Stainless steel sailboat

[LyndonJ]

Stainless steel 316 boat for sale here:

http://au.boats.com/boat-details/Auz...e-Off/21733761

I know of a few that have been built but the cost..............

[rugludallur]

Lets go over the facts regarding the building of a stainless boat:

1. You have to use anti-fouling so you have to paint everything below the waterline.

2. You can't really polish the stainless to a mirror finish (cost/time/glare) so you will always have a slight reddish tinge on the surface if it's bare.

3. Stainless is susceptible to crevice corrosion so any tight space like between stringers/frames and the hull skin needs to be sealed/painted to prevent corrosion, and since you need to glue/spray in insulation you might aswell paint it. Remember that active and passive stainless are quite far from each other on the galvanic scale.

4. Some stainless is quite susceptible to metal fatigue, there is a reason shipbuilding steels are different than regular mild steel, fracture toughness is an issue, especially in colder climates.

5. Welds in stainless are susceptible to carbon precipitation issues, especially if you don't use L type material and apply to much heat.

6. For proper paint adhesion you need to sandblast steel, this creates a rather rough surface for the paint to adhere but also a surface that would promote corrosion in stainless.

7. Stainless steel sections can be welded to mild/ABS steels with 309 wire quite easily so you can have stainless sections above waterline or for critical sections you could use 2205/318 duplex steels with 2205 wire which is much more resistant to crevice corrosion.

So you have to paint and insulate the insides of your boat, you have to paint it below the waterline and you can't have a shiny surface on your topsides because of glare.

Since you have to paint the boat you have to sandblast the boat, by sandblasting and painting you create a rough active stainless surface and if water were to get behind the paint it would corrode the stainless just as easily as regular steel because the water would be oxygen starved and therefor the stainless would not form a protective oxide layer.

The bottom line is that a stainless boat will probably perform worse in a marine environment than something built from ABS Grade A shipbuilding steel with stainless sections, 2205 below waterline and 316L above waterline.

Jarl
http://dallur.com

[LyndonJ]

I think it's quite a good application.

Outside you'd paint anyway, but the true problem with steel is inside, tight fitouts poor interior access and moisture traps eat a steel boat inside out. Stainless would be left delicously bare inside and would be corrosion free for ever.

Fatigue is designed for just as it is with alloy boats, in reality small metal boats have such low stresses relative to the material strength that it's not an issue if properly designed and built.

Fracture toughness is BETTER in colder climates Austenitic materials are generally recognized for their lack of ductile-to-brittle transition . Thats why they are used for cryogenic containers.

[rugludallur]

Lyndon:

What you say is correct but I still disagree and I will try to explain why.

Fracture toughness in 316/304 is better below about -100° C when comparing to grade A because of the ductile to brittle transition point but worse to that point, luckily by the time the temperature reaches -100C there isn't any liquid water around so it's not an issue. But I digress, my point was that some stainless steels have a brittle transition point not far below room temperature and if not properly done welds can have the same issues.

I agree with you on fatigue, mostly irrelevant since almost all small craft are overbuilt due to limitations in sourcing plates in thicknesses below 5mm.

I strongly disagree with you on not having to paint the inside of the boat, unless you intend to fully weld all stringers and frames and not have any insulation.

The biggest problem I see is preventing saltwater from getting between stringers/frames and the hull causing crevice corrosion and pitting.

Any insulation that's glued on also has the ability to retain stagnant saltwater and cause corrosion, also any wood bolted to the stainless can do the same.

For anyone that does not believe me just try taking a piece of plywood, screw in an A4 (316) screw, wet the whole thing in saltwater and leave it in a damp place for a year.

On top of this you will still get rust around the welded areas even if they were all welded with back gas unless you pickle them with acid.

I would rather spend a portion of the extra money 2205 duplex below the waterline than on a hull entirely out of 316.

My bottom line is that oxygen starved stainless can perform worse than regular steel so you have to make sure there is no crevice, nook or cranny of exposed stainless where seawater can gather.

Jarl
http://dallur.com