Mast Butt Adjustment: When, Why, How, and How Much?

By Eric Heim (Snipe Bulletin, Summer 2014)

Before we get into the details of why and when to move the mast butt in a Snipe, here’s a quick history lesson. The Snipe Class used to have a rule stating that the pin for the mast must be fixed 60″ back from the bow. This proved very challenging to measure and was deleted a few years ago.

Once the rule was changed, many top sailors started to experiment with changing the position. The general consensus at the time was that sails and tuning had been developed around the 60″ rule, so the boat balanced very well at that setting.

Since that time, rig tune and sail shapes have have evolved. For example, a really good sailor can take a mainsail that is not well suited to his weight or equipment and move the mast butt to help bring the boat back into balance. Moving the butt of the mast fore and aft is just another tool to achieve the desired feel in the helm.

Now that we’re done with the history, here’s the basic Physics lesson. Assuming all other factors remain constant (which they never do), moving the butt forward will shift the center of effort forward and cause the bow to bear off (lee helm).

By Eric Heim (Snipe Bulletin, Summer 2014)

Before we get into the details of why and when to move the mast butt in a Snipe, here’s a quick history lesson. The Snipe Class used to have a rule stating that the pin for the mast must be fixed 60″ back from the bow. This proved very challenging to measure and was deleted a few years ago.

Once the rule was changed, many top sailors started to experiment with changing the position. The general consensus at the time was that sails and tuning had been developed around the 60″ rule, so the boat balanced very well at that setting.

Since that time, rig tune and sail shapes have have evolved. For example, a really good sailor can take a mainsail that is not well suited to his weight or equipment and move the mast butt to help bring the boat back into balance. Moving the butt of the mast fore and aft is just another tool to achieve the desired feel in the helm.

Now that we’re done with the history, here’s the basic Physics lesson. Assuming all other factors remain constant (which they never do), moving the butt forward will shift the center of effort forward and cause the bow to bear off (lee helm).

Important points

2Making rig changes in the Snipe is a critical key to unlocking the potential speed of your boat. However, changes can easily make you slower instead. This past weekend while racing in Mission Bay, I moved the butt forward to reduce the helm (we were sailing very light, 114 kilos) but the boat went slower. The problem was that in a bay with flat water, you need to have leech tension in the sails to point well, and moving the mast forward caused the jib to hit the spreader when fully sheeted in. We tried dropping the sail down a little closer to the deck, but it was not enough. We decided to move the butt back to our base setting and increase shroud tension. Instantly, the boat was fast again.

Fiddling in the parking lot, I noticed that moving the butt forward did not have a large impact on rake (less than 10mm), but it changed the tension considerably (4 units less tension on the Loos gauge, or about 2.5 turns on the sta-masters). It would also stand to reason that moving the butt forward, effectively moves the shroud attachment point aft. Obviously, the neutral mark will change as well.

When and why to adjust the mast butt. The jury is still out, but at this point my theory is that moving the butt forward will be beneficial in waves to help keep the boat driving forward. In flat water, it may help to move the butt aft, which will increase leech tension and promote pointing. The new butt adjuster from Jan Persson is a pretty neat little tool that allows the change to be made on the water between races. (You’ll have to ease the jib halyard, vang, puller, and head downwind.)
Since we usually know if the water will be flat or lumpy before we leave the dock, the butt adjuster is probably not a required upgrade for everyone. As they are fitted to more boats, the top sailors will continue to experiment and the knowledge will trickle down through the fleet. That’s one of the reasons the Snipe has been around so long: our class shares information, while constantly developing and improving our boat speed.

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