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

http://www.tedbrewer.com/yachtdesign.html
academic.amc.edu.au/~gthomas/yacht/yacht_lec8_balance.ppt
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.