Yet another thread about spars from me 
But this time not so much asking for advice, but rather sharing an idea. I am sure I am not the first to come up with this, but surprisingly I've never seen it mentioned anywhere when it comes to building wooden spars (masts, booms, yards, etc.). Typically the suggested options are:
1) Solid spar made from 2 planks glued together and rounded over - cheap, fast, simple, heavy;
2) Birdsmouth - complicated and time consuming but lightweight;
3) Gluing together a hollow structure with copperhead joints or some form of square hole geometry - also quite complex and time consuming.
Some many years ago an idea popped into my head - who not do the 1# option, but before gluing the two planks together, use a round-head router bit to carve out a semi-circle groove along the length of each plank? For clarification, here are a few pics of how you would do it:
1. Get two planks
2. Use a router bit like this one (few bucks from Ali, even for really large ones)
then mount a router upside down on the table with a few guides, and route out the middle in each plank. If the dimensions are really large, you can make several routing passes with incrementally increasing cut depth.
3. Glue the planks together;
4. Taper the stock with some hand planes or whatever, and then round off as usual.
Obviously you wouldn't route the groove right to the ends of the planks to keep spars sealed, and you can also skip the groove in high-stress locations (ex. mast partner). So in reality the cross section could look something like this:
I exaggerated the dimensions of this spar in these pics for clarity.
I've actually built 2 spars (boom and yard) for a small dinghy I've built some 10 years ago with this method, and it was super easy, they both turned out very lightweight compared to solid pieces, and I've never managed to break them.
The way I see the pros and cons of this method compared to the others:
Pros:
1. Almost as simple to make as the solid spar, just need a router, though I suppose it could be done with tablesaw or even some hand tools too, and saves a lot of weight.
2. If you taper the spar, you are actually reducing the wall thickness, meaning you save a lot weight with very little taper while losing very little stiffness (because you can't taper too much before you run out of wall thickness). This seems to be much more efficient tapering than birdsmouth, where the wall thickness is often unnecessarily thick at the end of the tapered spar.
Cons:
1. Grain direction will inevitably be bad somewhere along the circumference of the spar, so you'd probably want to keep wall thickness bit higher than in birdsmouth to compensate. Of course you can build this spar from 4 planks instead of 2 with grain running at 45° on each plank, a little bit more complex but not really.
2. Since wall thickness is being reduced as the spar is tapered, I suppose it isn't the best choice for masts with very high compressive forces due to risk of wall buckling.
Can't really think of any other cons. Again, I'm sure I didn't invent this, but I couldn't find any info on this build method anywhere, and it really worked out for me when I tried this, so I'd thought I'd share this. If you have any comments, feel free to share.
But this time not so much asking for advice, but rather sharing an idea. I am sure I am not the first to come up with this, but surprisingly I've never seen it mentioned anywhere when it comes to building wooden spars (masts, booms, yards, etc.). Typically the suggested options are:1) Solid spar made from 2 planks glued together and rounded over - cheap, fast, simple, heavy;
2) Birdsmouth - complicated and time consuming but lightweight;
3) Gluing together a hollow structure with copperhead joints or some form of square hole geometry - also quite complex and time consuming.
Some many years ago an idea popped into my head - who not do the 1# option, but before gluing the two planks together, use a round-head router bit to carve out a semi-circle groove along the length of each plank? For clarification, here are a few pics of how you would do it:
1. Get two planks
2. Use a router bit like this one (few bucks from Ali, even for really large ones)
then mount a router upside down on the table with a few guides, and route out the middle in each plank. If the dimensions are really large, you can make several routing passes with incrementally increasing cut depth.
3. Glue the planks together;
4. Taper the stock with some hand planes or whatever, and then round off as usual.
Obviously you wouldn't route the groove right to the ends of the planks to keep spars sealed, and you can also skip the groove in high-stress locations (ex. mast partner). So in reality the cross section could look something like this:
I exaggerated the dimensions of this spar in these pics for clarity.
I've actually built 2 spars (boom and yard) for a small dinghy I've built some 10 years ago with this method, and it was super easy, they both turned out very lightweight compared to solid pieces, and I've never managed to break them.
The way I see the pros and cons of this method compared to the others:
Pros:
1. Almost as simple to make as the solid spar, just need a router, though I suppose it could be done with tablesaw or even some hand tools too, and saves a lot of weight.
2. If you taper the spar, you are actually reducing the wall thickness, meaning you save a lot weight with very little taper while losing very little stiffness (because you can't taper too much before you run out of wall thickness). This seems to be much more efficient tapering than birdsmouth, where the wall thickness is often unnecessarily thick at the end of the tapered spar.
Cons:
1. Grain direction will inevitably be bad somewhere along the circumference of the spar, so you'd probably want to keep wall thickness bit higher than in birdsmouth to compensate. Of course you can build this spar from 4 planks instead of 2 with grain running at 45° on each plank, a little bit more complex but not really.
2. Since wall thickness is being reduced as the spar is tapered, I suppose it isn't the best choice for masts with very high compressive forces due to risk of wall buckling.
Can't really think of any other cons. Again, I'm sure I didn't invent this, but I couldn't find any info on this build method anywhere, and it really worked out for me when I tried this, so I'd thought I'd share this. If you have any comments, feel free to share.
