Isolating Keel Bolts at Flange

Discussion in 'Metal Boat Building' started by SeaJay, Dec 1, 2009.

  1. SeaJay
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    SeaJay Senior Member

    Guys,

    I have a keel attachment similar to one that can be seen at:
    http://www.apm-keels.com/ (go to "keels" / "high strength fixed keels" and its the photo of the two guys loading a keel on a trailer). My keel is a steel weldment with cast lead inside. The keel bolts are not embedded in the lead as is often done, but rather, bolted through the steel flange. I plan to use monel bolts and am wondering how to galvantically isolate the bolts from the steel flange? Or is this even necessary or desirable?

    The heads of the bolts will be epoxy coated and the shank protected by sealant at the flange / hull interface. However, I cannot help but think that some water will work its way between the two dissimilar metals. Does anyone have a recommendation they'd like to share?

    Regards,

    SeaJay
     
  2. tazmann
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    tazmann Senior Member

    SeaJay
    Wondering if maybe hot dipped galvanizes bolts would be a better option?
    Tom
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Monel is way at the top of the galvanic scale, far away from steel, and you are right to be concerned about corrosion--in the steel, not the monel. A better choice would probably be stainless steel bolts so that galvanic corrosion is much less of a concern--the bolts and the keel are much closer together on the galvanic scale and do of similar electolytic potential.

    Another alternative if using Monel bolts is to install a suitable number of zinc anodes around the keel to protect the steel--the anodes will corrode before the steel will, if done properly.

    Eric
     
  4. SeaJay
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    SeaJay Senior Member

    Hot dipped galvanized? That's an idea. I'd have to increase the diameter of the bolts but that may not be a problem. However, I'd think I'd still have the same issue and maybe even a bit worse as I'd think the galvinization would tend to act like a sacraficial anode.
     
  5. SeaJay
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    SeaJay Senior Member

    Eric,

    Thanks for the reply. The design is a modification of a Dudley Dix design and he uses stainless steel bolts. Given the issues surrounding SS keel bolts that has been discussed often in these forums I was trying to think of an improvement. However, there is one great advantage in this arrangement in that the bolts are relatively east to inspect and infinately easier to replace than embedded bolts. I will, as you have suggested, be sure to use proper anode protection as my primary line of defense.

    In this thread it was suggested to use duplex 2205 grade ss bolts.

    http://www.boatdesign.net/forums/materials/monel-keel-bolts-24072.html

    I will do some research to see about the availablity of bolts in this material but is there another similar material I should be looking at as well?

    Thanks for your input.
     
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  6. gonzo
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    gonzo Senior Member

    If you use 316S type stainless, crevice corrosion shouldn't be a problem
     
  7. SeaJay
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    SeaJay Senior Member

    Well, I made another pass at the literature on this subject and the opinions appear to be all over the map. Clearly the subject is very dependent upon the specific situation; i.e. ss bolts through wood, ss embeded in lead, galvanized in iron, etc.

    There is a chart in the bottom portion of this webpage:

    http://www.estainlesssteel.com/corrosion.shtml

    that shows the relationships of various fastener materials with various base materials. I think this is an excellent reference piece for anyone interested in the subject. This information confirms Eric's thoughts on monel in my particular application; gives some credence to Tom's galvanized bolt suggestion (although confirms my concerns about the corrosion of the galvinization); and lines up with Eric's and Gonzo's ss recommendation.

    Of course this only addresses galvanic corrosion (which was the topic of my thread), and still leaves open the crevice corrosion and galling issue with respect to ss. Nonetheless, at this point the Duplex 2205 ss appears to be the front runner. It's yield strength, 65 ksi, is impressive. I see that it is readily available but haven't checked price against 316.

    Regards to all

    Sea Jay
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    Please explain how's that possible? Anyway I've allways thought that it's a problem common to all stainless steels..
     
  9. apex1

    apex1 Guest

    It IS Teddy...........
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    How??....crevice corrosion is just shielding of the oxide layer, thus starving the steel of oxygen. How is the grade of SS going to stop this occurring under a bolt head or washer?

