Under floor corrosion

[TAF]

I am wondering what can be done to help with some corrosion I noticed under the floor of my boat. I have a Wooldridge jet boat that I use often in saltwater. Every couple of years or so I pull the seats and floor out to do a thorough cleaning of all the dirt, crab ick, and gunk that eventually gets under the floor. This year I noticed that where the floor is supported there was some noticeable corrosion. It was worse at the places that see the most traffic on the floor. I checked all grounds and they are good.

What can I do treat these corroded areas with ? Its not that they are real bad, but I want to do what I can. I do not notice any excessive corrosion any where else. Its like the wood/support contact might be holding water in between and causing this to occur ? I use salt away to wash the entire boat each time it see's salt. Anyone else notice this occuring ?

[good times]

Can you show us some pictures of the corrosion? How old is the boat? thanks.

[TAF]

I don't have any pix of the corrosion right now. It is mainly on the surface of the supports, there is some deep pitting in the worst spots. I'll try to get some pictures as I want to put it back together so I can get it dirty again.

The boat is a 96 Wooldridge S.U.V.

I talked to the folks at Wooldridge and they mentioned checking grounds. They told me the only item that is "directly grounded" to the hull is the wiper motors.

[Grain of Salt]

Did this pitting occur where stainless fasteners held down the floorboards?


I had to cut out and replace the bottom of my alumaweld a few years ago...severe corrosion from a leaking battery cable splice the factory did and the engine being grounded to the hull...started as pitting on the floorboard supports around the fasteners...

[TAF]

Yes the corrosion is primarliy around the area near the fasteners that hold the floor panels down. How can I locate a source of a "leak"?

The boat has 2 batteries and a perko switch to select #1, or #2. I have been running it with only 1 battery and have the cable ends on the battery that is not there taped to prevent them from grounding to anything. I will check those for sure.

What is the best way to check for stray current ? I have an ohm meter and am going to see if I can find anything, just not quite sure how to check for the problem yet.

I don't want to let this continue.

Any suggestions are welcome.

[Chrome Bumper]

ALuminum and stainless steel needs to be exposed to oxygen to be 'stainless'. This is due to how a microscopic charges operate on the atomic scale. Basically an oxide coating doesn't properly form to protect the metal, aluminum is a very reactive metal without the oxide coating.

We have a similar problem under the rubber rub rail on our 35' launch. Also Stainless piping has sunk many a ship when the lines become passivated due to lack of oxygen.

Keep the area dry with ventilation, figure how to get air into the connection and use aluminum fasteners on aluminun and stainless on stainless are some options. Or go to diamond plate decking in aluminum.

But it could be electrical too. http://en.wikipedia.org/wiki/Passivation
http://en.wikipedia.org/wiki/Corrosion

[TAF]

Its interesting that as noted the corrosion is concentrated near the fasteners. I think this a big clue. I am going to the Wooldridge factory today to ask some questions as I am working about 2 miles from them by chance. I really need to rule out a electrical issue.

All that I have changed in the last year is running with 1 battery and taping the connectors for the second that is vacant. Both grounds for the batteries are attatched to the main engine block. *** leads run to the Perko switch. Can the switch be a prob? I have read somewhere here about one "leaking" when both were turned off ? I also installed the factory harness from the kicker (BF8 honda) to charge the 1 battery I am using.

If the use of stainless fasteners in a alum structure is part of the problem why would they use them from the factory? Is that enough to be an issue I am experiencing?

I sure won't be able to keep the floor dry ever as there is always some water from fish crab pots etc thats going to get in the boat.

My gut feeling is there is some current that is part of the problem.

I'm no expert, but I'll be learning about this, its really bugging me.

[TAF]

What I found out about the corrosion in my boat is.................

It is "crevice corrosion" that I am seeing under the flooring and the supports of the floor. It is worse in the areas in the boat that has less ventilation i.e. oxygen that can get to the areas affected.

The zinc on my pump does'nt help to protect the hull much as there is a rubber gasket that isolates the pump from the hull. I need a zinc on the hull too. (Got one now) I am using not sure what type of fastener material to use for attatching to the boat ? Alum I think would be best ? Y/N ????

The battery grounds hook to the engine block. The engine mounts are not isolated from the hull, so I am essentially grounded to the hull which is not good. Also the wiper motors ground to the hull also.

I need to isolate the engine from the hull. I am going to use some nylon sheet material to make gaskets between the mounts and where they attatch to the hull. Also going to run a seperate ground wire from the wiper motors to the battery ground point on the engine.

I also am going to see if indeed the boat is electrically "hot" by using the technique I found here that referenced the Pro Troll site.

I am thinking that sometime in the future it would be best to cut out the angled supports under the floor and replace them with new. It seems silly to build a boat with a wood floor located in contact with aluminum whch from all I read willl likely result in this type of corrosion. If kept dry it would not be a issue, but my boat does'nt stay dry. I can sure do more to provide more under floor ventilation when I get off the water and will do that also.

If you have a alum boat with a wood floor bolted to alum supports with SS screws you might want to pull it up and take a look. I had mine out just to do a thorough cleaning and discovered a surprise.

[Grain of Salt]

wow...you are reliving the exact thing I went through several years ago.

I learned alot...sorta had to become my own expert as nobody else had any answers...worst was the builder...either ignorant or malicious

Never charge a battery in the boat, remove it to charge...otherwise it will accelerate the problem.

As far as floorboard fasteners...some boats use aluminum rivets, but I like to remove mine once per year to clean so I didnt want to drill out rivets each time...there are lots of autobody fasteners made from plastic but I settled on a device called a well nut....neoprene body around brass nut...needs about a 3/8 hole to fit in, then fasten with a stainless machine screw...basicly isolates the al and stainless. they swell when you tighten em and have worked great...don't tighten too much or they will strip.

Isolating the engine so the hull isnt the ground is tough but worth it.
Unfortunately once you get some saltwater on it it will conduct small amounts of current even acrossed the plastic material...but only when the engine is running...not enough to worry about IMO

Disconnect all batteries when not using the boat. I like the Blueseas switch but perko is ok too. If you can take a volt/ohm meter and put the hot on the positive and the ground on the boat and still read current then you have a leak somewhere.

Zincs.....early and often....get the weld on type with an aluminum strap molded in...anything else will eventually corrode the fastener and fail to protect the hull...outboard zincs are a joke...most are zinc /aluminum alloys (too last longer!) you don't want them to last. And most are sized only to protect the motor, not the boat.
On my 16 foot sled I put 5 pounds on the stern and 5 pounds inside midway up the hull in the bilge.

Oh, and I skilsawed out 60 percent of the bottom and replaced it, box girders too. Hope that doesn't happen to you.

Think I might go back to wood boats...at least I expect rot.

Good luck