Originally Posted by bewitched
I don't disagree - whether the keel rips off, leaving the bolts, the bolts stay in the keel and are ripped from the hull, or a section of hull is pulled out with the keel - it still has the same result - the keel comes off.
The point is WHY does it comes off? I would like to argue that in the vast majority of cases it comes off because the keel is being put through something that was never in the designed to experience or accommodate in the first place (grounding, poor maintainance, modification)
Of the examples you had in your previous post, the only conclusion I could find of why the keels failed was for the Cynthia Wood, where USCG stated that the failure was due in large part to repeated groundings and improper repair.
Let's face it the reason a hull fails is abuse and stress to the hull itself, regardless of the reasons. We've been talking about keel lose through hull failure of sailboats, however, powerboat hulls fail for the same reason. There's a very distinctive "crack" when a powerboat's hull gets overstressed from repeatedly pounding it hard through heavy seas. I experienced this myself and it's very unnerving hearing your hull shrudder. In a sailboat when you hear it, it's all over.
Getting on with the "why"; here are a number.
fatigue and moisture: (from National Fisherman)
One report that is available is a six-year study of fiberglass
laminate fatigue on hulls carried out under the direction of Paul Miller, assistant professor of Naval Architecture and Ocean Engineering at the U.S. Naval Academy. The academy and the University of California
at Berkeley jointly did the work and were supported by the American Bureau of Shipping
, the fiberglass boatbuilder
TPI Composites (formerly Tillotson-Pearson) and Maricomp (a California marine
structural analysis company).
The study depended on locating one or more fiberglass boats that had been in service
for a significant period of time and whose service history
could be documented in detail for the hours on the water
, and wind
and sea conditions the hull was subject to. From these figures, the number and magnitude of strain cycles or wave impacts could be estimated with reasonable accuracy.
Then the boats' manufacturer had to be able to supply test samples and panels
fabricated to the original specifications to represent the laminate's original strength. Calibrated strain gauges went on the boats to measure the effects of fatigue on stiffness and strength.
The boats that met the criteria were two J/24 one-design racing
sailboats, which had been built by Tillotson-Pearson (now TPI Composites) of polyester resin and E-glass. Both boats were used at a sailing instruction and charter
company in the San Francisco
Bay area and detailed records of usage were available. Typical sailing conditions were 1-foot seas, 10- to 12-knot winds and boat speeds in the 5- to 6-knot range.
One boat, the "high-mileage" J/24, entered the charter
fleet in 1984 and had 11,300 hours of sailing, and 10.2-million wave encounters. The "low-mileage" boat began service in 1999 and had 740 hours of sailing and 600,000 wave encounters. The stiffness, or strength, loss in the high-mileage boat was 18 percent, and that for the low-mileage boat was 4 percent.
Bending tests showed, as might be expected, that hull stiffness declined with wave impact loads, though the stress damage was cumulative. For instance, with wave impacts at 12.5 percent of the laminate's ultimate strength there was no loss of stiffness, even out to one million stress cycles or wave impacts but after that point, damage did take place. And at 25 percent stress there was no appreciable stiffness loss below 200,000 wave impacts.
Microcrack fatigue growth is the reason the hull loses its stiffness. You can't see that damage, but it is there. Eventually, stress cracks will begin to appear, first on the hull or deck
exterior but also on the interior
. If the impacts are severe enough, and they continue long enough they will weaken the hull and cause structural failure. Stress also increases permeability and moisture penetration into the laminate, which introduces a whole new set of problems.
Add to this stress related to partial groundings or "smacking" the bottom and you've just added a whole new factor to the bigger picture.
The long and the short of it is that all the data says Endurance limits were found to be near 25% of static failure load, indicating that a fatigue design factor of four
is required for infinite service with this material; the Coast Guard requirement is only three
. A standard boat laminate of polyester will have a SF = four
. An epoxy
laminate designed using FEA for a competitive race
boat could have a SF as low as:three
x (1-0.3)x(1-0.15) = 1.8
If you want to read the report, here's the link below...you'll have to pay like I did if you want more info.
ScienceDirect - Composite Structures : Fatigue response of thick section fiberglass/epoxy composites
Here are some good links on the subject:
Composites Engineering Basics
An Overview of Boatbuilding Materials and Methods