stainless or silicon bronze mixing

[olesouth]

I have to replace about 200sq ft of bottom of a hull that has mahogany on it and I want to put bigger screw in screw holes.

question 1 . go with longer or bigger size. they are now 14x2 1/2

and next can I use stainless. NOW I know you guy's are going to say you can't mix metals but I see mixed metals on boat bottoms all the time. What I want to know is has anyone ever mixed screws. and then checkthem lets say 5 years or 10,15,20 years. and not just the stainless but the bronze also. I am not looking for opinions but facts.

[wizbang 13]

There are no facts. The only things black and white on a boat are black and white paint.
What I mean is, there are different qualities of , stainless, bronze, mahogany, and if you move the boat a half a mile to a different marina, the corrosion/electrolysis environment will be different. (might be different in the next slip)
Why did the bottom planking need replacing olesouth?
Anyway, I would tend to fill/ repair the holes and not increase the screw sizes. GB built very good woodies.

[Lew Barrett]

The reason you frequently see arguments against using stainless fasteners here is not just in respect to mixing metals. Read about crevice corrosion (link), which afflicts stainless steel fasteners (of various grades) and is the usually quoted reason for not using them in a bottom.

[Jay Greer]

Lew is absolutely correct! I can second his statement by saying that every boat that I have worked on that had stainless screws used below the water line showed evidence of pocket corrosion in the fastenings. Bad idea!
Jay

[Sayla]

Stainless only if they stay dry, silicon bronze if they get moist (carvel), though double hot dipped gal maybe better than stainless - if it's rotten then it sounds like it's normally a bit moist.

sayla

[Lawrie]

I concur with all of the above.
You asked for fact: I seen the crevis crack issues when pulling out ~ 10 yr old stainless keel bolt and a similar age fastening screw, therefore not about mixing, it is about not using SS when immersed, buried and exposed to galvanic potential.
I have had good luck with Iron keel bolts in iron ballast/oak. therefore galvanised is an option if you don't want to use bronze.

SS keel bolts well buried in tar can work if not exposed as often SS of these dims. is more easily obtained than bronze. in this case also - galvanised can be an option.
Bronze screws seem to be your answer

[JoshuaIII]

The only things black and white on a boat are black and white paint.

Even that! They make black paint with a gallon of white!

[ramillett]

The planks had bronze screws into the stem , the bow fitting used steel screws , and the oak stem was removed with a spoon And this was above the waterline . Bob

[wizbang 13]

Loves me some oak in a boat...not.

[gavinpascoe]

Not a very clear photo I'm afraid, but there was a steel bolt, 1/2" thick which had completely rusted out and infected the deck, deck beam and kingplank. The bolt fastend a bronze bracket which supported the transom/deck angle. The bolt was fastened through the kingplank. The deck was pretty much goo, while the kingplank and deck beam had the feel of papier mache, with about as much weight and strength. Mind you, there isn't much in the way of ventilation back there.