I think we think too hard. The actual planning limit should be force required to lift a hull.
Doesn't matter how fast that is. Even broached sliding sideways can only go as fast or
with as much force to trip the boat. I had dynamic stability analysis done on the
Visionarry design and it will trip (pivot) on the rudders. Not a big surprise.
Forces only get stupid on rudders if you go 20+ knots and turn the blade face to direction
of motion. You are going to break something. Thus rudder stops are necessary or a
watchful helmsman.
Otherwise rudder forces will always be substantially less as the boat whether crabbing
(slipping) to claw to higher point of sail or normal tiller response will only be generating
some effective angle of attack to boat direction (creating lift). Use the standard lift calc
formula or find several on-line calculators that have the density for water, air etc.
That being the planning or functional loads. Design loads use whatever factor you are
comfortable. SChinning uses I think 1.5X the loads for trip or side slip. Maybe easier to
design with some assumptions, total mass not higher than 5T, 60% or so in the WW hull
and calc lever arm center of WW hull to rudder distance. Say that is worst case design
loads and go from there. Should not be planning forces for some monster shaft, can't
afford the weight or the price of the bearings.
I will be using TidesMarine URB-3500 lower bearing (3.5") and taper to URB-3000 (3.0")
upper bearing. Shaft material 6082 Aluminum proof stress 280 N/mm2. Shaft size is
mostly for load distribution into bearing. Certainly can mess with carbon fiber shaft and
bog a steel ring for bearings. My pref for aluminum. Takes 173000 Kg to bend Al shaft.
Not going to happen. Not exceptionally heavy as it tapers into rudder itself.
JT
--- In harryproa@yahoogrou
>
> 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@> wrote:
> > >>
> > >> 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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