Epoxy Coating Mast

Did some final sanding on the mast and laid down the first coat of epoxy. I was planning to use the wet-on-wet technique Mik recommends but with temperatures in the low 60's the epoxy just would not go off fast enough so a bit of sanding in between coats will be necessary. It was easiest to hang the mast from both ends but the swinging proved to be a nuisance. The next coats will be done with the ends of the mast resting on something rigid. Spreading the epoxy with a foam brush was slow and cumbersome. I also tried skidding a foam roller as per Mik's instructions and that was slow as well and the swinging mast made it infuriating. I ended up just slathering the epoxy on with gloved hands like greasing a pole. It was fast and worked surprisingly well.

I did not bother to shape the base plug accurately since it is bedded in thickened epoxy and the mast base is not visible with the boat rigged. But it looks ugly and I just realized that with the mast de-rigged on the trailer the base of the mast is very prominent.  Oh well next time the base plug fits better.

Total time to date:  120hrs

Rounding the Mast

I cannot believe it's been a month since the last update. It feels like barely a week has gone by. The mast is almost ready for epoxy coating. I planed the ridges off to turn the mast section into an octagon.  Then I planed the octagon to a sixteenagon.  All this planing was pretty easy to do by eye.  Then I switched over to sanding.  I was not sure I would know when to stop so I penciled a wavy line on each of the 8 original octagon faces.  This acted like a guide coat of paint when fairing a hull in that I knew to stop as soon as the pencil line disappeared. This worked very well.

I made a semicircular sanding block to make sure I was sanding a round mast.  In order to get the right diameter I used a piece of 3" PVC pipe and wrapped a few sheets of printer paper around it to bring it up to the mast diameter.  Then I wrapper it with wax paper and laid up a few layers of scrap fiberglass mat over that.  (Cheap wax paper is porous.  Guess how I found that out.  Use name brand stuff.)  Once the resin kicked I cut it apart and used a light coat of 3M 77 spray mount adhesive to attache the 60 grit sandpaper.

I think I could have planed some more wood off the mast before switching to sanding.  The first sanding strokes were a bit tight but it got progressively easier as the mast became rounder.  Picture below is before the handle was glued on.

By stroke number 12387663346 the mast was getting pretty round.  The handle is made from the triangular scrap cutoff from the bottom of the dagger board.

Getting there.

Me likey round mast.

The tapered sections were sanded with the same curved block and as long as I did not sand too long in one spot the mast became nicely round, although slightly less perfectly round than in the un-tapered section.  Surprisingly the mast warped a bit during all this sanding.  It was very straight to begin with and now it has about 1/4 inch of bow in it.  This is insignificant but .... The sanding conicided with our summer heat and humidity breaking so maybe it is the lower humidity that is responsible.

Total time to date: 108 hrs.

Mast is Assembled

Removed the mast from the gluing jig Monday night. It's a pretty substantial stick. Looks a lot bigger once raised vertically.  Of course I had to run around the yard with it making vroom-vroom sounds.

It'll look a lot better once rounded over but the taper is already looking good.

Final Mast Assembly

So far so good on the mast assembly.  It looks like the mast is straight to +/- 1/8 inch or so.  The two halves separated nicely and there was ample epoxy squoze out from the joints.

I made plugs for the base, partners and tip.  The practice mast section I made earlier came in handy as I just traced the interior octagon onto some scrap timber and bandsawed the plugs slightly under-size in a few minutes.

 The mast base plug is a bit smaller due to the taper but I did not bother to lay it out accurately, just eyeballed it and it fits fine.  The mast tip plug started out as a tapered octagon but I quickly gave up on that and just hit it with a belt sander to get a tapered roundish shape.  All the plugs are set in thickened epoxy so the shapes and fit are not critical.  The base plug is about 1.5 inches thick.  The partners plug is two 1.5 inch thick plugs back to back.  And the tip plug is about 6 inches long.  That should be strong enough.  I drilled holes through the partners and base plugs for ventilation if need be.  I might make a little screw in plug for the base hole to use while sailing and leave it out when mast is in storage, maybe.

