Mast repair - scarf question

Discussion in 'Wooden Boat Building and Restoration' started by OrcaSea, Feb 4, 2015.

  1. OrcaSea
    Joined: Oct 2014
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    OrcaSea Senior Member

    Greetings, all,

    I am currently finishing the mast on my 16' '58 Crescent restoration.

    In the pic below you can see there is some wood damage to the bendy, solid wood (Doug Fir), three-piece laminated 2 1/8" X 2 1/4" tapered mast section where the spreader used to be (as some point a previous owner removed the spreader and aft shroud and simply ran the forward shroud to tangs bolted at this position). The damage is confined to about a 7-8" area and does not extend into the laminations.

    I plan on making a new spreader and returning to the drawing configuration.

    I know I have to match the wood and the grain for the repair. I also know that I will need to scarf the joint and not simply insert a rectangular mortise-and-tenon-style plug.

    My question is, how long should I make the tapered scarfed repair, especially given the stress on the area? There is no rot on the other side, so how much 'flat' should I have on the other side?

    Also, any tips & tricks for chiseling out the scarf for the best fit? I'm concerned that an ill-fit scarf would be worse than a well-fit mortise & tenon-style plug...

    Thanks!

    Curtis

    [​IMG]
     
  2. PAR
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    PAR Yacht Designer/Builder

    12:1 is the usual ratio for a scarf in a spar. Since it's a solid mast, you can go down to 8:1 with little regard to a problem, though the possibility of stress risers or a "hard point" being created is increased with the steeper slope of an 8:1 scarf.

    As to how to cut it out, well, really sharp chisels and a small shoulder plane will get the job done. Working inside a hole is always going to be difficult but one tip might be to only concern yourself about the fit of the edges. Make the edges fit tight, which is simply an aesthetic consideration. In side the tight fitting edges, the joint can be a bit sloppy, as thickened epoxy will fill the gaps just fine. In fact back cutting a joint to offer a place for the epoxy to live in bulk, while having a tight fitting edge is a great way to insure a joint stays stuck and looks good.
     
  3. OrcaSea
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    OrcaSea Senior Member

    Thanks, Paul, I was kind of thinking the same thing with regards to fitting the interior of the scarf. I couldn't think of a way to do that with great precision that didn't involve taking all day with a miter gauge and tiny, incremental cuts. I like to put a small 'step' on the edges as that final tapered edge is always so awkward to fit and a step with a little depth to it gives a nice, straight, firm line.

    I will split the difference at 10:1 and leave at least a 3" flat on the other side, so a 23" 'long side' through 2" spar and 3" short-side flat sounds about right?
     
  4. OrcaSea
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    OrcaSea Senior Member

    No, I think my math is wrong (art school - two degrees, ONE math class!).

    10:1 slope would be 40" X 2" (with two slopes).

    Right...?
     
  5. waikikin
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    waikikin Senior Member

    Hi OrcaSea,

    If you make your "gravo" diamond shaped you get to save a bit of length. When doing inserts like tat with hand tools I'll usually have around a 3/8 flat on the end/tips of the diamond ends- just easier to chisel... & seems less inclined to split at the ends later, you can stretch out to 12:1 or better no probs. If you have a router, a simple jig can make those blind holes easy especially with a template follower.

    Jeff
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You could build a jig with double ramps and a flat bottom router bit. You'd get nice edges and it would be quick, but I'll bet you could hack it out with a hand chisel in the time it took to build and setup the router jig. When doing the feather edge of the scarf, cut through the end of the piece with a razor, a 1/16" or so and bevel to the bottom of this razor cut. This will offer a shoulder to butt the other side against. Fitting in a Dutchman like this is a pain in the butt, but just concentrate on the edges and back butter with goo, to hide your sins.
     
  7. OrcaSea
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    OrcaSea Senior Member

    Alas, no router - gotta go old school. That's okay though, I like the challenge. I'll cut a foam board go-by so I can see inside the hole and just work my way out from the center until it seats nicely. I'm sure it will be rough enough to give the goo plenty to grab onto. It's just gonna take a while...

