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Thread: Confessing corrosion confusion... bobstay fittings.

  1. #1
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    Default Confessing corrosion confusion... bobstay fittings.

    At some point in the near future I must decide how my bobstay will attach to my Perigee's stem. After doing a bit of googling I see it is not uncommon to use stainless fabrications. This seems to be begging for crevice corrosion given the location of typical bobstay fittings... which gets me to thinking what is a better way? Are traditional fittings always bronze? Is bronze immune to corrosion? Not that I'm specifically looking for traditional... just figured whatever is traditional probably works. Has anyone come across a non-metal solution? A fitting built from 'frozen snot' perhaps? I've seen wizbang's comments on his approach to 'chainplates', west indian style... is there also a wood version of a bobstay fitting? I'd be very interested to hear of peoples experience with lower bobstay fittings of any sort?

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Goblin's bobstay stem fitting was wrought iron. Granuaile's was stainless. Neither gave any trouble.

    Trouble comes at the connection, all too often a normal shackle not even remotely stressed for the job. Even if all stainless, different stainless alloys can sometimes interact. (See http://www.ssina.com/corrosion/galvanic.html) Further, ever wonder why copper based bottom paint corrode at the waterline, causing conscientous boat yards to paint two coats in that area? Take a copper bar with a wire from each end going to a sensitive meter, hold vertically, and dunk up and down in salt water. You'll generate a current. Bobstay stem fittings and often the lower parts of bob stays can be subject to just this problem.

    This, by the way, is part of the reason why I am totally against the all too common effort to use chain instead of stainless cable for the bobstay. The connections between links is the perfect site for hidden galvanic and oxygen induced corrosions. Additionally, it's harder to inspect chain and correctly anticipate when it should be replaced.

    Back to the fittings, a fork from the cable to the fitting, either swaged or StayLoc/Norseman type, is best. Treat the area where the wire enters the fitting to prevent water intrusion, as you should with all lower stay fittings anyway.

    By the way, I observed no galvanic issues between bobstay fittings and Goblins wrought iron stem fitting.

    There are also many boats with bronze lower fittings. Assuming that like proper chainplates they are a correct bronze alloy and sized, they are the most trouble free. The interaction between the bronze fitting and the stainless clevis pin, bobstay fitting, and bobstay itself is not a problem.

    I've heard of but never seen or evaluated monel fittings for this purpose.

    Various composites for such fittings are very interesting. I'll happily leave this all to experimental engineers and chemists who feel that saving a couple of ounces here is somehow worth the effort. It's fairly easy to learn how to inspect wrought iron, stainless, or bronze and you can take scantlings from Skene's and other sources.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Kept a very close eye on Truth's fittings. Bronze fitting on keel, SS clevis, swaged to cable; no sign of trouble in 15 years.
    The idea of glass or carbon fiber reinforced fitting and high modulus synthetic cordage is intriguing; no corrosion worries

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Thanks for the responses. It sounds like maybe I am overthinking the corrosion issue given that your respective experiences show that in practice metal fittings are long lasting.

    Regarding chain for the stay, I very much like the look of it and the impression of something really strong (stay clear of my bow or be carved up by my bobstay)... but I take on board your warning about its potential to fail without notice. I saw the thread 'A new rig for Julia' and was inspired by the synthetic bobstay.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I use over size galv eye bolts for Woodwinds two bobstays. A wedge of hardwood above so it does not bend up and a wedge of hardwood below so lines do not get jammed. All faired in with epoxy that they do not look like a doofus used galv eye bolts.
    I use 7x7 ss wire with Molly's hogans and racking seizing. Not keen on chain rigging.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I love a Molly Hogan for any soft eye that will be big enough to actually form it. Properly seized with good wire (not fiber service and certainly not tape!) makes the Molly Hogan's breaking strength the same as the wire. For those here not sure what a Molly looks like: http://forum.woodenboat.com/showthre...plice-3-photos

    It's very hard to make this eye small enough to make sense around an eye for two reasons. Firstly, you must insert the thimble after making the eye, which means it will always be a bit loose around the thimble. But given the stiffness of the wire, making it anywhere near the size of a thimble will be incredibly hard unless you make the divided strand sets way too short to be properly spiraled down beyond the splice's throat for seizing. So you end up with a big splice drawn tightly to the thimble at the throat by seizing that's being stressed with an unfair load.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I bring the eye down to the thimble with nylon seine twine Ian. Haven't lost one yet. But you are right of course, it is the weak part.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I wasn't sure if I had the courage to ask about using eye bolts, but if Bruce has done it then that's a pretty good endorsement of it's legitimacy. Does the galvanizing hold up ok... doesn't it turn into a rusty mess?

