Scarfing Jig?

[thugdrummer]

I'm in the process of repairing a 40' spruce box beam mast that had some rot at the masthead. I've carved out the bad wood, and I've got to replace about the top 12 inches.

My question is, how do I cut the mast and the replacement wood so that I've got nice matching surfaces for the scarf? I've seen a description online somewhere, but unfortunately it wasn't very clear.

I'd appreciate any advice.

- John

[Jay Greer]

John, what you are dealing with is a very complicated and sophisticated project.
Even so, with patiance a good planning if you are not in a hurry, I am sure that the job can be accomplished by a person who is handy with wood working in general but not necessarily a spar maker.
There are pictures of the splicing project that show how I replaced a broken mast head for my H28 "Bright Star" in the forum files. First and foremost, you will need to build a stable support system for the mast in such a manner that it can be rotated during the various phases of cutting and fitting of the splice.
Bear in mind that 12:1 is the ratio for the scarf and that your own mast is, most likely constucted with the fore and aft panels rabeted so that the sides can fit into them with the shoulder of the rabbet acting as a stop when the sides are clamped in. The sides and the fore and aft pieces should not be spliced at the same point. This means that several feet of wood must be allowed in the staggering of the joint. Other wise you will end up with a weak mast head. My sides were almost three feet below the fore and aft stave splices at their joints. In addition a filler block will need to be placed within in order to give proper support to the back feathers of the scarf joints. The scarfs should be made so that the thin taper faces down on the outer surfaces as this gives the least exposure to the slant of the sun. UV exposue is the enemy of glued joints. So I always place the thinnest face downward to minimise sun exposure. And, moisture will drain off of the face rather than into it should the finish be compromised. You will need a good collection of chisels and carving gouges as well as rabbet planes to work down the faces on the existing mast. I made an investment in some special rabbet planes that are known as "Bunny Planes". These were a great help in cleaning out the rabbet joints on the existing mast. New pieces were repeatedly fitted with carbon paper in between the faying surfaces in order to insure a good fit. This is one place where I broke my own tradition of avoiding epoxy for mast construction as glue pressure and exact fit of the joinery was not as fine as I would have liked. I will be happy to walk you through the process further if you are in need of help.
One further point is that because of the complex and delicate nature of the existing areas to be shaped on the mast, a jig for that purpose
poses some problems of clearance logistics for either a router or power plane. One slip or mistake and a router or power plane can lunch the project.
Jay

[Jay Greer]

I forgot to mention that the sides are cut lower than the fore and aft pieces in order to give stability to the sides during working of the scarf joints. This way, the feather ends are locked into the existing rabets on the forward and aftrer staves. There is a bit of a challenge here to work the sarf down into the rabbet on the existing section of the mast. That is where the Bunny Planes come into play.
Jay

[Ian McColgin]

I've done this a bit more simply than Jay's method. 12:1 for sure but if you make a cloths pin scarf the total space used up is 6:1. ==>>===

Make sure you have the measurments exact before you take things too much apart, especially the landings for the tangs. Then lop of the top and bevel back the sides such that you have your cloths pin male surfaces point up. This makes a conventional across the face scarfing surface on the side staves and just an inverted V on the fore and aft staves. This is easy work. Use a long plane and get the two surfaces really really flat and clean. When it's as perfect as may be, circumsize the sharp edge. This will make the fitting much easier and will leave a small guide hole for drilling out a stopwater at the end.

Make the top of the mast, shape for the crane and halyard shivs. Basicly, duplicate the previous structure, including any solit parts, compression blocks, ets.

This is a good time to fully understand what caused your problems and correct if possible. If it was rot, it was likely rot from moisture intrusion from tang fastenings. That can be cured by proper bedding. Maybe it was the way the end grain under the halyard shivs absorbed water. That can be prevented by good sealing, CPES and then paint, during construction. Even if you finish the mast bright, all up facing surfaces should be painted white. Only the photog in the helio will see.

Remember to cock them such that the hoist of the main is about dead center back of the mast with the fall off-set a nudge to starboard (whether internal or external halyards) and the hoist of the jib about centered on the foreward face with its fall off-set to port.


Whole thing made up and a bit longer than the inverted V left on the top of the mast. Also, make the new section a little more robust all around so you can in the end plane to a perfect outside surface. Now match the V, making the female part in the new bit you made. After cutting the female notch in your top ===< move in on the exact fit by laying some carbon paper in the slot and tapping it. The carbon paper will tell you where a bit more planing or filine - inside that < gets narrow - is needed.

Remember to slather with unthickened epoxy to soak in all that endgrain before you thicken up a little and glue. Dry fit a time or two so you're sure your braching plan will work. I like a fitted trough for the end so gravity holds the new part in line with the old. You'll need to shim the old part a hair to allow for the new being a bit thicker. The sides of the trough keep it alligned fore and aft. Again with the shims to center the old part. And lots of wax paper or old plastic bags from the market, whichever is your favorite or handiest of free way to keep the structure from joining with the bracing.

