Bob Patterson 24 Feb 06 07:50
I made some masts last summer - 2X4 (top sides tapered)with 1X4 on each side of the bottom 1/3 (tapered ends). After reading your article, I think 3/4 inch ply on one side would make it very sturdy. Thanks
Shorty 21 Feb 06 13:05
I just heard back from West epoxy, and published their findings at PDRacer.com See the mast page.
peter lenihan 21 Feb 06 04:03
FYI. The mast for a Micro,as per the plans,has a safety factor of 6 if built right,with good wood and good glue,just in case anyone thinks there is something wrong with a Micro mast.....
John Welsford 19 Feb 06 18:34
Shorty there are fiberglass cloths which have almost all of their fibers orientated in one direction and not woven in and out over the threads crossing their path. That means that the stress bearing fibers are straight rather than kinked, and they contribute a lot more stiffness to the assembly. Some of these cloths are called "knitted" fabrics.
Kirk Charles 17 Feb 06 12:26
Ok Let's Get this Strength thing Right :-) TRUE: For identical materials stifness and strength vary by the same amount. (double the strength = double the stiffness Hooke's Law) OOPS: Neither Stiffness nor Strength vary by the Logarithm TRUE: Strength increases as the 4th power of Diameter as Michalak reported. TRUE: Strength increases by the 3rd power of thickness. HOW CAN THIS BE? If you double the diameter you double both the thickness and the width. For Example double the size of your mast and you get an increase of 8 times because you doubled the thickness AND an increase of 2 times because you doubled the width. Altogether an amazing 16 times stronger. WHAT DOES THIS MEAN? If you need a mast twice as strong you only need to make it 1.189 times bigger. (the 4th root of 2) A great investment of a little bit of wood. Knots are a Huge strength problem. You can cut them out and scarf in a good piece. WEST has publications on wood repair that show the cut angles and methods to make a joint as strong as good wood. Laszlo is right on with is comments on glass laminations. David, Thanks for bringing up this issue. Simple experiments like yours are the heart of learning. As far as safety goes, a mast will not break on a calm day, but only when a strong wind is blowing you toward the rocks.
Rob Rohde-Szudy 17 Feb 06 09:55
Gaetan is right that Michalak reported on the logarithmic relationship of stiffness to thickness. But I think his number is wrong. I'm pretty sure Jim said that stiffness varies as the CUBE of thickness. So double the thickness and it should be EIGHT times stiffer.
Also, this may or may not equate to 8x STRONGER. These are two different measurements that Shorty correctly separated - deflection at failure and force needed to cause failure.
To Shorty, thanks for a great addition to the Backyard Academy of Semi-Scientific Testing. We'll dedicate a wing to you one of these days.
Laszlo 17 Feb 06 04:45
The kind of fiberglass and epoxy used make a BIG difference. For maximum strength you want lots of fibers running vertically up the mast. You also want those fibers to be smooth, not folded over and under other fibers (the folds are stress concentrators which weaken the glass). So for maximum strength you should be using unidirectional cloth, not woven. Then, you need to protect the vertical fibers from mechanical damage, so an outer layer of light biaxial (again, not woven) glass should be applied. The force where the mast contacts the partner is a compressive force which causes the mast's fibers (wood or glass) to deform and break (exactly the same way that a knife blade cuts through a piece of wood). To resist that, you can simply wrap that area in glass tape which spreads the force over a larger area. For a really strong mast, you'll need both approaches - long fibers along the mast and localized reinforcement at high stress points. Finally, for the reader who was concerned about the glass peeling off a flexing mast, that's not a problem if quality epoxy resin is used and the layup is carefully done.