Base and partners plugs dry fit

Looking at the partners plug from mast base, dry fit

Mast tip plug dry fit

Mast interior coated with epoxy prior to assembly

Lots of thickened epoxy on plugs

I keep being surprised at how much epoxy this is taking.  I used about 12 oz total for the interior coating, bedding of plugs and joining of the two halves.  That brings the total epoxy used on the mast so far to about 26 oz.  That's 2 pounds of epoxy,  lot of weight!

Total time to date:  93 hrs

Fire the Lasaah!

Assembling the 8 staves into something resembling a straight stick would be pretty easy if the mast was untapered.  Since the GIS mast is tapered at both ends it's a bit difficult to eyeball straight, although probably doable with practice.  I tried using string and I think it would be OK but I'm allways in favor of applying maximum technology to solve a problem.  So out came the laser level and why limit yourself to one when your buddy has one too.  We set up two laser levels shining down the mast length at 90 degrees to each other.  This worked well but there was one complication.  The staves are tapered on only one side, opposite the bird's mouth cut so it appears that you can't just mark off a centerline down a stave and use that as a guide because each stave actually twists a bit as it bends to accomodate the taper. At least that's what it looks like to me.  Trying to think through this geometry makes my head hurt.  Anyway the aparent twist is not a lot but may be enough to cause a bend in the mast if a stave centerline is used as a reference.  To get around this we set the two laser beams to cross at 90 degrees in the center of the mast base. The horizontal beam was set parallel to the straight work surface that the untapered section of the mast was lightly clamped to.  The mast was then wiggled around such that the center of the opening at the mast tip coincided with the intersection of the two laser beams.  The actual intersection is not visible but can easily be eyeballed from where each laser line hits the mast tip.  Theoretically this setup aligned the base and tip concentric to each other.  The rest of the mast was eyeballed straight following the laser line.  Tomorrow, after the epoxy is set, I get to find out if all this goofing around with lasers made for a straight mast.  By the way I think just clamping the untapered section to a straight workbench or whatever and then eyeballing the rest of the mast would result in a maximum bend of probably not more than 1/4 inch at the tip.  At least it seems that way because the mast did not have to be moved much to align with the laser lines.

Epoxying the mast is a 2 person job, minimum.  At Michael Storer's suggestion we glued all but two opposing joints.  This will allow the mast to be split open for thorough epoxy coating and installation of  base, partner and tip reinforcing plugs.  The two unglued joints were covered with brown packing tape to prevent any stray epoxy from bonding them.  We turned all the staves bird's mouth up and spread epoxy, slightly thickened with wood flour, on each stave.  It took double the amount of epoxy I thought it would.  We started with 6 oz of epoxy with enough wood flour to increase the volume to maybe 7oz.  And ended up mixing a second batch.  Probably could have used a bit more as there are a few spots where barely any epoxy squeezed out.  Once all the staves were assembled I used brown packing tape to wrap the mast while Dave, my helper, held it together.  The packing tape worked pretty well.  The nicest thing about it is that it's thin enough to not interfere with the mast laying down flat on the staright reference work surface.  We also wrapped the mast with thin line and I liked the way that pulled the staves together.

One thing I would do differently is to cut the staves a bit long, maybe an inch or so.  I cut my staves to the exact length shown in the plans. After all of the tightening and wrestling with the mast it ended up with about 1/8 inch of stagger between stave ends.  The ends started out well aligned but the process of clamping the mast slid a couple of the staves a bit.  No big beal the mast will just end up about 1/4 inch shorter than specified after both ends are sanded flat.  This could be easily avoided by starting out with slightly over length staves.

Dry run aligning mast.  I used my foil shaping jig placed on the table saw as a flat reference surface.

