Monday, April 27, 2015

Folding Mast - Hollow, Sixteen-Sided (will end up round)

In Woodenboat Magazine #237 of March/April 2014, there is an article written by David McCulloch about building an "In-Mast Hinge" which he has designed and developed. I urge you to purchase a digital issue of this copy if you don't already have it, as Mr. McCulloch has written a good article, and there is an excellent coloured illustration.

 https://www.woodenboatstore.com/product/WoodenBoat-magazine-issue-237-march-april-2014-DIGITAL

David McCollough"s photograph of his "In-Mast Hinge" from the Woodenboat Magazine article. Note the detail in the upper-right corner.
This article inspired a customer of mine to inquire about including such a hinge in a replacement mast for a large, open daysailer he had recently purchased. After some discussion, and an inspection of the boat, I agreed to attempt the project. It was necessary for me to make modified drawings for the plates to fit the mast I was commissioned to build due to its different diameter from that shown in the article.

The location of pivot holes, and the radii of cuts made to all three plates alter depending on the geometry, which is itself dependent on the diameter of the mast. However, this is not at all difficult to work out, and just requires attention to detail.  For those who may be interested, here are some early progress photos: -

One half of the mast being laid up in a female station mould mounted on part of my 12 metre (40 ft)-long bench. The mast is made up of 16 staves, so this half-shell comes from 8 tapered pieces.

All eight staves glued up in the female mould. The first stave was fixed along a marked centreline in the bottom of the female stations, using 18 gauge polymer brads fired from a pneumatic branding gun through the stave and into the plywood edge of the station mould. Subsequent staves were laid up on either side of the "master" and glued using epoxy. The overwhelming reason for using epoxy is that it only requires contact pressure to form a good bond. Careful attention must be paid to priming the gluing surfaces with un-thickened epoxy before applying the thickened mixture. This method allowed me to fire polymer brads into the SIDES of each successive stave to hold it to the one before, because the epoxy did not require clamping pressure. The polymer brads stay inside the shell of the mast, and will of course never corrode. The bradder and polymer nails were purchased from Duckworks. After being glued-up, the inside of the half-shell was given three or four full coats of epoxy to ensure that the inside of the finished mast would always be protected from moisture.

Here is the station mould after the removal of the first mast half-shell. I used adhesive tape applied to the inside  of the cut-out section of each mould to prevent epoxy squeeze-out gluing the mast components into the mould. Alignment of moulds is very important, and you can see the blue chalk-line "snapped" onto the bench surface (now covered with epoxy drips!)

Two half-shells of the mast after being removed from the station mould visible in the top/right of the photo. The outer surfaces are still rough-looking due to epoxy marks. When first removed from the mould, the half-shells had lots of thickened epoxy squeeze out, which I largely removed using a heat-gun and scraper. Any gaps were filled with additional epoxy.

Here you can see the two shells clamped together with cable-ties and hose-clamps. The extra length of the staves has been roughly cut off using a handsaw and you may just be able to see that there is no glue on the opposing faces of the two shells. Note the heavy layer of sealing epoxy on the inside of the hollow mast.

This is a similar photo, but taken from the mast head. The trimming of the extra length  was done quite roughly, and the un-glued faces of the staves still need to be sanded to remove dags of cured epoxy, so the gaps in the un-glued faces are a bit open. This will change prior to final assembly. Note that the taper of the mast resulted in a reduced outside diameter, but the thickness of the staves has reduced as well. The idea is to keep the percentage wall thickness of the mast constant at about 20% of the diameter. This is something which can't normally be done with a "Bird's Mouth" mast. (see photo below)  
An off-cut from the tip of  "Bird's Mouth" mast I made. a while ago. Note how the wall thickness, which started off at the base being 17% of the diameter, has ended up being so large it almost makes the mast solid.  Compare with the previous photograph.

Three stainless steel plates to make up the hinged section of the mast (refer to the inset in the David McCulloch photo at the beginning of this post to see how this works). These plates are quite heavy, with the outer pair being 6mm thick, and the inner one being 8mm. I had these laser-cut, which saved a lot of time, and was not expensive.
This sketch shows the stainless steel plates extended and folded. The black lines depict the 8mm plate which is buried for half of its length into a solid mast stub which runs from a mast step near the keel to a short distance above the deckline of the boat. The red lines show the two 6mm stainless steel plates, which are fully buried in the base of the mast you can see in this article.
Here is a close-up of the insert from the heading photo in David McCulloch's article in Woodenboat Magazine #237.
 https://www.woodenboatstore.com/product/WoodenBoat-magazine-issue-237-march-april-2014-DIGITAL 

There needs to be a solid section at the base to hold the stainless plates. On the free-standing hollow masts which I normally make, I always insert a solid section from the base of the mast, to a reasonable distance above the mast partners. In both my normal free-standing mast inserts, and with this one, I terminate the solid insert with what is often referred to as a "Swallow Tail", the purpose of which is to ease the transition in stiffness from where the mast is solid (i.e. the hollow shell combined with the solid inset) to where it is a hollow shell. Here is a view of the early stages of the solid insert for this mast.

As you can see, there is the "Swallow Tail" section, and at the base, a cut-out slot to accept the stainless steel plates and an 8mm hardwood inset to hold them apart and allow the 8mm tongue of stainless from the mast stub to fit between. At this point I have planed a square blank into an octagonal section - it is on its way to becoming round!
At this point the blank has been planed from octagonal, to 16-sided, and them planed further to 32-sided, and then hand sanded to a round cross-section.. All of this sounds complicated and difficult, but if you start with an accurately cut square blank, and then mark carefully for the 8, to 16, to 32-sided planing work, you'll find that the work goes quickly, and is actually a satisfying and relaxing job. As with most boatbuilding work, it just a progression of simple steps. I can tell you I'm finding it more difficult to describe than it is to achieve on the bench. Other than the initial sawing of the square blank, I did all of this with a low-angle block-plane you can see in the photo before this one. It did not take long.

The hole you can see drilled through the filler block at the apex of the "Swallow Tail" is just something I do to prevent a crack propagating from the apex. This may be an over-kill, as the block will be contained within the mast shell anyway, but it only took a moment to drill - so better to be safe than sorry... 

Here is where the stainless steel plate assembly will eventually reside. Matching slots will have to be cut in the mast shells.


Checking the fit of the insert. The actual fitting will be done with the two half-shells of the mast opened up, and the plug will be laid into a bed of epoxy in one half, before the pair of shells are finally glued together to form a round, tapered mast. The tip of the mast will receive a similar (but smaller) plug to distribute loads from the shrouds and fore-stay, and to carry the attachments for the halyard blocks
I'll write more about this interesting folding mast experiment as the job progresses.


2 comments:

  1. Very nice work. Are you going to show the taper process. Do you cut the taper on the 90 side or the 40 degree side

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  2. Hey Ross! Nice work. Coincidentally, I just happen to have finished making the same hinge for my Welsford Navigator build. I had clipped the article a year ago when it came out. You can see my (not so professional) results on the last posting on my blog: http://johnsnavigatorbuild.blogspot.com

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