Hi,
Have a look at this boat. These guys have done well with much less
than four inches of rudder stock.
http://prao.
I have been thinking of this question a lot. Future will show how
our solution works. We built ours like the french did.
I we face problems, we will make remarkably smaller rudders, just
for steering, and add foils .
-Arttu
--- In harryproa@yahoogrou
wrote:
>
> 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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