For a similar stick, it took me about a working day to make the new top and while that glue was curing to attend to sharpening the end of the old. Next day I shaped the female end, arranged the bracing, and glued it together. It's the work of minutes when the glue has cured to plane down the sides, bore out for the stopwate or stop waters if you've internal halyards and can't have a dowl going right through the mast. From there it's normal refinishing and attaching the gear.

I've, by the way, used this general approach in rot problems in the middle of hollow and solid masts both square and round. I always orient the clothspin such that the knive edge of the male end (center of that edge being ficticious, as it were, for hollow masts) is fore and aft. That way I have the big glue surfaces on the sides where the bending is more significant. If it's a plug in the mast, I point the male parts up, so the plug itself has a male and female end. This is not really for any structural reason. If the joint is letting water in because the V points down, you have more problems with your glue job than could have been prevented just by pointing the V up! It just looks better to me.

When I made plugs for the rot in Goblin's 60+ year old hollow round masts, there was no way I was going to duplicate the amazing eight stave bead and cove job those old guys did with horsehoof glue back then. I happened on a variation of the bird's mouth a good 20 years before I ever read of it in WB - the real bird's mouth would have been easier - and then just did the cloth's pin as described. The only difference between the cloths pin in a solid stick and a hollow is that it's a little easier to see what your doing in solid. With hollow, just pretend.

For the sawing out part, I make a jig on either side of the mast or the plug to guide my eye. I use a japanese pull saw for this, not a power tool. A real pro working in a shop where production speed matters might well use some power and premade jigs if they do a lot of this particular repair, but even for pros the eccentricities of any mast repair are such that many will just
work with pull saw, hand planes, chisels, raspes and files.

The stop water bit does not, or perhaps only minimally, prevent compression stresses on the ==>>== from splitting the female side. That's what glue into the endgrain does and with epoxy does perfectly. Rather, it makes the fitting easier as you don't have a fine point confusing the dry tests with carbon paper. And a nice little dowel there gives the scarf a nice finished look if you leave the mast bright.

G'luck

[Jay Greer]

Being concered with cosmetics as well as structure, I chose to work the new sides into the original side scarf that was near the mast head any way. This also created a stronger mast head. Here is a dry fit of the sides.
Jay

[Zakkendrager]

John,
There are probably better jigs but this is one that I made cheaply to scarf 1 inch thick keel and chine members up to 12 inches wide for my Redwing. The concept can be adapted to almost any situation, I think, but a mast would take some ingenuity. The plunge router is mounted on a stiff platform slightly more than twice as wide as the jig. The base of the jig was a piece of flat butcher block; the sides were scrap 1/2" plywood cut to the scarf taper (1:8 in my case) and attached to both sides of the butcher block. The wood to be scarfed was clamped to the butcher block and the router slid back and forth, gradually cutting the scarf. Its a bit slow and I'm not saying its the most efficient but you get there eventually and accurately. As the scarf is cut, the piece being scarfed needs to be moved further into the jig (the downslope end) and reclamped 1-2 times to finally get it to a knife edge.

You can find 2 pictures of the jig at this link (in bottom row of photos) amongst other boat project pictures:
http://www.facebook.com/album.php?ai...4&l=8c46fb0f49

Hope this helps.
Ralph

[Jay Greer]

Not to detract from Ralph's excellent jig, I did consider making one, such as I use for scarfing new wood into old hull planking joints that have bad butt blocks. After due consideration, I weighed the time taken to fuss and tinker with making and fine tuning such a jig and realized that I could hand cut them quicker and more safely and more accuratly. One slip and the whole job is gnarfed!
Jay

[Raka025]

John,
There are probably better jigs but this is one that I made cheaply to scarf 1 inch thick keel and chine members up to 12 inches wide for my Redwing. The concept can be adapted to almost any situation, I think, but a mast would take some ingenuity. The plunge router is mounted on a stiff platform slightly more than twice as wide as the jig. The base of the jig was a piece of flat butcher block; the sides were scrap 1/2" plywood cut to the scarf taper (1:8 in my case) and attached to both sides of the butcher block. The wood to be scarfed was clamped to the butcher block and the router slid back and forth, gradually cutting the scarf. Its a bit slow and I'm not saying its the most efficient but you get there eventually and accurately. As the scarf is cut, the piece being scarfed needs to be moved further into the jig (the downslope end) and reclamped 1-2 times to finally get it to a knife edge.

You can find 2 pictures of the jig at this link (in bottom row of photos) amongst other boat project pictures:
http://www.facebook.com/album.php?ai...4&l=8c46fb0f49

Hope this helps.
Ralph

Anyone have updated pictures/link of this jig?

[Zakkendrager]

Sorry, I had taken the pictures off the facebook album. Here they are plus another scarfing jig I made using a planer for plywood.
Ralph

5scarfing.jpg

Cutting sheerstrake scarph with jig 1.jpg

Cutting sheerstrake scarph with jig 2.jpg

Scarphing sheerstrake 1.jpg