    I was thinking about some kind of non-metal fitting... so decided to experiment. The result is strong, chafe resistance suspect, and above all butt ugly. Pics below of the result... but at this stage I much prefer the wizbang idea.








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    Default Re: Confessing corrosion confusion... bobstay fittings.

    For a fitting like that, effectively made of glue, you could glue it in place. Laminate it right on / into the stem.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    wizbang, as I think about it, and I have seized the throat of a molly closed, if it's a under a constant strain, then even if the throat seizing is beginning to break it's not like the thimble will slip out any time soon. So long as one stays alert, it should be fine.

    A bad Molly Hogan can be quickly made but that's not a good thing. Making a proper Molly with the end strands carefully spiraled past the throat at least 12 diameters and then seized with 6 seizings 2 diameters long, seizings properly made up with ss wire and seriously tight, and then maybe leathering it . . . You have probably not saved any time over a Liverpool splice once you master it.

    The Molly retains the advantage that you don't need an expensive or hard to make rigger's vice.

    Anyway, it's hard to beat the StaLoc/Norseman type ends.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I make the "bad" one. Takes about a minute. Two racking seizing take a few minutes each.Service with nylon seine twine takes 5 or10.
    Same on all my shrouds, top and bottom.
    Only time I lost a shroud was from one being crushed by another,and that was an error in layout.
    i did some amateur logging long ago, pulling for trees outta the woods with a v dub bug And some funky 7x7.hoganed together. Just the one minute quick un lay and re lay. No seizing. I punished the crap outta a long quarter inch wire, saw what kind of abuse it could take, and came to trust it.
    I give the boat "fitting"a quick chipping and osphoing and epoxy touch up each year. I'll find a photo when I get back to my 'puter.
    Bobstay is important on Woodwind,in that I do not want to loose it (them) at sea, which is why I have the redundancy of two.
    One can use bronze of course , I'm just a fan of old school galv.
    These days I can afford to put norsemans on, heck I can go buy a modern Brian Toss rig if I wanted it. I don't.Same kind of stubbornness that I still make my own crappy sails.
    bruce

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    Default Re: Confessing corrosion confusion... bobstay fittings.


    Here. They look like normal fittings. The upper wedge of hardwood keeps the bolt from bending up, which would fatigue the steel pretty quickly.
    Three quarter inch eye bolts, very fat. One was added about 20 years ago, but the original is still there ,1984.
    No photos of my cheat splices.
    ugly ass eye bolts for whiskers too, eeewww.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I think the fg /epoxy fitting you made is pretty stinking cool. not ugly at all . But I would tend to not trust the screws.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Perigee is 18' LOA with about a 3' bowsprit, right? I'd consider using an ultra-low-stretch line spliced or knotted through a hole drilled low down across the stem close to the rabbet line. Cinch the other end to the bowsprit and spring it down a bit, if possible. No hardware, etc. to buy and wonder about. Easily replaced when necessary. Can be saved from chafe with a hose or 'parcelling and serving'. Aaargh!
    "... and the great shroud of the sea rolled on as it rolled five thousand years ago."

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Interesting discussion. I am good with metals and corrosion, but bobstays are not my bailiwick. What I have read is that the loads on them are incredible.

    I have to agree with Bruce. I like the look of sbr's FRP fitting, but the screws look too small to me. The shear strength of a 1/4" stainless screw is in the 1200 to 2000 lb range. I also think the thick tail should be a lot longer. No Z-axis strength, and poor inter laminar sheer, so more length to spread the loads.

