Hi,
My rudder configuration does not allow me to change to change the submerged foil area. Thatīs why I chose daggerboards. I think my steering foil area is now at its minimum. I would like to have only one daggerboard, but because of the mast I could not place it in the middle, thatīs why two daggerboards. The daggerboard (asymmetric) sections create lift, so I hope might use them to balance the boat even better.
Regards,
Arttu
Mike Crawford [mcrawf@nuomo.com] kirjoitti:
> 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:
> >
> > --- On Mon, 3/7/11, Rob Denney <harryproa@gmail.com> wrote:
> >
> > Rob.
> >
> > I mostly agree with what you say below. However, I keep coming back
> > intuitively to the fact that the rig is pressing the lee bow hard and
> > even though it has lots of buoyancy it still could be pressed down by
> > rig forces more or less. I know there has been a lot of discussion on
> > the subject and whether the dynamic forces on the hull will counter
> > that force or not.
> > 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
> >
>