On Mon, 31 Dec 2007 07:58:13 -0500, "Roger Long"
wrote:

The tanks I've seen have the restriction at the middle, be it two vertical
pipes or a necking down of the width, about half of the cross section area
of the tanks themselves so you would be talking about pretty big pipes. The
time it takes for water to get all the way from one side to the other is the
issue so, if you are using pipes which are longer and restrict flow more,
they would be even larger.

Your roll period sounds improbably short. If your boat has a typical
trawler hull form, you would have sold her and moved to Kansas after just a
couple of trips if the roll period is that short. I'm curious what it is
and it is fairly easy to check.

On a very calm morning, slack the lines and push the boat off the dock.
Step on and off the boat or push up and down on something. You will see her
move just a little bit. Get into the rhythm and get her rolling as hard as
you can. You can set up an impressive roll on even a 100 foot boat this
way. Let go and time as many rolls as you can with a stopwatch before the
rolling stops. Sight a landmark such as the top of a rail stanchion against
the background to identify the end of the roll. A complete roll is from one
side, over to the other, and all the way back. I suspect your roll period
is actually 6 seconds.


It is definitely not as high as 6 seconds. I did this excercise a
while ago and recall that the actual number was between 3 and 4,
possibly closer to 4 but I don't remember the exact value.

4 seconds is a good roll period for a highly damped hull form such as the
Kingsbury with a flat beamy hull. When checking the roll period, the boat
will roll just once and pretty much flop back upright. There is no
carryover of motion.

That closely describes our boat, perhaps as a result of the full keel
and hard chines. There is some carry over of motion in flat water but
the amplitude is small and damps down quickly.

The roll period matches the period of waves found in
lighter winds. Resonant rolling is thus small in magnitude. When the waves
get big, the boat is no longer in tune and there is little rolling. The
number of times a hull rolls after you let it go during the roll test is a
measure of the amount of damping.


The biggest "rolling at anchor" isue we have ever had was in Bar
Harbor, Maine. At high tide the ocean swells surge over the
breakwater and really get things going. You also have the "Big Cat"
fast ferry coming in at night from Nova Scotia leaving a powerful
wake, not to mention the 40 ft lobster boats barreling in and out at
all hours.

6 - 8 seconds causes the boats natural period to match that of waves
typically experienced on the days when you decide to head for port or more
protected water. If the hull form isn't well damped, rolling can be heavy.
The stability the produces roll periods in this range will usually not have
a lot of inherent damping due to narrower beam and more rounded sections.
This is why your trawler needs those dynamic roll stabilizers.

An 8 - 10 second roll period means your boat could well be dangerous and
should have a full stability analysis, especially if freeboard is low.

It sounds like your issues are primarily at anchor. Before tanks, I would
try this.

Cut a sheet of 3/4 inch plywood into two of the largest equilateral
triangles you can. Drill holes in the corners and make a rope bridle so
they can be suspended flat by a length of rope about 50% longer than your
draft and freeboard combined. Weight the sheets so that they sink.

Yes, the old flopper stopper trick. I built a set for our old boat
and they were quite effective. On the Grand Banks they appear to be
undersized however, with 3x the displacement. Equilateral stoppers,
more than 4 ft on a side, would be quite bulky to stow and deploy.
I'll have to think about it. One trick that I did get some success
with was to leave the inflatable RIB dinghy over the side, half
suspended from the cargo boom. It allowed the boat to roll easily
towards the dinghy but provided 300 lbs of resistance going back the
other way. You could hear the dinghy splashing around at night
however so it was not a big hit with the admiral, and probably tough
on the mast rigging over the long term.

The
weights can be removable to facilitate storage. Concentrate the weight
towards one corner so that, when the sheets sink, the side away from the
weight is nearly floating. Hang these over the side made fast to your
midsheet cleats. When the boat rolls towards one of the paravanes, the
weight will pull it down edgewise and it will sink quickly. The other will
be pulled up flat with great water resistance. These will probably do a
better and quieter job at anchor than a tank.

On our old boat they provided about 50% damping which was a noticable
improvement from its deep-V "snap roll" characteristic.