Lead for a yawl

Discussion in 'Boat Design' started by willfox, Dec 22, 2009.

  1. willfox
    Joined: Oct 2008
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    willfox Junior Member

    I am currently designing a modern yawl with a short keel with a bulb and winglets, I just wondered if anyone knew what percentage lead to go for (percentage of LWL that the Centre of effort leads the CLR) I have read from 10 - 15 % but would this be for a long keeled vessel? With LCB positions of around 53% this seems ever so far fore for a fin keel. At the moment the rig looks good at around 2%. Any suggestions?:confused:
     
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  2. TeddyDiver
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    TeddyDiver Gollywobbler

    You better stick to that 10-15%. There's no excact value or formula to give you. It all boils down to rig and hull form but perhaps the most important factor comes due the height of the CE and how much the boat heels. You can leave some allowance to resize your mizzen afterwards if it needs to..
     
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  3. sorenfdk
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    sorenfdk Yacht Designer

    Bulb, winglets and LCB has little or no importance.
    Read Larsson & Eliasson! Or study yacht design before you actually begin designing yachts!
     
  4. willfox
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    willfox Junior Member

    LCB obviously has importance as this dictates where the keel is placed to maintain level trim which in turn gives the 1/4 chord position from where the lead is measured, am I wrong? Was only asking for a bit of help, some people are so rude on this site.
     
  5. dskira

    dskira Previous Member

    In that case you have also to be careful for the ballast in your fin. You can't put it where the LCR is the best, but where your weight estimate will tell you.
    You can fair with wood/epoxy or fiberglass if needed to elongate your keel where needed.
    Keep, if your hull is very symetric 10% and it will be ok. And you can have 2%, but I don't know your hull. Distribution of volume is also important on the balance.
    All that also depend of your sail! So many factor! Rudder also take some part of it, depending of its design.
    What sorenfdk try to tell you is that yacht design is very complexs and it's part of studies, exepriences and sailing yourself.
    Now the best way for you, is to make a model and try several configurations. It will give you a lot of information. Respect the scale weight and scale weight distribution. It's all in the balance of all these factors.
    I had a fin keel boat with minus 2%, tought but was not too bad. the hull was very symetrical. I will not consider it again :D
    I will tell you a secret: nobody realy knows where to put the darn lead! its all try and error. As I said depend to many factor, including the sailor who tune up the boat ! So be conservative.
    Cheers
    Daniel
     
  6. willfox
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    willfox Junior Member

    Thanks Daniel, Thats very helpful of you. I am currently designing my dissertation yacht for a degree so any help is much appreciated. Thats true about the rudder. Had not figured that in. All the best.

    Will
     
  7. PAR
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    PAR Yacht Designer/Builder

    I know where to put the lead, but several factors affect it's exact location. Given your description 11% to 12.5% lead appears to be what is called for, though not knowing what things actually look like makes this just a guess (since you're guessing, guess 13% or less). The high aspect your appendages have the more lead you'll want. The longer the keel, the less (noticeable) effect the location of the lead will have on the boat abilities. There are several other things to consider as well, which as mentioned comes for experience, study of previously well balanced yachts of similar configuration and personal observations/testing.
     
  8. dskira

    dskira Previous Member

    You realy don't know where to put the lead. You assume with your experience and that is different. We all do that, but it is not an exact science or calculation like the other one needed in the design process.
    As I said, nobody "knows" everybody assume for the best, and in most case succeed.
    It's all as you said about balance. The lead on paper is a start.
    Two Japonese naval architect in one of the transction of the SNAME find the exact contrary of what we thought. The longuer the keel the more noticable effect of the lead they find. But it can be challenged easely. It was their finding. Nobody's wrong, nobody's right.

    And for our friend, I am biaised, the yawl is my favorit rig.

    Cheers
    Daniel
     
  9. PAR
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    PAR Yacht Designer/Builder

    I've own a few yawls and designed a few as well, it's one of my favorites too.

    I can nail down the lead within a couple of percent (less then 2%) with full information regarding the design. If it's a fairly common set of variables and shapes, then likely within a single percent of an ideal arrangement, which is well within the "tunable" range during trials. Yes, this does rely on experience, but also can be preformed reasonably well with equations, assuming the model isn't very extreme. If it is extreme, then it's a crap shoot.

    I'm not sure who the two NA's from Japan are, but their "results" fly in the face of lots of testing and observation to the contrary, most notably, Herreshoff, Skene, Gutette and Kinney.
     
  10. sorenfdk
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    sorenfdk Yacht Designer

    Exactly - thank you! I didn't mean to be rude, it's just that way too many people seem to think that the most important part of yacht design is downloading the right piece of software - it is NOT! You didn't mention that this design is a dissitation yacht for a degree (what degree, btw?), so I thought you were one of those. My mistake - please accept my apologies!

    BUT: Lead does NOT depend on LCB! A large LCB does NOT mean that the lead should be large (or small). LCB does - more or less! - dictate where you put your keel. Whether you measure your lead from the 1/4 chord is entirely up to you - there are many ways to do it! I prefer to also take the hull (especially the forefoot) and the rudder into consideration.
     
