Hi guys,
As always, I did some serious research before writing this topic, which only led to a dead end, but in the off-chance I shamefully missed another topic or a good source of information on this topic, let me know.
Without anything too specific in mind, I have been wondering about traditional and exotic rigs for a small sailboat I might or might not build sometime in the future. It is in the nature of small sailboats never to stay in one place for too long, and they tend to spend a good amount of their total mileage sitting on a trailer or on the car roof rack. Ground transportation doesn't favor spars that exceed the length of the boat (or the car), which is one of the reasons why so many sailors love rigs that fit inside the boat. There are plenty of good rigs that fit such criteria, and there are ways to get that extra sail area or aspect ratio while keeping spars short enough (like using Gunter rig instead of Bermudian). However, some traditional rigs demand very long spars, and there is no way around that. High-aspect lateen is one of them, requiring a yard which may very well exceed boat length, often by a factor approaching 2.
Which is why I am theoretically considering folding spar construction, where a spar is made up from 2 or more parts that can be quickly joined together for a sailing session and disassembled for storage inside the boat and transportation. With aluminum, it is pretty straightforward - you get some tubes, weld or glue a smaller one inside to act as an insert, and voila, it's done. As long as the wall thickness of the insert tube is considerably larger to compensate for the lesser diameter and to prevent this part from becoming a weak link, it is all good.
However, some traditional boats don't look too good with aluminum spars, and finding right sized tubes might be difficult in some parts of the world. So I'm wondering if it would be possible to make a 2 piece wooden spar. Call it a mental engineering exercise.
I have contemplated on these four options:
1. An aluminum sleeve on one of the joining ends of the spar. Easiest one to make. Obvious disadvantage of this method is that the spar would become thicker on the joint, meaning that lines would tend to jam there. Not an issue for lateen, unless you're using luff pocket, but definitely an issue for most other rigs.
2. An aluminum sleeve that is flush with the surface of the spar - would solve the jamming issue, but it would be more difficult to manufacture the "groove" at the joining ends of the spar to accommodate the aluminum sleeve. Also, the "insert" part of the spar would have decreased moment of inertia, making it a weak link. Unless the spar is a hollow one, and solid on the insert - if carefully engineered, the strength could be made to match the strength of the rest of the spar.
3. No aluminum - just a wooden joint. One end of the spar would have an "insert" (a deep groove), and the other part would have a long drilled hole to accommodate the insert. To make sure that the strength of the link and the rest of the spar matches, spar it must be a hollow one, and the wall thickness must be around 16% of spar diameter - at that ratio, the moment of inertia of the sleeve and the insert is equal. Problem is, 16% is a bit on the low side of the usual 20% that is recommended for hollow wooden spars.
4. A steel rod which connects both spar pieces through the center. Easy enough to make, but this rod must be pretty large to match the rest of the spar in strength, thus increasing weight by a very large margin.
(sorry for poor sketches, I'm away from my PC, so I just used the first online modeling tool I could find. Proportions you see don't represent my numbers, for example, the aluminum tube is drawn way too thick)
Now, of course, there are problems with all 3 methods I proposed:
1. Wood shrinks and swells with changes in moisture. The joint might become too loose or get stuck for good. Stuck being the more likely possibility.
2. Difficult to manufacture, because the joint must be a perfect match - too large a gap, and the joint won't be stiff, hence prone to breaking. This might be alleviated somewhat with extra hardware, but it would add complexity during assembly, and, of course, weight.
3. VERY difficult to manufacture because you have to predict how well will the joint match when you add finish to the wood - even these thin layers of varnish may add up and make the joint too tight.
Some other considerations:
1. Whatever the construction method, the joint must not only be as strong as the rest of the spar, but also retain the same amount of flexibility. If it's too stiff, then it might ruin the sail shape or force excess bend in other parts of the spar.
2. There must be some easy way to prevent the joint from working itself out (sliding out). Most sails would create compression when bent, but some backup would be nice, as long as it does not create anything on which lines might catch on to.
So, I've done my homework, but I'm hoping that some people here with experience could tell me which method would work best, if any. One might think that making a two piece wooden spar is a really unpractical idea due to the instability of the wood, and that you're far better off by using aluminum or other alloys. That may be, but I'd still like to entertain the possibility.
Please share your opinions and suggestions! Do you know any boats that use 2 piece (or more) wooden spars? I found very little information on the subject.
P.S. I am, of course, aware of the loop/bracket method used in square riggers (topmasts, stunsail yards), which is proven to work, but such construction brings it's own set of problems, which is why I suggest we leave it out of this discussion.
