-Hi Gardener,
My calculations come out a bit different ly form yours.
The centre of effort of a foil attached just near the top of the
blade is 2/3 down the blade. For a foil that has a significant shaft
above , the centre of effort is closer to the midpoint of the blade.
Take a 1m blade we have about 670mm centre of effort, for the same
blade with an extra 500mm of shaft, we have less than 1.1m CoE. ie
about 40%. The rudders on a Harry may experience greater sailng loads
as they are also acting as leeway preventers.ON the other hand,the
Harry rudders are relatively large to allow maneuverability at low
speeds. This may mean that the forces aren't quite as large as
imagined for the position and size of the rudders.
Rudders on all boats have to be built significantly stronger than the
sailing loads require. They may have to with stand being forced
backwards while kicked over 35 degrees if the boat is tetherd to a sea
anchor in a nasty sea.
I do not see that the Harry would have any more extraordinary loads
than other rudders on other boats, therefore the extra 40% should be
equivalent ie a shade over 10% extra in thickness, but to be extremely
conservative we could say that the extra sailing loads give three
times the bending moment of an equivalent rudder on another boat and
that's what we should use as a safe extrapolation for extraordinary loads
To resist say three times the bending moment, going up by 45%, more
than doubles the quantity of material at another 45 % depth. Plenty
strong enough if you go to an extra 50% in diameter.
If the rudders are supported to closer to the water than the forces
are less.
For better comparison, it may be worth comparing Hobie 16 rudders as,
with the mast raked right back, much of the leeway prevention is done
by the rudders and they are subjected to a lot of rough and tumble.
If we can get all their dimensions and extrapolate it should be a
better ball park figure.
6" steel seems much too large for such a light boat. This is thicker
than the rudders themselves and their strength is defined by a
relatively narrow bit of carbon tow on either side.
-- In harryproa@yahoogrou
>
> Rob,
> You mention moving the rudders to windward on boats that don't fly a
hull.
> We had talked about this a few months ago, because it would really
simplify
> the steering on boats wil telescoping beams if the rudders could be
in line
> with the steering station. At that time, you mentioned testing the
idea on a
> small boat, because the theory predicts that the rudders won't work
as well
> there. Have you had a chance to do that? Did I misinterpret any of
what you
> just said, or said before?
>
> I am particularly interested, because one of the comments Doug made
was that
> he would like to move the rudders further towards the ends. I am
assuming
> this means that he is not getting the steering control he would like
where
> they are currently placed.
>
> I hope you have not taken any of my comments as nay-saying. I would be
> thrilled once all the rudder questions are resolved.
>
> The 6" rudder stock calculation came from "The Nature of Boats". In
a past
> discussion, jjctaylor mentioned that his rudders would be 5.5 sq ft,
1m tall
> and 0.5m deep. On page 398 of my copy, there is a graph for rudders
> supported at the top, which shows that a 5.5 sq ft rudder at 30
knots needs
> a shaft 4.75" in diameter. I had to interpolate for 25 knots, but it
looks
> about 4".
>
> That assumes the rudder is supported at the top, and the center of
force is
> about 40% down the rudder, so I figured a moment arm of about 1'. If the
> support is 3' above the waterline, I make that a 4' moment arm, so
the force
> would be 4x that much. I noticed that the equation for rudder force
used the
> square of the speed, so I figured if I doubled the speed, that would
be 4
> times the force also. They don't have 50 knots on the chart, but they do
> have 45 knots. At that speed, they specify 6.25", so I probably
should have
> said you need a 6.5" diameter shaft.
>
> Anyway, big numbers.
>
> - Gardner
>
>
> On Fri, Feb 20, 2009 at 12:13 AM, Rob Denney <harryproa@.
>
> > G'day,
> >
> > What you say is mostly correct. I am addressing these problems as
follows.
> > 1) The rudders can be supported down to very near the water surface,
> > as long as they are well faired. The shaft above the water can be
> > much wider than the shaft inside the rudder wide as it sees water less
> > often. this can be improved even further on boats that don't fly a
> > hull by moving them to windward. Even if they were unfaired, the
> > drag is still less than that of two daggerboard slots in a cat.
> > 2) The support for beam mounted rudders is very strong. The problem
> > with encased rudders is that between the fuse breaking and the rudder
> > blade actually floating free there is half a second or so of very high
> > loads which destroy the structure. With beam mounted rudders they can
> > be fully supported from down to kicked up. Because the fuse for beam
> > kick up rudders only handles the fore and aft loads, it is quite small
> > and easily tuned.
> > 3) Beam mounted rudders allow simple variable draft. The 15m under
> > construction has max draft of 1.6m, min steering draft of 800mm, min
> > draft 200mm.
> >
> > The 15m will be sailing in 3-6 months, so we will have some
answers then.
> >
> > Could we see the calculations for the 6" rudder stock, please?
> >
> > Keep up the good work on sketch up. The boat looks good. I have
> > stayed out of the discussion, but if you have any specific questions,
> > please let me know.
> >
> > The bows and underwater area from the beams forward are solid foam
> > with glass over them and bulkheads/floors between them and the rest of
> > the hull. I think this is a better option than steeply sloped
> > (necessary for anything other than trees) very strongly built bows.
> >
> > regards,
> >
> > Rob
> >
> >
> > On Fri, Feb 20, 2009 at 11:46 AM, Gardner Pomper
<gardner@...
> > wrote:
> > > Hi,
> > >
> > > Since I seem to be the one voicing the most concern, I think I
should
> > > clarify the points that bother me.
