Sailboat Rigging: Blocking and Sealing the Mast Partners

How do you seal and secure a mast that goes through the deck?

23rd December 2010.
By Paul Grimes

Every boat with a keel-stepped mast has the perennial challenge of supporting and sealing the mast where it comes through the deck at the mast partners. Those who are new to sailing might wonder why the mast goes through the deck anyway – and on some boats it does mount right on top, which seems like an easier solution.

The mast collar – just before sealing it up

Here’s why most bigger boats have the mast go through the deck:

  1. In a stayed rig, there is a tremendous amount of compression in the mast created by the rig tension. There is no way that a deck could support that compression without a structure below, supporting it from the keel area. So, one approach is to mount the mast directly on the keel.
  2. The mast is a column in compression, and supporting it in two places at the bottom adds a lot of stability – either allowing for better control of the section, a narrower shroud base for tighter sheeting angles on overlapping jibs, a lighter mast section, or a combination of all three.

Rubrail used as mast blocks

Boats that mount the mast on the deck do have some advantages, though:

  1. On smaller boats, the mast can be on a hinged step to allow it to be raised and lowered with a bridle system.
  2. Stepping the mast on the deck eliminates leaking at the partners.
  3. Some consider it a safety issue – with less potential for damage to the deck or interior if the mast breaks.

Pulling the mast sideways

On our J/35, the mast is stepped belowdecks and must be supported and sealed at the partners. The best way to do this with any mast is by using a pourable rubber called Spartite. With that system, you install the mast, get it positioned perfectly, create a dam with foam and clay at the bottom of the gap between the mast and the collar, pour the rubber between the two, and let it harden. Oh, and one more thing – be sure to rub plenty of petroleum jelly on the inner face of the mast collar. Otherwise, when you try to remove the mast the next time, the collar and a good part of the deck might come with it!

In our case, I wasn’t confident enough about the “positioned perfectly” part of the Spartite process to go with that system the first season. Instead, I found some strips of vinyl rubrail material that Waterline Systems sells for blocking J/105 masts in their partners. This turned out to be just about the right size for the space between our mast and collar as well (with a little sanding here and there).

Forward rubrail piece installedd

I cut this material into sections and hammered it in place with a mallet. The forward and aft pieces went in easily, but the side pieces were tough. To get those in, I used a line tied around the mast, led outboard to the forward jib lead and back to a winch, to pull the mast one way or the other. It was just enough to make the pieces fit.

To seal the partners, I had envisioned using a mast boot and hose clamps, but the lugs for the vang toggle are so low on the mast that there just wasn’t enough room. Luckily there’s a better solution anyway – stretchy tape from Spartite or Holt-Allen used for exactly this purpose. It self-bonding, and stretches so much that the bottom can go around the partners, while the top makes contact with the mast. The key is to keep wrapping, keep stretching and keep pressing it against itself – fantastic stuff.

Pulling hard to stretch the tape

Is it perfect? Well . . . no; water can still trickle down the sail slot in the extrusion, or come down inside the mast after entering through halyard exits, but this got us going for year one, with a mast pretty well secured and pretty well sealed at the partners.

Editor’s Note: This article is part of an ongoing series on used boat repairs. Previous posts include:

To Buy a Boat or Not to Buy a Boat

Used Boat Ads

How to Talk with the Broker

Looking at the Boat

Reaching an Agreement

Choosing a Name

Keel Repair and Fairing

Working with Boatyards

Rigging Your Mast

Adjusting the Mast Step

Paul Grimes is an engineer and marine surveyor living in Portsmouth, RI. For more information, visit the Grimes Yacht Services website.


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