Two piece (folding) wooden spars - is it a practical idea?

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.

 

You forgotten one way (at least). Running two parts of a spar parallel in the middle where they joint together. Like upper masts of a square riggerd or the the lateen sprits in the days of old...

BR Teddy

 

Think you've done a good job of highlighting the issues and problems with the various solutions. And yes, really this is why in the end folk have mostly come to the conclusion that the disadvantages of a one piece wooden spar outweigh the disadvantages of the joint, or else elect for one of the solutions as you mention, like gunters or traditional topmasts, that involve two full size spars overlapped externally. I think in the end the wall thickness of a wood spar makes this pretty much inevitable. If money is no object and aesthetics important then carbon fibre with external wood veneer works.

 

I mentioned that one in my Post Scriptum part But this presents a lot of problems on it's own, because the spar now becomes jagged.

EDIT: I now read it again and thought that maybe you meant the same thing as I've shown in the video a little below? Tapered joint?

Maybe, but I'm hoping that there may be some solutions I'm now aware of.


What about old-time fore-and-aft rigged lateen vessels? Car transportation wasn't an issue back them, but I think storing a very long yard in the off-season must have been a problem too.
I watched this video on lateens, and at around 1:28 it is pretty clear that they used multiple pieces of timber for spar, lashed together:

https://www.youtube.com/watch?v=QAByJmEsOdg

Of course, that is not an arrangement that lends itself to quick assembly and disassembly. But if it could be done - two parts that overlap with tapered ends, and are secured with buried-in fasteners, maybe it could be reasonably practical? I wonder if just a single fastener on each end of the tapered joint would provide sufficient strength, ensuring the joint is not a weak link.

 

I would think some kind of scarf overlap might produce a stronger join. A metal band each end of the scarf?

There are also these sweet carbon ferrules:
http://duckworksbbs.com/hardware/misc/saw-frh/oar.htm http://duckworksbbs.com/hardware/misc/saw-frh/grnlnd.htm

 

You mean, like a sliding band? That could probably work, but there's still the issue of wood swell (the metal/carbon band could get stuck). It would also protrude outside the diameter of the spar. But apart from that, it seems a sound solution.

Still, what about hardware? A big bolt on each end of the joint, wouldn't it provide enough strength?

 

One other thing to keep in mind is the torque on the joint. In a lateen, there can be huge torques near the middle of the spar. The joint isn't just loaded in bending.

 

Torque? You mean along the spar? Where does that come from?

 

I think you are overthinking a non problem. A small diameter increase at the joint has not been a problem for many mast builds. A local solution is to mold a fiberglass sleeve to fit over a long scarf. You can bevel the end of the tube for an easy slip of rigging over it if that bothers you.

I developed a method for making a fiberglass tube directly over a spar so that it will slide off easily when completed. You can see a video here. My contribution was the plastic strips on the inside, without which the tube is very difficult to remove from the spar or mandrel.

https://www.youtube.com/watch?v=JO_-sbbBRBM

The inside diameter of the tube is adjusted by the number of plastic sheet layers over the spar.

 

Absolutely fantastic. I will have to make one of these someday. Never thought it could be so easy...

Anyway, what about wood instability? How would you address the problem of wood expanding or shrinking to a point where the joint gets stuck or too loose?

 

Of course both parts would be well coated with epoxy in three coats minimum , especially the end grain of the scarf.

 

Well, I took that for granted. But as we know, no matter how much you protect the wood, it will still absorb moisture and expand... Or won't it? Have you made any spars that needed such fittings? Did it work alright, without getting stuck in the fiberglass sleeve after some use?

 

I assume that the mast will not be kept under water and that it will only be exposed to ambient air. In use the upper section will have the sleeve permanently glued on so no rain or condensation will run into it from below. In use the lower section will shield the scarf area from water and even ambient air will have a tough time penetrating. I have made a kayak double paddle this way and had no issues. I know of some masts made this way also.

Frankly, I doubt that there would be a serious issue even if all these precautions were not taken. If you are looking for an iron clad, money back, hope to die guarantee, no one offers that for any advice on this or any other forum. The removable section can have more epoxy coatings to make a closer fit with even greater moisture proofing and that will depend on how close you fit the ID of the tube when making it.

No splicing system other than a sleeve of some type offers any hope of success for joining removable mast sections.

 

I see, great information, thank you. What about the sleeve length? Do you have any rules of thumb for sizing that length in relation to the spar length or thickness?

 

Your #2 sleeve is a good one provided a suitable diameter and strength aluminum tube is available. The other choices all suffer from stress concentration at the weakest point and may fail. You have not said but I have assumed you were interested in only free standing masts. Bending stress of a free standing mast is maximum at the partners or deck opening and goes down from there to the mast top so the higher the splice, the easier to engineer it. No rules of thumb are meaningful since it will depend on the loading expected. That is a function of sail area, center or force, righting moment, wind velocity, as well as height of the splice above the partners, etc, etc.

Some extension of the sleeve beyond the scarf is needed but it is your choice as to how much you trust your work. Many like an extreme scarf in normal unsupported work or 12:1 which I consider excessive and I mostly use 8:1 in plywood and lumber joints. Both are excessive for this job as a 2 1/5" spar would then need a scarf of 20"which seems too much to me as a couple inches of sleeve is needed on either end with a bit greater on the free end. If the strength of the sleeve is strong enough, a scarf can be dispensed with completely which makes the sleeve shorter and will still work. A taper of the sleeve thickness near the joint and will strengthen it.

I did a repair scarf for a guy sailing the Great Loop a few years ago who had broken his too small mast near here. I replaced the lower section, made a 4:1 scarf and wrapped it with a couple layers of Knitex and epoxy. This served well from North Carolina through the northern stretches, Great Lakes and home to St Louis.

Some of this is experience and some is engineering estimates. You have to expect to be the beta tester on some things.