Replacing lead Keel bolts

[Stumble]

Cousin,

The generally recommend design spec for keeps is that each bolt should be able to support the entire keel alone, at a minimum of a 5 times safety factor.

If you are really ok with that safety margin you might as well go down to 1/4" bolts. There is no reason to use anything close to the size you are currently planning on, and 1/4 will give you about the same strength as your design.

[cousineddy]

Actually, that 5 times safety factor works for the 1/2 hanger bolts. 1400 x 5 = 7000 per bolt, which is about what i calculated was required for a 1x safety factor. So, having a 13700 lb holding capacity gives me an over all safety factor of 2x. I get a warm fuzzy with that margin.

[casey1999]

Originally Posted by Plumbean

While probably most boats are made with "j" bolts for their keel bolts, there are plenty that used the pocket method, mine included. Here is what they looked like when we dropped the keel and replaced them all with silicon bronze.

My lead keel attachment is just like yours (windows). My original keel bolts are stainless steel and appear ok (34 years old), but maybe one day I will need to replace.

Would silicon bronze be preferred over stainless? Titanium is also becoming available at a resonable price, any thoughts?

What is your boat make?
Regards

[Stumble]

Originally Posted by cousineddy

Actually, that 5 times safety factor works for the 1/2 hanger bolts. 1400 x 5 = 7000 per bolt, which is about what i calculated was required for a 1x safety factor. So, having a 13700 lb holding capacity gives me an over all safety factor of 2x. I get a warm fuzzy with that margin.

I think you misunderstand. Each bolt assembly however anchored should have at least a 5 times safety margin.

Your assembly's week point is the attachment mechanism. So you have to design to that. If the max load the keel will generate is 7,000lbs, then you need an attachment mechanism, and bolts rated to 35,000lbs. Your design is woefully undersized.

[cousineddy]

The 13700 holding capacity is per bolt. There are 6 bolts. Total holding capacity is 82200 lbs. That oughtta do er.

[Plumbean]

Originally Posted by casey1999

My lead keel attachment is just like yours (windows). My original keel bolts are stainless steel and appear ok (34 years old), but maybe one day I will need to replace.

Would silicon bronze be preferred over stainless? Titanium is also becoming available at a resonable price, any thoughts?

What is your boat make?
Regards

Casey: Looks the same because it is the same boat! Aquafibre built S&S34. Someone had encapsulated the bolt heads in the bilge and they had corroded due to the lack of oxygen. On some there were only 3 full turns of the threads left on the bolt in the bilge. The rest of the bolt looked fine, just the part in the bilge was corroded. Silicon Bronze is more expensive but won't corrode like stainless. I only wanted to do this job once, so decided to spring for the silicon bronze.

[cousineddy]

Titanium is the best choice. It will never corrode in salt water. The only substance that can penetrate its protective oxide coating is a strong acid, like hydroflouric.

Silicon Bronze is more corrossion resistant than stainless, both 18-8 and 316 in salt water. No advantage in fresh water. Just gotta make sure you get good quality bolts. We tested silicon bronze against brass at work for a non-magnetic application. They were only half as strong as they were supposed to be. So, we abandoned them.

[Brewgyver]

Eherlihy, I'm wondering if the root cause of the damage to those bolts is more stretching than corrosion? Anybody?

[Stumble]

Originally Posted by cousineddy

The 13700 holding capacity is per bolt. There are 6 bolts. Total holding capacity is 82200 lbs. That oughtta do er.

As I said above the 5:1 safety margin is per bolt. Assuming each bolt alone is holding the entire keel. The justification for this is that when the boat is rolled onto it's side, and being slapped by waves, there is no telling how unequal the forces are going to be applied. It very well may be that just the front bolt will have to briefly hold the entire weight of the keel, plus it's momentum on the boat, even if everything else is in good shape.

[Stumble]

Originally Posted by Brewgyver

Eherlihy, I'm wondering if the root cause of the damage to those bolts is more stretching than corrosion? Anybody?

It's a pretty easy check, just try and thread a nut down on the existing threads. But I don't see anything that looks like elongation. The pictures are pretty typical of general corrosion eating away at the bolts.