ROCKER
That might be a good point about the luffing being caused by the
leeward hull.
Having no rocker means more fore/aft righting moment, but at the
expense of the bows digging in further than the rest of the hull.
With the stern up, that could add to any luffing tendency.
With some rocker, the fore/aft righting moment will be caused by
more of the hull depressing more evenly into the water, without that
corner at the extreme end. Even though the stern would lift more on
a rockered hull, the lack of the bow corner digging in could
potentially reduce luffing tendency. It would certainly may also
make the boat a bit easier to turn.
Does anyone have software that will calculate where the hull's CLR
will be with a rockered hull versus a straight hull when the bow is
depressed?
VICIOUS CYCLE AND TANDEM KEELS IN SHALLOW WATER
The vicious cycle also makes sense, particularly because things
get a lot less efficient as soon as water stops flowing smoothly
over those foils.
I personally can't see the need for a third foil, though, unless
you've got to sail in really shallow water. As I recall, Blind
Date's ratio of submerged foil area to sail area is not as great as
Elementarry, particularly at shallow depth. That would be enough to
substantially increase the leeway force on the foils in terms of
newtons per square centimeter.
If the rudders are anywhere near their limit for smooth flow in
the default setup (partially retracted so the ratio of foil area to
sail area is down) a, 20% reduction in steering moment could be
enough for steerage and leeway to start breaking down. This change
would require more helm for the same effect, and any change when
you're near the limit will set things off. If the rudders don't
have a fair and slippery surface, the cycle starts even sooner.
A single keel or leeboard would put the boat back into more of a
balance in terms of submerged foil area to sail area. A tandem keel
is probably less ideal, but is also probably the only real option if
you're going to try to sail in less than meter of water.
TWO FOILS IN DEEPER WATER
As for the *need* for a third foil, or other leeway prevention
system, I'd say that's debatable. Many trimarans sail with just two
foils, a single daggerboard and rudder, and do just fine. And those
with twin dagger boards and twin rudders tend to have the windward
foils retracted when racing, and are still operating on two foils.
Granted, the daggerboard doesn't turn, but especially at speed,
the steering angles should be slight enough to make this a
non-issue. If there's enough submerged foil area, then it won't
matter whether you have a single foil turning four degrees or two
foils each turning two degrees. Given the size of the Harryrpoa
rudders, you might get even less drag, since even less helm should
be required.
The key, then, would be to make sure there's enough submerged
area. At that point, there's a good argument to be made for two
foils instead of three because there will be less drag due to a
third appendage interacting with the water surface.
So, if you have the ability to get Elementarry's ratio of
foil-to-sail area, or even surpass it, the two-foil system should be
ideal, particularly if the rudders are relatively far apart.
If you need to sail well at shallow depths, a third foil, or a
keel, might be the most realistic solution. If there's not enough
submerged foil area with the rudders alone, you'll have to find
another way of getting it.
This would not be a Harryproa issue, though. It's something
12-meter multihull in shallow water would face. A trimaran or
catamaran would also have to deal with the same problem of not
having enough submerged foil area, particularly if it has a single
daggerboard and rudder. A Wharram wouldn't really run into the
problem, due to its V-shaped hull section, but it would face other
performance issues.
The Harryrproa might even have an advantage in that, because
leeway prevention is spread across two rudder/foils, neither would
have to be as deep as the daggerboard on a larger cat or tri.
---
Of course, Arttu might argue against this, having sailed his
larger proa in deep water, and still preferring to have a
daggerboard. It would be hard to prove him wrong.
That said, I'd like to see the comparison of: a) the optimized
proa with the daggerboard and rudders, and b) the same proa with
only two foils, but with the same submerged foil area.
I think that sometimes the discussion gets misdirected because
we're dealing with a somewhat unique boat form. In the end, it
could be something as simple as how much foil is actually below the
surface of the water.
- Mike
Arto Hakkarainen wrote:
Things that lead me to this theory were two sources that support each other, namely what Rudolf reported here about the experience on new rudders and Todds model testing. In BD reducing the sail force seemed to help (no luffing without jib wven though that moved the CoE back). Also on many of the videos and tests Todd made with his models they did the same many times: heeling due to too much sail -> bow pressed down -> luffing in more or less uncontrolled way. With all the righting moment of visionarries boat hardly heels but the bow may still go down. Big boats behave differently to models. Both cases support my intuition that the problem is caused by bow pressing down.The visious circle you described supports different foils for leeway prevention and steering doesn't it? That way the board takes loads and you don't need to have too much angle on rudders. I know it is against your idea of minimising the number of foils...Still I acknowledge that there are many people here with more experience so please correct me if I'm wrong.Arto
Arto,
Bow down trim does cause weather helm, especially on boats with no
rocker. However, the bow down in this case is caused by the drag from
the partially rotated rudders. A vicious circle. On Rare Bird, even
at 17 knots on a broad reach, the steering remained light and the bow
was not noticably submerged, although it was difficult to tell because
of all the spray. I think it would be less on a light boat with the
same hull, but could be wrong.
rob