  11. willfox
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    willfox Junior Member

    Hi Søren,
    Thats ok, yes, sorry, your right. I measured the lead from 1/4 chord and didnt take into account the rudder and underwater profile. I am now taking the 2d profile area an taking the centroid which i have been taught in the past. I now have 8 % so will play around with the sail positions a little. I am at Southampton Solent University studying the BEng Yacht and powercraft design course. Really enjoying it and the lecturers are really good.
    I did not think LCB had any direct impact on the amount of lead, only that the keel position for level trim was influeced therefore having an impact on lead.

    Thanks PAR and Daniel for your contributions. I love a Yawl too. My design has alot of influence from the S&S stormy weather....Probably the most beautiful yacht ever!

    I am trying to reduce the amount of draft using a bulb with winglets which wil increase the effective draft. This therefore gives much a low aspect ratio foil. Does 2.5% of area sound ok for keel area percentage of sail area. Think Larsson recccomends 3.5%. Guess this may lead to greater leeway angles but have gone for a higher t/c at 0.14 to make the stall angle greater. Is my way of thinking correct?

    Best regards

    Will
     
  12. sorenfdk
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    sorenfdk Yacht Designer

    Actually, I use the 1/4 chords of the keel, rudder and hull to come up with a sort of weighted average position of the CLR. This method was described by (IIRC) Austin Farrar in a RINA-paper many years ago.

    As for the keel, the relationship between the keel's lateral area and the sail area as described by Larsson & Eliasson (and others) is based purely on statistics. If you think a little bit about it, you'll find out that the sideforce needed from the keel (and therefore also the keel area) is depending on the righting moment. So when your keel area is very different from what statistics tell you it should be, you can (and should) check it with some (rather straightforward) calculations.
     

  13. Tcubed
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    Tcubed Boat Designer

    Complexities of lead

    Let us consider some of the factors involved yaw torque of a sailing boat;

    The sails have a pitching (yawing, since the sails are approximately vertical) moment about the quarter chord line dependant on their camber.
    The thrust vector from the rig gets more and more laterally offset from the resistance vector with increasing heel angles.
    This is the dominant cause of yaw torque.
    Clearly the larger the vertical separation between the aero and hydro dynamic centers the greater the yaw torque effect will be when heeled. To compensate more lead is required. This also steepens the yaw torque/heel angle curve, requiring additional coefficient of rudder volume.

    The underwater foils typically have zero camber and can thus be considered to have zero pitching moments about their quarter chord lines (very close approximation under normal conditions)
    The hull will develop a yawing moment depending on the curvature of the water flow lines on windward and leeward sides. Most hulls tend to yaw away from the direction of heel. Relatively narrow and deep double enders are often heavily assymetrical in this regard, compounding the yaw-heel effect of the rig. [1] A notable exception is the scow which will yaw towards the heel, partially compensating for the yawing effet of the heeled rig.
    In full keel designs the analysis is greatly complicated because one is forced to consider the hull and keel as a whole while with most fin keelers it is reasonably accurate to consider the total forces to be the sum of the forces created by the individual components.

    So there are three reasons for lead:
    Mainly to counteract the rig/hull lateral offset, to counteract the pitching forces due to sail camber, and to counteract (hull shape depending) rounding up tendencies of the hull itself.

    Balance is considered for the close hauled case. There are additional factors to be considered at other sailing angles.

    I consider that lead recomendations based upon type of boat and expressed as % of LWL to be rather arbitrary and to be recognised as a rule of thumb method. They may work very well for someone who has experience with a certain type of boat and knows that x% LWL lead worked well the last time but is not very applicable in a more general sense.

    A more rigorous approach is to consider the total aerodynamic vector force and yaw moments and the total hydrodynamic force and yaw moments.
    The rig must be so positioned such that these match up in such a way as to produce the correct range of weather and lee helms.

    Due to the lateral variability of the thrust axis w.r.t. resistance axis there is no way to have a boat perfectly balanced at all heel angles.
    A typical compromise is to tolerate a slight amount of lee helm when ghosting so that the weather helm is not too extreme at large heel angles, with the boat showing ideal weather helm in the range of moderate to optimum heel angles. I define "optimum" heel being the heel angle of greatest speed, ie the heel angle which if passed, demands a reef in order to keep the boat sailing as fast as possible.



    As you can see it is not possible to tell you a definite answer witout a good look at the plans.


    ___________


    I would tend to agree with earlier comment that greater keel chord 'softens' the range of acceptable rig positions. I would be very interested to see the japanese guys analysis though.

    14% t/c is very fat! And no it will not nescessarily increase the stall angle.
    Lower aspect ratio will.
    Do you really want to put all that buoyancy down low like that? Increase your frontal area?
    Think about it.

    2.5%, 3.5% keel/S.A. ?
    These numbers are hokey. What happens when you're down to the last reef on your trysail and got spitfire straining? 80%?
    The efficiency gains of trying to pare away as much wetted surface as possible are very minimal beyond a certain point. Overloaded foils in adverse conditions, however can simply stall and you won't be worried about perecntile increases in efficiency but rather just maintaining headway and control. I presume you are drawing a displacement type of boat?

    Also yawls are the most forgiving in a sense since it will suffer the least from having to trim the mizzen at a sub-optimal angle in order to correct the helm, since the mizzen is but a small fraction of the total sail area.




    ______________

    [1] - [Harrison Butler did considerable work to correct/mitigate this. His theories are largely considere obsolete nowadays, yet i would remind readers that his boats were renown for their excellent balance and handling]
     
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