What is weather helm?

Wikipedia gives a nice definition:

Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward (i.e. ‘to weather’) in order to counteract the effect. Weather helm is the opposite of lee helm.Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward (i.e. ‘to weather’) in order to counteract the effect. Weather helm is the opposite of lee helm.

What causes it?

I’ve attempted a very simple definition, so if you’re after more detail then this article, and this one (which is more in-depth) are worth a read.
The fundamental cause of weather helm is the relationship between two forces which act on a boat:

  1. The “centre of lateral resistance” (CLR) which acts on the hull.
  2. The “centre of effort” (CE) which acts on the sails.

CLR: Standing alongside a boat, the CLR is the point somewhere in the middle which when pushed makes the boat move sideways (its geometric centre). You can demonstrate this with your phone on a table. Push the middle then push instead at either end – the bit you pushed will pivot away from you.

CE: This is the geometric centre of the sail plan. Very simply, imagine finding the centre points on each sail and drawing an imaginary line between them, that’s your CE. You need to move it along the line depending on the size of the sails, but you get the general idea.

To make things a bit more interesting, weight, volume and forces play roles too causing these two locations to constantly change, but these are the basic principles.Weather helm occurs when the CE is behind (aft) of CLR. This means the power in the rig is now putting more effort aft which pushes that part of the boat away from the wind, which in turn causes the bow to turn into the wind and pivots on its CLR.The opposite, lee helm, occurs when the CE is forward of the CLR.

I’m no artist, but here’s a basic illustration which I hope helps.

How can you reduce weather helm?

You need to adjust the relationship between the CLR and the CE by either moving the CE forward, or the CLR back.

How to move the CE forward:

  • Ease mainsheet, either by moving the traveller to lee (away from the wind), or letting the mainsheet out.
  • Flatten the mainsail by tensioning the outhaul, cunningham and halyard.
  • Reef the main (a must in high winds)
  • Rake mast forward (if you have a running backstay, ease tension on it).
  • Increase twist in main and jib.
  • Make jib larger compared to mainsail (if you have a sail wardrobe).

How to move the CLR aft (which you may do more if racing):

  • Shift the weight of the crew aft (put them in the cockpit).
  • 
If the boat is heeling, move crew’s weight to windward (sit them on the rail)
.
  • Raise centerboard (if you have one).

A little weather helm is actually a good thing

Boat designers do build in a certain amount of weather helm. Why is that? Underwater, your rudder acts like a foil, in much the same way as your sails do above water. Imagine sitting straight into the wind – your sails flap around without generating any lift. Conceptually the same is true of your rudder. When underway, if you rudder is dead straight, water runs equally past both sides and it doesn’t generate lift. A few degrees from centre helps lift the boat to windward.

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