    Confused? :confused:
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    Sea Jay,

    Back in the old days, bronze alloys were the preferred keel bolt material, but in modern times, bronze became more expensive than stainless steels. Boatbuilders, always trying to save a buck, adopted stainless steel for keel bolts, much to the chagrin of old-time builders and marine surveyors. I like to specify silicon bronze for keel bolts when I can, which is a widely accepted alloy. Copper-nickel alloys such as 90-10 CuNi, are also very good. However, if your boat is built out of aluminum, then bronze and any copper-based alloy is a no-no--your aluminum hull will fizzle faster than an Alka-Seltzer.

    Stainless alloys of 304, 304L, 316, and 316L stainless are very common in keel bolts. They will last years, and eventually they will corrode, the non-L versions faster than the L versions. But they will last a long time, and longer if you protect them well and maintain/replace them when necessary.

    Another alternative is Aquamet 22 and its sisters.

    http://marinemachiningandmanufa2.liveonatt.com/aquamet22.nxg

    I am not sure of their more generic alloy designation, but Aquamet 22 (and Aqualoy 22) are the best, most highly corrosion-resistant alloys that are used for propeller shafting--same environment as your keel bolts, under the water and near the surface, also in contact with a wider variety of materials when you consider the whole drive train. You could have bolts machined out of Aquamet these materials. You would have to check the price.

    Eric
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    Soy yo :D
     
  13. SeaJay
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    SeaJay Senior Member

    Eric,

    According to the reference I posted above, and along the same lines as your comments on Monel bolts, bronze bolts would likely accelerate the corrosion of the steel flange (I realize your comment regarding silicon bronze was a general statement.) I've also considered Aquamet 22 and here is a reference that states that it is similar to 17-4 and 15-5 stainless.

    http://www.eng-tips.com/viewthread.cfm?qid=3731&page=8

    (It is also a pretty good general discussion on ss corrosion in saltwater. I found it interesting that one poster said that all things considered, mild steel fasteners can be a good choice as their rate of corrosion is so predictable)

    From my earlier rePossible alternative grades to grade 2205 stainless steels are given in table 5.

    Table 5. Grade specification comparisons for 2205 grade stainless steels

    Grade
    Why it might be chosen instead of 2205

    904L
    Better formability is needed, with similar corrosion resistance and lower strength.

    UR52N+
    High resistance to corrosion is required, eg resistance to higher temperature seawater.

    6%Mo
    Higher corrosion resistance is required, but with lower strength and better formability.

    316L
    The high corrosion resistance and strength of 2205 are not needed… 316L is lower cost.

    Corrosion Resistance
    Excellent general corrosion resistance; superior to Grade 316 in most environments. Excellent resistance to localised corrosion including intergranular, pitting and crevice corrosion; the CPT of 2205 is generally at least 35°C. The grade is also resistant to chloride stress corrosion cracking (SCC) at temperatures of up to about 150°C. Grade 2205 will often perform well in environments which cause premature failure of austenitic grades. It has better resistance to sea water than grade 316.


    I think my take-away from this discussion is that for my particular situation, and despite the noted issues with stainless, 2205 or an Aquament type product is probably the best choice. This choice would also be predicated upon following these precautions: a) Protecting the bolts from water intrusion as much as possible b) proper cathodic protection and c) an appropriate inspection schedule.
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    I agree, all smart choices. Yes, Aquamet 22, as I recall now, is similar to 17-4 stainless (I don't necessarily check my library every time as I write stuff like this). We are using 17-4 stainless for shafting material is a wind turbines that I have been working on this year, and we chose it because it is relatively easy to get, has very good corrosion resistance (high chromium content), and it is going to be used in a Gulf Coast environment (very corrosive atmosphere, as anyone living in Florida can attest). I am sure we readers will all be curious to know what you find out about costs.

    Eric
     

  15. SeaJay
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    SeaJay Senior Member

    Apex,

    Could you please explain why you feel Aquamet is a bad choice???

    Regards,

    Sea Jay
     
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