Tendency to broach during beat or close reach

Discussion in 'Sailboats' started by Joakim, Oct 17, 2008.

  1. Joakim
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    Joakim Senior Member

    What makes quite a few modern boats to broach during beat or close reach? This seems to be a quite "new" phenomenon only seen in boats designed after about the mid 90's. Even very tender 80's boats do not have this tendency although they might easily broach with spinnaker.

    I have discussed this with several sailors and a few designers, but haven't got any clear answers how to "see" this tendency from specification and drawings.

    This doesn't seem to be much related to high stability, which only seems to delay broaching to higher winds. No heeling no broaching is quite obvious.

    It seems that these boats can be sailed with rather much heel, although maybe not efficiently, but when they heel rapidly in a puff they broach and this can not be stopped once started (easing the main during the puff is not enough, it has to be done before the initial heel). One example of such boat is First 31.7.

    The thoughts so far

    1. Efficient and deep rudder helps somewhat, but at the same time extreme rudders stall more easily.

    2. Round bottom and narrow waterline seems to be better than wide and flat with much better form stability. Heeling changes more dramatically the lateral forces?

    3. Wide transom seems problematic, but not at all always due to lifting the rudder off the water.

    4. There might be some inertia issues coming from heavy bulb + high mast -> a lot of inertia, which keeps the initial heeling and turning going on.

    5. Longer keels and/or V-shaped hulls have much better course stability.

    6. Current masts are much stiffer and have shorter free lengths above forestay thus they don't depower at puffs.

    7. Diffrences in sailplan. Jibs are much higher nowadays.

    Joakim
     
  2. Crag Cay
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    Crag Cay Senior Member

    Do you mean 'broaching' or do you mean 'rounding up' when over pressed in a gust of wind?

    My top two would be fat transoms and also improved (less stretchy) sail materials and better boom vangs (exacerbated by stiffer hulls, sheet material, masts and rigging) meaning the leach of the main doesn't open by itself. I think it's the drive off the back edge of the mainsail that is a big part of the problem. A good guy on the main traveller seems to be able to keep even the most extreme modern designs on their feet.

    Back when, the dacron sails, wire rigging, 3:1 vangs and rudimentary travellers (where it was common to dump the mainsheet in the gusts) all transpired to ensure the main leach never had a chance to round the boat into the wind. Even on the IOR two tonners with hydraulic vangs, there was probably enough give in the hulls, rig, sail cloth, multiplatt sheets and boom sections, etc to ensure the leech could depower on its own to a significant degree.
     
  3. Joakim
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    Joakim Senior Member

    I don't sail in English, so what's the difference? I mean uncontrollable turning to the wind with some extra heeling (not necessarily more than ~30 degrees).

    There are many upgraded 80's boats and also several new designs which do not have this problem with new high tech sails, sheets etc. I don't think that's the key. Actually I think stretchy sails are worse, since stretching spoils their form.

    With good use of sheet and/or traveller there is no problem.

    Joakim
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    In my opinion, being in the boat design business since 1978 and living through a number of different boat design trends, the problem is most closely associated with the wide sterns and transoms. A boat with a wide stern and transom will quickly gain severe weather helm as she heels, especially if she goes over in a gust. This is because as the boat heels over, she picks up volume aft which raises the stern, lowers the bow, and shifts the lift on the keel from the windward side to the leeward side. That is, because the boat's trim is forced down by the bow, the bow goes down and away from the wind. This forces the angle of attack on the keel to switch to the lee side, and that forces the boat to start righting and heading up into the wind.

    This trend of really wide sterned boats started after the advent of the BOC and Vendee Globe Around the World Races. The BOC is a 4-stop race with about equal time sailing upwind and downwind. The Vendee Globe is non-stop and so is primarily a downwind race. Particularly in the Vendee Globe, downwind running dictated wider sterns and transoms to support fast downwind sailing. The BOC picked this up, but with less success. Because of the upwind legs, many boats struggled. It is aciomatic in IMOCA designs that these boats do not go to weather worth beans.

