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Who Will Be The March 2011 Know It All Winner?
Email your answer to:
nnudelman@westlawn.edu
Want to see how much you know? Want to show everyone else how much you know? The
first three people to submit the correct answer to the following question will win a West-
lawn tee shirt and cap, and will also receive a Know It All certificate. The answer and win-
ners to be published in the next issue of
The Masthead.
The standard formula for propeller shaft diameter in inches, as found in ABYC P-6, in
Propeller Handbook,
and in
Boat
Mechanical Systems Handbook,
is:
Where:
SHP = shaft horsepower SF = safety factor
St = shear stress, psi
RPM = revolutions per minute
Can you explain where the constant 321,000 comes from?
321, 000 x SHP x SF
3
Dia., in. =
St x RPM
Re: The Know It All contest on the September 2010 issue of
The Masthead
, about the stray current
corrosion of a single bronze seacock . . .
On January 7, 2011, KIm MacCartney wrote:
While I concur with the basic premise of the individuals’ responses, I believe the answer is flawed. For there to have be
only an attack of the one fitting, the boat was not properly bonded. All of the thru hulls should have been bonded and
therefore equally damaged. The anode(s) on the shaft should have been completely consumed and non-existent. The
bronze prop would have taken on an orange tinge as the less noble metals were consumed. The stainless shaft may ha
an appearance of worm attack; literally worm-like holes in the metal, particularly in the area of the stuffing box as the
chromium oxide film was destroyed and the ferrous component attacked by the stray current.
Stray current corrosion is not selective and attacks anything metallic that is connected to the hull bonding system. For
the one thru-hull to have been damaged, the boat was not bonded in accordance with the ABYC standards.
I really enjoyed Dave Gerr’s article on galvanic corrosion. I have sent the web link to all of our underwriters and claims p
sonnel.
Keep the excellent and informative articles coming.
Kim I MacCartney
Principal Marine Surveyor
ACE Recreational Marine Insurance
Dave Gerr’s response:
Kim:
Thanks very much for your comments on the answer to the September 2010,
Masthead
Know It All contest. You are cor
rect. Though the answer presented was accurate as far as it went, it should also have included the fact that the corrosio
was concentrated on this one fitting because either the fitting wasn't properly bonded or the entire boat wasn't properly
bonded.
In recognition of your extraordinary sagacity, you are now officially a Super Know It All. A Know It All certificate, Westla
tee-shirt and cap are on their way to you.
Cheers,
Dave