    I actually cut a 1/4" step into the ends. I've done this before in a place or two, and it always looks nice and I don't have to worry about the this edge splitting or lifting up or not being even.

    Do you suggest wood powder or silica (I'll pre-coat the surfaces with straight goo) as filler, or does it matter? I'm leaning towards the wood unless silica has better properties for tying the wood all together? My reasoning is that wood filler will match up better with regards to shrinkage & expansion...?

    I decided to go ahead with a trapezoidal shape with a 10:1 slope and 2" of exposed flat on the other side rather than triangle; I like the idea of the spreader bolt entering and exiting through the same piece of wood, leaving about an inch of meat on either side of the hole on the opposite side. I left 1/8" excess on both sides so I can plane it down to match after it's glued in. If I'm careful with measurements it should look okay.

    Thanks, as always!

    C
     
  8. PAR
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    PAR Yacht Designer/Builder

    I'd use a milled fiber and wood flour mix, with enough silica to control viscosity. Cotton flock could be substituted for the milled fibers, it's just I like a longer fiber in the mix with flour and silica.

    If you have a bit of hardware mounting in this area, why not incorporate the repair with a "stand off" of some sort, offering some additional "meat" for the fasteners.
     
    Last edited: Feb 13, 2015
  9. OrcaSea
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    OrcaSea Senior Member

    Sorry for the late reply; dang need to make a living...

    Paul, could you expound a bit on what a 'sand off' is? I think I know what you mean, but given what I have discovered I want every advantage in my corner.

    My repair has expanded a bit (don't they always?). Seems the mast is actually hollow, except for a 1-foot section where the spreader and forestay passes through (or fastens) to the mast. The solid wood was all rotten, so I removed it. Long story short, instead of one scarfed splice there will be two (maybe three) more elaborate ones on two or three sides of the mast. Luckily, the damage is quite high in the mast and located in the solid reinforced area - just a couple feet from the top, otherwise I would have just considered making a whole new mast.

    There are about 70 1/8" holes where the (unsealed) luff track was fastened, and I'm a little leery of the quality of wood on that particular side, but my explorations in the repair area seem to show reasonably solid wood associated with the track fasteners. None the less, I will drill and plug with dowel and when the new track goes in I will invest in some 4200. I have developed a strong disrespect for people who don't properly seal fittings & fasteners... ;)

    Curtis
     
  10. PAR
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    PAR Yacht Designer/Builder

    Sand off was a typo and I've corrected it to "stand off" which should be self explanatory.

    It's common to have hard points within a mast, typically at high stress areas, like the gooseneck and rigging attachments. This should be tapered (called swallow tails) bits of wood, glued internally, but most importantly a significant drain hole drilled through these hard point areas. It's not unusual for the drain to collect dust, bugs, etc. in time, clogging up, which traps moisture, with it's eventual unfortunate outcome.

    Instead of doweling the mast, consider using a "bonded" fastener, using epoxy instead. This will prevent moisture from getting in, even if you don't have a good bedding under the track and fasteners. Bonding also greatly improves the pull out strength of the fasteners too.
     
  11. SukiSolo
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    SukiSolo Senior Member

    I would personally use cut core plugs in Doug Fir as per the mast construction, rather than long grain dowel. If the holes are very 'soft' go up a bit in size so the new cross grain plugs are fully secure. Even better if you can match the grain spacing of the existing, if in doubt go a bit tighter, especially if you want to reuse.

    Cross grain cutters tend to go in size from 9/32" through to 10mm and 12mm (3/8" to 1/2") just remember to cut them on a pillar drill.....;) If you have the wall thickness and the cutter is long enough it is also possible to cut them from a pre sealed (on back face) correct thickness piece of timber. This way you can drive the plug in flush and know the internal end is 1. Sealed and 2. Flush with the hollow internal wall.

