I will depend how you sail your HP.

Both boards down or one down and one up.

I tend to favour what i have already had working before which was only using rear rudder at a time.

But if able to leave both down all the time and simply fix one in straight position and unfix the other when doing a shunt then this is simpler and less work.

Maybe on a long continuous tack (shunt) may be improved with only one down? Not sure.

The idea of steering with the a front rudder though is not easy to grasp how that works.

Doug

--------------------------------------------
On Thu, 21/3/19, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:

Subject: Re: [harryproa] Re: Flat bottom hulls?
To: harryproa@yahoogroups.com.au
Received: Thursday, 21 March, 2019, 10:34 AM


 












Björn:
    The
difference between a
unidirectional and bidirectional foil comes as you
deflect it from
straight.... the classic extended teardrop shape of a
symmetrical
unidirectional foil will produce less drag as it is
given an angle
of attack relative to the flow of liquid, and it will
not reach
the angle of stall / flow separation nearly as
quickly..   As you
approach the stalled angle drag increases rapidly. 
The rounded
leading edge is the most efficient, combined with the
long sharp
trailing edge that allow the streamlines to rejoin
aft. 
Aerodynamics and hydrodynamics are the same thing,
just in a
different realm as far as Reynolds numbers.  A look
at Abbot and
Doenhoff, The Theory of Wing Sections is useful, as it
shows the
LD (lift drag ratio) at different angles of attach for
many
airfoil sections, and also AOA as compared to stall
angle.   One
well known designer has designed
"luffboards" for his catamaran
designs, which are airfoil shaped boards like dagger
boards on the
inboard sides of the hulls as an option.  He uses the
NACA 2012.  
Not a plans option... just an "idea" offered
to builders.  It's
pretty common to use aircraft foil designs for rudders
and
daggerboards these days.   The LD is far
superior...... That means
the "sideways lift" created has the least
drag on the boat, and
that is what one wants, particularly when sailing
close to the
wind...... especially in light winds.   End plates
both top and
bottom have been shown to have a huge effect on
efficiency...... a
shorter foil can do more = less drag....... eliminate
cavitation
for the most part.


    Rather
than a bidirectional foil,
the best choice would be to design in such a way that
the foil
could be rotated 180 degrees easily..............



       
                               
                           
                               
   
    H.W.








On 3/20/19 11:07 AM,
Björn
bjornmail@gmail.com
[harryproa] wrote:




 







I'm not convinced a
unidirectional
uncambered foil has less drag than a
cambered
bidirectional. But I guess it
requires some
analysis. A cambered foil can
develop a higher
coefficient of lift, so can be
smaller, which
might balance the higher coefficient
of drag.
But it could also be that for
hydrofoils this
potentially higher coefficient is
not that
significant, because of effects like
cavitation
and suction of air from surface
piercing foils?



Regarding reefing, I
think some
kind of system with ropes and
pulleys must be
used on a Harryproa. Otherwise, how
else would
you reach to the end of the boom to
attach the
reef if the boat is stopped with the
wind abeam?
(And then, how to tie in the loose
sail in that
position?)




Regarding tillers on 50'
boats, it's not
completely strange. Most of the
old Swedish
coastal cruisers where steered
with a tiller..
Check out the classic SK55,
"La Liberté",
which is 54 feet long.


https://vimeo.com/26804093


https://en..wikipedia.org/wiki/Skerry_cruiser


(The "Formula"
created some beautiful and
interesting slim boats. Some of
them had a
length to beam ratio of 9:1 (on a
monohull!),
slimmer than the hulls of some
modern cruising
catamarans!)



Also the modern Pogo 50
http://www.pogostructures.com/en/fiche-bateau/pogo-50/


Some data and graph's on
that boat
performance here:
https://www.fastsailing.gr/the-yachts/sail/high-performance-line/pogo-50/





Tiller is the proper way to
steer a
sailboat, imo.  











On Wed, Mar 20, 2019 at
10:00 AM '.' eruttan@yahoo.com
[harryproa] <harryproa@yahoogroups.com.au>
wrote:




 







| Or you could "dive"
under the boat's 1'
draft and put a larger patch
on.



