Curing Weather Helm

Discussion in 'Boat Design' started by stuart_paget, Aug 17, 2007.

  1. stuart_paget
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    stuart_paget Junior Member

    Hi there,

    I know this is an old chestnut but I would like to describe my problem and thoughts and see how these go down. I have a 20' gaff yawl with 4' bowsprit and 4' bumkin, beam of 7'9" with draft of 2'3" keel up and 4'6" keel down.. With full main I quickly get serious weather helm in F3-4 winds with the mizzen furled. There is a lot of lead in the bilges and 4 ' swing keel. Reefing the main early helps a lot but slows the boat down and the weather helm kicks in before the lee rails even get a look at the water! Also on a broad reach the tendency to suddenly break off on a broach can be a little alarming.

    I would like to go faster with full sail with the weather helm dramatically reduced.


    The present rudder hangs off the transom on 3 pintles the last of which is fixed just aft of the propellor. It has been suggested that I need to to have a lifting extension on the rudder for more grip and more resistance to weather helm. I am about to experiment with a second headsail as an easy option to try and when new sails are being made this year I will be asking the saikmaker to peak the mainsail as high as possible

    Does this sound sensible?

    Stuart
     
  2. lewisboats
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    lewisboats Obsessed Member

    Sounds like you are doing the right things...you might try reducing the rake of the mast to move the CE a bit more forward too. A combination of moving the CE forward (Peaking the sail higher and more forward, adding additional head sail area) and reducing the rake...or even negative rake should help. As a last resort...you might re-step the mast a bit forward...but that is a big job.

    Steve
     
  3. Guest625101138

    Guest625101138 Previous Member

    The physics of weather helm are reasonably simple and I expect you already know this. But I will give my view.

    The centre of pressure on the sails is aft of the centre of lateral resistance. The problem is compounded as the boat heels because the centre of pressure moves leeward and this encourages even greater weather helm. Another factor as the sails load up is that the forward drive pushes the bow down and lifts the stern so the centre of lateral resistance goes further forward. Normally all of this provides inherent safety by depowering the sails as the boat rounds up but it should be nicely controlled.

    Some thoughts on getting the centre of pressure further forward. Another head sail as you suggest. A longer bowsprit with a bigger headsail. Ensuring the mast is not raked aft. Could even rake it forward. Is is possible to step the mast closer to the bow. Sail shape - a fuller cut on the jib maybe and the belly of the main closer to the mast. (My view on peaking the main higher will lift the centre of pressure a bit and will be adverse. If you mean getting the peak further forward then this should be beneficial).

    Some thoughts on getting the centre of lateral resistance aft. Shift weight to the stern. Make a bigger rudder - deeper should be more effective than longer and use a NACA0020 profile - fat rudders are better than thin ones at the limit. (I don't like stern hung rudders because they ventillate and lose efficiency but you are probably stuck with this set up). Check where the swing keel stops - having it with some aft rake when fully down will move its centre of pressure further aft. Check the shape of the keel. It is a lifting foil no matter what it looks like and moving the maximum chord thickness further aft will move the centre of pressure aft - not a simple job. You could possibly reshape the keel so the area is further aft.

    Moving weight (normally human ballast) to the windward rail towards the aft keeps the sails closer to vertical and pushes the stern down and bow up. THis will reduce weather helm.

    There is an interesting story here on a rudder:
    http://byyb.org/gaffrig/ond05/article scott widmier.htm
    If I was dealing with weather helm I would want something substantially deeper than what is shown here. It is also likely that your prop interferes with flow over the rudder and reduces its effectiveness. Getting more rudder lower than the prop will reduce this problem. I like a folding prop on a sailing boat that is well forward of the rudder.


    The broaching sounds like an exciting ride. I guess the same as being overpowered when reaching with a spinnaker up. You might lose interest if it just goes faster without the thrills.

    Rick W.
     
  4. Pericles
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    Pericles Senior Member

  5. stuart_paget
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    stuart_paget Junior Member

    Thanks Rick,

    The cat boat analaogy is quite close. My rudder is a small barn door and is no where near the NACA 0020 being a slab of plywood! If I make the rudder deeper the question is how deep do I go?

