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Weather helm is a much much more complex issue than the simple relationship between The combined geometry of the sails (CE) and the geometry of the hull (CLR).
The 'dynamics' (aero & hydro) as developed by the sails shape and hulls shape are equally important .... and there are conditions that 'mimic' so called weather helm --- eg. the boat skidding sideways and giving undue 'side pressure' to the rudder.
The first aspect to sort this all out to be sure that the wake is coming off the boat's stern in a more or less straight line (no more than about 5 degrees difference than the centerline of the boat. If more than about 5° then the boat is skidding and developing undue side forces on the rudder which may feel like and may 'mask' weather helm. Chief causes of 'skidding' is the keel or centerboard held at a large heeling angle which reduces the lateral resistance (via trigonometry). Next major cause of skidding is a too loose rigging tension which allows the rigging/rig (chiefly the forestay) to sag drastically to the leeward side of the boat ... making the 'lines of force' as generated by the sails to be not parallel to the centerline of the boat.
Sail SHAPE has a strong influence on 'weather helm' as the 'balance' is determined by the 'actual' or 'adjusted' position of where the maximum draft occurs in the sail ... adjusted primarily by halyard (and/or 'cunningham') tension. This effect of correcting weather helm by adjust where this 'position of maximum draft' (POMD) occurs is quite important when using 'stretchy' sails such as those made from woven dacron - ie. the tighter the luff/halyard the further forward the POMD, and vice versa.
So, sail shape, how precisely one 'raises' and 'sets' the sails, how well the rig is tensioned, how fast the hull is moving through the water also determine 'helm balance'.
For the specific case of sailing (on a sloop rig) with just a 'small' jib up, in all probability you had the jibs clew trimmed in too tight towards the boats centerline and the boat was 'skidding' off to leeward. Most probably if you would have 'opened' the jib a bit you would have developed better 'dynamics' for less helm pressure. Next time you sail with just a jib (or any other combo) when you 'think' you have 'weather helm' the first thing you should do is LOOK AT THE WAKE COMING OFF OF THE STERN ... if the wake is coming off the stern in an almost dead straight (not greater than 5° angle) then you have 'weather helm', if more than about 5° then you are 'skidding' off to leeward. Once you are skidding the keel or centerboard will not be operating in a correct hydrodynamic flow and will be developing 'strange' patterns of turbulence and most certainly the keel/centerboard will not be 'lifting' or 'resisting' correctly - destroying or greatly altering the effect of static 'center of lateral resistance' CLR Just like the sails, you also must 'fly' the keel/centerboard (and the rudder) correctly !!!!!
Rx Its NOT weather helm if the wake isnt coming nearly straight off the stern. If the wake is not coming straight off (greater than about 5°) AND there is a lot of 'helm pressure' then the boat is probably 'skidding'.
If youre flying just a small jib on a sloop you have to keep the clew quite far from the boats centerline ... or skid !!!
hope this helps
Last edited by RichH; 06-10-2011 at 12:35 PM.