The GOOD, the
BAD the UGLY of the 9.2 KEEL BOLTS
Just finished the 9.2 keel bolt project. The bolts are actually .752"
rod, threaded 2" on each end. ( 3/4" by 10 per inch, but not a normal
SAE thread) They extend down into side pockets on each side of the keel
except the front and aft ones that are on centerline, the keel is too
thin to put in pockets. The the photo to the left, shows the 3 keel
pockets that are visible and the erroded / corroded bolt in the front has
broken. This allowed the keel to pull away from the hull. I was
fortunate that this is not SilverSwan. I found all of my bolts
almost as good as new. That is the beauty of a fresh water boat.
I extracted 3 bolts and examined them and also had a
metallurgist inspect them. No pitting, no crevice corrosion, no
galvanic corrosion. They looked like they were installed last
month.
Not bad for 24 yr. old bolts, but a fresh water boat for its entire
life. Cast iron and mild steel have almost the same
galvanic
potential, stainless is a little further up the scale, 430 is much
closer than 304.
The pockets were originally packed with can foam and then faired with
an epoxy mixture. Water ran out of the pockets when I ground down to
them. Easy to find the pockets, they can have a little
surface
distortion, or with a strong magnet. I used a strong
refrigerator magnet to check the distorted area to confirm that they
were the pockets.
I used an angle grinder to get to
the pockets and then pried out the foam from around the bolt and nut. I
tried to just open the faring to minimize the amout of work. I only had
2 weeks to seal the bolts and fair the work areas at the keel pockets.
I used a dremmel tool to clean the inside of the pockets down to the
cast iron. I tried a filled epoxy blend ( the dark red-brown in
the photo on the right) for ahesion to the bare
cast iron prior to the whole job. I took a lot of abuse and the
epoxy itself failed, not the bond to the cast iron.
I ran a dremel tool with a wire brush attachment over the threads to
clean them up. I brushed McLube ( Krytox dry film lube) on all threaded
sections and also coated the threads of the nuts. Rotational friction kills
clamp load when tightening. MoS2 dryfilm lube would also work well. The
bolts if mild steel, worst case, would have a proof load capability of
around 55,000psi. or 24,700lbf. It sounds like overkill until you
consider the bending moment applied to the bolts when the boat heals.
Even with a worst case moment applied the bolts are only subjected to
around 7,000lbs in shear and tension. The amount of bolts is more for
load distribition to the cross ribs and the thickened hull section.
The original packing,sealant was cotton balls with a white sealant that over
the years had turned to dust. I had 3 definate leakers and the possibly existed of a couple weepers. The inside of the bilge around the bolt locations were wire brushed and wiped clean with acetone. In a couple spots the bilge was not flat with respect to the bolt, so I utilized thickened epoxy and a plastic form to put the bilge on an even keel. The brown filled epoxy is evident under the right bolt next to the cross stringer.( below) This was also the bolt that I threaded with
a SAE die to clean up the threads. The new nuts would then work.
Sealant ( 4200 fast cure) was injected into the bolt to hull gap,
packed in best possible with a bead built up around it, a little
bead on the back side of the load washer and carefully placed over the
threads to keep them clean. I used a stack of 3 where possible to keep
the sealant out of the threaded area. I tried to use new 3/4"x10 nuts ,
but the nuts jammed and would not assemble. The right-most stud was
modified by running a SAE 3/4x10 die over it on both ends. It did make
a little pile of shavings and did an excellent job of cleaning up the
threads that I buggered up with a cold chisel removing the froozen
nuts. I carefully cleaned the threads in the old nuts and coated them
with the McLube, dry lube.
All bolts were snugged down well
and a final load applied 24hrs later to seat the keel to hull. A small
bead of sealant oozed out around the bottom of the stack of washers. I
estimate that I applied 120 to 150 ftlbs of torque to tighten the
bolts. This should result in approxamately 10,000 to 14,000 lbs of
clamp load for each bolt. That should keep the keel snug to the hull.
I decided to protect the nut and threads from direct contact with the epoxy by placing a little
sheet of plastic overthem and then the pockets were then packed with highly filled
epoxy. The local boating store no longer carried the WEST
SYSTEM in favor of the MAS EPOXY brand. I did not have any
experience with it for cure times, so I chose the med cure rate. As
usual, I really wanted a little faster cure time due to the colder
weather.
This filled epoxy will be very easy to dremel out or use a chisel if I need
to get back in. I wanted to be able to tighten the nuts in the bilge if
necessary without having to manually secure the nuts in the pockets
with a wrench. ( the problem that started this whole project) The keel
area was faired with several layers of filled epoxy, and then the usual
sealant followed by VC17. I used the original VC17, clear resin
with copper powder mixed in. The old bottom paint looked like an old
penny, the new bottom paint just like it was fresh from the mint.
I'll wait a week and splash her the next Sat, AM just in time for our
CLSC
Long distance Wyss Race.
I single handed and arrived 2 minutes after the
start.
Total time for this quick project was 4 man days. We do not have the
ability to drop the keel and completely re-bed. If in salt water, I believe that this would be far better.
A quick note on stainless bolts, it is subject to crevice corrosion.
This occurs when the stainless is bathed in salt water without flow. It
stagnates and the water is depleted of O2. The result is that the
stainless cannot oxidize to protect itself, pitting begins and develops
a + and - charge in the pit that is self feeding thanks to the NA+ and
CL- ions. If you decide to use
stainless bolts, consider the 400 series instead of the 300 series to
reduce the galvanic cell potential. ( contact folks that do this for a
living) You may have to re-bed the keel to
hull joint
to prevent water weepage as well as packing the nut pockets in the
keel. I don't know how to encapsulate the bolts with a removable
compound should service ever be necessary.
The biggest problem experienced:
I ran a die over the threads of one of the bolts.
It was a
¾”x10, and removed a little material. It did a great job of
cleaning up
the
threads. Normal ¾”x10 nuts would now fit, before they acted too
small.
I made
me believe that a special tap and die were used for the keel bolts not
the
standard 6G, 6H thread classification commercially available today. I
used a
cold chisel to bust off a couple of the frozen nuts and new ones did
not fit.
If you know the
reason why let me know.
The prop on SilverSwan is a 14Dx12P RH Martec
Eliptical. I snapped a pic for reference. She will cruise at 6.5 knots
running at 2900RPM which is well within the limits of the 2GM20 Yanmar
engine. I burn a tank a year, about 12 gallons. I add cetane and bio
treatment in the fuel for the winterization. I will still go out
if the weather is over 50F. I just have a few extra gallons of
antifreeze on board to treat the engine when we return to the dock.
Keel bolt repairs on some boats is difficult
due to the bolts being "J" bolts molded into the lead. In this case the
old bolts are left in place and new ones are added, by the process of
"sistering". They just drill, tap, epoxy new studs in place doubling
up. Through the Islander 36 site I found Don Huseman (310-547-4604) who travels around the USA doing just this. He quoted in the ball park of $2,000 to travel with tools and do 8 new bolts. Considering the alternatives of being in east Tennessee, no marine service centers, it looked like a good alternative if necessary.
(HusemanD@netscape.net)
In the case that the old ones leak, it is common practice to
encapsulate them with epoxy. When I found a loose bolt that did
not tighten I feared the worst and considered this method. I
believe that the pockets are an OK method and easy to service if you
have a full service marina with a travel hoist. It then would be
possible to remove and fully re-bed the hull to keel joint.
Link to MAS EPOXY http://www.masepoxies.com/
Eric
SilverSwan #19
Cal
9.2