    I don't know why you can't find rigging and fittings made of duplex stainless steel. It is stronger and more corrosion resistant than 316 and should cost less. 'Should' is one of those concepts that never works out. Probably too hard to swage the duplex grades, but fittings would be great.
    http://www.jklopman.com/articles/stainless.pdf
    https://theriggingco.com/2016/11/28/...s-rod-rigging/
    https://www.atimetals.com/markets/oi...nSubfor300.pdf
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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I prefer not to use eyebolts to secure a bobstay at the waterline no matter what kind of material it is made of. The reason is that the eye of the bolt is subject to an angular load due to the curvature of the stem. This, most often causes the bolt shank to bend and create a crevice that weakens both the bolt and the stem.
    I prefer a cast fitting that allows for a better spread of the load over both a heavy through bolt at the center plus several fastenings above and below it.
    I shy away from stainless steel in this area as it can often be subject to pocket or crevice corrosion. I have seen such fittings in a state simular to a Swiss cheese! Remember that if your bobstay fails there is the possibility of losing the entire rig which, God forbid, can fall straight aft and possibly injuring or killing those who are in cockpit!
    Hard to see in this photo is the bobstay plate that I made up of bronze bar stock for "Red Witch". Both it and the bob stay fitting are above the water line. The fitting has a 3/4" bolt bronze welded to the back and two 3/8" fastenings above and below it. There is a matching back up plate on the after side of the stem.
    Jay
    Last edited by Jay Greer; 10-06-2017 at 02:33 PM.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    This sort of fitting would also work in that application. Home made, this is 3/4" bronze plate, two 1/2" bronze bolts and a 3/4" hole, compact and powerful. (It is a small boat, the stem and cutwater are only about 9" through right there)


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    Default Re: Confessing corrosion confusion... bobstay fittings.

    All these responses and ideas are sensational... really gets the creative juices flowing.

    Wizbang, since you think the fg thing is 'stinking cool' I should add a bit more information. When laying up the glass I used the occasional strip of double bias to hold everything together, but by far the bulk of the material is strands of glass oriented in the longitudinal direction (to lie along the length of the stem). I didn't have any unidirectional cloth on hand, so I pulled apart some really heavy woven roving and then set the rovings in one at a time. Probably sounds painstaking, but in practice it only took a few minutes per layer to get all the strands neatly in place and as close together as possible. I suspect doing it this way resulted in a slightly better glass to resin ratio than if I would have had unidirectional cloth handy. Anyway, the thing is most of the glass is oriented to get max strength.

    I agree with comments about the lack of surface area and the screws being small (3 1/4" and 2 a bit smaller). My thought was to bond in place with epoxy, and screw as well to compensate for the smallish surface area. Of course, if I had thought ahead, this type of fitting could have been integrated into the hull when I was applying the sheathing to the hull. Then loads could have been spread over a really big area. I guess that is still possible... but the aesthetics could be tricky.

    Rbgarr, your idea seems so simple and so obvious. I like it... but there must be some down side, otherwise why isn't this approach seen everywhere? Is it only because there is nothing for the marine industry to sell, or is there some engineering issue? For my Perigee I would build up more outside stem timber to accommodate a hole rather than bore through the planking which comes well forward on the stem timber... but that's just a detail... the concept seems good. Someone please tell me why this idea is not as good as say a wizbang eyebolt or a jay or canoeyawl masterpiece?

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I suppose I would tie the decision with how you are going to do the bobstay.
    Wire or synthetic. Hope we have talked you out of chain.
    Then, will you use a bottlescrew or not? And will it go near the water or stay up by the bowsprit? Will the bowsprit end use a soft eye or a metal fitting? If the stem is laminated, rbgarr's plan is sound. You can make boost the strength of the fg fitting by bonding hanger bolts rather than screws.
    This is the story with small boat projects, what started out as a chat about corrosion got much bigger.
    There are as many ways to do it right as ways to do it wrong, well, almost as many.

    As there is no inner stay , your bobstay is very important. Small boat yes, but heavy. (1700 lb ballast)She has only one set of shrouds with no backstay, the standing rigging must be sound , as there is not much redundancy in the rig.
    bruce

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I have to agree with Ian that saving weight with a composite fitting is pointless. Fabricating a composite part is well within the capabilities of most of us, although understanding the load paths and properties might not be. Fabricating a bronze or stainless part is not so easy for those of us without the metal cutting and welding equipment. A stainless thimble might be wrapped into the composite to resist chafe. I would make it longer, wrap some roving around to prevent splitting and taper the thickness slowly as it gets farther from the loop. You might be able to form it in place so matching the curve of the stem isn't an issue.

    As Jay said, eye bolt load ratings are much lower at any angle and they should never be used for a side load. A DIN 580 eye bolt may be a bit better, but...