    By not lowering the keel all the way I can move weight aft and a folding prop is a possibility, I have a stout 3 blader at present.

    Pericles and Steve, because of the furling gear, getting the rake further forward is not really achievable but I'll see what I can do.

    Yes there are thrills at present but total concentration is required at all times to prevent accidental luffing and ti does get a bit wearing on long beats...

    Stuart
     
  6. Guest625101138

    Guest625101138 Previous Member

    Stuart
    A slab of plywood will have a narrow operating range. I think you would be surprised by the power you can generate with a better shape rudder. I also liked the way that the fellow in the article improved the rudder balance but it also made it more difficult to lift out.

    As far as rudder depth goes I would be thinking of the depth of the fixed keel. Was that 2'3"? The keel should protect it. Make it say 1ft long and 2"+ thick to get close to the NACA0020. Does not have to be exact to the NACA but a nice rounded nose, maximum thickness about 30% from the nose, smooth/fair surface and a sharp trailing edge. This alone should give much more progressive control. It is probably getting away from the original design but that is cost of getting something that works better. It should not take a lot of effort to improve on the slab.

    Rick W.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You should evaluate if you actually have weather helm, by noting how much rudder deflection you really have. Sail close hauled in 15 knots of wind and you should have no more then 5 degrees of rudder deflection (ideally 3 to 4) to hold course. It would be better if it was less, but a hard mouthed gaffer, in building winds can get head strong. If you're able to hold course with 5 or less you have another issue.

    If you need to apply more then 5 degrees then you really have weather helm and you only have a few options, but several remedies. The CE needs to move forward or the CLP needs to move aft or a combination of the two.

    Headsails and decreasing mast rake are the usually choices, to begin addressing weather helm. If these don't work then you may need to add area aft to the deadwood or skeg or possibly rudder changes.

    Lastly, it's quite common for gaffers to get hard mouthed in these wind strengths. It's their way of telling skipper to reef 'er down. Beaufort force 3 is 8 to 11.5 MPH (perfect sailing wind) and force 4 is 12.5 to 24+ MPH (most coastal craft will find need to reef at some point in force 4 and Small Craft Advisories will be issued). The gusts usually associated with building winds can make you feel over powered (you are) so you need to reef.

    Sailing along in big winds, straining your rig and sails isn't going to make you faster, this is likely governed by your waterline length anyway. You'll just pile a big stack of water, abeam of the boat. This boiling water may have the appearance of moving fast but you're not, you are just dragging water along for the ride. Sailing on her lines, in correct trim will get the most out of her. I would investigate this before moving your rig around. I mention this because weather helm is often the incorrect diagnoses, it needs be verified, then addressed systematically.
     
  8. stuart_paget
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    stuart_paget Junior Member

    Thanks Par,

    I am still waiting for some decent weather to get out and test this. Mast has now been raked as far forward as tabernacle will allow i.e. very slightly aft, but I look forward to putting your test into action.

    Stuart
     
  9. alan white
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    alan white Senior Member

    well, today I launched my own gaffer, with the mast a full 13" behind it's original position (where it balanced perfectly). It was cat rigged, and now it's sloop rigged, but I haven't added the jib yet.
    Results were based on 10 kt occasional breezes, gusting to 12 or so, guessing. I would have expected a bit more weather helm, but apparently, a 12 ft long keel on a 15 ft boat is so forgiving of sail position that I noticed nothing different from last year when the mast was 13" forward, even though the new tiller is 16" shorter. The tiller position (I remembered to check) appeared to be at about 5 degrees. This is, of course, without jib, which ought to lower that figure.

    Alan
     
  10. woodwind
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    woodwind Junior Member

    Ah well - some input from a practical point of view.

    As far as my experience goes, weather helm is dominantly determined by angle of heel. So, in effect, reducing angle of heel will remedy weather helm. This is more pronounced in 'modern' hull shapes, which tend to have a broad stern (what I call 'fatass'), compared to more classical designs, and notabely, a lot of scandinavian modern boats.

