I recently decided to replace all of the
thru-hulls on my 2007 Beneteau 40. None of them were leaking, and in fact they
mostly looked brand new at six and a half years old, but I have heard so much
about the sub-standard metals used on Beneteau thru-hulls that I decided to
just take care of it. I'm glad that I did.
Beneteau OEM Thru-hulls: Looking Good but Deeply
Flawed
The photo shows one
of my original thru-hulls. It looks pretty good, doesn't it? Unfortunately, it
wasn't. Beneteau, like most if not all production boat builders, uses mushroom
style thru-hulls with straight (NPS) threads. These are bolted to the hull with
a threaded backing ring, and then attached to ball valves that have tapered
(NPT) threads. There are, however, a number of serious problems with this
installation:
Straight-thread thru-hulls should never be mated with tapered-thread valves. You end up with surprisingly little contact between the two components, which makes for a weak connection and a great potential for leaks. Of course you can't buy tapered thru-hulls nor straight threaded ball valves, which is why the production builders get away with doing it incorrectly.
That tiny threaded backing ring is no substitute for a real backing plate. When you grab the thru-hull and give it a good shake, it actually moves, flexing the hull. This is made worse by the round well or recess into which Beneteau installs their thru-hulls - see the photo again. The Beneteau hull consists of an outer shell plus an inner shell, both of roughly equal thickness, strongly bonded together. Where there are such recesses, however, you get only the outer hull, which is perhaps a bit more than 1/4" thick. In other words, the hull is thinnest where you'd want it to be the thickest. The diagram shows a cut-away view of the situation.
Beneteau does not appear to use bronze thru-hulls. Some parts on my boat, such as the hose barb adapters, were almost certainly plain brass, and were tinged with pink when I removed them - a clear sign of dezincification. Others may have been DZR brass or something similar.
The thru-hulls are surprisingly thin - far too thin. One Beneteau dealer told me that my particular generation of boats - from 2008-2010 or so - used thinner metal than usual, and that they switched to thicker materials after receiving many complaints.
For some reason, some of the components attached to the ball valves were firmly glued or epoxied in place. In the photo, for example, this shows up as a thin red line of glue at the top of the ball valve between components. Upon inspection, I suspect that they were using some metric threaded components that didn't mate properly without the glue.
You can't see it in the photo, but the thru-hull in the photo terminated in a 1.25" hose barb onto which was attached a 1.5" hose. Beneteau used hose clamps to crush the 1.5" hose down to 1.25". This connection never leaked, but I was really disappointed to see such sloppy work in a bit of plumbing that spent all of its life below the waterline.
Removing the Existing Thru-hulls
There are a number
of different ways to remove an old thru-hull. My boatyard usually chisels or
grinds off the mushroom head from outside the boat, which is easy for them to
do but has the potential to gouge the hull and/or compromise the epoxy barrier
coat. I chose instead to use a multi-tool from inside the boat to cut through
the threads just above the backing ring, and then to use a very large socket
and an impact driver to quickly "spin" off the threaded backing ring. The cutting might have been a challenge
except for the very thin metal and, I suspect, its partially de-zincified and
weakened condition. For me, it cut like butter in a few seconds and the impact
driver spun off the threaded backing rings instantly with no drama.
New Hardware - Backing Plates This Time
I could have
replaced the old thru-hulls with identical hardware made of top quality silicon
bronze and, very likely, had no problems for ten years or more. I instead chose
to "do it right", by which I mean that I used the Compass Marine
method here:
http://www.pbase.com/mainecruising/backing_blocks.
The right half of the diagram shows the setup in cut-away view. Like the
excellent Compass Marine how-to article, I used 5/8" thick G10 board (from
eBay) for my backer plates, Groco flanged adapter plates, and replaceable
bronze studs threaded into the backer plates. I deviated slightly from his
method by adhering the backer plates with epoxy thickened by long-strand
"kitty hair" additive for even more strength. Also, I found that a
cheap Harbor Freight tap and die set worked perfectly - no need to buy
something more expensive.
Other Thoughts To Share
Using the Groco adapter plates will make future replacement - if needed - a breeze. I can remove and replace a mushroom fitting without touching the inside of the boat, and I can replace a corroded ball valve without breaking the seal on the mushroom.
With the hull, the backing plate, and the layer of reinforced epoxy between them, the new thru-hull openings are almost 2" thick - much better than the 1/4" original depth, and deep enough that I didn't have to trim the mushrooms for length.
It is absolutely critical that your new backing plates are aligned properly so that your new thru-hulls seat flush against the hull - and they won't be unless you work at it. The Compass Marine article glosses over this topic somewhat. I ended up test-fitting everything dry and then using temporary wooden shims to get the alignment just right before applying epoxy. It worked perfectly.
Because the new thru-hulls are taller than the originals, I needed to re-run quite a bit of plumbing. This was not an easy task. If you embark on the same project, allow time for this.
Disenchanted with the Beneteau hardware, I decided to replace every metal fitting attached to any thru-hull, and to switch from the OEM hose clamps to a top quality clamp with a rolled edge. Overkill, but I like it.
G10 board is extremely difficult to cut and drill. Your table saw will work, but the blade will dull quickly. A drill press works very well to drill the holes, but you need something quite powerful - I ended up using a friend's when my 0.75 HP press just wasn't up to the task.