View Full Version : bobstay chain, galvanics, solutions?
01-24-2005, 05:11 PM
does anyone know any tricks for protecting the bobstay on a boat that sits down on her lines? i can't adjust the trim enough and i really don't want to move the fitting ( or change the staying angle)..
i have an oversize, huge stainless fitting at the bow, and i've been shackling galvanized chain to it and bolting a zinc through the chain...the galvanic connection through the chain can't be great as the zinc only seems to adequately protect a couple of links and there's two shackles and 2 to 3 links underwater...plus it eats zinc like there's no tomorrow..
stainless chain scares me..fatigued it could fail spectacularly without warning (?)...at least i know when the galvy is getting rough
are there any other solutions out there?? would served dacron double braid over a plastic thimble be insane? too stretchy i think...
John E Hardiman
01-24-2005, 05:36 PM
Originally posted by Murray Campbell:
does anyone know any tricks for protecting the bobstay on a boat that sits down on her lines? ...Not much, even with stainless chain, the chain will always corrode where the links are loaded (always wears of any coating...paint, galvanization, passivation). Maybe an alpha iron, monel or inconel chain would work, but the cost would most likely be too great. That leaves wire or synthetic. I would go synthetic with a non-metalic eye.
Edit, sorry, by synthetic I mean a good high-modulus low stretch polyester (dacron) single braid that is served.
[ 01-24-2005, 06:45 PM: Message edited by: John E Hardiman ]
01-24-2005, 09:22 PM
Stainless under water isn't a good idea to begin with. There are a few solutions to the galvanic corrosion problem, but you're still left with stainless under the water.
What does the rest of the set up look like? Cranze iron, turnbuckles etc...
01-25-2005, 01:16 PM
the rest of the set up is a galvanized cranse iron and turnbuckle....
as for sizing dacron, i wonder what the working load is on a bobstay? would matching the strength of the jibstay be about right or does it need to be a lot heavier??
to avoid having any shackles underwater (or avoid underwater splicing and serving every time i want to replace it!) i was thinking of passing the line right through the bow fitting and then leading both ends to thimbles at the turnbuckle...serve everything, leather the part passing through the bow fitting, and then perhaps even sieze the two parts together ?
01-25-2005, 02:28 PM
Stainless cable is always better than any chain as chain has both stretch and corrosion problems.
None of the fibers are at a point where they'd make anysense as a bobstay, even assuming in such a short piece you could rationally splice in an eye at each end and could prevent bottom growth.
Whether just above or just below the waterline, you'll have some air-induced galvanic action for the same reason the bottom paint goes first at the waterline. Just the in and out of air and water on one metal can set up a current.
SS is pretty noble and I've not seen sudden hard to detect failure if you make it all SS and keep it simple. A Norseman or Stayloc at each end of the bobstay is good. Forget having a turnbuckle. You should be able to size this correctly, maybe heave down the bowsprit when first setting it up but the turnbuckle for the jib stay will do all the tightening you need.
While SS does not appear to like being sealed up anoxicly in epoxy, it appears quite happy with red lead, so you could insulate with that before applying bottom paint to the below waterline parts.
If you can't get red lead you might just be better off leaving it bare and scrub the ginge off very frequently, like once a week.
01-25-2005, 03:05 PM
What if you replaced the (oversized) fitting on the stem with galvanized?
You would be replacing fewer components. Galvanized is less expensive to replace (bobstay, turnbuckles).
You can paint it with anti fouling, and keep an eye on it during your annual maintenance.
Would it be in keeping with the rest of the boat?
01-25-2005, 04:12 PM
i always appreciate the breadth of opinions here, thanks...
galvanized would certainly suit the boat but doesn't seem to be as inexpensive as it was back in the day....i was asking around before and found only one place in vancouver that did hot dips and i think their minimum was on the order of hundreds of pounds of metal...maybe it's different down in port townsend?? i really need to get some chain plates done too..
point taken about spliced eyes too close together
back in the day of running bowsprits weren't they set up with rope and a purchase?