> > > 1) rudders need to be effective at low speeds for shunting -
this makes
> > them
> > > larger than usual
> > > 2) rudders are the primary leeway control - also makes them larger
> > > 3) harryproas are fast - increasing the force on the rudder
> > > 4) long lever arm - most underhung rudders are supported within
an inch
> > or
> > > so of the top of the rudder. For harries, we are talking about
2-3 feet.
> > > This increases the level arm for the center of force from about
1' to
> > about
> > > 4', quadrupling the force on the rudder. From crudely applying
the info
> > from
> > > Dave Gerr's "Nature of Boats", that means the rudder stock has to
> > increase
> > > by a bit more than 50% in thickness.
> > > My very rough calculations for a 5.5 sq ft rudder at 25 knots, would
> > require
> > > a solid stainless steel shaft 6" in diameter.
> > > 5) rudders have no protection - underhung rudders often have
skegs, plus
> > > mini keels, giving obstacles 2 other, fairly solid things to hit
before
> > they
> > > hit the rudder. Even if we went with underhung rudders on a
harry, the
> > > bidirectional nature of a proa makes these bumper guards
impractical.
> > > So, to summarize my concerns, the rudder supports (shaft,
bearings, etc)
> > are
> > > subject to a MINIMUM of 4 times the load on a comparably sized
catamaran,
> > > probably considerably more. Plus, they really need to kick up, since
> > there
> > > is no protection. Since they are new, an even bigger safetly margin
> > should
> > > be built in for the unexpected (like Doug's mishap). You don't
want the
> > > rudder to let go the first time you are in a gale, surfing down
breaking
> > > waves.
> > > Now, on the other hand, I desperately want them to work. The
combination
> > of
> > > kickup rudders, light weight and long hulls makes the draft of a
> > harryproa
> > > typically about a foot or so. This makes even other catamarans
seem like
> > > deep draft boats. I have actually been in many places in the
Bahamas and
> > > Florida keys, Cheasapeake Bay, where even a 2' draft would have
allowed
> > me
> > > to really have a lot of fun.
> > > In addition, I really do like the idea of nothing tearing a hole
in the
> > > boat. I wonder if it isn't worthwhile to slope the underwater
profile of
> > the
> > > bows so that if you did hit a submerged object, like a log or a
shipping
> > > container, that the hull would just ride right up over it,
instead of a
> > > sudden impact.
> > > Anyway, I think I am really going to have to see some numbers, and
> > > especially some real on the water experience before I trust the
rudder
> > > systems. I expect I will have the rest of my design all ready to
go by
> > the
> > > time that happens.
> > > - Gardner Pomper
> > > York, PA
> > >
> > > On Thu, Feb 19, 2009 at 9:02 PM, Robert
<cateran1949@
> > wrote:
> > >>
> > >> -The sailing loads are less on a Harry so the loads on the rudders
> > >> shouldn't be that much more than on other multihulls. It is
simply a
> > >> case of beefing them up enough. The enough bit is what we are
> > >> learning. The Vis rudders work but twist a bit under high loads and
> > >> can be a bit of a handful at high loads while partially lifted.
Bain
> > >> has had no troubles with his. I think the difficulties in the
present
> > >> Vis system can be overcome with more tow to stop any twist and
a cuved
> > >> blade rather than a straight one to keep the rudder closer to
balanced
> > >> at all depths of immersion.
> > >> The beam hung rudders work but Doug hadn't designed for loading
from
> > >> a different side while surfing. and didn't put the reinforcing
to take
> > >> tension. This is simply a bit of extra glass.
> > >> Going to Harriette type rudders, if you don't have the kick up
system,
> > >> then it is no different in principle to spade rudders, only
having a
> > >> 30-50% longer lever arm. The kick up system adds complexity but
> > >> doesn't seem insurmountable. The point is that the boats are
trying to
> > >> get away from holes in the hull that can shatter the boat if
you hit
> > >> anything hard at speed. You can have crash boxes or kick up
systems or
> > >> tear on the dotted line type systems, but something has to give. It
> > >> would be quite easy to put holes in the hull like most of the
faster
> > >> boats with their centre boards and spade rudders, but Rob likes to
> > >> eliminate what he sees as a potential weakness. There have been
quite
> > >> a few boats that have been sunk or immobilised from hitting things
> > >> with their rudders or centreboards.
> > >>
> > >> I reckon the beam hung rudders will get sorted out pretty well as
> > >> there is nothing intrinsically difficult about them, just
making sure
> > >> there is enough carbon in the right places. My only objection
is for
> > >> telescoping crossbeams. Farrier had to recently organise a patch up
> > >> job on his rudders due to unexpected loadings. Rudders are a
concern
> > >> for most boats.
> > >> My preference is either modified Vis types on outriggers or
Harriette
> > >> type in a flare that gives plenty of bury. I am still working
on the
> > >> kick up system to my satisfaction but I think I have a system that
> > >> would work and I am tempted to have a disposable, crushable bottom
> > >> third of the blade.
> > >>
> > >> -
> > >> >
> > >> > I am glad to hear about someone actually sailing these things. We
> > >> need more
> > >> > real life harry stories. The rudders are always a concern. It
sounds
> > >> like
> > >> > you really stressed them out. Other than the loose gudgeon,
can you
> > >> think of
> > >> > other stuff to make them stronger. I don't accept operator error,
> > >> because
> > >> > that is exactly what I am likely to do. I need a boat that
can deal
> > >> with me
> > >> > being wrong.
> > >> >
> > >> > Good luck on your trip.
> > >> >
> > >> > - Gardner
> > >> >
> > >> >
> > >>
> > >
> > >
> >
> >
> >
>
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