SATURDAY AFTERNOON MASTWORKS #2

[bob mitchell]

CHECK THIS MAST SEAM SOLUTION OUT!!!!!!!!!!!!!!!!!

Just got off the phone with John Thomas at Gougeon Brothers (1-866-937-8797). They are so good at any technical questions concerning epoxy - they have about 6 technical specialists who are there just to help and discuss issues and situations. So of course I wanted to talk about how to deal with the 1/16 inch gap that would be created when I saw my mast staves apart. He was very enthusiastic about a ribbon of carbon fiber and epoxy up the seam, and also confident in a ribbon of biaxial fiber glass. THERE IS AN EXCELLENT ARTICLE ON FIBERGLASS AND CARBON FIBER REINFORCING TAPES AND THEIR APPLICATION AT www.westsystem.com. and plug in carbon ribbon. SHORT EXCELLENT ARTICLE WITH INFORMATION THAT APPLIES SPECIFICALLY FOR MY MAST REBUILD. A MUST READ!!!!!!!

West System 702 unidirectional carbon tape cured with a 60/40 resin/fiber content has a tensile strength of 200,000psi, while their 713 unidirectional glass cured with 60/40 resin/fiber content has a tensile strength of 78,000psi. The carbon tape is electrically conductive - that's scary. The cost of the carbon fiber is about, literally, 10 times as much. Just for reference the tensile strength of sitka spruce paralell to grain is 1150 psi (although there are several choices of force orientation in the table on page 111, UNDERSTANDING WOOD, by Bruce Hoadley - a must have basic reference for woodworkers). That, of course brings up the issue of forces on masts, both compressional and tensional.

STOP THE PRESS------ CHANGE THINKING MODES..................,
LET ME INTERRUPT WITH A DIFFERENT SOLUTION FROM JIM AT GOUGEON BROTHERS!

Jim suggested that I should set my saw blade the slightest bit shallower so when the cut is made there is still a thin piece at the bottom of the kerf connecting the two staves. That thin remainder serves as a stop for epoxy that will be poured into the kerf grove, as well as keeping the mast together and aligned, saving hours of work. In other words if the stave is 1 1/8" thick set your saw blade depth at 1" to 1 1/16" deep. After you make the cut clean it out good and fill with west system epoxy. WOW! Boy does that simplify the issue. He did not feel like the glass would make a significant difference, and I will go with that. (if you happened to cut through you could probably figure out a way for backing). Given the choice of a 1/16' kerf versus a 1/8" kerf, he said it would not affect the strength of the joint, and the 1/8" would be more likely to clean and prepare the sides of the joint. Once again, be sure to leave a little of the joint at the bottom so the spacing and position will be maintained. I guess I am assuming I will not cut right up the masthead, and there is the solid blocking in various areas that will help hold things in place. If this method works on my mizzen as good as it sounds I may do my main as well. WILL TEST TONIGHT ON ONE OF MY OLD PIECES OF MAST.

Back when I thought I was going to have to build my masts I researched and wrote a treatise on mast building for myself. Here is a section on Mast Characteristics, force, etc. that is relevant.

