Mast Chocks – Traditional vs Technical
Like many construction features on a sailboat, there is more than one way to support a keel stepped mast at the deck collar and seal this opening from rain or seawater. In the past 6 months we have chocked our mast 2 different times with two different techniques. First we used the more traditional approach using hardwood wedges for chocking and a mast boot to seal out water. Most recently we used the Spartite polymer “cast in place” mast chock and sealing ring.
Our boat is a 1981 Whitby 42 center cockpit ketch. The main mast is keel stepped passing through a robust fiberglass deck collar on the cabin top. We sailed over 10,000 miles with the hardwood wedges that were in place when we purchased the boat. They were likely quite old as many of them were splintered on the ends from being hammered into place over many years of mast installation and removal. Since the boat was based in Maine, this was done each winter for storage on the hard.
Last Spring, near the end of a particularly lively 3 day passage from Belize to Florida, I noticed a creaking noise coming from the area of the mast collar. Unfortunately, our mast passes through the deck into a box shielding the mast from the interior. A quick inspection of the mast chocks is not possible because the top of the chocks are covered by a mast boot that is not easily removed and the bottom side is inside the mast box. There is a floor to ceiling panel of the mast box that can be opened by removing about a dozen screws. To better troubleshoot the creaking sound at the mast, I opened this panel and discovered that one of the hardwood wedge mast chocks had fallen through the opening allowing all the other chocks to loosen and the mast was beginning to move within the mast collar. As you can imagine, this condition could be particularly dicey on a long offshore passage. As a quick remedy, I reinstalled the mast chock that had fallen out of place and tightened all the chocks with a rubber mallet. I re-used the mast boot that was in place but noted that the water tightness was likely compromised.
Before departing Maine in the Fall of 2017 for our trip to the Eastern Caribbean, I rebuilt the mast chock to assure that it would be robust for our planned offshore passage. I was able to purchase a half dozen new hardwood wedges from the Landing Wooden Boat School in Rockland, Maine. I also needed to replace the sealing material used for the mast boot. When we purchased the boat, the mast boot was a two piece construction. The inner layer was a soft, sticky black membrane material similar to ice and water shield used in home construction. This material was reusable to a degree but after years of manipulation, had developed a few holes. This layer was wrapped around the mast and secured top and bottom with giant hose clamps. The outer layer was a vinyl covering that wrapped around the mast and deck collar and was closed with Velcro. I could not find the exact membrane material but I realized from my construction experience that Vicor window and door flashing was very similar and likely a fraction of the cost.
Traditional wood wedges supporting the mast
Grace Vicor window flashing makes a great sealing barrier for a mast boot. A white vinyl boot covers this for aesthetics.
With the new materials in hand, I disassembled the mast chock in Rockland Harbor and reassembled with the new wedges and sealing material. Overall, I was pleased with the outcome but not fully convinced all our problems would be solved. The new sealing material looked great so I was not concerned with leaks. The bigger issue with the wedges I used is that they could still fall down into the deck collar if they loosened up over time. A better arrangement with wooden mast chocks is to cut them in a way that the top of the wedge has a lip that will stop the wedge from falling too far down into the collar. This requires custom cutting each wedge after measuring the gap between the mast and collar. The wedge must be cut so that it wedges in firmly without bottoming out on the lip at the head of the chock. I did not have the time or the resources to custom cut wedges so I buttoned up the project with the less than optimal wedges and headed south.
At the Annapolis boat show, I revisited the project by chatting with the many riggers with booths at the boat show. After several good discussions I decided that a Spartite mast chock was a great alternative to traditional wooden chocking. The Spartite Mast Wedge Replacement System comes in two sizes, small and large. For our mast, I purchased the large kit. I did not get a chance to install the Spartite mast chock before heading south, so the kit stayed in storage until we found a good spot to complete the project. One of the things recommended for installation is that you have a calm anchorage so that the mast is not swinging with wakes and surge. We found just the spot in the lagoon in St Martin.
The first step in the process is to remove all of the existing chocks. I was worried that my mast might move within the deck collar when I did this. I was pleasantly surprised that my mast moved less than 2mm when I removed all of the chocks. I left all the rigging tight in spite of the recommendation in the instructions to loosen rigging.
The Spartite kit includes all items required to complete the project except for standard masking tape. The Spartite mast chock is cast in place using a 2 part polymer compound. To create the form to pour the liquid into, you need to first create a dam within the deck collar to form the bottom of the chock. At the top you also create a circular form of masking tape to create the top lip of the chock. The Spartite kit includes foam insulation and modeling clay to pack between the mast and the deck collar to form the bottom dam. Standard 2” masking tape is used for the top form. Once the forms are complete a very important step is to apply the Vasaline supplied to the inside of the deck collar. The Vasaline is used as a mold release so that the polyurethane chock does not bond to the inside of the deck collar. It is absolutely necessary to have this mold release so that when the mast is removed at a later date, the polymer chock comes out of the deck collar. As an option, it is also recommended to coat the mast so that the polymer chock may be slide up the mast after the mast is removed. Once the forms are in place and the Vasaline applied, the last step is easy. The kit includes 2 mixing buckets and pre-measured cans of the 2 part polyurethane compound. You simply mix the two parts together and then pour the liquid into the form around the mast. The material hardens quickly and within a few hours it is firm although the center remains curing for a couple of days. The instructions recommend not sailing for a couple of days to assure the material is fully hardened.
Based on our experience, we would strongly recommend the Spartite solution. It is simple to complete and the result is a mast chock that will not loosen in rough conditions. It is also completely watertight. No mast boot is required for sealing purposes. We fabricated a canvas mast boot for cosmetic purposes.
Masking tape creates the top of the form with modeling clay sealing the bottom
Modeling clay between deck collar and mast creates a seal so that polymer can be poured in from above
Cured polymer creates a one piece solid mast support replacing wedges
