Wood mast and boom

[StoneCat]

I have seen several great sites on building a birdsmouth design mast but have not seen anything regarding splcing the staves for long masts, ie lap joint etc. Any advice out there ?

I plan on building a gaff rigged spruce mast and boom for an 18 ft Catboat.

thx
Mike

[Ian McColgin]

Make 12:1 scarfs. For neatness, put a bit of a flat on the sharp outside edge, with a little matching drop on the top of the outside slope such that you have maybe just a little - 1/32" or so - glue line that is normal to the mast grain. That glue line will be much nicer looking on the finished mast.

I'd make up the scarfed staves all in advance before any milling or tapering you might do.

With stuff this long, I recommend building a nice straight spar bench to work on. You can set up a standard jig - really not much more than a couble of planks, to hold the router at the correct angle and in a straight line as you run it down the now well supported staves.

G'luck

[Ken Hutchins]

I prefer a 2:1 edge scarf and finger joint. This is how I made the planks on my boat instead of using butt blocks, and $$$$ of bronze bolts in the joints.
http://www.imagestation.com/picture/sraid81/p0a86206838a1833cc24385a518ebe544/fafd4aba.jpg

[JimConlin]

Seems to me that the appeal of a long scarph joint in spar stock is that there is lots of side-grain glue surface. A 12:1 scarph will have glue surface of 12 times the cross-section of the stave. Machined finger joints only give six or seven times the cross-section of the stave in side-grain glue surface, and at a steeper angle, too. The end-grain bonds, if you count their strength at all, add just a bit more. I'd think that, in tension, the strength of the joint would be proportional to the area of the side-grain glue surface and that the finger joint would be significantly weaker.

For planking, finger jointing is an elegant solution, especially with a short scarph.

Perhaps someone with a better background in structural engineering will chime in and put us all right.

[JimConlin]

After further reflection, maybe i was wrong in my earlier post. I was wrong once before. In 1967, I think. ;)
The 2:1 side scarph that Ken shows in his photo increases both the length of the joint and the effective length of the 'fingers' in long-grain direction. That is elegant.

I'd still want to tensile-test it before trusting it in a hollow spar.

Jim

[Ken Hutchins]

When I made the first joint in a plank I did a test to insure strength. How I did this was while standing on the ramp to the barn holding the plank 1-1/8 thick white ash 36 feet long over my head with the plank across the ramp I started moving the plank up and down so as to make the ends whip. The ends were whipping up and down in excess of 10 feet for about a minute. Had to stop because my knees began to get wobbly. There was no sign of failure in the joint. All of the planks went on without any failures even in areas of extreme twist, and the planks were perfectly fair to the others which indicates the flex at the joints is real close to solid wood. So based on my non scientific tests I think this joint would be just fine for a mast or boom. The depth of the fingers is the hypotenuse of the angle which amounts to about 1/2 inch.
http://www.imagestation.com/picture/sraid81/p0a86206838a1833cc24385a518ebe544/fafd4aba.jpg

[Paul Scheuer]

I once toured a plant that made wooden garage door panels. They did finger jointing on a massive basis, routinely using 6 inch pieces. They used finger joints to rejoin pieces where they had removed knots. They had test data and absolute confidence that their joints were stronger than grown wood.

[Dave Williams]

Ken,

How did you cut those 2 to 1 tapers in long stock?

Dave

[Ken Hutchins]

Dave, The important thing is that the cut edges MUST be smooth and square prior to finger jointing for the joint to be good.
First line up the 2 parts one on top the other using match lines for length and by eye for alignment along the length, keeping in mind that planks are not straight so I was looking for a nice sweet fair curve thru the joint.
Then draw the angular line on the top part. Next transfer the ends of that line with a square down the edges of both parts. Remove upper part and draw the angular line on the lower part using the edge marks that were transferred from above.
Next draw a line parallel to the angular line on one of the parts offset by the depth of the finger joint cutters - 1/4 for my set. The offset goes towards the scrap end.
Then band saw or carefully skill saw leaving oh maybe 1/32 for cleanup. For cleanup I use a router with a straight bit running along a straightedge. Perhaps a good miter or radial arm saw with a good blade would make a cut good enough, but having neither of these I clean up the saw cut with the router. I have several routers so no bit changing.
To help keep the routers square to the surface and provide a run-off extension for the finger joint guide I hot melt glue scrap pieces on the edges of the parts prior to sawing. The scrap piece on the pointy end has angular notches on the outer edge for the clamps when doing the glue up to put the force of the clamps square to the angular cut.
Here is a photo showing a sample setup of the parts. After a few joints I was able to cut, finger joint and glue up a pair within an hour.

http://www.imagestation.com/picture/sraid159/p89bdff2fe6aee12d84094450890872a6/f4f43710.jpg

[DerekW]

There's some good stuff here, Mike, but it may be overkill for what you are planning.

If you are building your spars 'birdsmouth' fashion for a boat that size then your individual staves will be pretty small in cross section; maybe 1/2" by 1 1/2" ?

