These are my figures. 6 tonnexmeter righting moment would give about 3
tonne leeway prevention with all sail up or 5 to 6 tonne if 3rd reefed
and flying a hull. At a guess of about 1m coe of rudder gives a bit to
play with, especially considering much of the leeway prevention is
taken up with the bow once you start pushing things. I am presuming
that the stocks were tested to a given deflection rather than busting
them.
Robert
--- In harryproa@yahoogrou
>
> G'day,
> >
> > 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?
>
> Haven't tested it, but can not think of any reason why it wouldn't
> work, given that cats steer equally well with either rudder. I still
> plan to. One of the advantages of the beam hung rudders arrangement
> we are using on the 15m is that it is very easy to move them along the
> beams if required.
>
> > 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.
>
> Doug's rudders are much further aft than mine and he cannot use the
> front one. I have no problem with one, except in light air.
>
> > I hope you have not taken any of my comments as nay-saying.
>
> Of course not. I value your ideas and welcome the opportunity to
> explain wty I do what i do.
>
> >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.
>
> Big numbers indeed. Does he say how they break (twisting, sideways or
> fore and aft)? And under what circumstances (helm hard over at 50
> knots, falling sideways down a wave, going backwards at high speeds)?
>
> My take on these is:
> As harry rudders are nearly balanced (shaft at 30% of the chord, they
> see low twisting loads. If they are high, either the steering lines
> slip, the wheel shaft key breaks or the tiller is pulled out of your
> hand.
>
> Fore and aft the fuse breaks and the rudder kicks up.
>
> Sideways, the 15m/50'ter shafts are 85mm fore and aft with 20 mm/0.8"
> carbon walls. This can easily be increased from the top of the
> rudder to the bearing, which is clear of the water. Rudolph tested
> his and they withstood (from memory, could be wrong), in excess of 10
> tonnes. Our rudders rotate when going astern so don't see the same
> loads as normal rudders. If they don't rotate for any reason, the
> fuse will break and they will kick up.
>
> The bottom bearing on the 15 is about 300mm/12" above the water.
> Also, harrys are fast, but I doubt you will ever exceed 25 knots, so I
> would use that as the speed input. Finally, I would check and see
> what the load cases are that Gerr uses, and his safety factors.
> Please report back if you do any of this. Also, the loads on the
> shaft are very dependant on how far apart the bearings are as the load
> on the bottom bearing is that of the shaft, plus that on the top
> bearing. The closer they are, the bigger the bottom bearing load, so
> check to see what he allows for this. Would also be good to know what
> he suggests for daggerboards on 50' cats, as these arguably see much
> higher loads than harry rudders, yet have a 100mm wide x maybe 5mm
> thick piece of carbon down each face.
>
> regards,
>
> Rob
>
>
>
> > - 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@
> >> >>
> >> >> -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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