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Boat Forum / Building / April 2007epoxy failure by flexion?
Hi group, If one were to sheath an unstayed, solid (grown) wooden mast with unidirectional fibreglass and epoxy (painted over for UV protection), would the sheathing eventually embrittle and fail? I'm thinking that an unstayed mast goes through far more flexion cycles and flexes much further than a stayed mast. Thoughts? -Maxime Camirand Max, Warning: The following answer may be too Engineerese for the general public, my appollogies in in advance. The real value of any modern woodworking epoxy is in it's capability to saturate into the surface of wood and the fact that it remains flexible. Yes, an unstayed mast will have higher flexural loads than a stayed mast, but if you do the glass/epoxy correctly it will be the loaded element and the wood will largely just hold it to form. As it is not a case where the wood core could flex and the glass/epoxy would not, there should be little shear load available to cause the separation of the two. The matrix overlay will have to be carefully designed and assembled to achieve stiffness as and where required and then taper as required to not cause an abrupt change in stiffness at as undesired location. Unidirection glass tows would be and interesting way to do this, but you must be careful to cause hard spots as the section reduces (tapers). Any place that parallel fibers overlap, this is a possible issue. I have had several applications that were in sunlight for many years without exhibiting any characteristics that caused me any concern at all. Was this rig designed to use a solid wood unstayed mast (like many old Catboats)? If that is the case, what do you hope to gain? Matt Colie > Hi group, > [quoted text clipped - 7 lines] > > -Maxime Camirand epoxies can be formulated from super brittle to max flex. Marine epoxies tend to be very brittle (no no wants a flexing boat). You just need to select the correct epoxy which you will not find in a marine supply store. paul oman progressive epoxy polymers >> Hi group, >> [quoted text clipped - 7 lines] >> >> -Maxime Camirand Thanks for the replies, Paul and Matt. I understand that once you get to a certain thickness of layup, the fibreglass begins to take the load while the wood inside acts simply as a core. I also understand the concept of hard spots. I don't plan on carefully designing this sheathing, though. This rig is designed for a solid wood unstayed mast. What I hope to gain from sheating isn't an increase in strength, it's protection from rot, reduction of maintenance (no need to oil the mast) and elimination of mast chafe. I was planning on one layer of cloth only. That's why I imagine that I'll be in a situation where the mast is flexing as much as the wood will allow it, and the fibreglass is simply following. That's why I could be exceeding the epoxy's capability to flex. There's also the shear force underneath the layup to consider. Maybe there are different materials I could consider? I've heard of plastic cloth and other resins (besides polyester, which is out for obvious reasons), but I don't know much about them. Paul: You mentioned that flexible epoxies are available, but not from marine stores. Where could I find it? Regards, -Max Camirand Paul > Max, > [quoted text clipped - 27 lines] > > Matt Colie Paul (part 2) I have found for max reduction in 'brightwork' seal with solvent thinned epoxy then topcoat with varnish then optionally a 2 part clear poly with UV blockers (hard to find) If pigmented surface I would use a pigmented moisture cured urethane as the primer and sealer then enamel over that. the next step up would be your 1 coat of glass over either the thinned epoxy primer or the moisture cured urethane, in which case, use a flex based epoxy paint. any version of these systems will handle the flexing of the mast. The right 'primer' over the exposed wood no matter what you do, is probably the critical step. There are several 'grades/classes' of primers - commercial primers are not will explained or understood very well outside the painting contractor market. Most folks just think in terms of water or oil based primers and perhaps also epoxy primers. Note that area specific VOC regulations are making it harder or impossible to often get the best products for the job. It is not science or technology that develop new coatings but rather lawyers and regulatory groups. What you can apply in MI is different from what you can apply in NY or CA. so location makes a big difference in your options. email me privately for specific product names and locations paul oman pauloman@pauloman.com Progressive Epoxy Polymers, inc. >Thanks for the replies, Paul and Matt. > [quoted text clipped - 58 lines] > > > Hi group, > [quoted text clipped - 5 lines] > > Thoughts? Fiberglass and whatever over solid wood is not good. Painting a solid mast is not good (can't see what is happening to it).  SignaturedadiOH ____________________________ dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico > > Hi group, > > [quoted text clipped - 7 lines] > > Fiberglass and whatever over solid wood is not good. Really? Built 1985-6: http://hem.bredband.net/b262106/Boat/build.html Sailing 2006: http://hem.bredband.net/b262106/Boat/video.html SignatureMartin Schöön "Problems worthy of attack show their worth by hitting back." Piet Hein >>> Hi group, >>> [quoted text clipped - 9 lines] >> > Really? Yeah. Can't say I consider 8mm as "solid wood". SignaturedadiOH ____________________________ dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico > >>> Hi group, > >>> [quoted text clipped - 11 lines] > > Yeah. Can't say I consider 8mm as "solid wood". It is a small boat. Bigger boats use thicker wood and you have things like wood cored dagger boards sheathed in glass/epoxy. -- Martin Schöön <martin.schoon@gmail.com> "Problems worthy of attack prove their worth by hitting back" Piet Hein |
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