[Public-list] Travellers...Propane Tank

Meinhold, Michael J MICHAEL.J.MEINHOLD at saic.com
Thu Mar 31 05:12:35 PST 2005


Roger -
  What you're calculating is the additional rotational moment of inertia,
and it's 16,000 lb*ft^2.  It's not really a load on the structure of the
boat, though it does affect the load. I can only guess, but the gyradius of
the A30 might be 40% of the length. The boat's moment of inertia would then
be about (30 ft * .40)^2 * 9000 lb = 1.3 x 10^6 lb*ft^2 . Your 210 lbs of
water thus increase pitch moment of inertia by about 1.2%. 

You're correct about the dilemma and tradeoff between transverse and
longitudinal weight distribution, and between performance and safety. That's
why I own an Alberg - the designer made the right choices!

The center of bouyancy is just the center of the water the boat displaces.
The center of gravity is the center of all of the mass on the boat.


Mike
Rinn Duin #272

-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org]On Behalf Of Roger L Kingsland
Sent: Wednesday, March 30, 2005 3:36 PM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-list] Travellers...Propane Tank


Mike Lehman
~~~_/)_/)~~_/)~~~
Wrote;
"At least the forward water tank is not in the overhang of the boat."

Roger replies;

Mike,

As the Budweiser ad says, "true."

In a never ending quest to confuse myself (and learn just enough to be
dangerous), I have been reading up on dynamic loading and discovered that,
unlike static loads where the moment arm is weight times length (from center
of buoyancy),  the moment calculation for dynamic loads is weight times the
length SQUARED.  So, if the fore/aft center of buoyancy is 16' aft (pretty
darn close to the middle of the boat), a full water tank (30 gal @ 7 lbs)
has a dynamic moment arm of 210 lbs. times about 8.75 feet squared or a
whopping 16,000 lbs.  If a 25 lb propane tank is located all the way aft,
the moment arm would still be about 5,000 pounds.  This is a scary problem;
so much so, I have decided to saw off 8 feet from each end of my boat, which
means I now have to completely rethink the layout of the main cabin!

While keeping the fore/aft weight near the middle is desirable to prevent
"hobbyhorsing," it gets more confusing because the opposite seems true for
transverse weight distribution.  The dynamic stability (ability for boat to
resist wind gusts, side wave impacts, roll going downwind) improves as the
roll moment of inertia (weight times square of distance outboard from center
of gravity) goes up.  Of course, with our boats, it isn't possible to move
weight outboard without moving it up which raises the center of  buoyancy,
which compromises the static stability, which confuses me even more, which
makes me want to get back inside "the box" and take a nap after, of course,
bolting the engine to one side of the boat amidships and the water tank to
the other.

Mike Meinhold, did I get any of this even close to right?


Roger Kingsland
Chief Boat Boy, Rubber/Scraper, Numbers Cruncher, Source of Confusion and
Check Writer
Alberg 30 #148, PERFECT intentions
N40-33.92, W79-51.25

"You don't have to think outside of the box, but it doesn't hurt to peek
over the edge once in a while."
Leonardo Da Vinci (or somebody like that)


 1112274755.0


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