In a message dated 2/13/03 5:03:50 AM,
trawler-world-great-loop-request@lists.samurai.com writes:

<< Greetins, Midlifemadness again, (Phil & Vicki), a couple more questions
for the experienced trawler world.  I have come across something I have not
seen in other trawlers we have been looking at. Bilge Keels ? Are these good
thing or bad? what are they for? Is the craft unstable and thats why they are
there? >>

I used to live on the Welsh island of Anglesey on the Irish Sea and I can
tell you all about motorboat bilge keels. There are two purposes for bilge
keels on a power vessel. The first and most important is to keep the boat
level when the tide ebbs. On a number of British moorings, the tidal range is
measured in fathoms, not feet. In our case the tidal range in the Menai
Strait was between 4 and 6 fathoms (26 to 30 ft.) depending on the phase of
the moon. A boat would sit on the hard for half a day.

In a perverse way this was looked on as a convenience by local boaters. If
you time it right, you could slog out to your boat wearing a pair of mud
boots, stow the boots in a locker, have a nice cuppa tea, and by the time you
finished the boat would be floating and ready for a pleasant day on the
water. After you picked up your mooring on return, you would heat the kettle
again and sip your tea or stronger stuff until the boat settled. On with the
boots and a stroll to shore. Dinghys were considered unimportant but a tide
table was essential.

The other main purpose of bilge keels is to minimize rolling in beam seas.
The keels increase the frictional damping along the longitudinal (fore and
aft) axis. If the keels are unweighted, they don't change the roll period,
just make the motion stop quicker.

Bigger bilge keels, more properly twin keels, are used in sailboats in lieu
of a single keels in high tidal areas. If hydrodynamically shaped they offer
some lift to windward, but, since there is greater underwater surface, twin
keeled sailboats are fractionally slower than their single keeled
counterparts. In Europe a number of boats are supplied in both versions.
Sailoffs show that the single keeled models have better light air performance
but both are about equal when the wind picks up and skin drag is less
important. We have a 30 year old English twin keeled sailboat as a family pet
and can attest to their advantages and disadvantages. Michael Kasten, a NA
oft referred to on the TWL, is also a fan of twin keels or bilge keels as
passive roll reducers.

I'm sure that's more than you want to know.

Larry Z

Count me in favor of bilge keels for roll reduction in
round-bottomed trawlers.  I added bilge keels to Pooh at my last
haulout, about 8 months ago.  The roll-reduction, while not
dramatic, has been very useful.  I used to wince and take strong
evasive action when passed by large sportfishing boats on plane.
Now, I just grumble quietly to myself.  I estimate the roll
reduction at 30%, but have no data to back that up.

Pooh is 46' long, beam 15', draft 4.7 ft, and weighs about
33,000lbs loaded.  The bilge keels I built of PVC foam with glass
and epoxy over.  The keels are each about 8' long, 1.4" thick,
and 5.5" wide.  They are mounted a little aft of amidships, and
are about 12" below the at-rest waterline.  If I had it to do
over, I'd put them a little deeper, maybe 18".  In beam seas and
with some wakes, they come out of the water and make a slapping
sound.  But overall, theyu've been a big success.

=====
Mark Richter, M.E., aboard M/V Winnie the Pooh,
"Mark's Mobile Marine" electrical systems repair & consulting.  Homeport Stuart, FL  Verizon cell 772-631-7408, free after 9PM & weekends.

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