Mast is strongest in fore-aft plane and the loads on the mast are highest in the sideways plane. So more rotation will bring the strong plane of the mast more in line with the plane of highest loads. Pretty the forces on the upwind leg are the highest and here the sideways forces are 3 to 4 times as large as the fore-aft forces.

In reply to mast rotation. In big winds the way the stays are attached to the mast also help rotate it. This means that the amount needed to properly rotate the mast can be pretty small. In light winds you need the mainsheet to keep the mast properly rotated but here the mast loads are way lower. So in general the load from the boom (with an adjustable mainsheet angle as you have) can be considered pretty small. Unless you trim the mainsheet angled way back adn have the rotation limiter take up alot of load.

Wouter

Macca,

There are quite a few hollow timber/glass/resin masts out there. Most of them on larger multihulls like this one.

It is definately not a rare occastion and they tend to hold up well.

Wouter

I'm proposing horizontal rings (loops) of unidirectional fibre cloth (glass or carbon) around the circumference of the mast so the fibres run perpendicular the to length of the mast (the vertical). It is very important that you understand this correctly. Having the fibres aligned along the vertical will screw up the bending characteristics. The perpendicular (horizontal) loops will hardly affect bending.

Personal I would just make a sleeve on the inside and glue both parts to it and laminate the outside. I'm not the experienced guy in this aspect but I guess steaming some ply into shape will go a long way to make the sleeve. You will have to cut out the damaged ends and insert an new section here.

Most easy method I can think up (but I'm not the most experienced at this) would be to find some 1 mm thick ply steam the first one and bend it into the end of the mastsection. Then add more layers of 1 mm ply by bending it over the first one (steaming) and glueing them into place. This way you can build up any thickness of wall by adding layers. After the first 2 or 3 layers have been glued the insert will have been locked into shape. I would then take out the insert and glue some more layers on the inside finshing it off with some glass or carbon laminate. Maybe glue a web in there as well. Then I would glue the insert in to the mast sections (both ends) and then build up the part of the section that is replacing of cut off parts by adding more layers of ply or when it is already thick enough, adding layers of glass.

A way of making a poors mans scarve is to have the additional layers of ply on the inside reach up less far inside to old mast sections then the previous ones. This way you get a starecase like scarf. Of course this whole approach rest on using thin ply sheets and being able to steam them properly into shape. I heard from Phill that such sheets can be shaped with a blow drier as well. I would just test this and see of spraying some water before blow drying helps to shape these thigns

In the way of strength, we must not underestimate this composite of thin layers of ply. Before glass and other high tech fibres came along they used this veneer method to build life rafts. The advantages were, lightweight and yet strong construction and it would remain water tights and dependable even when stored for long times in dry places. This method was not used in a much wider sense (except for some race boats) as the labour involved for building a full boats was quite alot.


I will say one thing for the position of the damage. It is in quite a good place. The stresses and bending are pretty low on that spot. Meaning that you ca't really screw up on the repair or the bending of the mast. You have the liberty to induklge yourself in some "better save then sorry": reinforcements of the damaged area without worrying much about affecting overall behaviour of the repaired mast in the way of sailing.

I would just "risk" repairing it. And I'm not the most handy guy with laminates. Seems very doable for a reseanable handy person without the need to maintain tight margins. It seems like a rather cheap job as well. It is mostly labour. And you'll most likely end up with the right mast/sail combo again. Buying a new mast is an option you can always persue later when somehow everything fails.

Wouter

Dave,
Where are you located?

I would absolutely fix that mast. The repair technique Wouter described sounds like it will work. Create an insert out of 1 or 2mil ply - run it beyond the break on either side, stair step an extra ply up near the middle of the break. Dont overdo the strength of the repair, you dont want to create a hard spot in the mast.

The only part of his advise I dont agree with is horizontal loops of carbon uni. This really doesnt do much to reinforce the mast along the length of the mast, it would keep the wingmast section in its section and not allow it to deform, but I'm not sure that adds much to the mast, and thats not how it broke, it broke pretty cleanly in the horizontal plane, so you need to run reinforcements vertically.

I'd sand off the eglass or half the eglass and add a layer of 5.7 oz uni, with the strands run vertically. You might want to add one layer of weave overlapping the repair. If you do add a layer of carbon weave over the repair go with some light weight fabric here, like 100g/in2. The only issue you create is that that part of the mast is going to be stiffer than the rest of the mast and might cause issues in other sections.

The loop are indeed not a cure for all. All they do is prevent early warping of the wall itself under load. This has the beneficial effect of suring up the mast making it appear stronger. By definition it is not stronger then it was before but it will move any damage mechanism to higher stresses then before. The net result is that the mast will be more resistant against abuse and overloading.

How can I explain this better. The loops themselfs don't add any strength but they do allow the already existant parts of the mast to take higher loads/stresses before any failure mechanism results in damage. So in practical terms they make the mast stronger.