    In the ensuing press, magazines started touting "powerful stern sections" as a desirable feature in fast sailing yacht design. In my opinion, this is one of the biggest hoaxes in yacht design--all-around sailing ability, and particularly upwind sailing ability, suffers. However, as the trend started to grow, boat builders discovered you could get a lot more room in a boat, both in the cockpit and down below, if the boat had a wide stern. And so the trend grew even to the present day--you still see this in marketing.

    But it has all been at the expense of good sailing characteristics--the wide sterns create unwholesome sailing behavior because of the imbalance they cause when heeling. Any boat that is narrower and more evenly balanced fore and aft in cross section and in volume distribution is going to sail much better than a fat *** boat.

    And that's all I have to say about that.

    Eric
     
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  5. daiquiri
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    daiquiri Engineering and Design

    I perfectly agree, plus reps from me for the very clear explanation. :)

    It is a known feature of the wide-stern sailboats, at least since the times of historical races from the end of 18. century between "skimming dish"-type and cutter-type boats.
    Today, we see a lots of hulls which are a kind of mixture between the two concepts, with narrow bows typical of the cutters and wide, skimming-dish type sterns.
    The result is a completely unbalanced longitudinal distribution of buoyancy, with the volume distribution curve shifted all-aft.

    That is also the answer to the second Joakim's question - you can recognise this tendency to some extent by an examination of how the boat's cross-section area distribution curve changes vs. heel angle (keeping the trim constant, for the simplicity of calculations). Ideally, it shouldn't change with increasing heel angle. In the real world this condition is generaly not possible to obtain with the common hull forms of practical use. So the curve's peak will usually move aft as the heel angle increases, indicating you that the boat will tend to pitch the bow down and could have the problems described in the initial post. An extremely varying curve then indicates a potentialy dangerous boat under a sufficiently big heel angle.
    Note that it is a static analysis and doesn't take into account the wave system which is created when the hull is moving. Therefore it is a method which can be rightfully criticized, but it does give you a good starting point for the evaluation of a design.

    I particularly agree with Eric when he criticize the strong emhasis given by certain "specialist" press to these so-called "powerful" stern sections, a nonsense expression which sounds cool and maybe helps the sales but says nothing about the negative by-products of that design solution.
     
  6. Earl Boebert
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    Earl Boebert Senior Member

    This topic was extensively studied by model yachtsmen in the 20's and 30's, because without radio control there is no recovery from a yaw induced by a hard puff. I presented a paper on the topic at the 2007 Chesapeake Sailing Yacht Symposium. PM me with your email if you'd like a PDF copy.

    Cheers,

    Earl
     
  7. daiquiri
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    daiquiri Engineering and Design

    That's interesting. I recall (actually, I believe to recall) that back in 20's and 30's there was Admiral Turner's method in use for studying this, by balancing the submerged sections on the razor edge... Is that correct?

    I would like to have a copy of your work, if possible. I've sent you a PM. :)
     
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  8. Earl Boebert
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    Earl Boebert Senior Member

    Yes, part of it was Turner's work. I was lucky enough to locate his original articles, which appeared in Model Engineer in the 1920's. Many of his assertions are controversial, but I hope I give a balanced treatment in my paper.

    Cheers,

    Earl
     
  9. john.wakely
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    john.wakely New Member

    Earl

    I have just registered today and would also like a pdf copy of your paper

    Thanks

    John
     
  10. Joakim
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    Joakim Senior Member

    But many of these "problem boats", like First 31.7, are very fast upwind, especially in stronger winds. I don't know about the Open-like designs and was talking mainly about cruiser racers.

    I was also surprised to see how well Sun Fast 3200 (wide transom two rudders) managed on 15 m/s beat.

    Joakim
     
  11. Brent Swain
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    Brent Swain Member

    Extremely assymetrical ,wide assed hull shapes are the main problem. If a boat is designed to maintain the same position for its longitudinal centre of buoyancy when heeled , she can be very docile on the helm. If , when she heels, the bow drops and the stern rises she will be very cantankerous on the helm , have no directional stability, and insist on rounding up whenever she heels sharply.
    Solution? Avoid hulls with extremely wide sterns and lean bows , who's maximum waterline beam is well aft of amidships.
    I remember a South African boat in the round the world singlehanded race named 'Allied Bank" that was shaped like a wedge.. She was so lacking in directional stability that by the time she got to Capetown, she had to drop out because she kept burning out autopilots .
    Twin rudders have given them some control , but nowhere near that of a properly shaped hull.
    Brent
     
  12. Paul B

    Paul B Previous Member

    I've read quite a few posts by you that are totally inaccurate. You have made strange claims about all offshore racers now using aluminum hatches, and other equally ridiculous comments.

