Sunday, March 7, 2010

How To Cut a Mast Boot

(Those of you with deck-stepped masts can skip this post. )

A keel-stepped mast must pass thru the deck. Where it does so, some kind of seal must be made to prevent a deluge of rain water from running down the mast into the cabin. Traditionally, this has been accomplished with a mast boot - a piece of flexible, waterproof material which covers the deck penetration. It is a simple, low-tech solution, one that has withstood the test of time. But: how to cut the fabric so that it drapes smoothly from the mast down to the deck?

It's not hard. Here's how:

To begin, you will need three dimensions:
  • The circumference of the mast. Let's call this 'm'.
  • The circumference of the raised deck ring onto which the bottom of the boot will be clamped. We'll call this 'd'.
  • The height up the mast, measured from the deck, that the boot is to extend. Note that what follows is based on geometry that assumes that the deck ring and the mast have circular cross-sections. If yours is not (and it probably isn't), then add an inch or so to the desired height to allow for some final trimming. We'll call this 'h'.
Make these measurements and write them down.

Now, we need to do a little math - get out your calculator. We need the diameters of the mast and the deck ring:
  • Dm = m / 3.1416
  • Dd = d / 3.1416
Make these calculations, and write the results down.

You will be cutting out a shape that looks like this. We already have 'h', so all that we need now is R1 and R2.

To get R1, we need to do a little more math, using the numbers we have prepared above. Use this formula:

R1 = (h * Dd) / (Dd - Dm)

That is, multiply h by the diameter of the deck ring, and then divide the resulting number by the difference between the diameter of the deck ring and the diameter of the mast.

R2 is much easier:

R2 = R1 - h

With R1 and R2, you are prepared to lay out the pattern on a piece of suitable material (we used white naugahyde purchased from a local fabric store). You will find that R1 and R2 will turn out to be pretty long - you will probably need to do the layout out on the dock. Mark the pivot point on the dock, and set some weights on the fabric (wrong side up) at the right distance. Then make a series of marks using a tape measure at distance R1 from the pivot. Similarly, make a second series of marks at R2.

Connect the marks with smooth curves, and cut it out! Cut a long enough portion of the arc so that there will be an overlap of a few inches (you have the circumferences...). Take it up to the mast and make a trial fit, arranging things so that the overlap is at the back of the mast.

If your mast is not a circular cross section, the top and bottom edges will be wavy instead of straight. Put on the hose clamps top and bottom, and adjust everything as necessary for a nice fit. Using a ballpoint pen, mark at the edges of the hose clamps to get a straight line. Pull off the boot and cut at the line you just marked. When you reinstall the boot, it will now have an even top and bottom.

Do not neglect the seam at the back where the two ends of the boot overlap - it needs to be sealed, or rain water will still find its way below. We used 3M 5200 - it works great!

Finally, seal the top edge to the mast with rigging tape.  Update:  We've found that aluminum tape is a better answer for sealing the top edge.

Now you have a traditional and functional seal between the mast and the deck. Pour yourself a grog, and feel the fellowship of shipwrights who have done this very same task for centuries - you've earned it. Arrrr.


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