stuffing box choices for aluminum inboard boat

The electrical connection is by the water, nothing can be done about it. Coating aluminum can cause more serious problems. If a tiny paint piece chips off, all the galvanic action will be concentrated in that area and will create a hole. Sacrificial anodes are the appropriate protection.

 

Epoxy can more easily chip than a flexible-stretchy polyurethane. I have used Black PL roof and flashing polyurethane from Home Depot and 3M5200, they both stick fine underwear to wood and metal.
I have mixed 1/32 milled fibers 30 to 40% (fiberglass) into Black PL(60%), and result was a tougher harder rubber coating, I really liked the result applied by 6 inch putty knife, smeared onto surfaces about 1/16 to 1/8 thick. Did a 5 year test, and it was the same when it came out. So this past haul out, I used it more extensively, been fine for the past year.

If you cut the exposure of metal to water with decent coatings and use anodes, then should be fine, even if some coatings get scraped off. There are no maintenance free boats.

 

I recommend to install anodes in the bilges too. Often there is severe electrolytic corrosion in the inside.

 

Looks like magnesium anodes in salt water are no good.
http://www.performancemetals.com/images/pdfs/Aluminum Anodes.pdf

 

Thanks for the additional info - very interesting.
For the first summer, hopefully this summer.... The boat will not be finished, so it likely won't spend all summer in the water.
I anticipate having to put it on the trailer, or dry it out between tides, a few times to change props, etc. I will not just set it and forget it!

 

The reason not to use a bronze stuffing gland is that small amounts of copper will drip off it as it corrodes even slightly and will precipitate as copper into the bilge. This will cause corrosion of the bilge plating.

 

thank CDBarry - I knew it was to reduce potential issues. but didn't know the reason ie filings in the bilge.
I did fabricate my own stuffing boxes (prop and rudder) from aluminum pipe, with flanges welded on them for bolts/ tightening.. working good so far.

 

Please note that carbon/graphite (as found in natural rubber compunds for hoses) has a great galvanic potential versus aluminium! This means that a "rubber" hose in the stuffing box should NOT be Rubber/graphite, but a syntetic quality!

A simple, yet well functioning version is Volvo Penta's "all-in-one". (locally called Black Max after a popular condome......)

 

Sorry, I have to disagree Baeckmo!
Galvanic corrosion is a function of electrical current. Put the test pins of a multimeter close together on any rubber hose and measure the resistance. You'll see it is at least several 100 K-Ohms, usually more than 1 megohm.
A galvanic potential of 1 Volt creates a current of 0.00001 Amp (0.01 mA or 10 uA) over a 100K resistance; almost zero, so no corrosion.

 

Check out Tides Marine.

 

The point is that in some natural rubber qualities the rubber component seems to degrade in the saltwater environment, leaving the graphite intact. The potential of graphite is ~+90 mV in moving salt water, versus ~-780 mV for aluminium (relative to a saturated Calomel electrode).

Now, with a highly conductive graphite layer pressed to the alu by clamps, there will be a comparatively big contact surface and a short contact track in the water, leading to intense corrosion.

 

right on- if you look at the galvanic tables Graphite and aluminum don't mix. Years ago I saw a beautiful example when I fellow used a graphite lubricant that dripped into the bilge. Nice large pock marks in the aluminum . But... it could have been something else!

 

Graphite, copper or brass even in fittings will destroy an AL boat. I have zincs in my bilges and water tanks just in case. If you own an AL boat you have to be anal about galvanic corrosion. I leave all my change in my car, no change is allowed on THE boat. All plumbing even seacocks are pvc, aluminum or stainless. No steel screw, all stainless. No Graphite lubricant, only silicone. And if I don't trust the marina, i DONT PLUG IN.

 

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