    FYI:

    “Allied Bank” open class BOC 60. Winner & Monohull recordbreaker of 1990 “Twostar” doublehanded transatlantic race E-W 15.3 Days.
    Winner & recordbreaker of Cape Town–Sydney leg of 1991 BOC single handed round the world race. She held 17 hour accumulated overall lead in 1991 BOC when she had an ice collision on leg 3 near Cape Horn, putting her out of the race.


    So this design that was so unruly (according to you) was actually beating the snot out of the rest of the fleet when she had a collision. If she was so directionally unstable she would not have been doing so well.
     
  13. DGreenwood
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    DGreenwood Senior Member

    Take a look at this video of a modern Open 60 under some challenging conditions. The video is an amalgam of different shots in differing conditions. Take note of the intentional twist in the main seen in the aerial shot at the beginning of the film. Approaching 15 knots and tracking like it is on rails. Granted the sailor is very skilled and the work is heavy and wet. Also he is not really broad reaching and has no spinnaker up. The fact that it can track that well, at that speed and in those conditions is a testament to the boats ability to hold a course. Open 60s spend a lot of time reaching in the southern ocean while under autopilot and they have to track well. Don't kid yourself, these shapes don't have to track badly...they just have to be sailed really well.

    http://www.beltchiztour.com/en/list-sailing-videos-page-1.html
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    Hey Dennis, those are great shots! Thanks.

    Two comments: First, the twist in the main indicates that the skipper is easing the load to reduce weather helm. If the mainsail were to be properly cambered and trimmed, the boat would have a wicked weather helm in those conditions and a tendency to round up. Anyone who sails can experience this effect--we did many times on our transoceanic trips. When the wind built to be too great for proper balance, even with reefing, we would take up the slack on the topping lift to twist the top of the mainsail so that it would spill wind. The helm dramatically came back into balance and it was much easier to hold the tiller and for the wind vane to hold course. Try it, it's neat, and a very under-appreciated sailing tactic.

    Second, did you notice the spray coming up over the bow time and again? These boats with plumb stems are very wet indeed. I have always contended that such spray consumes a lot of power, in the form a drag on the hull as well as impact when it strikes the boat again. Boats would be faster, drier, safer if they could deflect the spray. This does not mean that the stem needs to be raked. On my open 60 design Project Amazon from the 1998 Around Alone, I deliberately shaped the plumb bow with a knuckle forward and up high to deflect the spray. I also had lifting strakes full length in the hull to improve planing performance. These are exactly like planing powerboat strakes, much more pronounced than the knuckle in the hull as seen in this video. It is interesting that lifting strakes are showing up on other competition round-the-world racers now. I attach a drawing of Project Amazon's hull lines in perspective showing its features.

    Thanks again for the video.

    Eric
     

    Attached Files:


  15. water addict
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    water addict Naval Architect

    Lots of speculation- understandably, as there can't feasibly be a test program to quantify all this. So I'll speculate a bit too as it's an interesting topic...
    I think the wide sterns contribute for sure, but mainly because as the boat heels, a good part of a centerline rudder emerges from the water. Thus it isn't doing anything, and can more easily ventilate on the low pressure side. Eric, I don't get the negative lift argument on the keel. That would make the boat side-slip to leeward, not round up? Wide sterns with twin outboards rudders seem to fair a lot better.
    Also, rigs have gotten taller, so as the boat heels, the CE of the sailplan is now more to leeward than a shorter rig. This creates a longer couple arm tending to round the boat up (driving force on sailplan coupled with hydro drag). Further, in more club-type performance designs, The trend appears to be a bit less "lead" between the sailplan CE and CLR. This makes the boats perform well upwind in light to moderate stuff, where the vast majority of club racing takes place.
     
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