On 2008-06-09 20:32:40 -0400, said:

As I discuss in another post, the conventional wisdom is that external
keels are better than internal keels.

Though we have an external (bolted on) keel, I'd say that the above is
ONE conventional wisdom; there are other conventional wisdoms.

Each is a compromise.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

On 2008-06-10 11:59:06 -0400, " said:

On Jun 10, 3:31 am, jeff wrote:
I guess its time to repost this page:

http://www.cliffisland.com/boat.html

BTW, the boat is an early J-35 (fully cored), and apparently it had
grounded previously.

Yuck. It looks like the hull failed, not the ballast keel/sump joint.
I suspect the "outsideness" of the ballast was not a contributing
factor.

Internal or external, with construction that looks that light, repeated
groundings would have torn any keel off. The strips of glass look SO
thin!

We have bolted-on cast iron -- not my favorite material, but what we
have. The reinforced area that I can identify looks to be about 4-6
times that J's. It's about 8' long and an nearly 2' across. We've got
at least an inch of solid hand-laid glass in the three spots around the
keel I've drilled holes, and a good inch-plus of reinforcement that the
bolts go through. (Not sure of the material as it's at least
encapsulated and no one I know has had reason to investigate.)

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

On Tue, 10 Jun 2008 19:10:59 -0400, "Gregory Hall"
wrote:

And, the Mac 26 can get to that shallow entrance a lot faster than any
sailboat other than perhaps a racing multihull.

A scows are good for 25, but not in big waves.

Casady

On Jun 10, 1:36 pm, RichH wrote:
....
once 300 series stainless gets loaded above its endurance limit it
typicallly only lasts approx 1 million load cycles - doesnt matter if
its rigging, keelbolts, chainplates. If the endurance load factor (at
about 30kpsi) is exceeded, 1 million cycles is about all you get...

At stresses less than that the bolts will essentially never fatigue,
right? I'm looking at a worked example of an ABS keel bolt
worksheet. Since you're using psi I'm converting to USA units. 10
keel bolts of 0.83" (at the thread root) support a 7,175 lb keel with
a cg 2' below the joint. Even assuming half the bolts aren't doing
anything the maximum static stress on those bolts are going to be an
order of magnitude below their endurance load. Hydrodynamic loads on
the keel max out at about the same order. Day in and day out you'll
never approach the endurance limit of ABS sized bolts. On top of that
your typical designer is going to use the next size up off the shelf
rod. As you'd expect with those kind of scantlings keel bolt failure
is extremely rare. Fatigue isn't normally an issue.

... That sailboats constantly have to
have rigging replaced, on some - keels & rudder shafts, etc. keep
falling off ... would tell any prudent engineer/designer that
'something is wrong' in the 'typical design'.

Rigging is a different story with very different compromises. Keel
failure is so rare that I think each case needs to be looked at
individually. There is no evident systemic problem with keel bolts as
a class.

-- Tom.

Tom --- simply go back to basic structural engineering basics.

All metal has a service life based on fatigue. Even below the
endurance fatigue limit, applied cyclical stress will develop
microcracks between the grain structure. Such propagation is minimal
but continuously additive to applied cyclical stress. At or above the
endurance limit the fatigue become more 'predictable', below the limit
the fatigue is 'not so' predictable. The inevitable failure for ALL
metals in cyclical stress service is embrittlement and crystaline or
fatigue failure. Fatigue failure at scantiling design values with
Safety Factor of 4X (typical ocean service) still but rarely happen.
Only when the stress design approaches FS=6 does fatigue failure
become rare; but rare doesnt exclude some failure. I repeat: The
inevitable failure for ALL metals in cyclical stress service is
embrittlement and crystaline or fatigue failure ... this WILL
eventually happen to rigging, rigging supports and keel bolts on
boats.

At stresses less than that the bolts will essentially never fatigue,
right? *I'm looking at a worked example of an ABS keel bolt
worksheet. *Since you're using psi I'm converting to USA units. *10
keel bolts of 0.83" (at the thread root) support a 7,175 lb keel with
a cg 2' below the joint. *Even assuming half the bolts aren't doing
anything the maximum static stress on those bolts are going to be an
order of magnitude below their endurance load. *Hydrodynamic loads on
the keel max out at about the same order. *Day in and day out you'll
never approach the endurance limit of ABS sized bolts. *On top of that
your typical designer is going to use the next size up off the shelf
rod. *As you'd expect with those kind of scantlings keel bolt failure
is extremely rare. *Fatigue isn't normally an issue.

... *That sailboats constantly have to
have rigging replaced, on some - keels & rudder shafts, etc. keep
falling off ... would tell any prudent engineer/designer that
'something is wrong' in the 'typical design'.

Rigging is a different story with very different compromises. *Keel
failure is so rare that I think each case needs to be looked at
individually. *There is no evident systemic problem with keel bolts as
a class.

-- Tom.

On Jun 10, 7:02 pm, RichH wrote:
All metal has a service life based on fatigue. ...