    Edit: [quote]Edit; It is interesting to note and compare the strength of a standard bolt vs. the "working load" of an eye bolt.[edit/]
    The way a bolted joint works in metals is such that the bolt does not see cyclic loading unless the applied load exceeds the preload. The eyebolt probably sees a load cycle every time it is used.
    Last edited by MN Dave; 10-06-2017 at 12:50 AM.
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    Default Re: Confessing corrosion confusion... bobstay fittings.

    About the "Eye Bolt"... Aside from the load derating at an angle, the material used will have much to do with how many times it can cycle the load. A bobstay is constantly applying cyclical loading, with resulting fatigue.

    (Note the material used in this chart is forger steel, probably not the first choice for a marine application, but undeniably strong).

    Edit; It is intereseting to note and compare the strength of a standard bolt vs. the "working load" of an eye bolt.

    Bolt strength chart is here http://www.almabolt.com/pages/catalo...oadtensile.htm
    Last edited by Canoeyawl; 10-05-2017 at 12:31 PM.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Quote Originally Posted by Small boats rock View Post
    Rbgarr, your idea seems so simple and so obvious. I like it... but there must be some down side, otherwise why isn't this approach seen everywhere? Is it only because there is nothing for the marine industry to sell, or is there some engineering issue? For my Perigee I would build up more outside stem timber to accommodate a hole rather than bore through the planking which comes well forward on the stem timber... but that's just a detail... the concept seems good. Someone please tell me why this idea is not as good as say a wizbang eyebolt or a jay or canoeyawl masterpiece?
    Truly low stretch/high strength line is fairly new and not favored all that much by some. That's about all there is to it.
    "... and the great shroud of the sea rolled on as it rolled five thousand years ago."

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Regarding saving weight with a composite fitting... no argument there, though the motivation for thinking about such fittings was with a view to eliminating potential corrosion rather than saving weight. Still... if weight was saved it wouldn't be a bad thing right?

    The comments about side loadings and cycling loads on eye bolts make perfect sense... Of course there is the theory on which we're probably all agreed, and then there is the practice as demonstrated by woodwind's eyebolts which have stood the test of time... maybe due to Bruce's addition of hardwood chocks/bolsters/thingamajiggies... or maybe because the eyebolts are so massive that the loads involved are not enough to flex them at all... we can't be sure of the reason, but we can be sure that in Woodwind's case the method has worked.

    As has been pointed out, Perigee's rigging is rather lacking in redundancy (assuming the standing rigging is the only thing holding the mast up)... In the case of my build, I have been taking the approach of building a potentially free standing mast with the rigging then being a comforting extra. The mast has about 1/2" oversized diametre (up from 3.25" on the plan) which doesn't sound like much but it has added a substantial amount of meat to the stick at the price of only around 3 or 4 kg extra weight aloft. The lower portion (through the partners and continuing a few feet above deck) is heavily glassed (may or may not add more grunt), the mast has over 3' of bury (representing nearly 17% of the total length of the stick), and the step and partners are of chunky proportions. There are bulkheads immediately before and aft of the step and partners... and... the step and partners are bonded to the bulkheads, floors, and deckbeams. I don't know if all this is enough that I would deliberately carry all sail without the standing rigging... but it surely is at least some kind of back up in case of a standing rigging failure. Might buy me enough time to respond to a failure before the mast goes by the lee. I wouldn't consider actually going the whole hog and having a free standing mast because, I don't trust them as much as a stayed mast, and because in my (possibly wrong) opinion a free standing mast would distribute loads better if it were tapered but that doesn't suit Perigees gaff rig. Alas, I am getting off topic... I think the point about possible rigging failure was to make sure it doesn't happen by having among other things, a good bobstay fitting and good bobstay and to that end this thread has so far been really helpful. You guys all rock nearly as much as small boats

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Loads equivalent to the entire displacement of a vessel are possible, to be made at times, on the bob stay. Theroetically it should be capable of bearing the entire weight of the boat.
    Jay

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    I can't post the link here, as this from my phone, but Murray from Classic Marine UK has some excellent articles online about rigging and relevant loads. He states bobstay loads can be 3 times displacement, depending on the length of bowsprit. My boat is 12 tonnes, so could see 36. Hellava heave.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Quote Originally Posted by lupussonic View Post
    I can't post the link here, as this from my phone, but Murray from Classic Marine UK has some excellent articles online about rigging and relevant loads. He states bobstay loads can be 3 times displacement, depending on the length of bowsprit. My boat is 12 tonnes, so could see 36. Hellava heave.

