What is “lead” in sailboat design. Joe self designs a rig and finds something interesting.


Joe Koenecke and his brother built an Electric Quick Canoe.

They then put together a sailing rig and leeboard to see how it would sail.

Homebuilt Electric Quick Canoe by storerboatplans with homemade sailing rig by Joe Koenecke

On returning from the first trip Joe sent me a question

Hey, Mik

I have a sailing question. The first time we sailed the QCE, we had trouble sailing upwind, and coming about. We made two changes: added battens to the sail, and moved the leeboard forward.

The leeboard had been positioned on the “center of effort”. But when we moved it
forward, we sailed better. I’m not exactly sure what effect the battens had, except that the sail seemed to have a better shape.

Any thoughts on why she sailed so much better with the leeboard forward of the “center of effort”?

I replied

BINGO

You have discovered a design element that is called “lead” (like I lead her to the cafe).

The centre of effort is an approximation.  In reality the front part of the sail develops much more force than the back.  Also in the gusts as the sail distorts the centre of effort moves back. Additionally the CE and CLR method is a 2D approximation of a 3D situation.

So we all have a fudge factor called lead where we move the leeboard or centreboard forward to compensate for the approximation.

Brilliant Observation!

MIK

The CE is the geometric centre of the sails.
The CLR is the geometric centre of the boat that is in the water – I find that with a daggerboard boat with a shallow body that you can ignore the boat completely and just use the centreboard or leeboard position.

The centre of sails is where they sails are trying to push the boat sideways.
The centre of the underbody of the boat is what resists the boat going sideways.

If their locations line up the boat will go straight ahead and sail nicely.
If they don’t the boat will want to turn all the time and the person steering will have to fight it.

In general you want the CE just behind the CLR by this amount called “lead” so that the boat will have slight “weather helm” want to turn slightly up towards the wind. This helps windward performance. The opposite situation “lee helm” is almost always bad for overall performance.

Here is the drawing of a new 12 foot dinghy I am designing. The circle with the cross in it is the CE of the sail, see how it is slightly behind the centreboard. I have quite a lot of data from boats I have designed to guess at the “lead” for a particular design based on feedback from sailors and my own experience.

New 12 foot plywood sailboat by storer boat plans

In general something around this size – or a PDRacer or an OzRacer will have an effective “lead” of about 6″ or 150mm. It needs to be slightly more for a leeboard boat.


2 thoughts on “What is “lead” in sailboat design. Joe self designs a rig and finds something interesting.

  1. The relationship between CE and CLR is certainly approximate and variable, but in design of conventional rigs and hull forms lead usually goes the other way, with CE ahead of CLR. See:

    http://www.tedbrewer.com/yachtdesign.html
    http://academic.amc.edu.au/~gthomas/yacht/yacht_lec8_balance.ppt
    http://legacy.sname.org/standing/education/ISWSCYDC/first05.pdf

    In this case, with most of the lateral resistance in the appendages,it’s possible that Joe omitted the rudder from the calculation, placing the effective CLR much farther aft than calculated. Moving the leeboard forward might improve balance, even if the true CLR remained abaft the CE.

    • Hi Peter,

      Excellent point. This way is my own personal one … every designer has their own “recipe” because it is not science – really a 2D simplification of a complex and changing 3D situation.

      But you are right … I need to be clear about my assumptions, which I was a little .. but not enough.

      My method only works with shallow body hulls with relatively narrow centreboards or leeboards.
      I ignore the rudder because there is not rational way for knowing how much load the rudder is really carrying so I set up a couple between the CE and the centreboard.

      Lead is never scientific because …
      Some designers ignore the rudder (me as one example)
      Some designers will include 10% of the rudder area (a bit of weather helm)
      Some will include all of the rudder
      Some will use the centre of the centreboard/keel or leeboard/s
      Some will use the midpoint of the immersed leading edge of the centreboard/keel
      Some use a point 30% back along the chord of the keel.

      CLR is not a real measurement, but an arbitrary point chosen by a specific designer using a personal guideline on the basis of experience.
      CE is also not a real measurement even though the method is relatively standard, unlike the method for CLR.

      Anyone who has sailed an old style windsurfer or sailboard will know in the big gusts the centre of effort moves a couple of feet in a relatively small sail. In light to moderate winds you can have your hands on the boom close together. In the stronger winds you had to move your hands about 30 inches apart and sometimes had to push out with the front hand and pull in with the back showing the CE had moved behind the rear hand (as far as the coupling of the forces was concerned)

      It is arbitrary but one designer will be consistent with one method. And all experienced designers will end up with about the same relationship between CE and CLR all based on different assumptions. Happiily a designer that has a chance to design more than a couple of boats gradually gets a better and better feel for the amount of lead.

      If I include the rudder then my “lead” goes the other way with CE in front of CLR on paper, though the boat doesn’t notice any difference … but how much loading to choose for the rudder?? It is completely arbitrary. I think it is an “intellectually unclean” argument that the rudder “must” be included because nobody knows how much load it will carry.

      I am not saying the other are “wrong” … just explaining my bias.

      But the main thing is to be consistent. If others want to include the rudder … why not? 🙂

      So my Assumptions in my method are
      1/ Rudder can be ignored because we don’t know it’s loading
      2/ The hull must be shallow without a deep forefoot or skeg – most light shallow boats fulfil this criteria
      3/ Use the centre of the centreboard
      4/ For a boat PDR or OzRacer Or Goose or Goat Island Skiff size the My CE needs to be behind the CLR by 6″

      When I design a new boat I work it out more carefully than this, but if I am far from this measurement the alarm bells start ringing 🙂

      thanks hugely for the effort Peter, also your links will help others who want to understand bigger and heavier boats

      Best wishes
      Michael

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