Is your boat trim?

“There seems to be something wrong with our bloody ships today” – Admiral Beatty

– Not really a case of bad trim, but never the less you don’t want to get caught out!

 

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The centre of effort of a sail is the point at which the resultant forces of the wind may be considered to act just as the centre of gravity of an object is the point at which the entire weight of the object can be considered to be concentrated. Lateral resistance is the resistance the water offers to the sideways movement of the hull. The centre of lateral resistance is the point at which this force may be considered to act. If you were to stand in the water alongside the boat with its plate down and press squarely upon the centre of lateral resistance the boat would bodily sideways without any forward motion or tendency to turn. For the purpose of the following the centre of lateral resistance may be treated as a pivot, just like the pivot of a weather vane. Forces acting forward of it will tend to cause the head to fall away to leeward. Forces acting aft of the point will tend to cause the bow to turn to the opposite direction, that is to weather, unless this tendency is checked with the rudder. Unfortunately there is no simple and satisfactory method of finding either the centre of effort or the centre of lateral resistance either by theory or experiment. Given the method of standing in the water and pushing on the hull till it moves without turning only gives the centre of lateral resistance at rest or when moving very slowly. dinghy-with-marks Both the centre of effort and centre of lateral resistance have a marked tendency to move forward as the wind velocity and boat’s speed increases, and this is true of the centre of effort; hence the tendency for weather helm to become greater with a freshening breeze. It is usual to find the geometrical centre of the sail plan and of the immersed profile of the hull (excluding rudder) and apply corrections to allow for the above phenomena. Bisect two sides (jib or foresail) and then draw lines to the opposite corners. Their point of intersection will mark the theoretical centre of effort. The same procedure may be used for a mainsail if all its sides are straight. But for a gaff main or Marconi sail with rounded foot or leech a better method is to cut a scale model of the sail from stiff cardboard and balance it on a straight edge, mark the line of balance; by finding two such lines, the point of intersection is the centre of effort of the mainsail. It is necessary to find the value of x in feet or inches in the accompanying sketch y may be found by measurement, and x from the algebraic equation x by area of mainsail equals (y – x) multiplied by area of jib. By cutting out the underwater profile of the boat, including the plate, but not the rudder, the centre of lateral resistance may be found by balancing as in the case of the mainsail.

Unless your boat will sail to windward with only a small pull on the tiller it is not correctly trimmed. Get the boat upright sails hauled on , the plate down and all the crew in position then let the tiller go, it should move away from you very slightly and the boat should slowly swing head to wind. If you have to push the helm away from you to keep your course, or, if in the above base the boat turns away from the wind so as to come broadside to it you have LEE HELM and this condition may be dangerous. If you have to pull the helm toward you with considerable force, or if on letting it go the boat shoots rapidly head to wind you have excessive WEATHER HELM ! Whilst this condition may not be dangerous it makes sailing uncomfortable and causes a drag on the rudder with consequent slowing of the boat.

Lee Helm – If centre of effort to far far in advance of centre of lateral resistance.

Cures:

1) Move centre of lateral resistance forward

  • Shift centre plate forward or lower centre plate
  • Move crew forward or to leeward

2) Move centre of effort aft

  • Set a smaller jib
  • Move the mast aft, shift the mast aft, or tilt it further aft
  • Set the jib further aft on the bowsprit
  • Use more area of the mainsail by easing outhaul and cunningham

Excessive Weather Helm – The centre of effort is not sufficiently advanced of the centre of lateral resistance, or may even lie aft of it.

Cures:

1) Move centre of lateral resistance aft

  • Shift centre plate aft
  • Slightly raise the plate (this applies to metal plates only)
  • Move crew aft ( but watch that transom does not drag on the water)

2) Move the centre of effort forward

  • Reduce area of main sail by reefing (particularly for heavy weather)
  • Use a larger jib
  • Tilt mast forward
  • Set job further out on bowsprit
  • Step mast further forward