Next message: Ron Rogers: "T&T: Re: Cooler/Freezer conversion malfunction"
>While the temp of water cooled exhaust is relatively cool maybe the
added warm salt water on the aluminum is causing accelerated corrosion.
... Other thought I had was a acidic reaction from the exhaust itself
but I would think the exhaust would have to be right on the aluminum for
the reaction you desribe.
The problem is greatest when a fairly heavy deposit of diesel soot is
allowed to collect on the exposed aluminum adjacent to the exhaust
outlet. In the case of my friend's aluminum dinghy, it was suspended
off the transom directly above the center-mounted exhaust. This
arrangement trapped exhaust in a big "pocket" behind the transom, and
the dinghy (and transom) quickly assumed a coating of soot. This soot,
when combined with the water and spray splashing up onto it, formed acid
which attacked the unprotected aluminum on the bottom of the dinghy.
I shouldn't think there would be any worry at all about an aluminum
dinghy carried on a cabin top, or towed behind a boat. Whether or not
there would be a potential problem of diesel exhaust-induced corrosion
an aluminum dinghy carried on a swim step would be a function of how
well the exhaust was dispersed by the airflow behind the boat and how
"dirty" the exhaust actually was.
On our boat, which has relatively low-power Lehman 120s, the only time
I've ever gotten any diesel soot on the lower side of our step-mounted
Livingston is when I ran the boat for about 20 minutes at a high
throttle setting to determine whether or not I had a cooling problem. I
didn't, but I was puzzled the next time we used the dinghy to see faint
parallel gray "stripes" on the starboard side of the dinghy. I finally
realized this was exhaust residue that had come up through the swim step
slats. The exhaust had not been blowing noticeable black smoke during
the test, but there was obviously a lot more soot coming out at the high
throttle setting than at the normal cruise setting. So it may be that
many trawlers would have no problems with diesel exhaust corrosion
attacking an aluminum dinghy carried on a swim step. My friend has a
36' lobsterboat with a 350 hp Cat engine, and he's always had a soot
buildup problem on the transom. The addition of the dinghy hanging off
the back and effectively putting a "roof" over the exhaust has simply
made the problem way worse.
______________________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington
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