Searunner History, was Re: [MHml] small multi's

[multihulls at steamradio.com]

In a message dated 1/26/01 9:17:00 AM, mproj at earthlink.net writes:

<< The keel was ripped off and I've wondered about putting som form of
tabs on the amas in addition to replacing the keel in order to improve 
pointing
ability....any experience, advice, experts to ask, etc? >>

A boat's underwater foil(s) resist leeway, provides lift to point higher, and 
determine its tracking (ability to sail in a straight line).  In general, the 
best pointing is accomplished with a deep underwater foil with a narrow 
chord.  The best manuverability is typically accomplshed with an underwater 
foil with a narrow chord on the centerline of the boat.

With underwater foils on the amas, the boat will turn like a car following 
its front wheels, unless the foil on the outside of the turn can be raised.  
In other words, the tri will turn like a cat.  If those ama fins are deep 
with a narrow chord, the boat could point well (depending upon other 
characteristics of the rig and hull).

If the underwater foil is on the centerline, the boat will tend to pivot at 
the center of lateral resistance as it turns.  Of course, the boat will have 
forward motion and inertia, so it will follow a curved turn, but the turn 
will tend to be tighter than a configuration with ama fins.  A long shallow 
configuration will tend to track well but be harder to turn while a deep 
narrow configuration will point higher and respond more quickly to the helm.

I don't know what the advantage of three underwater foils would be.  There 
are some monohull designs that use twin keels so that as the boat heels, one 
is straight down in the water and providing maximum lift.  This tends to be 
used to acheive a shoal draft without totally sacrificing pointing ability.  
Since a multihull is not going to heel to the extent of a mono, it would make 
no sense.

Fin amas on a trimaran are typically an attempt to keep the draft shallow 
without intruding on the accomodations with a centerboard trunk.  Jim Brown 
came up with a slick concept of putting the center cockpit over the centerb
oard trunk.  The result is to maximize the accomodations and pointing ability 
in a boat that can actually be run up on the beach.  The disadvantages are 
some bumping from the centerboard when it is not loaded (like when you are 
trying to sleep at anchor) and a split cabin with a trip through weather to 
the head from the aft cabin (my Cross designed tri has the same problem: 
she's an older design with the companionway to the aft cabin on the starboard 
side rather than forward through the covered cockpit like the Mark IIs.)

It was unclear to be whether you have a Cross or a Brown, but I'm guessing a 
Cross as I know he designed a 38 footer.  If you want to improve pointing 
ability, you could explore a deeper, narrower keel.  The connection to the 
hull would experience higher loads, so I would not do that without 
engineering assistance.  You could also add a staysail to your rig for upwind 
sailing, possibly leaving the mizzen down.  You might, however, experience 
some trouble tacking with this configuration because the center of effort 
would be way forward.

Greg Barker
San Luis Obispo, CA, USA
Cross 42 Cherokee