade-off between the aerodynamic gains and these practical issues, or at least the possible means of finding alternative solutions to the issues. An RC sailboat, for instance, is hardly affected by any of these issues and could quite possibly be adapted to make use of the concept.

    About a year ago I set out to quantify these aerodynamic benefits, for interest sake if nothing else. I developed a crude velocity predicting program based on Tom Speer's Vortex 95 sail analysis spreadsheet and I have now also started cfd analysis to compare the results with. The cfd streamlines revealed very interesting phenomena that reduce some of the theoretical gains, but I will discuss that later once I have a few nice graphics to help illustrate the flow.

    In the meantime, please contribute with any real world test examples, data or simply just opinions on the subject.

    Here is a link to one of the only general applications I could find on the web:

    http://www.setsail.com/end-plating-main-and-mizzen/

    In the article, Steve Dashew explains how the deck seals he used on his yacht, Beowulf has improved performance both on and off the wind.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Well, this part is not actually true. Check this discussion and the attached example pics: http://www.boatdesign.net/forums/hy...ail-equivalent-winglets-51107.html#post699541

    There is a number of performance and competition yachts which have the genoa sail run all down to the deck, to minimize the gap. It is a nearly impossible task when it comes to the main sail, due to its position relative to the cockpit.

    But the main problems here is - it takes a lots of compromises to close that last tiny amount of gap, when compared to practical benefits one can obtain. So, imo, quite often it is actually not so worth the effort. :)

    Cheers
     
  3. gonzo
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    gonzo Senior Member

    What are the real advantages on a heeling boat? The foot of the sail is below the sheer.
     
  4. Will Fraser
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    Will Fraser Senior Member

    Thanks Diaquiri, you quite rightly emphasise the fact that closing that last little bit of the gap is where the real gain lies. If it is still mostly not worth it, then I am very interested why that would be, given how good it looks on paper.

    There are other ways to reach a compromise between the ideal closed gap and what is practical in real life. There is still some benefit even if only part of the foot chord is sealed properly. The deck sweeping genoa is in fact nothing other than that - the front portion of a sloop "sail unit". I am in fact more interested in finding improvement for traditional rigs with their low aspect ratios and lack of pointing ability.

    A number of cat-rigged dinghy deck layouts could spare a wedge shaped flat area aft of the mast for closing off with a semi-rigid board attached below the boom. The deck itself is also not the only surface against which the sail can be sealed off: flat bimini tops would serve the same purpose.

    Steve Dashew's performance gains (pointing 5-7 degrees higher for a given boat speed, 5% increase in speed when reaching) is testimony to what can be achieved if the concept is considered early in the design stage, even on a boat with a layout that is not ideally suited for it. So when saying that closing the gap is difficult and involves compromises - which is true - it is important to put it in perspective and also ask at the same time: which alternative improvement option could provide the same gains? At what cost and weight?

    The pic below is of a 10ft Spindrift dinghy by B&B Yachts with a modified sail. It showed 42% more drive and 8% less heeling moment (that is for the sail, hull and sailor combined) than the stock sail scenario when pointing 20deg to the apparent wind. Not a realistic point of sail, I know. I was just testing to see what happens when pushing the limits a bit.
    The modified sail also showed 7% more drive dead downwind than the stock sail. The total sail area was kept the same - I have to check the model again to see if I kept the luff length the same or the total mast height...
     

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  5. Will Fraser
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    Will Fraser Senior Member

    The pressure differential across the sail still cannot equalise under the foot of the sail, so there will be no vortex and induced drag normally associated with a gap at the foot.
    The pressure on the sail effectively "spills" onto the hull and topsides and, to a lesser extent, the surface of the water as well. The marked increase in high pressure to windward and low pressure to leeward causes an increase in leeway force on the hull but also adds a bit to the total drive. The cfd simulations I have done to date have all been on level hulls. The low pressure that forms on the leeward topsides is actually problematic, since it causes all kinds of unstable regions of circulation that can curl up and mess up the flow around the lower leeward part of the sail- the hull in fact behaves as an airfoil with a very sharp leading edge in a deeply stalled condition with separation bubbles forming and detaching the whole time.

    When there is a gap under the sail, the flow across the deck seems to stabilise the detached flow which just sits there and circulate right next to the bow. - See pic below.

    I honestly don't know what effect heeling would have on all of this - it would very much also depend on the hull shape and where the sails are placed relative to the bow.
     

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  6. gonzo
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    gonzo Senior Member

    OK, the end plate effect has to provide some gain.
     
  7. messabout
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    messabout Senior Member

    Way back in the day, there were some serious experiments with this concept on AC type boats. The boom had a horizontal plate which closed the end gap....presumably. The end plate had sail tracks to allow the sail to assume its designed camber at the foot. The device was given the name; Park Avenue Boom.

