Re: TWL: Ideals and Compromises
Terry,
You probably should consult a naval archetict on this one. In my
opinion it would increase stability and probably decrease the peroid of
roll.
What I said in my origional post was:
"Are you implying that more draft makes a boat "rock steady", or at least
less prone to rolling? Deep draft with a high ballast ratio will make the
peroid of roll faster and snappier. On the other hand a flat bottom barge
with a beam of 50% of its length and shoal draft will be very steady. A lot
depends on hull form, as well as draft."
I should not have been so emphatic about the deep draft part. I was trying
to make a point that different boats and hull forms will have different
peroid of motion. My experience in similar hull forms in sailboats is that
boats with deep keels, and very high ballast ratios seem to have a snappier
motion. I realize that this is against pendulum physicis. However the
motion of a boat in a seaway is not the same as a pendulum in air (although
certianly the general laws apply). I had six different boats 30 feet, 45
feet and about 53 feet. One of each had 35% ballast ratio in relitatively
shoal keels (4 to 6 1/2 feet draft). The other set of boats had very high
aspect ratio keels, with 50 % of the boats weight in the lower part of the
keel with keels 6 to 10 feet deep--racers.
These high ballast ratio boats, with deep keels had very snappy and
uncomfortable motion and I was prone to sea sickness--where as the other
boats I virtually never got sea sick. There was some general difference in
hull form--but less than in most displacement trawlers.
Another example I use is if a sailboat that loses its mast and rig, the
motion becomes much more rapid and snappier--I realize that these are
contradictions to the simple laws of physics in air. I am told that
catamarrans have a snappy motion, even though they are very stable--to a
point.
It is entirely possiable that the difference is explained by hull form, but
that does not explain the sailboat without the rig developing a snappy roll.
If you put ballast in an existing hull, it may affect performanace, it will
increase the draft and it will should increase the stability. If I was
passagemaking in a slow displacement boat, I probably would want ballast in
a keel--as deep as you could get--that is not as deep as a racing sailboat.
The motion of a boat in a sea way is a complex issue.
Regards to all.
Bob Austin,
30 foot single diesel aft cabin Carver
Pensacola, FL.
At 04:11 PM 12/21/2001 -0600, Robert Austin wrote:
If you put ballast in an existing hull, it may affect performanace, it will
increase the draft and it will should increase the stability. If I was
passagemaking in a slow displacement boat, I probably would want ballast in
a keel--as deep as you could get--that is not as deep as a racing sailboat.
The motion of a boat in a sea way is a complex issue.
REPLY
I had no end of trouble teaching buoyancy and stability in Power Sqaudron
class until I hit on the idea of using a plastic model and fishing weights.
In less than 15 minutes I was able to demonstrate the effect of
increasing ballast, shifting the same weight from low to high and fore or aft.
The plastic model was a Lindberg model of a Moran harbor tug measureing
about 10" long.
For anyone wanting to gain a better appreciation of the relative effects
of adding or subtracting weight, shifting it hiogh or low this is about as
good as it gets for very littel money. Of course, if you need specifics of
adding a 500 lb dinghy on top of your hard top 14 feet above the weater
lin; well then you have to get out the calculator.
Cheers
Arild Jensen
The Electronic Navigator
<<Another example I use is if a sailboat that loses
its mast and rig, the motion becomes much more rapid
and snappier--I realize that these are contradictions
to the simple laws of physics in air. >>
No mystery here. The sea forces acting on the
sailboat's hull are the same before and after the
dismasting. These forces are resisted in the roll
axis by the boat's roll moment of inertia. The
contribution to the boat's roll moment of inertia of
each piece of the boat is that piece's mass times the
distance from the CG raised to the 4th power. The
mast isn't very heavy, but it is a long way from the
Center of Gravity. Raise this distance to the power
of 4 and you get some really big numbers. In the roll
axis, the mast may well contribute more to the moment
of inertia of a sailboat than the rest of the boat
combined. Lose the mast, and the roll moment of
inertia gets much smaller, like shortening a pendulum.
Thus the roll period gets shorter, for a quick, jerky
, uncomfortable roll.
=====
Mark Richter, M.E., aboard M/V Winnie the Pooh,
custom Morgan 46 Pilothouse Efficiency Trawler.
"Mark's Mobile Marine" electrical systems repair & consulting. Homeport Stuart, FL
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Richter-Pooh@rocketmail.com writes:
In the roll
axis, the mast may well contribute more to the moment
of inertia of a sailboat than the rest of the boat
combined. Lose the mast, and the roll moment of
inertia gets much smaller, like shortening a pendulum.
Thus the roll period gets shorter, for a quick, jerky
, uncomfortable roll.
Excellent explanation Mark. A few years ago, I unstepped my mast (35
ft tall on a 27 ft sailboat) to see how she would behave as a wannabe
trawler (to explore some of Ontario and New York's beautiful canals).
There was some moderate chop and a wee bit of wind out on the big
lake. Would have been a pleasant sail with some splishing and
splashing against the hull. But without the mast, it became instantly
uncomfortable very uncomfortable.
The stick went back up the very next day!!!
George, wintering in Frenchman's Bay, Lake Ontario
P.s: Scaramouche is a 6 ton boat with a hollow modified (Brewer-bite)
full keel, 4.5 ft deep and lead-filled .
Years ago, we did a lot of boating in Ontario, Quebec and NE USA.
Often, we would see sailboats with stepped masts cruising the canals and
waterways. Invariably, anything bigger than a passing canoe would rock
the heck out of them. They could sometimes be heard complaining
vehemently at marinas and anchorages, and they often would give the slow
down signal to boats that were already going quite slowly. Almost as
often they could be seen giving another internationally recognized sign
when they felt that the passing traffic did not slow down sufficiently
<Grin>.
I thought they were just grumpy blow-boaters, embarrassed by the loss of
their phallic symbol. Now, thanks to Mark and George, we know the
truth. Question is, now that we know this, will anyone slow down? Our
Whaleback makes so little wake, even at cruising speed that we'd have to
pass them stopped (VBG).
Best to all,
Maurice & Louise-Ann
AKAMA, KK48 Whaleback #2
Raffles Marina, Singapore
-----Original Message-----
From: owner-trawler-world-list@samurai.com
[mailto:owner-trawler-world-list@samurai.com] On Behalf Of George Geist
Sent: Sunday, 23 December 2001 00:15
To: Richter-Pooh@rocketmail.com
Cc: trawler-world-list@samurai.com
Subject: Re(2): TWL: Ideals and Compromises
Richter-Pooh@rocketmail.com writes:
In the roll
axis, the mast may well contribute more to the moment
of inertia of a sailboat than the rest of the boat
combined. Lose the mast, and the roll moment of
inertia gets much smaller, like shortening a pendulum.
Thus the roll period gets shorter, for a quick, jerky
, uncomfortable roll.