    It would be interesting to see the math.

    Jeff

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Actually it was L Francis Herreshoff that gave me that tip on the amount of load on a bobstay. Gawd that was nearly sixty years ago!
    Jaybird

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    My Perigee's lower bob stay attachment has now progressed somewhat and I thought perhaps a brief update here might interest some. Much thought was given to every comment in this thread and my decision involved lots of dithering back and forth (wizbang said 'stinking hot, caoeyawl's fitting is awesome, rbgarr appeals to my inner always have to be different swim against the flow tendency, etc etc...), but to cut to the chase, I ended up wanting to try rbgarr's idea from post 14 above. The simplicity of it is very attractive. My version is slightly modified in that I bored two holes fore and aft... but essentially the same idea. Two pictures below show the holes bored from the outside, and an inside shot to show how I have built a 'splash box' (pictured without its watertight lid in place) to prevent water ingress into the forepeak.

    Jay's comment about loads on the stem fitting is one of the reasons I took a modified approach. The stem timber is 7 inches moulded but only 3 inches sided. By boring holes fore and aft the bob stay can be passed through the entire stem. The holes are slightly angled away from each other so as to spread the load a little further over the inside of the stem.

    So I guess it is obvious now that I have also chosen to heed Ian's words of wisdom about chain. Instead I expect to use a 'synthetic' bobstay (isn't synthetic just a fancy word for rope?). I think a 7x7 wire could also be passed through the 'stem fitting' just as easily since I have rounded the edges of holes quite a lot (probably not visible in photos). Incidentally, in the Perigee plan William Atkin details a 1/4" rod for a bobstay.

    To allow lots of rigging options the splash box has been sized to allow an 8mm line to be formed into a loop with two double fish knots (as in a prusik loop) and the knots to sit inside the splash box.



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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Interesting solution. Is the line or splash box going to be bedded/covered somehow to keep water out? How will it be inspected to check for chafe on the line or water intrusion into the stem if chafe or crushing breaks the epoxy seal?

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Will you seal the line with some 3M 4200 or similar?

    I got thinking about the leverage that adds up to the rather large loads that the bobstay is subjected to. If I am not mistaken (again), the loads on the bobstay have to be ratios of the sines of the angles between the forestay and the bobstay and the bowsprit. If the angle between the forestay and the bowsprit is 60 and the forestay tension is 1000 lb, the vertical load on the bowsprit is sin(60) x 1000 = 866 lb. The vertical load should be balanced by the tension on the bobstay, so the sine of the angle between the bobstay and the bowsprit times the bobstay tension also has to be 866 lb. For 30 the tension is about 866/sin(30) = 1700 lb. For 20 and 10 degrees, the tension would be 2500 and 5000 lb. That would explain why you want the bobstay mounted as low as possible on the stem.

    Stay bowsprit bowsprit total
    angle tension vertical compression
    Forestay 60 1000 866 500
    Bobstay 30 1732 866 1500 2366
    Bobstay 20 2532 866 2379 3245
    Bobstay 10 4987 866 4911 5777
    Bobstay 5 9937 866 9899 10765

    EDIT: Brucesummed it up well enough, so why bump this comment to the top of the list?
    The load path is likely to be through the stem to the sheer clamps and keel. I think the planking connection is too flexible to share that much of the load.

    Where it comes to Dyneema bobstay loops, you are not alone
    http://forums.sailinganarchy.com/ind...-other-things/
    http://i.imgur.com/W4e7AOS.jpg

    https://j109.org/forum/viewtopic.php?t=1046
    http://forums.sailinganarchy.com/ind...bon-bowsprit/&

    http://www.ybw.com/forums/showthread...bstay-fitting&
    http://i33.photobucket.com/albums/d7...ps941rrqed.jpg
    Last edited by MN Dave; 10-19-2017 at 11:47 AM.
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    Default Re: Confessing corrosion confusion... bobstay fittings.