Two laser levels 90 degrees apart used as reference lines to ensure a straight mast (I hope).  And yes, I removed the one clamped to the garage door before opening the door but forgot to remove the bit of ply it was clamped to .......

 

The laser lines set up to intersect at the center of the mast base.

 

Lasers are cool.

Even cooler with the lights out.

Total time to date: 90hrs

Bird's Mouth Mast

This bird's mouth mast business is cool.  Below is a picture of the trial sample mast section I made while setting up the table saw for cutting staves.

A bird's mouth mast is constructed by gluing together multiple wood strips (usually 8) to form a very strong and light hollow tube.  I chose to use douglas fir because it's readily available at Home Depot and 10 foot 1x2s happen to be slightly bigger than the stave dimensions needed for the GIS mast.  The pieces are scarfed together to yield eight 15.5 foot long staves.  The fir strips range in weight from 1.5lbs to almost 3, yes I weighed a bunch of them while suffering many a strange look from other customers.  If I had picked the heaviest wood the mast would weigh well over 30lb.  I was tempted to use all of the lightest pieces but they were significantly more flexible than the heaviest ones so I split the difference and got half medium heavy and half medium light.
A free standing mast is basically a cantilever beam which means that the stress in it decreases from base to tip.  The mast can therefore be progressively weaker and lighter from base to tip.  This is often done with a taper.  The GIS mast is very slightly tapered along its length but the majority of the taper occurs near the tip.  In order to maximize strength and minimize weight I used the heavy stiff wood on the bottom sections and the lighter on top.  Scarfs are distributed over a 5 foot long section about half way up the mast.  I'm hoping this will provide a smooth transition resulting in a mast which is plenty strong where it needs to be but as light as possible and with a low center of gravity.

Very simple but effective 10:1 taper jig set up on the table saw.  It's made from an appropriately tapered piece of 3/4" MDF with a block of wood glued to one edge.  The wood block allows clamping of each stave to the taper jig then the whole works is fed along the fence.

Gluing up the scarfs.  I let the epoxy absorb into the end grain until it looked like no more was soaking in and then lightly clamped the staves together using a long straight edge for alignment.  The joints fit so tightly that I did not feel the need to thicken the epoxy.

After ripping the staves to the correct dimensions the table saw is setup for cutting the bird's mouth.  For an 8 sided construction the bird's mouth is a pretty straight forward 45 degree cut.  45 x 8 = 360 cool huh?  Each stave is run through once, flipped end for end then run through again.  It is critical the the stave is not allowed to move up or away from the fence.  Feather boards insure this does not happen.  (what's up with all the bird references?) My vertical feather board is far from perfect but it worked well enough.

 

 

Now comes the tricky part.  The mast is tapered at both ends therefore each stave has to be tapered accordingly.  The side opposite the bird's mouth is planed to the profile specified in the plans.  The mast base taper is slight and I just marked a straight line on each stave and planed to shape.  The designer recommends lofting a curve for the taper profile but the difference between a straight line and a curve is minimal in this section so a straight line it is.  The tip taper is much more pronounced and a lot more wood has to be removed.  I made a template for the tip taper by lofting the specified profile on a long piece of 1/4 inch ply.  Then it's just a matter of tracing the same profile onto all 8 staves and cutting to shape.  I used a small band saw for the cuts and followed up with a hand plane.  I found it exceedingly difficult to maintain the planed edge at 90 degrees so a scrap of wood clamped to my plane made a reasonably good guide.

Stave marked and being cut on bandsaw.

All staves tapered.

Mast tip stave shapes.

I have read that bird's mouth mast construction is harder to explain than do and I think it's true.  All the steps went quite well and the staves almost self-assembled into the mast shape, a very rewarding moment.

Mast weight at this point is 19.5lb (8.9kg) including all the line holding it together..  I'm pretty happy with that. Final planing to round will remove a bit more weight but the epoxy will add some so I should come out around 20lbs.  I did a mock stepping and it's very manageable at this weight.

Total time to date: 85hrs