Replacing a Mast Boot Over the years I have tried several methods to create a watertight mast boot. As a temporary fix, I once used duct tape to form a mast boot. It was inexpensive and actually lasted about 10 months before it started to leak again. I tried using a couple of rolls of mast boot tape, which is a wide self-bonding tape. It was quite a bit more expensive and lasted only a year and a half before it started leaking. I also tried Spartite, a two part liquid that forms both a mast wedge and mast boot. It worked well, but is expensive and if not applied properly, can permanently bond the mast to the boat. While we were in New Zealand last year, we pulled the mast for a refit. When we re-stepped the mast we could not reuse the Spartite boot, and I began looking for another solution.

At the suggestion of a local rigger, I decided to try a one part waterproofing compound that is marketed for roof and gutter repairs. It is a thick, rubbery substance about the consistency of honey that can be brushed on. It will adhere to metal and painted surfaces, and once it dries, it remains flexible and is UV resistant. It seemed ideal for my application.

My first step in making the mast boot was to create a mold to support the waterproofing compound until it could set. I used my ever present duct tape to make the mold, but any wide, adhesive backed tape would work. I started the mold by making a wrap around the deck collar and continuing to make wraps upward until I had a cone shaped mold reaching from the deck collar to the mast. See Figure 1. Next I put a wrap of masking tape around the mast 1”-2” above the top of the mold, and applied masking tape to the deck below the mast collar.

Figure 1.

Making the mold I found the waterproofing compound at a local building supply store. The product itself was not hard to find, but it was difficult to find in a small quantity. For Nine of Cups, our 45' cutter, I needed less than a quart, and the usual quantity available was in 1 and 5 gallon size containers. At the end of this article, I have listed a few sources that will sell and ship the waterproofing compound in 1 gallon or smaller quantities if you cannot find it locally.

Using disposable chip brushes, I applied three thick coats, following the manufacturer's re-coat schedule. For the product I used, I was able to apply a new coat after eight hours of drying time, so I applied two coats the first day and the last coat on the second day. After it had dried beyond the tacky stage, I carefully removed the masking tape, using a razor blade as necessary. I then let the waterproofing compound cure for a couple of days.

Since the waterproofing is UV resistant, the mast boot is now functionally complete, but it doesn't look finished. I opted to make a cover from Sunbrella, using a small piece left over from our bimini project. The shape of the cover is a bit odd as shown in Figure 2. After a bit of trial and error, I came up with a simple formula that can be used to make the pattern.

Figure 2. Cover Pattern

First I measured the circumference of the mast, c1, and the circumference of the mast collar, c2. I estimated how high on the mast I wanted the cover to reach and put a pencil mark on the mast at that point. Then I measured the distance from the base of the mast collar to the pencil mark on the mast which is “ht” in the formula. You can calculate the two radii, r1 and r2, using the formulas below.

r1 = (c1*ht)/(c2-c1)

r2 = r1 + ht

For example, on Cups the mast has a circumference of 26” and the mast collar has a circumference of 31”. The distance from the bottom of the mast collar to the top of the cover is 7”. Plugging these values into the formulas:

r1 = (26*7)/(31-26) = 182/5 = 36.4”

r2 = 36.4 + 7 = 43.4”

Before marking and cutting the fabric, I made a template from paper to make sure the pattern was correct. This is shown in Photo1. Once I was convinced the template was accurate, I used it to draw the pattern onto the fabric. I added an extra 1” to one end of the pattern as shown to allow for an overlap, then added 1/2” to the entire outline to allow for a finished edge as shown in Figure 3. Next I folded the 1/2” of material along the outline and hand-stitched it. Marcie keeps a roll of basting tape on hand, which is narrow double-sided tape. This works well to hold the material in place while stitching it. The last step in the fabrication process was to hand-stitch mating pieces of velcro onto the ends of the fabric where it overlaps.

Photo 1. Paper Template

Figure 3. Fabric Pattern

The cover was installed by wrapping it tightly around the mast and collar and pressing the velcro strips together. I wanted to add a whipping with 1/8” white nylon line to the top and bottom to give it a finished look. In previous versions, I used a French spiral hitch and once I even finished it with a turk's head. It was a lot of work and I don't think anyone but me ever noticed. This time I used an easier method. I used a hot melt glue gun to attach one end of the small line to the cover about 1/2” from the top, then made 12 wraps of line tightly and neatly around the mast working upwards. I put a small dab of glue on each revolution and secured the end to the mast and the wrap just below it with another dab of glue. Using the same process, I also added a whipping at the bottom around the mast collar. The end result looks nice and I can remove it quite easily when necessary.

Photo 2. Finished Mast Boot

The mast boot has now been in place for over a year. It has been exposed to heavy seas and cold temperatures in the southern New Zealand and Tasmanian waters as well as the more tropical temperatures of Vanuatu and Fiji. So far it has shown no sign of leaking. I did find that the whipping at the bottom of the cover had a tendency to trap water between the cover and the mast boot, leading to a bit of mold growth. I have since removed the lower whipping.

I have also learned that the manufacturers of some of these products do not recommend its use on stainless steel because it will not adhere well. This should not be a problem in this application. If the mast collar is stainless, the membrane will form a tight, flexible boot over it, shedding water downward.

Published in Good Old Boat Magazine, Issue: 89 - March/April 2013


Figure 1. Making the mold Figure 2. Cover Pattern Figure 3. Fabric Pattern Photo 1. Template Photo 2. Finished

Mast Boot Sources:

Kool Seal Elastomeric Roof Coating, for data sheet; to purchase Liquid Rubber Highbuild S-200 Waterproofing, Duram 195 Waterproofing Membrane, Liquid Rubber EPDM,