01-25-2005, 04:13 PM
How about a length of compatible Stainless Steel rod, with forked ends threaded or welded onto it, to raise the chain connection high enough above the water to minimize galvanic interaction? The rod could even be full length, but that may not be desirable from an economic point of view. Any decent machine shop could make something up like this for you at reasonable cost – and it can be as robust as you like.
I think it is best to avoid stainless chain or cable. They have a tendency to fail without warning. Galvanized chain isn’t all that expensive and you could easily replace it every few years without great cost or hardship, and it will show you plenty of rust long before it fails. The same goes for galv. cable, if you prefer to go that route.
Rope, such as Vectran 12 single braid, would be very strong, but then again it will give no warning before breaking. If your mast staying is at all dependent upon the bobstay, I think it best to avoid it as well. If it is not, then no problem - gaff cutters, with fixed forestays to the stemhead, have long used rope tackles for their bobstays.
A turnbuckle would be insufficient to tension a rope bobstay, in my opinion, and running the rope directly through the bobstay fitting, as suggested, could lead to severe chafing problems as the rope stretches and slackens under load. The see-sawing action through the stainless steel fitting will make short work of the fibers, guaranteed!
01-25-2005, 04:33 PM
If i don't know what type of stainless the original bow fitting is made of is there much concern with mixing different stainless alloys underwater? or am i worrying too much?
rig is bermudan and the forestay doesn't go that far up the mast so i think the bobstay + jibstay is pretty important to keeping everything standing
01-25-2005, 04:56 PM
Assuming that the chain plate has been there a while, it'll be compatable with the SS in staylocks and norsemen and with good SS cable.
I'd not go with a solid bit to get it up out of the water. Very little theoretical additional security at the cost of greater complexity and more different things to fail.
The following applies if you've a staysail stay, most likely set from the stem to maybe 2/3 up the mast. Of the eight dismastings I've experienced, none were due to jib stay out on a bow sprit failure. I've had two busted bow sprits, both the result of collision. In neither case did the mast come down as the backward pressure of the sail on the mast above the staysail stay does not really matter. In fact, if your rig opposes the foreward thrust of the staysail stay with jumpers rather than running backs, it's likely that you'd not even have an alarming bend in the unlikely event of loosing the bowsprit. If you don't have runners or jumpers, your mast is stiff enough to stand up without the jib stay.
01-25-2005, 08:58 PM
British fishing smacks have a galvanized chain from the stem (which is usually well underwater while sailing) up to a mulitple part purchase at the cranze iron.
Google "colne smack" and there is a great web site that has good photos of the arrangement.
I am thinking of some arrangement like that on my boat to keep the mooring pendant chafe out of the picture.
All of the suggestions seem to leave you with stainless underwater, which I think is a bad idea.
I send my galvanising to Seattle, and they have a minimum, and I can usually come up with the minimum with the help of my friends.
Any photos of your boat? How mighty is your bowsprit?
01-26-2005, 12:48 AM
Cheap and Simple: Cut 12” to 18” off of your galvanized chain bobstay at the waterline end when you feel that it has corroded beyond what you consider is safe. Replace that section with a slightly shorter length of new galvanized chain and shackle it to the existing standing part. Replace the short length of chain annually, or when it looks like it needs it. Don’t bother with zincs. Total yearly cost: about 5 bucks at the fisherman’s supply store. Total time to renew: 15 minutes!
At the risk of contradicting myself, I would recommend using high quality, drop forged stainless steel shackles for joining the chains. They are considerably stronger than galvanized for the size that you will require to fit the existing chain and to match its breaking strength. You could consider using stainless steel chain for the short length, but I would still advocate for galvanized for “peace of mind”.
