The fore-aft position of your mast plays a very large roll in the handling of your yacht. Too far aft, and the yacht will exhibit a lot of weather helm; too far forward and your yacht will exhibit a lot of lee helm. As you can see, you can use this feature to tune your yachts handling and create a balanced machine that will glide through the water effortlessly.
Masts all look straight, but they actually bend back just a bit. Cruising yachts tend to rake aft less, racing yachts rake aft more.
If this is the first time you are setting your rigging, and you have no idea if the yacht has weather helm or lee helm, you are best to start at a happy medium. A good starting point is 12 inches aft for every 50 feet of mast height. You can calculate your aft rake by multiplying your mast height from the deck by 0.24. This number will be the number of inches your mast head will be located aft of your mast at the deck.
You might be thinking "How can you figure out how to set your masthead 12 inches aft of your deck junction when it is 50 feet into the air?"
The answer is very simple, you use a plumb line!
Attaching a gallon jug of water to the main halyard, will provide you with a very ideal measuring device for setting the mast rake. The gallon jug weighs around 8 pounds when full, which ensures that it will pull the halyard straight and taught.
The jug will act as the plumb weight and will transfer the position of the masthead down to the deck. Using this, you can easily adjust the headstay and backstay to move the jug of water to the ideal placement on the deck. When the jug is positioned at the correct distance aft of the aft side of the mast, you know that the mast head is also in the correct position fore-aft.
After this step is completed, your next step will be to sail test the yacht and see how it responds.
If you feel that you are not able to balance the sails properly, it might not be an issue with your sail trimming, but instead that your mast needs to be moved a bit more. If you have too much weather helm, your mast is too far aft and needs to be moved forward. If you have too much lee helm, your mast is too far forward and needs to be moved aft.
If you have a backstay adjuster, these adjustments can be made while sailing to further improve your yachts performance. When sailing upwind, you can tighten your backstay and pull the mast back a bit. This will tighten your headstay and grant you better pointing ability while also pulling the mast aft, giving you beneficial weather helm.
When reaching, you can ease the backstay adjuster to move the mast a bit forward and reduce your weather helm. This will also cause the headstay to sag and create more of a belly in the luff of the headsail, greatly increasing your sails power.
On a run, you can ease the backstay even further and push the masthead forward to induce lots of lee helm that will help you power along downwind. The headstay will also sag a lot and that will cause the sail's luff to bulge out like the luff of a spinnaker. This will create a lot of power and pull your yacht downwind with speed!