On Tue, 19 Dec 2006 14:21:17 GMT, "Roger Long"
wrote:

Still, it's a big, messy, and dangerous job. I'd focus on that instead of
the results.

That's very true and an important point. I know people who have tried
similar mods and been very disappointed.

The whole wing keel concept was really applicable only to the old
style 12 meter Americas Cup boats because of a design quirk in the
rating rule. As others have pointed out, the best bet for a cruising
boat is something called a scheel keel, or possibly a bulb.

http://www.cs.brown.edu/people/jfh/p...AQ/node16.html

Wayne.B wrote:
On Tue, 19 Dec 2006 14:21:17 GMT, "Roger Long"
wrote:

Still, it's a big, messy, and dangerous job. I'd focus on that instead of
the results.

That's very true and an important point. I know people who have tried
similar mods and been very disappointed.

The whole wing keel concept was really applicable only to the old
style 12 meter Americas Cup boats because of a design quirk in the
rating rule.

This is nonsense. It may be true that the particular design used in
the 12's wouldn't be appropriate for a cruiser, but the basic concept
is valid and quite useful. Simply put, the water that flows under the
tip of the keel reduces the effectiveness of the bottom foot or so of
the keel. The wing keel prevents this flow and keeps the last foot of
the keel effective. This permits reducing the draft by roughly a foot
in many cases. In addition, the ballasted wings are essentially a
bulb. It should be noted that the primary purpose is to reduce draft;
the deep keel will normally be more effective overall.

The quirk in the 12 meter rule was a draft limit of about 8.8 feet
(16% of WL plus .5 meter) so the wing allowed Lexcen to get better
upwind performance out of that draft. The story gets a bit more
complicated, but that's the most important part.

As others have pointed out, the best bet for a cruising
boat is something called a scheel keel, or possibly a bulb.

The Scheel Keel is certainly a neat design - it has the same design
goals as listed above for the wing keel, but it has less drag (or is
it more lift?).



http://www.cs.brown.edu/people/jfh/p...AQ/node16.html

This is a pretty simplistic view of the topic.

Jeff wrote:

This is nonsense. It may be true that the particular design used in the
12's wouldn't be appropriate for a cruiser, but the basic concept is
valid and quite useful. Simply put, the water that flows under the tip
of the keel reduces the effectiveness of the bottom foot or so of the
keel. The wing keel prevents this flow and keeps the last foot of the
keel effective. This permits reducing the draft by roughly a foot in
many cases. In addition, the ballasted wings are essentially a bulb.
It should be noted that the primary purpose is to reduce draft; the deep
keel will normally be more effective overall.

After the America's Cup where the winged keel appeared, a lot of shoal
draft winged keels appeared on cruising yachts.

Winged keels are actually very HARD to get right. The added weight
down low helps, but the extra wetted surface and drag are
considerable. You have to get the angle of the wings right and
without doing studies in a tank, that's hard to do.

My copy of Principals of Yacht Design has an interesting study of
shoal keels. Delft University researchers used a VPP to compare keel
shapes. Results of sailing an Olympic trial course in the computer.

Times are in decimal hours:

Deep Fin keel 3.96
Shallow keel + centerboard 4.06
plain shoal draft keel 4.13
Scheel keel 4.10
Winglet keel 4.04
(short wings, aft swept leading edge)
Winged keel 4.01
(wide wings, reverse taper)
Deep Elliptical keel 3.96

So a good winged keel can be nearly as fast as a deep fin keel. But a
bad one (as many were) can be quite slow upwind due to extra drag.


If your husband is a ME then adding a simple bolt on lead bulb is the
best bet to keep performance reasonable. I'd try to keep the bending
moment on keel root the same as the original keel. I'd also use a
body of revolution using a 64 series NACA foil to define the bulb
shape. He can use Profili to get foil sections in DXF format, export
into Autocad or any 3D modeler, and revolve to get a bulb shape with
the correct volume. Then plot out full size cross sections and make
templates to form the bulb. Make a bulb plug in foam or wood. Get
somebody else to cast it for you or DIY. Glenn Ashmore's RUTU site
has a good description of keel casting.

I don't like Sponberg's "beavertail" keel. I think there's a lot of
drag in the tail that isn't justified as an endplate astern of the foil.

Evan Gatehouse
(also a naval architect and ME)

Evan Gatehouse2 wrote:

Winged keels are actually very HARD to get right.

This is a good point. I would agree about wings being difficult. A bulb
that doesn't do any lifting on it's own would be pretty hard to screw up
hydrodynamically unless you were obsessing about the last 1% of performance
for racing. Think about the drag of a lead wing that gets bent back on
itself after touching bottom


I don't like Sponberg's "beavertail" keel. I think there's a lot of
drag in the tail that isn't justified as an endplate astern of the
foil.

I could agree with that for racing but this is a cruising application. They
probably wouldn't notice much difference if they cut the bulb off flat on
back unless they had an identical boat to sail next to.

The most critical drag issue if they are going to sail anywhere that there
is kelp or lobster pots is having stuff slide off the keel easily. This is
always a problem with bulbs and the Scheel keel is expecially good in this
regard. I look at the Sponberg idea as a way to get the required volume in
the bulb without having to put if forward where it makes pot warps and weed
more likely to stick.

I'd try to emulate a Scheel keel (may be still patented) if you can get the
required area in but the Sponberg looks like it would shed most floating
stuff as well as any bulb.


Evan Gatehouse
(also a naval architect and ME)

--
Roger Long

Wayne.B wrote:
....
The whole wing keel concept was really applicable only to the old
style 12 meter Americas Cup boats because of a design quirk in the
rating rule. ...

The Oz II keel had a lot of things going on including inverse taper,
and very thick winglets cum bulbs and, as a whole, it could only have
worked well on Oz II. When we saw it I think most of us though, "wow,
that keel is on upside down!" It is the upside down or inverse taper
part of that keel that was most notable and is least transferable to
non-meter designs. There is a theory that the inverse taper on the Oz
II keel smoothed out the spanwise lift for the entire boat including
its deep, narrow canoe body. If true, this would only work on oddly
shaped boats like late generation 12 meters and so, I think it's
reasonable to say that upside down keels are only applicable to 12
meters because of the rating rule.

Winglets and bulbs are a whole other kettle of fish, and they can work
well on many designs.

Anyway, for the OP, let me second the opinion that
http://www.marsmetal.com/newpages/torpedobulbs.html is worth a look.

-- Tom.