    The 'splash box' has a lid (not shown in the pic) which screws down with silicone sealant. Inspection can be done by removing the lid, and this will be possible afloat in calm conditions since the holes inside the stem are a good 6 inches or so above the LWL. To inspect further it would be best to temporarily remove the bob stay, and then it is possible to see right through the holes which are 1/2" diametre. I have wondered about possible damage to the holes and or stem where the stay passes through. The holes have so far been coated with 8 layers of epoxy resin. No idea if this will be sufficient to protect the stem... the only way to find out if this approach works is to give it a go. If problems arise in the form of damage to holes or stem, possible solutions could include putting a sleeve into holes, and/or a backing plate to protect the stem as would be used if one were installing a through bolt. Worst case scenario, if this experiment proves too problematic the holes can be completely filled and a more conventional fitting used instead. Whatever happens it will be interesting to see how this pans out, but it may take a couple of years before I can report back with observations of the experiment put into practice.

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    Default Re: Confessing corrosion confusion... bobstay fittings.

    Quote Originally Posted by Small boats rock View Post
    The 'splash box' has a lid (not shown in the pic) which screws down with silicone sealant. Inspection can be done by removing the lid, and this will be possible afloat in calm conditions since the holes inside the stem are a good 6 inches or so above the LWL. To inspect further it would be best to temporarily remove the bob stay, and then it is possible to see right through the holes which are 1/2" diametre. I have wondered about possible damage to the holes and or stem where the stay passes through. The holes have so far been coated with 8 layers of epoxy resin. No idea if this will be sufficient to protect the stem... the only way to find out if this approach works is to give it a go. If problems arise in the form of damage to holes or stem, possible solutions could include putting a sleeve into holes, and/or a backing plate to protect the stem as would be used if one were installing a through bolt. Worst case scenario, if this experiment proves too problematic the holes can be completely filled and a more conventional fitting used instead. Whatever happens it will be interesting to see how this pans out, but it may take a couple of years before I can report back with observations of the experiment put into practice.

    Make the cover out of plexiglass or lexan. Then you can see what's happening without removing the cover.

    Jeff

  33. #33
    Join Date
    May 2012
    Location
    UK. Cornwall, Suffolk.
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    3,502

    Default Re: Confessing corrosion confusion... bobstay fittings.

    Personally I would not use wire or any kind of rope that has to pass an angle, however rounded, at the exit of your stem.

    Ive been thinking about the internal bracing for my bobstay fitting; plumb stem, 14 foot bowsprit.

    image.jpg

    Please excuse the crappy drawing...

    In the top sketch, (plan view), the fitting is attached to the stem which, pulling outwards, results in the planks being pulled apart. Same if a rigid wedge is used as a big washer, even if the legs of the wedge (think breast hook shaped) are bolted to adjacent frames (not drawn).

    In the lower sketch, a 5mm x 75mm flat bar is used to spread the loads to adjacent frames, but being a little bit flexible, results in the frames and hence planks being pulled inwards, where you want them...

    What says the forum?

  34. #34
    Join Date
    Oct 2009
    Location
    South Puget Sound/summer Eastern carib./winter
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    13,544

    Default Re: Confessing corrosion confusion... bobstay fittings.

    You have a main (inner) stay?
    Is the boat carvel planked?
    Is the boat gaff or bermudian?

    If the boat has an inner stay, and is not a tight raceboat... I can't see the planking being tweaked from the bobstay attached to the stem.
    This call for the bobstay to be extra strong from an engineering /numbers point of view, I don't get that either. yeah, I'm ignorant of mathematical engineering. Just do not see bowsprits and bobstays breaking unless they are rotten or neglected or abused.

  35. #35
    Join Date
    Mar 2011
    Location
    Mukilteo, WA
    Posts
    1,397

    Default Re: Confessing corrosion confusion... bobstay fittings.

    I"ve done the engineering, and on most of our boats the loading isn't anywhere near displacement. Yes it is the highest loaded wire on the boat, but still.... My boat is 25000 lb and my bobstay load, with a big genny (which I do not fly) is ~8000 lb. On a carvel boat, anything near displacement or 3x displacement would pull the boat apart. On a raceboat with big genoa and massive backstay tension, sure its possible.

    For example, Bruce's eyebolt would have departed his stem long ago, and every chain bobstay out there would explode at those higher numbers. Not that I would advocate chain.

    I would recommend not using a turnbuckle on the bobstay. It is just extra expense and another set of joints to corrode. I used a gasket of Dyneema parcelled and served with no turnbuckle. Measure carefully, but you do still have a turnbuckle on the headstay probably.


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