Galvanic action on fastenings

[bischoffboatworks]

Hello. A friend found a loose garboard on his wooden trawler. It was suggested that the iron in the boat was dying and that he should refasten the hull. He did so with 4000 Silicone Bronze screws (adjacent the iron nails).........an act that brought almost universal objection from the boats previous owners. They were right. Even in fresh water, rusting of the iron heads is accelerating and he's fearful he might be starting to see some softening of the wood around a few of the screw heads (along the waterline). They have set up little galvanic cells with the dissimilar metals. I want to save this boat for another generation or two of boaters. He is thinking of pulling the SiliconeBronze (replacing with HD Galvanized?), repair / replace any damaged wood and restore the hull to the condition it was in after he worked on it 2004.

What do you think? Is galvanic action the reason for the problem. Is this the correct repair solution? Thanks for any input. KB

[donald branscom]

Hello. A friend found a loose garboard on his wooden trawler. It was suggested that the iron in the boat was dying and that he should refasten the hull. He did so with 4000 Silicone Bronze screws (adjacent the iron nails).........an act that brought almost universal objection from the boats previous owners. They were right. Even in fresh water, rusting of the iron heads is accelerating and he's fearful he might be starting to see some softening of the wood around a few of the screw heads (along the waterline). They have set up little galvanic cells with the dissimilar metals. I want to save this boat for another generation or two of boaters. He is thinking of pulling the SiliconeBronze (replacing with HD Galvanized?), repair / replace any damaged wood and restore the hull to the condition it was in after he worked on it 2004.

What do you think? Is galvanic action the reason for the problem. Is this the correct repair solution? Thanks for any input. KB

Bronze and steel have at least a 30 point difference.
The steel fastenings will be sacrificed.
Silicon bronze not silicone bronze. The silicon makes the metal harder.
For screws that is important.
That is why aluminum car pistons have at least 22% instead of 20 percent like engine cases. It also limits expansion and contraction rates when the aluminum is hot.

[donald branscom]

Your buddy has another option. I took a nail set on my Seattle-build iron nail fastened boat and went at it. I sunk all the nails a good 1/2". I then "bunged" them with epoxy thickened with microballoon. The idea was to isloate the nails from seawater stopping galvanic action. Silicon bronze screws were used to refasten the hull into both the old frames, and full length laminated sisters. The labor to remove the old nails and frames was obscene, and un-needed. The old frames were oak, and thus had iron sickness local to fasteners, but otherwise, do contribute structurally to the hull, although they are now only "supplemental" as the new frames take the primary loads (new floors also).

Removing the nails usually means replanking and perhaps reframing as the nail has expanded and won't come out in one piece.

Excellent suggestion Brian.
And maybe just a tooth pick in the hole or one of those little wood sticks used for shish Ka bobs.

[redbopeep]

Brian, what did that end up doing to your boat in terms of weight (% added?).

We also had iron nails/oak frames (with iron sickness around the fasteners) and ended up making the decision to go ahead and replace frames because we wanted full-strength frames and wanted to not sister. We plan on this boat going offshore and didn't feel like we had the options that you used. Had we planned on the boat staying near-coast and not using it for cruising, though, we might have done as you describe.

[gaff cutter]

I have a boat similar to Brian M's, a bit older (built in 1942) and also originally fastened with iron nails (cedar on oak). The oak frames were in good shape so I pulled the nails, getting about 85-90% of them out, as well as replacing galvinized screws at plank butts. Like Brian, I sealed the nails that I could not completely remove (they had broken when pulled). However, where nails were completely removed I filled the holes with epoxy and wood plugs and then redrilled for silicon bronze screws. I've completed the project (a few thousand nails and screws) but am considering replacing drift bolts with bronze as well.

[Ian McColgin]

Look at representative fastenings for deterioration and the number of each. Get rid of whichever will be easier, but get rid of all of one or the other. Quite possibly get rid of the iron at this point.

I'm a big fan of refastening with trennels, given that you need a larger hole anyway. Especially if there's good bronze next to bad iron, this has some real appeal. There are other threads on this.

Given that your friend mixed metals, better make sure he used actual bronze screws and not brass or something. Get one of his box lables or check the source.

[bischoffboatworks]

Thanks so much everyone for the input. As always, this is a great place for quick info with some good options.

[redbopeep]

Brian,

We briefly considered sistering as you did. In our case, we figured that leaving the white oak frames in the boat (3"x3"x average of 12' long x 42 lb/cu ft x 110 white oak frames) comes out to over 3,400 lbs of extra weight that we didn't need. Of course, that's if you left all the old frames in place and added new ones right next to them. There's not room to do that. When cruising, ;) I'd prefer to have that 3,400 lbs in food, fuel, water, tools, any number of things besides old rotted frames. Also, in our case, we really didn't have room to leave the old frames in place with new sisters right next to them-- midships this boat has not only the 3" frames on 12" centers but it has Crocker's "intermediate" frames that run from bilge stringer to bilge stringer so that the boat looks almost like solid frames for about 18 feet. We have 18 portholes that wouldn't fit between double frames and would have had to move to a different location than that in the plans if we'd put in sisters.

Originally, we'd thought that we were going to leave the "good looking" frames (about 40 of the 110 frames didn't "look" bad) and re-fasten into them with trunnels (as someone else here has mentioned) or the new bronze fasteners. However, we decided to do destructive testing on one of our "best" looking frames and learned that the iron sickness had weakened the white oak's holding power to about 1/3 of what was expected. When we cut the frame apart, we could see the black discolorations extending much further into the frame than we'd originally thought it would be.

I have no idea if the black iron sickness we saw could harbor additional rot that can move to other wood members in the boat--we just decided that leaving this wood in the boat, for us, made no sense. We, like you, used laminated frames to replace the old frames.

And, yes, I agree with you that there is nothing economical in time/money about this type of project! In our case, the benefit that outweighs the effort and cost is that we know that we're getting a final product that will work great for our intended purpose of returning this boat built for cruising to actually cruising. And, I'll take my 3,400 lbs and fill up on provisions!

Here's a picture of the new frames being bolted into the floors, in this area with intermediate frames, there's so little room between floors (since the intermediates go along next to the floors across the keel) that a right angle drill is used to pre-drill the holes in the new purpleheart floors.

http://farm1.static.flickr.com/130/355502881_644a9df026.jpg

Here's a picture of some of the new laminated frames being sealed up before getting installed in the boat:

http://farm1.static.flickr.com/127/351273792_ca06a1d363.jpg