This is a report on my experiments
with a stabilising and steering device called a 'hapa' or 'seadog'
(because it pulls to windward like a dog on a leash). The basic
principle of the thing is that you put a foil on a string or some other
flexible connection and attach it to the boat on the weather side. If
the attachment point is far enough to weather and/or high enough the
pull of the foil balances the sail's heeling moment (Fig 1a). 
The
combination of a flexible connection between foil and boat and a foil
that tracks the water surface makes this balance self-correcting. If the
boat does heel for some reason the resultant force from the foil will
pass above the centre of buoyancy higher than the centre of effort of
the sail, and the sail will pull the boat upright again (Fig 1 b). I
found during trials at the Loch Lomond meeting that righting moment was
only limited by the strength of the structure. Once it started blowing a
Force 4-5 my mast (from a children's windsurfer) started bending more
than I was comfortable with, so I eased off before I discovered its
limits. It actually took me a while to realise there was something
distinctly odd to the feel of the boat. A monohull heels when hit by a
gust. On a beach catamaran I can feel it plant its lee hull more firmly
in the water before it accelerates. What is odd about my boat with the
hapa is that it doesn't feel any different when a gust hits. The
water noises get a bit louder as the boat accelerates, but that’s it.
If I had had a stronger rig and no need to ease off, then presumably the
boat would have felt more 'solid' as the foil stabilisation becomes
stronger and weight shifts have less effect.
Another distinctive feature of the
hapa is that because the foil is not rigidly attached, it can follow the
surface without having to overcome the roll moment of inertia of the
whole boat immediately. This should be a major advantage over a rigid
foil attachment in a seaway, but there wasn't enough of that on Loch
Lomond, so I have no personal experience.
There are at least two ways of making
a foil track the surface. One is to use a foil shaped so that the
direction of the resultant force changes with the immersion of the foil.
The other, which I used, is to have control surfaces that 'feel' the
surface and change the pitch angle of the foil. This is shown in figure
2.
As can be seen on the side view, the down force from the T-foil acts in
between the upwards components of the strut and canard foil. If the
canard leaves the water, perhaps coming out of the back of a wave, the
moment produced by the strut and line pulling up aft and the T-foil
pulling down further forward makes the whole foil pitch down. If the
canard pushes into a wave it will rise because it has a larger angle of
attack than the strut.
Finally, I attached the hapa to the
boat far enough forward that the boat would turn into the wind if left
to its own devices. A separate line pulls the whole hapa back so
that I can balance the centres of effort of sail and foil for any course
from hard on the wind to a broad reach (Fig 3) 
. I can use the hapa for steering between these courses, and don't
bother with a rudder. Having only a single sail I do need a paddle for
tacking or gybing. Actually, the boat tends to turn around the hapa, so
I had to paddle through the wind when tacking, rather than using the
paddle just as a rudder. In stronger winds sail pressure was too high
for that, so I took the sail down, paddled round and raised the sail
again. With the junk rig lowering and raising the sail only took
seconds, so that was not a big problem. I guess a boat with two sails
could tack under a hapa by backing one sail. Or else, a rudder and a bit
of board, so the boat has something to turn on, might do the job.
However, rather than attacking the problem head on I favour sidestepping
it by using a two-way hapa and rigging the boat as a proa. Then I don't
need to tack or gybe. That also saves having to retrieve the hapa on the
lee side, or having it slow down the boat quite uselessly. But that is
for next year, and I might use a different boat. I have to be very
careful not to impose excessive loads on my folding canoe. Most loads
can be contained within the rig and hapa, but some still go into the
hull. And as righting moment is, for all practical purposes, unlimited,
the loads could easily become too large. If anyone else wants to
experiment with this concept, make sure your boat is up to it. For those
who want to read more about this I recommend publications available from
Amateur Yacht Research Society, BCM
AYRS, London WC 1 N 3XX.
Link
to page of photos of Hapa in use at Loch Lomond