01-26-2005, 12:25 PM
but ouch my head hurts now!
don, the short chain replacement is what i have been doing so far, maybe it will continue for a while...i just don't like the extra shackle...too many shackles including the two required to step down in size from the fitting at the bow (chainplate??) down to 3/8 chain (and they are corroding too at different rates!)
paul i agree about the stainless underwater...i''ve got stainless pintles and gudgeons too...but they've been down there for probably 30 years now and i hope they'll wait a bit longer until i can afford a long haul-out (knocking on wood!).....(the boat is kind of an Atkin's Thistle)
(cool smack site, i like the mooring pendant chafe solution...i've thought about running the rode through a snatch block off the cranse iron but it always seemed like it could be a lot of force out on a long lever that way)
ian the stainless cable is starting to sound pretty good....and maybe a spare one for under the bunk....yes i do have jumpers
8 dismastings??? nerve-wracking.....i'm impressed that you still have anything to do with boats! ;)
01-26-2005, 02:20 PM
Yeah, but I'm including such ludicrous dismastings as the time some kids in my sailing class were too light to capsize their Opti for a drill (this in the days when Dad's built their kids' Opti's) so I came alongside in the tender, stood up to grab the top of the mast, and yanked it off at the partners.
And surfing a dory on a broad reach onto a beach where the pitchpoling also stove in the bow.
And managing to break a rotten mast at the dock as I hauled on the hallyard from a nearby finger float in an attempt to careen her off the mud.
And the all time first, when I was six and had my four year old brother as crew on a day when the wind should have stayed very light on-shore but anyway we gybed the Cape Cod Knockabout all standing with spectacular results. At least the on'shore wind pushed us to my very annoyed mother's embrace.
The others were more what you'd think of: I a crew and bit of a ringer in races and not aware of the owners' maintenance habits - two lower shrouds leading to buckling, one upper, and one backstay. I hate the sound of ping. These were all the SS studs of SS factory installed turnbuckles. Led me to a love affair with dry cleaning fluid and a good magnifying glass.
01-26-2005, 02:58 PM
FWIW, I have it on good authority that someone turning up at certain Vancouver zinc baths clutching small boat parts in one hand and a flat of beer in the other may receive a grudging welcome...
01-26-2005, 04:26 PM
A couple of final comments: Before you commit to stainless steel cable and Stayloc or Norseman type fittings Murray, please check with the manufacturers first to see if these products could, in fact, be used in an application where they will be constantly immersed in salt water. Better safe than sorry.
Secondly, stainless steel and galvanized mild steel are not that far apart on the galvanic scale and should be relatively compatible - at least according to my copy of Skeene’s. So, if you are experiencing a high rate of corrosion near the bobstay fitting, it may be possible that there is a lot of stray electrical current occurring near your boat. Marina’s are notorious for this and it can seriously accelerate electrolysis in boat fittings. The fact that your zincs are eaten up so quickly may be a sign that this is the true culprit. You might want to test your own boat’s electrical system for good measure as well, just in case the problem lies there.
Check out these articles in WoodenBoat:
Kirschenbaum, Jerry, author and illustrator:/"Electrolysis and Corrosion, Part I," 23:30
Kirschenbaum, Jerry, author and illustrator:/"Electrolysis and Corrosion, Part II," 24:78
Kirschenbaum, Jerry, author and illustrator:/"Electrolysis and Corrosion, Part III," 25:61
PS sorry for the adding to your headache!
01-26-2005, 05:06 PM
Emphasising Don's points - the galvanic corrosion is not likely to be between the dissimilar metals - it's air-induced. SS is far more resistant to this. This is shown by your observation that the chain is failing, not the chainplate.
One real problem with chain is that it's hard to get a proper chain shackle. A chain shackle has and oval pin so it will fit into the link and still have plenty of beef in the direction of the strain. What you've got is a system seriously compromised by the shackles.
I've used both Norseman and Staylock in this application and in years past both were just fine. There have been some rumors of changes and you might check the Brion Toss web site for more up to date info but I believe both products are still reputable. While I've heard of other terminals, I've no experience on which to comment.
Even more than with the lower terminal on a shroud, the proper goobering of the wire as it's placed on the cone and into the fitting is really important to prevent moisture intrusion and consequential failure. It's easy. Just pay attention.
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