As always, I did some serious research before writing this topic, which only led to a dead end, but in the off-chance I shamefully missed another topic or a good source of information on this topic, let me know.
Without anything too specific in mind, I have been wondering about traditional and exotic rigs for a small sailboat I might or might not build sometime in the future. It is in the nature of small sailboats never to stay in one place for too long, and they tend to spend a good amount of their total mileage sitting on a trailer or on the car roof rack. Ground transportation doesn't favor spars that exceed the length of the boat (or the car), which is one of the reasons why so many sailors love rigs that fit inside the boat. There are plenty of good rigs that fit such criteria, and there are ways to get that extra sail area or aspect ratio while keeping spars short enough (like using Gunter rig instead of Bermudian). However, some traditional rigs demand very long spars, and there is no way around that. High-aspect lateen is one of them, requiring a yard which may very well exceed boat length, often by a factor approaching 2.
Which is why I am theoretically considering folding spar construction, where a spar is made up from 2 or more parts that can be quickly joined together for a sailing session and disassembled for storage inside the boat and transportation. With aluminum, it is pretty straightforward - you get some tubes, weld or glue a smaller one inside to act as an insert, and voila, it's done. As long as the wall thickness of the insert tube is considerably larger to compensate for the lesser diameter and to prevent this part from becoming a weak link, it is all good.
However, some traditional boats don't look too good with aluminum spars, and finding right sized tubes might be difficult in some parts of the world. So I'm wondering if it would be possible to make a 2 piece wooden spar. Call it a mental engineering exercise.
I have contemplated on these four options:
1. An aluminum sleeve on one of the joining ends of the spar. Easiest one to make. Obvious disadvantage of this method is that the spar would become thicker on the joint, meaning that lines would tend to jam there. Not an issue for lateen, unless you're using luff pocket, but definitely an issue for most other rigs.
2. An aluminum sleeve that is flush with the surface of the spar - would solve the jamming issue, but it would be more difficult to manufacture the "groove" at the joining ends of the spar to accommodate the aluminum sleeve. Also, the "insert" part of the spar would have decreased moment of inertia, making it a weak link. Unless the spar is a hollow one, and solid on the insert - if carefully engineered, the strength could be made to match the strength of the rest of the spar.
3. No aluminum - just a wooden joint. One end of the spar would have an "insert" (a deep groove), and the other part would have a long drilled hole to accommodate the insert. To make sure that the strength of the link and the rest of the spar matches, spar it must be a hollow one, and the wall thickness must be around 16% of spar diameter - at that ratio, the moment of inertia of the sleeve and the insert is equal. Problem is, 16% is a bit on the low side of the usual 20% that is recommended for hollow wooden spars.
4. A steel rod which connects both spar pieces through the center. Easy enough to make, but this rod must be pretty large to match the rest of the spar in strength, thus increasing weight by a very large margin.
(sorry for poor sketches, I'm away from my PC, so I just used the first online modeling tool I could find. Proportions you see don't represent my numbers, for example, the aluminum tube is drawn way too thick)
Now, of course, there are problems with all 3 methods I proposed:
1. Wood shrinks and swells with changes in moisture. The joint might become too loose or get stuck for good. Stuck being the more likely possibility.
2. Difficult to manufacture, because the joint must be a perfect match - too large a gap, and the joint won't be stiff, hence prone to breaking. This might be alleviated somewhat with extra hardware, but it would add complexity during assembly, and, of course, weight.
3. VERY difficult to manufacture because you have to predict how well will the joint match when you add finish to the wood - even these thin layers of varnish may add up and make the joint too tight.
Some other considerations:
1. Whatever the construction method, the joint must not only be as strong as the rest of the spar, but also retain the same amount of flexibility. If it's too stiff, then it might ruin the sail shape or force excess bend in other parts of the spar.
2. There must be some easy way to prevent the joint from working itself out (sliding out). Most sails would create compression when bent, but some backup would be nice, as long as it does not create anything on which lines might catch on to.
So, I've done my homework, but I'm hoping that some people here with experience could tell me which method would work best, if any. One might think that making a two piece wooden spar is a really unpractical idea due to the instability of the wood, and that you're far better off by using aluminum or other alloys. That may be, but I'd still like to entertain the possibility.
Please share your opinions and suggestions! Do you know any boats that use 2 piece (or more) wooden spars? I found very little information on the subject.
P.S. I am, of course, aware of the loop/bracket method used in square riggers (topmasts, stunsail yards), which is proven to work, but such construction brings it's own set of problems, which is why I suggest we leave it out of this discussion.
But this presents a lot of problems on it's own, because the spar now becomes jagged.