Rich,

No argument there. But, failure after, say, 10 million cycles doesn't
seem to fit the profile of most of the keel failures that I know of.
There is a theory that suggests that many of the racing boat keel
failures are preceded by a hard grounding that leaves no immediate
signs of harm but weakens the system. There may be something in that,
but the problem I have with it is that racing boats go to ground all
the time (as a class they are deep) and while many of them will have
had a hard grounding in the recent past few of them will drop their
keels. In any case, all of the keel failures I know of that have not
happened at the moment of going aground have happened in pretty new
boats. I do know of cases where people have removed the keels of
older boats, inspected the bolts, found signs of corrosion and
replaced them but I don't know of any actual failures underway. To be
sure this is just hear-say, but I've been aboard or near bunches of
boats that have had rig failure and many boats that have broken their
rudders (including the entire Hawaii J/24 fleet and three time on a J/
29) but no keel failures except for grounding and accompanied by major
structural failures...

-- Tom.

The additional problem with 'bolt on' keels is the concentration of
stress at the root of the keel and the mating structure. The Bavaria
line is the most stunning example of this stress anomaly wherin new
boats are losing their keels ... not as a failure of the keel nor its
attachment bolting but rather the mating FRG structure. This, to me,
is simply an unforseen 'concentration' of stresses which lead to a
very weak, prone to failure structure.
With an encapsulated keel system the lines of stress are allowed to
follow a 'more open' or less concentrated pathway over larger cross
section of the structure --- inherently safer but at a greater cost of
excess material/weight. My engineering eye usually is in an extreme
'wince' whenever I see a sharp inside corner anywhere near a
cantilever structure --- as thats the prime location of concentrated
lines of stress. Encapsulated keels usually always have 'smooth
transitions' in this critical area and thus avoid this 'stress
concentration'.

The real problem with critical stress design is that the 'calcs'
sometimes dont match with the actual alignment of stress, causing
unforseen 'stress risers' in the design that inherently weaken it.
Anytime the lines of force/stress come 'close together' (concentrated)
the basic values of ultimate tensile, etc. strength simply 'go out the
window'. All the 'strength values' have to be amended whenever there
is a 'discontinuity' in the surface of a structure ... as the lines of
stress are more located in outer surface of a 'shape'. The only way
to prevent such anomalies is to do (very expensive and very long term)
dynamic load testing to failure in a test rig -- as is done on
critical components (wing roots, etc.) on advanced aircraft, cranes,
gantrys, etc. etc. Obviously this extremely expensive process of
dynamically validating a design to actual failure is well beyond the
financial limits of a boat builder.

On Jun 11, 6:50 am, RichH wrote:
The additional problem with 'bolt on' keels is the concentration of
stress at the root of the keel and the mating structure. The Bavaria
line is the most stunning example of this stress anomaly wherin new
boats are losing their keels ... not as a failure of the keel nor its
attachment bolting but rather the mating FRG structure. ...

You're onto something here. I saw a Bavaria in the Bay of Islands
that had gone aground and split the hull right aft of the keel right
along the line where they remove the core to "reinforce" hull at the
keel joint. It also looks like the J in the link Jeff posted had a
similar split but at the front of the keel right where the core is
replaced. I can't really see it well in the photos, but it looks like
the panel sandwich had a peel failure, too. Also, with the J the keel
ripped the hull open right along the reinforcement line (the bolts
didn't fail, the panel failed). So, yes, I think you're right on the
money with the stress riser issue. Just to be pedantic, the problem
would be the same even if they glassed over the metal part of the
keel. So, I think of these kinds of failures are hull panel failures
rather than keel joint failures and don't see them as a problem with
bolt on keels per se. But I'm willing to concede the point.

-- Tom.

The simple fact is that a cantilever ( a keel is a cantilever) is
inherently WEAK .... needing FOUR TIMES the strength to be equal of
any other 'simple beam in flexure' structure. Add a safety factor of
4 for offshore or 'hard usage' and you wind up with a minimum of
needing 16 times the strength ... then add fatigue/endurance, etc.
etc. etc. and you wind up with a very complicated structural
element. A cantilever with smooth transitions at the 'root' of the
cantilever is MUCH stronger simply by 'good structural design
practice'.

"RichH" wrote in message
...
The additional problem with 'bolt on' keels is the concentration of
stress at the root of the keel and the mating structure. The Bavaria
line is the most stunning example of this stress anomaly wherin new
boats are losing their keels ... not as a failure of the keel nor its
attachment bolting but rather the mating FRG structure. This, to me,
is simply an unforseen 'concentration' of stresses which lead to a
very weak, prone to failure structure.
With an encapsulated keel system the lines of stress are allowed to
follow a 'more open' or less concentrated pathway over larger cross
section of the structure --- inherently safer but at a greater cost of
excess material/weight. My engineering eye usually is in an extreme
'wince' whenever I see a sharp inside corner anywhere near a
cantilever structure --- as thats the prime location of concentrated
lines of stress. Encapsulated keels usually always have 'smooth
transitions' in this critical area and thus avoid this 'stress
concentration'.

I am sure you are right about this. IMO the problem has become much worse
because the more modern designs of yacht are much more flat bottomed than
older designs such as my Catalina 38. On my boat the transition is quite
smooth but on modern boats it is almost 90 degrees.
Last year at my marina I looked at two almost new French built boats which
had suffered heavy groundings. The keels had not come off and all you could
see outside were some relatively small cracks in the outer glass layer.
However the flexing in the upward direction was such that major components
had failed inside the boats. The keels had to be removed and the boats took
all winter for the interior to be stripped out, repaired and put together
again.