At that size it is pretty easy to knock together a jig to cut the scarfs with a handheld circular saw; basically a trough to hold the stave, and a table with a fence pinned to it at your chosen scarfing angle. Mine was done from shop scrap in about fifteen minutes. The birdsmouth construction is supportive of the stave alignments - I susupect a 1:8 ratio is adequate. Our unstayed gaff mast of similar size has survived thus far.

For assembly, make up some 'scarf-support' jigs from scrap; plates a few inches longer than the overlap, with a scrapwood fence running their length. Clamp one strip of stave down with its unscarfed face uppermost, so the scarf face and the clamping jig form a socket into which the gooped up second strip of stave can be slid. Clamp.

I put together several staves from multiple sections this way, gluing and clamping all the joints in one session; the resultant scarfs are proving durable,the process was easy, and the 'tooling' was next to nonexistant.

I have pictures somewhere if needed.

cheers
Derek

[Keith Wilson]

For a small mast, it's really easy to make the scarfs with a hand plane. Just clamp the two halves of the stave together and plane the bevel on both pieces simultaneously; it takes about five minutes. If you get the planed surface halfway flat, they have to line up right. It's very easy to make a birdsmouth spar from relatively cheap stock by scarfing out the knots. The last one I made was an 18' mast from a lumberyard 2x10 - about $25 worth of wood, not a knot in her anywhere, and I can step it with one hand. smile.gif

[JimConlin]

More on the strength of finger joints from the USDA FPL Wood Handbook http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch11 .pdf (http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch11.pdf)

Page 11-8:
"Fingerjoints have the potential to reach at least 75% of the strength of clear wood in many species if properly manufactured. These joints are adequate for most applications because most lumber grades used in the manufacture of glulam timber permit natural characteristics that result in strength reductions of at least 25% less than that of clear wood."

Page 11-10:
"The highest strength values are obtained with well-made plain scarf joints; the lowest values are obtained with butt joints. This is because scarf joints with flat slopes have essentially side-grain surfaces that can be well bonded to develop high strength, and butt joints have end-grain surfaces that cannot be bonded effectively. Structural fingerjoints (either vertical or horizontal) are a compromise between scarf and butt joints; the strength of structural fingerjoints varies with joint design.

No statement can be made regarding the specific joint strength factor of fingerjoints, because fingerjoint strength depends on the type and configuration of the joint and the
manufacturing process. However, the joint factor of commonly used fingerjoints in high-quality lumber used for laminating can be about 75%. High-strength fingerjoints can be made when the design is such that the fingers have relatively flat slopes and sharp tips. Tips are essentially a series of butt joints that reduce the effectiveness of fingerjoints as well as creating sources of stress concentration. Generally, butt joints cannot transmit tensile stress"

What this suggests to me is that for masts in particular, where the member will be highly stressed and where care is taken to select clear, straight-grained stock for maximum strength, finger joints compromise strength to a meaningful degree and a scarph is preferred. For other applications where the member is less heavily stressed, they have a lot of promise. For building up long planks with sweep, Ken's use of finger joints has got to be much easier to align than a flat scarph would be. Good stuff!

Please forgive me for hijacking this thread.
Jim

[Ken Hutchins]

It would be interesting to do a comparision test of the common finger joints on a 90 degree butt the handbook is refering to verses the 2:1 edge scarf as I have done. I would bet that my joint is a lot better than 75%.

[Nicholas Carey]

Originally posted by JimConlin:
After further reflection, maybe i was wrong in my earlier post. I was wrong once before. In 1967, I think. ;)
The 2:1 side scarph that Ken shows in his photo increases both the length of the joint and the effective length of the 'fingers' in long-grain direction. That is elegant.

I'd still want to tensile-test it before trusting it in a hollow spar.Our Sponsor ran an article a few years back about a guy in England who specialized in yacht restoration that puts planking stock together like this (edgewise finger joints on an edgewise scarf).

I can't remember what scarf angle he used, but the article said that Lloyds certified his planks as 'continuous' so the ordinary staggering rules didn't apply. [He was of course gluing up with epoxy].

And if its good enough for Lloyds...I suspect its good enough for me.

The article quoted him as saying that another benefit of the technique is that you can construct a crooked 'plank' to match the spiled sweep needed: it saves wood, 'cause you can use narrower stock, and you get more continuous grain.

[David Tabor (sailordave)]

Good idea Ken. Interesting how we adapt our techniques to the tools we have (or not). As I was reading your post I thought that was really involved to cut the angle, then I saw you don't have a miter saw.... Ah ha.
Me, having a miter saw, I would just cut the angle (I make jigs to help cut things at angles greater than 45 degrees) and once the two boards were cut, clamp them together and check the alignment with some monofiliant line or the good old MARK IV Eyeball (with version 2.03 upgrade cheaters :D )

Otherwise I think it's an elegant solution and as Nicholas said you can "make" a curved plank!

[StoneCat]

Wow, you guys are unreal.
Although I have a fair amount of structural engineering experience, not much in marine applications of wood construction. You gentleman have taken care of that, although its 10 F with 30 knot winds right now, brrr, and more snow on its way, I'm very excited about turning this cat back into a sailing one. I will take advantage of all the good advice. thx all, Mike