The fact that the loops themselfs don't add strength means that they also don't affect the bend characteristics much at all. This is not a side effect but fully intended property of adding the loops. The mast design will behave as before but still is it will be able to take higher external loads (gusts, sheet tensions etc) I feel we are after the last thing.

Most masts do not fail because of some stress exceeding some maximal material property, but rather because the masts enter into a instability failure mode. Buckling is one such example. The material should be able to handle the buckling loads quite well but the fact that it bends out of colom seriously lowers the stresses at which damage occurs.

I feel that with this mast the instability failure modes are to blame for the damage and not the mast strength per see, the damage is not in a particulary highly loaded spot. Suring up with loops seems like an attractive option in this situation.

But I fully admit that adding loops is optional

Wouter

Wouter,
Is that what think happened to this mast? That the walls warped and the section failed?

So if you are correct in saying

then I would recomment a sanding off the eglass and replacing it with one layer of 5.7oz weave carbon - it will resist both warping and bending failures much better.

Dave,
Building a carbon wingmast would be easy if,
1. you knew the tolerances to build it to and
2. you had a wingshaped male mold.

You can get a good catamaran architect to tell you (you would probably have to pay him) the layout of the cloth and how many layers and how to orient them and such. Applying the cloth is the easy part.

The hardpart in my opinion is the male mold.
You could make a male mold out of lightweight foam and wrap it in carbon.
You could make a light weight ply section like the one you have any wrap it with carbon.
You could make the male mold out of machined polished aluminum and wrap it in carbon and remove it, but by this point you'd be in the mast building business.

I'd say it would be easier to fix the one you have an wrap it in carbon to make it stronger.

Bill

theoriticly you would not want to use carbon because of the different flex characteristics it has compared to the rest of the mast. there is a good chance it would work anyway tho.
kevlar has about the same flex characteristics of glass...if you use kevlar burry it deep under glass because it cant't be feathered down like glass or carbon. epoxy, s-glass, and wood should be good repair materials for the job. consider radiating the fibers of the cloth rather than lining them all up in the same direction. you may not be able to put enough material on the inside to take the loads that run inside the mast, but it's worth a try.

for me building a mast seems a little tricky because of the mast track. i'm thinking high density foam core shaped on a cnc machine and glass it w/carbon & s-glass. then gluing the track on with urethane glue. the track could maybe be a tube that has a groove cut into it.
sound realistic?

make a quick easy FEMALE mould. it is the only way to do it. the engineering is easy. the mast track is easy. dont get worked up because it is a mast.
i snapped an aluminium spar and had no option but to build one. i threw a glass mast together with a bit of help in a weekend. cost me about $200 canadian and lasted for four years.
i am not saying that this should be done here, but it is an option that should be explored.

I wish I could find some info on the "elastic modulus" of wood.
I see the design problem with wood/glass (and even more so carbon/glass) being that you are expecting the wood to take the compressive load and the glass the tensile, which really can't happen efficiently. The glass skin will be buckled way before the wood is loaded up to failiure. However, In Reality, the wood supports the Glass laterally to prevent local buckling,(Like the rings Wouter wants to add?) and allows the glass to take compressive load. Can anyone come up with some numbers?
I suspect therefore that the glass/wood mast is not that much better than an epoxied wood mast. (In other words, the wood is probably typically able to handle enough tensile load to crush itself on the opposite side, and so you are essentially adding glass to do a job it is not particularly suited to, unless sandwiched or honeycombed to resist local"surface" buckling.
To return to the original question: Maybe it would shed some light to imagine the mast having failed by the glass skin first failing on buckling, and the fibres breaking, while none the less the wood kept the mast up. Going onto the opposite tack, the glass fibres parted in tension. The wood typically would not be asked to take this tensile load, because the glass skin is so good at it, and the design may not have intended it.

OK boys, here's the latest. My buddy Joe Poire, who races (and breaks!) I14s regularly, stepped up and said "lets fix it' now big deal". Spliced it in much the fashion recommended above (Wouter?), wrapped the break in carbon weave, then a 30" wide glass weave. 3 guys, 7 hours, a heat gun, and a fair amount of beer, and I once again have a one pc mast. Still in debate about whether to throw it up again and watch it closely, or laminate one layer of carbon uni from base to hounds (which I'm leaning towards).
Upon completing the repair, we walked the mast out to the trailer and set it in the cradles. Turns out the repaired break landed directly on the support cradle, which generated the suspicion that the mast may have been damaged at some time while in transit at some point (I trailered the boat from SF to WA last Nov, don't know how much travel it saw before me). There wasn't any damage visible on the outside of the mast, but the base is quite heavily reinforced around the foot and gooseneck, so maybe this extra weight hanging some 6' aft of the mast cradle and bouncing around over some thousand miles created some damage or stress internally. I don't know, a bunch of theories have been posited here, several quite detailed and most believable, often conflicting.
I'm very grateful to all of you for all the advice, education, and offers of replacement masts. I'll keep you posted as to whether she stand tall.

Dave