    Then follow PAR's wise words and bond in the fasteners in the new plugs, or choose a different spacing. Visually it may be better to use the old but soundly repaired holes, especially if you have a clear top finish.
     
  12. OrcaSea
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    OrcaSea Senior Member

    Thanks, guys, for the advice - always appreciated!

    It's funny that you mention insects, Paul, as that is exactly what I ran into. The rotted solid section contained a large number of very old pupae rolled up in cocoons made from leaves. I thought to myself, 'This is how sci-fi movies begin...' and put on my respirator! ;) Many were embedded in the wood plug, and their decomposition no doubt assisted in the rotting of the wood.

    The track fasteners were pretty small (it's a light, hat-shaped track), maybe #8's, if not smaller (it looks like the original builder actually used SS sheetmetal screws). After I am done with the splice I will investigate the holes further and see what I am dealing with as far as the wood integrity goes. Unfortunately, I don't have a drill press (yet) though I know I will need one soon. I am watching Craigslist for that and a 14" band saw, as well, but those are hard to find...

    I do have a question about finishing the mast. I am going to use a water-based stain and let it dry for a month, or so. Then what I was planning was using an epoxy base under varnish. I'm reading a lot of good things about it, as it allows for easier refinishing down the road; just scrape/sand to the epoxy base and reapply the coats of varnish. The question is, on a mast application will the epoxy be flexible enough, or will it crack/delaminate?

    Curtis
     
  13. SukiSolo
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    SukiSolo Senior Member

    Well, epoxy coated and sheathed foils don't seem to crack and they flex - even more when you stand on them righting the boat!.

    Personally I'm still a bit wary under varnish on topsides etc because of UV degradation. The epoxy will be fine on the mast in terms of application etc but my personal preference would be paint white to resist both UV and any extra heat. If you use a specific UV resistant varnish (which are available) that may be OK too. If the repair scarf is a little let us say less than perfect and it is not easy to get a really sweet join, then painting may be a good option. You know where the trouble is likely to be ie screws etc so you can always check them after some time. If your worried you need to see through the finish to check on the wood then epoxy and varnish, but expect it to yellow quite a lot. Interestingly when I've repaired foils (epoxy coated) that have been painted the wood surface is still bright unlike a varnished epoxied foil because the UV has not affected it.
     
  14. OrcaSea
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    OrcaSea Senior Member

    Thanks, Suki, some good food for thought there.

    One consideration is that this is a trailered boat, and the mast & boom will reside in the garage, on-season and off. I think you're right that, given time, it will likely yellow, but as it is not a marina boat and the mast & boom will almost always be indoors it might be years and years. And I do love the look of wood on a boat :) If the epoxy gets ugly in a few years white paint will look great! LOL

    One little trick from the R/C world (where a lot of basswood & balsa is used) is to put a drop of thin CA glue in a fastener hole where it gets absorbed into the fibers and hardens/strengthens the area. That, along with goo on installation, might be a good compromise to plugs as the area at the fastener holes seems pretty solid. I thought I could drive screws fast at Boeing, but driving 70 screws before the goo sets off might be a trick ;)

    For being all hand-laid out, cut & planed I'm happy (and frankly, a little surprised) at how well the scarfs are turning out thus far. There is one that I'm not terribly proud of, and if I have the appropriate length of wood I will likely re-make it.

    Thanks, again, for the advice :)

    Curtis
     

  15. SukiSolo
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    SukiSolo Senior Member

    I'd use the trick suggested by PAR and others, put epoxy on the screw threads. To remove use a heated driver. If you are careful with the type of epoxy and study the melt temperatures you can use a 'high' temp one for sheathing (West for example is about 230 deg C) and a low temp one for the screw threads (Araldite is about 150 deg C) which gives a useful window to get screws in/out with a hot driver without damaging the main coating.

    Absolute minimum is to saturate the screw holes and threads with varnish when fastening in. Usually OK for daysailed dinghies, stuff left afloat requires more durable ie epoxued solutions.

    Wood starts to char at around 225/230 deg C btw.
     
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