    So how to reduce heel just by trim, without loosing too much power?

    a) Make sure your headsails (jib, genoa) are not cut too full, or deep.

    b) Allow for twist in the mainsail. Twist accomplishes 2 things:

    - vertical lowering of the point where forces act, thereby reducing the 'lever' length when heeled

    - moving the 'pressure point' of the whole sailplan fore, which helps counteract weather helm.

    Sorry for my unprecise wording, but this is not my native language.

    In my observation, allowing twist and reducing power on the mainsheet to a point where you get a noticable 'backbulge??' (pls. help me out with words!) on the first third of the main aft from the mast does not really loose you a lot of power forward, but does significantly reduce heel and weather helm.

    Happy sailing,

    George
     
  11. starktr
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    starktr Retired

    Weather Helm

    Built a Glander Tavana 33, 30 some years ago, from a bare hull. Heard weather helm was a major problem with the design. Installed the center board a foot or so aft of Doley's recommendations and never had a problem. Averaged 9Knots Fort Jefferson to St Pete hard on a fresh cold front driving off of waves diving occasionally pounding into troughs w/Aries driving all the way. Frequently day sailed out of Marathon into the Stream when nasty fronts where blowing. Only way I know to learn anything. Friends who came along called it Submarine Duty. My 51Morgan had a semi balanced rudder. Very controllable even driving hard with the rail lee awash. Plus hydraulic steering minimizes helm feed back. If it had a weather helm problem I never knew about it. Went to multihulls after that.
     
  12. alan white
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    alan white Senior Member

    Since then, I've had the opportunity to sail in twenty knots with gusts to upper twenties. I still haven't gotten a jib, and the boat is definitely being slowed now by the angled rudder (but not by much). The difference isn't felt until it really pipes up.
    The aft relocation in the mast position was a full 10% of waterline length.
    The boat most forgiving of CE shifts is narrow, with similar lines fore and aft, and has a long keel.
    Boats with assymetrical underbodies (wide, flattish sterns and narrow, pointy bows), short keels, and wide beams will not only be sensitive to CE shifts, but will also be far more sensitive to heeling.
     
  13. gramos
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    gramos Junior Member

    thanks ,all , we have lifted the errant weather helm yacht out , now and she has a fairly large rudder area , considering a temporary trailing edge addition /10/15 % , but i discovered that the main is 15 plus years old with 4 transatlantics ! so as the weather helm has been getting progressivley worse , the fullness could be the main cause ? he is putting a new main on before next trans atlantic in november 08 so could this be the whole cause of the problem ? it is a traditional timber hull built in the late 1960,s and has a large sail area . now , another point of interest is that the decks were raised 15 years ago , by 8 inches !and extra ballast added. the owner feels she is trying to sail faster but rarely reaches 7 knots , due to his fighting the wheel and dragging the rudder sideways through the water . there is also little mast rake .
     
  14. PAR
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    PAR Yacht Designer/Builder

    Worn out sails are a common cause of progressively a increasing weather helm feeling.

    I'll bet you'll find you have to remove all the changes you've made to the rig (mast rake, etc.) to get her "back in the groove".

    As I mentioned earlier in this thread, most cases aren't actually weather helm, but a feel like it.

    I can pretty much assure you that a 15 year old sack for a sail will get real hard mouthed on the helmsman. Change the old rags for new and retune the rig, she'll be her old self again.

    You may have other issues creeping into the mix, such as the additional weight and windage of the raised foredeck, plus the burden of additional ballast.

    Most yachts built in the era of yours will have speed limitations based on waterline length. If she was designed to compete under CCA then you're not very likely to increase her abilities by much (fractions of a knot at best). These vessels were reasonably burdened (and damned comfortable) particularly by today's standards.

    Your rudder issues may be somewhat relieved with the new sails. If additional rudder area was added and other sister yachts have required similar modifications, then you possibly could address a different rudder plan form and/or section. These aren't changes to make out of hand, but should be done by a skilled eye, who's versed in the qualities of this yacht.
     

  15. FAST FRED
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    FAST FRED Senior Member

    I have a stout 3 blader at present.

    This surely doesn't help a marginal rudder waterflow.

    FF
     
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