Keel bolts

[wheels 544]

OK, their are a lot of close too's in all the above comments. But it is far more complex than mist of you have reasoned.
Myjane, sorry if I understand your comment wrong. But taking it as written, 304, or any SST for that matter is not " crap" due to being a lower number, if that is what you meant. 

In general to all,
The numbers do not note quality, but the particular type of SST due to the mix of metals that went into making the alloy. Even within the numbers, there is a difference and designated by a letter or two. for instance, 304 and 304L, 316 and 316L. Actually, you can have three grades within these numbers called Dual grade.
So for 316, this grade has excellent mechanical properties. 316L has slightly less carbon content and this aids welding, but does not have quite as good mechanical properties.
316 is the choice in Marine applications, because it has better resistance to chlorides and Salt is Sodium Chloride.
When we say, Oxygen environment for SST's, we are talking at a molecular level. Even underwater has enough oxygen to keep it stainless.
Whether a SST works well under water or not, is usually due to two other issues. Corrosion is caused by two very differing animals. Simple corrosion due to oxidation and electrical corrosion due to galvanic processes. SST protects itself by creating a layer just one molecule thick of chromium oxide on it's surface. This stops oxygen from then touching the surface and causing more oxidation. The CrO is a barrier to oxygen. As long as oxygen is present, the CrO is also always present. But as soon as oxygen is taken away, the CrO decays and then other metals in the SST begin to corrode. Funnily enough, you have to have oxygen to corrode. It's is just a case that you need more oxygen to make CrO than you do for other oxides. So in a completely starved oxygen environment, you won't actually get rust anyway. that is why in some cases, the surface of SST may rust, but not go any further. However, just really low levels of oxygen will cause rapid corrosion and eat a SST bolt away quickly. In timber, if it is damp, can also create a slightly acidic state and that only serves to increase the oxidation rate as well. The second corrosion type is Galvanic and this is actually the cause of crevice corrosion. In the older days when only Imperial SST bolts were available, the threads were cut with cutting dies. this ripped the surface and crevices formed. Today, threads are formed by rolling and the issue is much improved in that respect. 
Any heating or welding on SST changes the Crystaline make up of the metal and this can be enough to create enough of a difference in the metal, that in just the right chemical soup of water/oxygen, the metals can start eating each other. One reason why SST chain tends to eat away at the weld points on the links.

Re the main topic of this thread, if the keel bolts are epoxied into a GRP hull, they are pretty good at lasting as long as no water gets between keel and hull. For a timber boat, it is a different story. The timber will be damp enough to cause some form of a problem on a 30yr old bolt and replacement is highly recommended....by me anyway.

 

[Myjane 40]

Ta wheels ,,,, Yes but in tantalised timber , tanalith reacts to s/s as is the bolts I pulled out had dulled off , most of the suppliers in the building industry don’t really know what they are buying , , they just order s/s bolts and charge heaps for them I found a place on the north shore the tool warehouse there s/s bolts you would never know what What coding they are but are under half price the building merchants , every body goes there , your proberly going to shoot at me , but the more steel in the s/s the longer they will last , old Galve bolts just corrode through the galv and seam to last well , in the old days we used to greese galv bolts And they last years I give up

[Crazyhorse 47]

There's another answer to identification right there. Lead will "meld" (solder) to some Monel, but not SST.

I answered the the question of can you identify by looking at the two and that is very hard to do. But there are other tests to identify accurately and even identify what species of Monel or SST.

The simplest is the scratch test. Monel is soft and easy to file or nick with a hacksaw blade. SST is not. But that will not tell you the type. To do that requires the use of Nitric and Phosphoric Acids and I figured that a bit beyond the realm of the original question. A drop of Nitric does nothing to SST. It instantly reacts on Monel. A drop of 45% Phosphoric Acid will slowly bubble on SST. Then it gets a little more complex for identifying the types and probably not needed here.

Yep, you CAN solder stainless steel (Ive done it). Google it!

[Crazyhorse 47]

Not sure if it would work on keel bolts but a density meter might work? Seen one used on concrete. If there were differences in density between the bolts, it probably would show up?

[rmiker 15]

Yep, you CAN solder stainless steel (Ive done it). Google it!

Yes you can solder SS. Soldering ss was still an accepted construction method when I started my apprenticeship: From distant memory I think we used high tin solder though..

 

Usually you can tell the difference between monel and ss visually. SS appears more silver due to the chrome content: monel is duller and has a more golden hue. Monel is also magnetic where 304 and 316 aren't. However some SS's are magnetic so visual was the main in the old days

[smithy09 50]

When I fitted the new bulb to the Marshall we got 2205 stainless steel studs machined up to bolt it to the fin. We pulled the old 2205 studs out. They had been in there around 10-12 years and were in perfect condition. 2205 is designed to be able to be submersed in seawater and be corrosion resistant.

[wheels 544]

Of course you can solder to SST. I did not say you can't. Re read my post for exactly what I did say and in what the circumstance is. We are talking plain old lead and no flux. You can solder to darn near anything if you have the right materials.

Yep smithy, 2205 is far superior in many areas, most especially corrosion resistance. And it is lovely stuff to machine. The only reason why it has not really been seen much as things like Keel bolts is that it has not been around all that long. It's was developed by Sandvick only ruffly about 20yrs ago. So as you said, the only real way of using it is to machine shaft up into your own bolts. 
It is very commonly used for prop shafts.
 

[vesper1 3]

It's best if they are all tight carrying equal strain, you won't break a 19mm bolt, Put your shoulder into it on the end of a 3/4 inch drive ratchet or similar. Monel is the best totally unaffordable these days.

[TimB 7]

SAF2205 is a Duplex Stainless Steel. there are lots of them. They were developed for the oil industry in the 60's to prevent Stress corrosion cracking in high temperature heat exchanger tubes on oil rigs in the North Sea. Using seawater on the cooling side so hot wet and lots of  chlorides.

 

Much stronger than 316 but must be careful welding not to let the (inter-pass) temperature get too hot or the structure changes and it is no longer duplex. Duplex means the molecular structure is both austenitic and ferritic. 304 and 316 are austenitic.

 

An another interesting things is that its coefficient of expansion (with temperature) is almost identical to that of mild steel where as 304 and 316 are almost double.

 

I was involved in building two 30m by 5m diameter pressure vessels (10 bar pressure) for the pulp and paper industry back in the nineties. we saved about 30 tonnes per vessel by changing from 316 to 2205.

 

Tb

[Battleship 100]

We used 2205 on our rudder stock, lovely stuff they use it for hydraulic rams on big diggers, It allowed us to make the rudder much thinner and still be safely over spec. The engineer told me the 2205 had double the flex resistance of 316. And yes we had to get a welder who knows his stuff to weld the webs on.