chris casey 16 Feb 06 21:54
hey shorty your article brought back a lot of memories, i built the micro and the "mast" that tim is standing next to in the picture. for some reason when building the boat i was afraid to try laminating a mast up, and couldnt find/afford a piece of sitka spruce big enough for the job so i bought that. i was worried about the knots, and considered fiberglassing it like you were thinking, but was afraid i wouldnt be able to make a nice job of it. i was also afraid that being a free standing mast if i did fiberglass it all the flexing might cause delamination or stress cracks in the fiberglass. ive got some pics of that mast really bending, im sure now that the better way would have been to laminate one up, i hope tim was able to make a new /better one and is still enjoying the boat, keep up the testing i really enjoy your articles thanks, chris
Pete Leenhouts 16 Feb 06 21:21
This was an interesting article, for several reasons, one of which was that I had discussed today this very question with my instructor here at the North West School of Wooden Boatbuilding (Port Hadlock WA). One of the three boats my section is building is the Ian Oughtred-designed "Caledonia" sloop; it will have a gunter rig. The wooden mast (which I am making at the moment) will be 19'6" long. It was felt that fiberglassing this mast would increase it's resistance to bending, but that the bending would eventually spring loose the fiberglass. The Caledonia mast is stayed with rope shrouds, and is 3 1/4 inches thick at it's widest. So while the mast will be coated with several coats of epoxy, it will not be fiberglassed. Let's see some more experiments! Just my two cents. s/Pete
Gaetan Jette 16 Feb 06 17:59
According to an article Jim Michalak wrote back in February 1st, 2000, the strength of a mast increases 16 times every time the thickness doubles. You might want to check it out for your next experiment. This article was posted only once, it seems.
Steve Lansdowne 16 Feb 06 16:06
You show when the wood breaks, but not that your reinforcement of an actual mast is really needed to keep it from breaking. I suppose if your mast is a "seat of the pants" design, extra reinforcement may help, but on a professionally designed boat this extra glass/epoxy reinforcement may not really be needed.
Shorty 16 Feb 06 09:40
-- bends 6.5 inches / breaks at 6 1/8" of water in bucket
Yes, that is correct, and yes that is wierd that 1 layer of glass would bend more, but break with less weight (less water in the bucket). There are numerous problems to my experiment, namely my very small sample size, which the quality and grain of the wood could have a major effect. To get definitive garage-level-experimentation results, a much larger sample would need to be used with very close attention to the thickness of each piece, which includes the epoxy thickness.
If anyone is interested in re-doing this experiment and taking it further, then drop in to the PDRacer discussion group and I'd be glad to share further comments on how it can be improved, plus the other guys in the group will have other great ideas too -- we enjoy this kind of real life experimentation.
-- check with the W.E.S.T. epoxy
Thanks for the suggestion, I just emailed them. I'll post any response I get at the PD group.
-- appeared that glassed one side only / different than tube / you would make bigger wooden mast
Yes, you are right, I only glassed one side. The leeward side of the mast is in compression, and I wanted to see the effects on the tension side of the mast. Re-doing the experiment with sticks that were glassed all the way around might prove different results -- any chance you would do the experiment?
As for the compromise between glassing or just making a bigger mast, both seem to be good choices, but you have to take into consideration all of the factors. For me, I was searching for a smaller, stronger and easy to make mast so it would be lighter and to provide less of a wind shadow to the luff of my sail. A thicker wooden mast would certainly work just as well, and might be a better alternative for your situation.
Now something that would be really cool is to make full sized masts, and break them. Full size being "plugs" the full diameter, but would only need a couple of feet long. Possibly put a steel tube in the ground (like concrete it in), the plug goes half way into it, then a breaking arm is attached to the top half of the plug. Like a steel tube with another one welded on at 45 degrees, so barrels of water could be suspended from it. Like the experiment I did, we are only looking for the relative difference, not measuring exact PSI etc.
hoz 16 Feb 06 09:34
I learned the embarrasing way. I attempted to make a take apart mast for my sail canoe. The joins were 1" unreinforced wooden dowel. A sweet young beauty was watching me rig up and asked what I was doing. "Just watch this" I replied as I began to let the boat drift out from the lee of the dock. The winds were force 3 and gusting. As I entered the wind stream the mast broke off immediately at the dowel.
All I heard was laughter from the dock...
lee martin 16 Feb 06 08:22
If you want an entire spectrum of results check with the W.E.S.T. epoxy people who have researched everything you can think of and put it in their book on boat construction. These are professional methods using stress testing machines for comprehinsive results.
john trussell 16 Feb 06 07:41
I can't tell from your write up, but it appeared that you glassed one side of your test pieces. This is somewhat different from glassing the bottom 30% of a mast where you are creating a glass tube with a wooden core. I think that such a structure would be extremely stiff for its bottom 30% and as flexible as usual for the top 70%. If a mast like this breaks, I would expect it to break just above the glass.
I think I'd be more inclined to use clear wood and/or make the mast thicker.
Bob Means 16 Feb 06 02:48
I'm a bit confused of your tables. With just epoxy it bends to 6.5 inches and breaks at 6 1/8" of water in bucket. With one layer of class & epoxy it bends to 6 3/4" and breaks at 5 3/8" of water in the bucket.
Is that right?