    Of course there have been many a sail built with a boom shelf which presumed to act as an end plate. Various class rules made them illegal because the shelf material was calculated as additional sail area.
     
  8. Will Fraser
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    Will Fraser Senior Member

    The closest I have been able to get to a direct comparison is the 1976 Little America's Cup for C class cats, as described in Garret's book "The Symmetry of Sailing".
    It is unfortunately impossible to draw any conclusions as to the contribution of the gap seal itself on Aquarius V (soft wingsail), since the rigid wing on Miss Nylex also seems to also have had "some" sort of sealing - the only indication was that the seal on Aquarius V was "more effective". Besides that, the weights and friction drag differed greatly between the rigs.

    Has it been tested on a Moth perhaps? Or modern C-class cats?

    For the purposes of this discussion, I am not trying to punt the idea as some panacea to the woes of sailing. I am merely trying to understand the pitfalls of real world application so that I can take it into account in my own design - the most immediate which is nothing more than a humble sailing kayak. I use it on an river that is 200ft wide and would like to be able to pinch to avoid excessive amounts of tacking when going up wind, all the while keeping the heeling moment to an absolute minimum.
     
  9. Petros
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    Petros Senior Member

    there are many efficiency gains by reducing the "end gap", but sailing is for sport or recreation. In most sport sailing they have rules, and they tend to keep designs static. For recreational sailing a "deck sweeper" is a real nuisance, it tends to knock your guests overboard.

    there is no practical reason to own a sailboat other than recreation or sport racing (both for those with too much money), so you are either stuck with rules that limit the innovations in sail design, or do not want to be inconvenienced by sails always in your face.

    Most of the America's cup competitors have deck sweeping jibs. I would hate to get caught forward when they come about however, not the most practical design for enjoying a day on the water in nice weather.
     
  10. Will Fraser
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    Will Fraser Senior Member

    Not for much longer.

    Consider the case of Trillizas, the demo model for wind-assist wingsails being developed for ferries.

    I have no data on the sail except its height of 40ft. A rough estimate of proportions gives me a sail of 200sft with an aspect ratio of 8, foot chord of 6.6ft and tip chord of 3.3ft. If the gap is scaled according to the assumed foot chord length, it gives 2 inches.

    A quick calculation shows that that gap represents an additional 25% possible reduction in induced drag as well as up to 10% more drive when motoring (7kts) with the true wind (15kts) 20deg off the bow.

    It hardly looks as if there were any practical issues preventing them form creating a proper seal. Even as an afterthought it would not take more than a bit of duct tape and a couple of pool-noodles to effect a near perfect seal.

    So is it just ignorace or are these types of gains really just not worth it?
     

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  11. whitepointer23

    whitepointer23 Previous Member

    You have given me a lot to think about when I start sailing my ketch. I would love to experiment and see if your ideas work well. thanks for an interesting thread will.
     
  12. Will Fraser
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    Will Fraser Senior Member

    Thanks Pointer, I look forward to hearing about your findings!

    What ketch do you have?
     
  13. Rastapop
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    Rastapop Naval Architect

    Could you elaborate on these quick calculations Will?
     
  14. Will Fraser
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    Will Fraser Senior Member

    It was done on Tom Speer's spreadsheet - Vortex 95 - that was made available elsewhere on the forum.

    As inputs, it takes the sail plan, area, deck gap, lift coefficient, angle of attack, twist, wind speed etc. It then calculates the spanwise lift distribution, total lift generated as well as induced drag and a host of other figures.
    In my modification of the spreadsheet I added form drag (parasitic), and broke the drag and lift vectors up into drive and leeway forces (based on a user selected point of sail).

    I then compared the results for the wingsail with and without the gap and simply expressed the difference as a percentage.

    The 25% difference in induced drag seems to stay consistent over quite a range of Cl values (I tested at 0.4, 0.8, and 1.2). The difference in drive obviously gets more pronounced at the higher Cl values, since the induced drag gets proportionally more compared to lift.
    And that is exactly where sealing the gap has the most effect. It is also exactly where almost all sailing vessels operate - i.e. maximum Cl.
     

  15. Petros
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    Petros Senior Member

    good luck with the wind powered ferry. Usually the cost of the rigging, maintainance and lower times from port to port make burning desiel fuel cheaper. maintaining rigging is very costly, all of it wears out, and takes more crew to operate. Any work boat must also delvier the "payload" in a reasonable time to make max untilization of large up front investment and on-going operating costs, the faster you delvier your paylaod, the better return you make on your investment. So unless they hope to charge extra for the novelty of it, likely it will cost more to build and operate than a conventional ferry.

    Many have tired, but none have proven financially viable. About the only money to be made with sailboats is for recreational charters, where customers will pay the extra cost to get the sailing experiance.
     
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