Mast Chocks

If you are new to Pacers and/or come from a class that does not use mast chocks you may find them somewhat of a mystery.  You may have even asked yourself “What on earth am I supposed to do with these?”  Stand by for a crash course in the use of mast chocks!

by Alan Riley – 2847 AMEN

What Are Mast Chocks?

Mast chocks are small t-shaped plates put in front of the mast at the deck level.  They are usually made of plastic or ply-wood and are of different thicknesses.  On my boat I have 3 plastic Ronstan chocks, each of a different thickness.  I like the plastic because the chocks are less prone to splitting and rotting.

What Do They Do?

Mast chocks are used to help control the amount of mast bend you have, and therefore how much power you have in the mainsail. In general:

Less bend = straighter mast = fuller mainsail = more power

The main control that bends the mast on a Pacer is the boom vang. Pulling on the boom vang has two effects:

  1. It pulls the boom down, thus tightening the leech of the mainsail.
  2. It pushes the boom forward at the gooseneck level.  This forward push bends the mast.

If you want plenty of bend low in the mast don’t use many chocks.  More of the boom vang’s effort will then go into bending the mast low down than in tightening the leach of the mainsail.  This is a good setting for when you are overpowered in stronger winds.

If you want maximum power, put in more chocks. The mast will be restricted from bending, so boom vang’s effort tightens the leach more. A sail set on a straight mast with a tight leach gives maximum power, just what you need in light/medium winds.

You can also use mast chocks to induce pre-bend into the mast.  This can be particularly effective in light winds and flat water.

Mast Chocks And Your Pacer

How many chocks you need on your Pacer depends principally on:

  1. The distance between the front of the mast and the mast gate.  The bigger the gap, the more chocks you will need!
  2. How much luff curve is built into your sails.  More luff curve requires a more bendy mast, so less chocks are needed.  Less luff curve means more chocks are required to stop the mast bending too much.
  3. How heavy your combined crew weight is:  heavier crews need more powerful sails, therefore more chocks.

The first thing to do is to determine the neutral position. Fill the gap in front of the mast so that the chocks are touching the mast, but not pushing back on it.  My sail maker recommends this as the best set up for the sails I use.  I have generally found this to be a good average setting in light to moderate conditions.

Second, determine the minimum number of chocks you should use.  From the neutral position, take one chock out at a time and pull the boom vang on to the maximum tension you will ever use it. Keep going until you start to see large creases running diagonally from the clew of the mainsail to around the middle of the mast. Even with your downhaul on, the creases do not disappear.  These creases are overbend creases and mean your mast is now bending more than the luff curve built into your mainsail.  In other words, you have taken out one too many chocks and are now bending your mast too much for this sail. Put one chock back in and you have found your heavy wind chock setting!

For light wind sailing in flat water you often need to flatten your mainsail a bit. Using the vang will flatten the sail, but it also hooks the leach in light weather. At any time a hooking leach is slow.  Instead, try taking the chocks out from in front of the mast and instead put them in behind the mast. (Note: not all Pacers can do this because of the way their mast gate is constructed.)  This gives you a flatter sail with a loose leach – just what you need for lighter weather.

The above advice is “orthodox” thinking. Our current dual National Champion is a bit more radical and always puts his chocks in behind his mast to induce pre-bend. His theory is the mast tip does not bend much as it is untapered and the mast needs to bend somewhere, so he lets it bend at the bottom.  This certainly seems to work for his mast/sail combination.

My Advice

Having determined your neutral and heavy wind setting, go and for a sail and play with your chocks. Start with the neutral position and sail upwind, preferably with another boat acting as a tuning partner.  In the neutral position I sometimes find that the boat feels “bound up”, particularly when a gust hits.  The boat travels well in the lulls, but when hit by a gust it staggers and goes slower.  Keep taking chocks out to let the mast bend until you feel the boat “unlock”.  The boat feels “in the groove” and accelerates in the gusts, rather than staggering.  I like to call this “easy” speed.  Unfortunately, there are no hard and fast rules for this since it depends on the wind strength, your mast/sail combination.  My “easy speed” setting varies depending on the weight of my crew. My heavier crew needs more chocks than my lighter one. So get out there, have fun and play!

Alan was the runner up at the 2011/12 Nationals in Canberra and a previous State Sabre Champion. He knows what he is talking about and is keen to pass on his knowlegde. Well done Alan!