SOME INTERESTING MAST CHARACTERISTICS

A mast's general behavior as a physical system is modeled by a COLUMN under compression. There are essentially two types of columns, short fat ones (concrete pier) and long skinny ones. (arrow, mast). THIS IS RELEVANT because short fat ones FAIL due to the load or compressional forces exceeding the COMPRESSIONAL STRENGTH of the material, while long skinny ones (officially known as a "slender columns") FAIL BY BUCKLING", i.e. when loaded to the extreme it abruptly bends out of its straight line shape and suddenly collaspes before reaching the compressional limit of the material. In 1757 a mathematician named Euler worked all this out in a formula that you geeks can look up if you want to, but the main point that I, as an aspiring mast builder, found interesting is that it is the MODULUS OF ELASTICITY (resistence to bending), NOT the compressive strength of the material, that determines the strength of a slender column (mast). Now you can contemplate why I am considering putting a ribbon of carbon fibers running up the corners of my mast. This also leads me to question the desirablity, in general, of flexibilty in a mast.
So when you envision the mast, stepped in the keel, the wind blowing, you can see there will be a tendency to bend and break. Therefore you add the stays, which take the bending load from the mast as tension in the stays, and instead of the mast bending it is compressed into the mast step. So the mast is a slender column under a compressional load. One of the factors affecting the strength of the mast is the slenderness ratio, essentially a relatiionship between the length of the mast and the diameter (simplified), which says for a given diameter the taller the mast the weaker it is - therefore we add SPREADERS and stays, which effectively reduces the "unsupported panel" length, i.e. prevents bending. We have a single spreader on our newporter 40, but I am sure many of you have seen masts with double and triple spreader systems. The "unsupported panel" term can also come up when talking about taper and diameter, where in general it is stated that the maximum diameter of the mast should be in the middle of the longest unsupported panel.
A great analogy for a mast and stays is an arrow in a bow - athough admittedly a little bit sideways. The arrow is the mast, the string the stays, and the bow the keel. When the arrow is released huge compressional forces are exerted on the arrow by the string, bending the arrow somewhat similar to the way the mast under load would flex. Although the exact nature of the bending of a slender column is affected by whether or not the ends are fixed, hinged, or free, the fundamental analogy is valid.
If you would like to see this live you can go to YOU TUBE and view the three short (30 seconds) clips "High speed video bow and arrow" - worth doing if you really want to empathize with your mast.
In lighter, poorly built fiberglass boats the compressional force exerted by the mast on the bottom of the boat can be strong enough to separate the deck from the bulkhead below. The whole boat essentially bends like a bow. Gerr, in the "Nature of Boats" (an incredible book covering tons of fundamental boating design areas in UNDERSTANDABLE LANGUAGE) has a good discussion on rigging the rest of your boat so the loads from the mast and stays can be transferred appropriately. Additionally, in that same chapter, Gerr gives some excellent and easy to understand rules for spreader location and stay sizing. Skene, in his "Elements of Yacht Design", did several different quick and dirty load calculations using Euler's formula for masts which I chose not to expend the effort to follow, but examples are there if you are interested. Also, another great reference is Douglas Phililps-Brit's "Rigs and Rigging of Yachts", a book written in 1954. This is a time when hollow wooden masts were the rule rather than the exception, and he has an excellent chapter on masts, including a design graph that allows the sizing of mast scantlings based on the characteristics of the hull and displacement. Sort of esoteric, but cool old diagrams. " A variety of factors influence the stresses which a mast has to carry. The fundamental one is the displacement of the boat, this being the chief determinant of the stability, and hence the load which the spars and rigging will have to bear.....The chief effect of hull shape on mast loading lies in the initial resistance to heeling, and the quick motion of a stiff yacht, which produces greater momentum in the mast during a roll, and the more sudden destruction of this momentum at the end of each roll."

[bob mitchell]

Also, since I was dealing with a two sided right angle piece of scrap mast (as opposed to the whole mast section of four sides) I did not clamp adequately. If I have the material I will often do quick and dirty tests to see what I can learn, and see what serious test design I need to really establish what is going on. Although this was not a serious test, some potential problems (leakage and low visibility) did show, and I did gain more confidence in the epoxy joint. I do know I want a joint I can see into. The question of whether I will take the mast completely apart or just repair the split section with a cut a just shy of the bottom of the seam kerf that will leave the mast assembled remains unresolved at this point. Fortunately I have one more ten foot section of the old mizzen to run tests on. Incidently, a ten foot four sided section of an old mast hs a lot of strips of sitka spruce in it.

[bob mitchell]

So, I am a little late with the post, but I did test the 1/8" kerf cut down the seam to be repaired. It was not as easy as it seemed, the main problem being that you can't really see what is happening in there.

So what we are looking at here is the two pieces of mast stave which I sawed apart with the circular saw on the guide shown earlier (1/8" kerf), then epoxied with west system epoxy, and then broke apart a few days later. The "plywood rabbet" which is characteristic of Newporter masts is clearly visible on the bottom piece. The biggest problem was the seam leaked, and the wood was very absorbant, leading to not enough epoxy, as pointed to by the right arrow. On the other hand, the left arrow shows where there was plenty of epoxy and the wood literally was ripped apart (as oposed to epoxy failure. At this point my conclusion is that I will probably do a 1/4 inch kerf, but still try to leave a bottom of wood to keep the epoxy from leaking, and add a sliver of 1/8 to 3/16 wood filler. I'll post my test results for this configuration when I complete the test.