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What's New from IBEX?
Miscellaneous Musings from the Technical Director
Monday, 06-NOV-2006 by Donald
MacPherson - HydroComp Technical Director
This is the first of my two November musings, and
I'll share with you some thoughts about last week's IBEX show in Miami.
The second musing will probably be all about the upcoming METS show in
Amsterdam next week. You gotta love the October and November trade show
travel in the marine business...
I enjoyed the show, the weather was OK, and it was a productive visit
for us. Vin, Mat and I had a good time discussing new features in Rhino
and RhinoMarine with Larry Leibman (Proteus
Engineering) and Steve Baer (Robert
McNeel & Associates) over some pretty good Irish food and beer.
(It wasn't Galway, but for Miami, not too bad.) It was also great to run
into colleagues that I have not seen in a while, such as New Hampshire
native turned Florida import Dan Mielke (OneDesign
Ltd.), early PropExpert user Dave Fessenden (Cummins
Atlantic), and especially a colleague from my days in Wisconsin, Tim
Graul (TGMD).
And, I would be remiss if I didn't also mention seminar manager Barbara
Jean Walsh (IBEX)
who makes that special effort to let me know when the University of
Maine trumps the University of New Hampshire in ice hockey.
It is a bit out of character for me, but I thought I'd list a few things
I saw at IBEX that made me furrow my brow in wonder.
1. There were no naval architects or designers exhibiting. I'm not sure
why, but I found this very surprising. FLIBS?
2. Similarly, there was only one hull software program exhibited this
year.
3. Don't people know how silly they look wearing cell phone ear pieces
for the entire show. Buddy, it doesn't make you look like a secret
agent. In fact, it makes you look like something out of Star Trek. Now,
if that's the look you're going for...
4. Giant green aliens. Truly - there were giant green aliens.
5. Companies still calling it a "cavitation plate". Guys, it
does nothing for cavitation, it helps stop ventilation of the
propeller, not cavitation.
And now my personal favorite - the on-board real-time propeller slip
gauge. Anyone who knows me, or has taken one of our propeller seminars,
could guess that this would get me agitated. Propeller slip is an
out-dated way to compare propeller performance based on the advance
speed of the geometric pitch helix versus boat speed. It never really
actually told you anything, but when all propellers more-or-less
performed in a similar way, you could use slip to make a guess at the
relationship between pitch, speed and RPM. These days, however, slip is
something that I wish were removed from our vernacular (like "hull
speed"). More damage is done with slip than any benefit that could
possibly be gained. Let me give you some of the reasons why.
Propeller slip uses pitch in its calculation. What pitch? Face pitch,
hydrodynamic effective pitch, some average pitch, maximum pitch, pitch
at the 0.7R, nose-tail pitch? Well, you get the idea.
Boat speed is not the speed of advance into the propeller. The effect of
wake fraction is completely neglected in propeller slip, and it does not
necessarily correlate from boat-to-boat. Add a propeller tunnel to a
boat and wake fraction is different, so the slip should be different for
the same boat speed. Installation with a shaft angle of 15 degrees
results in a different speed into the propeller than an angle of 8
degrees. (Remember the cosine rule?) Again, same performance, different
slip.
Perhaps the most obvious example is that a propeller with enough cup can
theoretically have a slip of zero or even a negative value. I have seen
numerous occasions where a perfectly acceptable propeller was considered
"incorrect" by someone quoting a speed versus slip chart out
of some book or article. I have also seen people purchase a propeller
based on slip and get it very, very wrong.
Now someone comes up with the idea that boat operators need to know this
real-time. I conducted an informal survey at IBEX and found no one, save
the maker of this gauge, who could see any useful purpose for it. (But,
the salesman did say that people were waiting in line to get one, so
maybe we just don't "get it".) Of course, there is absolutely
nothing wrong with a company creating any product that the market
requests, but there is the potential, if not the likelihood, for this
product to be used to reach conclusions about a propeller's performance
that are not valid. All I ask is that anyone dealing with propellers
learns what propeller slip really is, what it isn't, and what knowing
this number will - and will not - tell you.
Michael
Porter, Marine Design wrote on
01-DEC-2006:
Wherever did you find "good Irish food"??? In Miami Beach??
Donald MacPherson,
HydroComp wrote on 04-DEC-2006:
It was the Playwright Irish Pub on Washington Avenue. Nothing fancy,
just a pretty good sports bar especially for soccer fans.
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