Do you know your keel bolt details

[vyv_cox]

I was pretty confident that in the past you have made the following comments (but now doubting my memory!)

- A galvanised chain/galvanised anchor was stronger than a stainless equivalent of the same dimensions
- That even 316 wasn't suitable for use below water level or where crevice corrosion was probable

The only stainless steel chain I have tested had almost exactly the same strength as much of the Grade 30 galvanised. It had a rather brittle appearance, which might lead to problems with snatch loads, but strength was well above the minimum requirement. Anchors are somewhat different because steel ones use heat treatable materials for shanks for example, and no stainless steels can reach those tensile values.

316 is widely used underwater for prop shafts. They will suffer crevice corrosion, several examples on my website, but generally they last well. Bolts are particularly susceptible to crevice corrosion, each thread root being a crevice, but use of sealants that keeps the water out can overcome this problem.

Based on the above I have been of the opinion that SS keelbolts were a) weaker and b) more prone to failure

They probably are a little weaker but the designer should have taken that into account by increasing their diameter and number. Experience has shown us that stainless steel keelbolts are just as reliable as carbon steel ones, although they certainly can fail, dramatic photo on the website. Again, the answer is plenty of good sealant. If you see rusty stains in the bilge, assuming it is not the result of permanent internal water, then it is likely that the water is coming up the bolts, time to investigate.

(This is cut and paste from an American boat manufacturer's site because they describe it better than I can but they in turn appear to have sourced the information from AZOM)
CONCENTRATED CELL OR CREVICE CORROSION
This corrosion is common between nut and bolt surfaces. Salt water applications are a severe problem because of the salt water’s
low PH and its high chloride content. Here is the mechanism:
• Chlorides pit the passivated stainless steel surface.
• The low PH salt water attacks the active layer that is exposed.
The absence of oxygen inhibits the re-forming of the passive layer.
These three factors work together in a vicious cycle, repeatedly attacking the same small area. If the metal is under tensile stress- like from an over torqued keel bolt nut, the pit formed can transform itself into a crack. When a crack forms the process repeats and accelerates as
the surface area of the ‘active’ layer is now much larger.

I would welcome your comment on the above and then I will then sober up and settle down on a new position of certainty

(often wrong but always certain as the saying goes)

Crevice and pitting corrosion occur by the same mechanism. I would have said that the one above, with initiation by chloride attack, is a description of pitting. Crevice corrosion, as far as I know, does not need any pits to develop, the crevice is there by design, in bolts, fits of propellers, beneath washers etc. See the 'classic diagram' on the website.

 

[Nicholas123]

I bought a Moody 346 a few month ago. I have previously been a long, encapsulated keel type yacht owner. I was surprised that there is virtually no information on the keel bolts on this yacht from either the designer or the MOA.

Asking around the marinas here I was amazed that I couldn't find one person with a bolt on keel who knew the dimensions of his keel bolts.

Is this information not important? Don't skippers care if there keels are held on with tooth-picks?

How many on this forum know the dimension and nymber of their keel bolts and whether they are adequate for the job. How much wastage is OK for instance.

I am seriously considering getting rid of this bolt on keel boat and returning to encapsulated for my next boat.

I think you are panicking unnecessarily.I believe there are far greater problems associated with encapsulated keels.I heard about one bloke who had the entire contents of one of his encapsulated keels drop out while sailing back across the channel.Only his incredible resourcefulness saved the day.

 

[silver-fox]

The only stainless steel chain I have tested had almost exactly the same strength as much of the Grade 30 galvanised. It had a rather brittle appearance, which might lead to problems with snatch loads, but strength was well above the minimum requirement. Anchors are somewhat different because steel ones use heat treatable materials for shanks for example, and no stainless steels can reach those tensile values.

316 is widely used underwater for prop shafts. They will suffer crevice corrosion, several examples on my website, but generally they last well. Bolts are particularly susceptible to crevice corrosion, each thread root being a crevice, but use of sealants that keeps the water out can overcome this problem.


They probably are a little weaker but the designer should have taken that into account by increasing their diameter and number. Experience has shown us that stainless steel keelbolts are just as reliable as carbon steel ones, although they certainly can fail, dramatic photo on the website. Again, the answer is plenty of good sealant. If you see rusty stains in the bilge, assuming it is not the result of permanent internal water, then it is likely that the water is coming up the bolts, time to investigate.


Crevice and pitting corrosion occur by the same mechanism. I would have said that the one above, with initiation by chloride attack, is a description of pitting. Crevice corrosion, as far as I know, does not need any pits to develop, the crevice is there by design, in bolts, fits of propellers, beneath washers etc. See the 'classic diagram' on the website.

Thanks Vyv

I have now modified my position of certainty.