Ya, but that 1' draft becomes,
perhaps 3', if
you put a hole in the hull of a
reasonably
loaded boat? Not hard to get under
3', if you
can wait for things to get calm. I
guess
that's better than 2'
becoming 6'+. Store the
hull repair kit in the on deck toy
box? Or
should we assume a sump is running
to keep the
water level down?



I guess the crush bows will help
mitigate the
holed 'draft' from getting
too deep.



Seems nice one has a separate
isolated hull
you can shelter in if needed,
until repairs
can be made.



A 3' extension on a hole saw
seems
impractical. And kevlar might make
that a
nightmare. But two overlapping
holes make a
shape that's easy to slip a
patch through that
will cover the hole easy. Also the
patch is
easy too!



If I have tanks in a bilge, the
hole will only
happen directly under them, so
I'll need to
remove them to fix it. So I need
removable
tanks.



Rocks on a coast are one thing. At
least one
is close to land. A hole at sea
seems worst
case. Do we agree? I guess its
better to think
on this now, than start then.



| There's always something
that's going to
want to gouge the hull;



Ah, so tie on the tires on before
beaching.
Derp! I am rather slow.



Gliding a multihull onto a beach
seems
classic. But I guess it is kinda
dumb.



| >> The shallower the hull,
the easier
these are to put in place.

| >

| > Why is a shallow hull
easier? Shorter
jack?

|

|   The boat is still floating
when you put
these in place. Trying to get
beaching
bunks/tires situated on a deeper
V-profile is
going to be more problematic than
a shallower
flat profile. 



Since we are fitting protection
before
beaching, this all makes sense
now.



| > Does one have to goto the
mast to hook
the luff downhaul to the reef
point?



|   Some boats <one does not
have to, but
yes>

|   That said, an ultra-simple
rig with
dyneema hoops and a wishbone boom
would
probably work better without
trying to get
fancy with a single line.



If one had a 'large rig',
that was regularly
reefed, perhaps run the extra
line?



|   The key is that the boat is
in neutral,
the sail is in neutral, and
you're in a
protected cockpit area (not up on
an exposed
bow).



So, 'going to the mast' is
a different thing
on a HP, vs many other boats. A
casual
occasional thing.



| Thankfully the ferry gave him
some room at
the last moment, but if he
hadn't, the
sailboat was going to be in a
world of hurt.



I thought 'right of way'
favored sails over
motors? Like in aviation lighter
than air,
beats gliders, beats motor
(typical) planes.
Is this not true?5



I guess if the ferry don't
yield, you are
hitting something either way. One
can sue the
ferry though.



|   A bidirectional boat with an
unstayed rig,
however, could just stop



This is such a huge safety
feature.



| > Is it reasonable for me to
suggest that
a slow boat is less safe?



|   It's a wash.  Speedier
is safer in that
you have more options in how to
get to safety
or how to get past the edge of a
squall.
Unless you push it too far with
that
supersized rig and then end up
having a really
bad day.



|   It's like knives. 



Excellent. Thank you.

But the supersized rig is only
supersized from
the point of view of observed
'expected'. The
HP can have a larger rig, and be
safer,
because the design has a larger
heeling
moment, as you well know. And you
are right,
when I push it too far and flip
the thing, I
imagine many will blame the rig or
the design,
and less so the sailor.



| > When you have a moment,
would you
explain where you are with the
bidirectional
blades and the one way
blades? 



|   Unidirectional foils will be
more
efficient.  But you'll have
to rotate them 180
degrees each shunt.

|   Bidirectional foils will be
less
efficient, but you can configure
them to give
a touch of lift to windward, and
they only
have to turn a few degrees..



|   Personally, I'm willing
to give up a bit
of performance for convenience. 
Plus the
bidirectionals are almost made for
tillers. I
love tillers and really don't
want to deal
with a wheel. (Though I have to
say the
tilting wheel on the 50 and 60
that can be in
the cabin or out on the deck is
brilliant).



Yep. The efficiency calls to me,
but I can see
the simple winning out.



Rob had said (IIRC) the
bidirectionals have a
few degrees of needed
'cant', that shifts
between directional changes. It is
a simple
thing, but he was working on it to
get right.



The wheels seem huge to me. And
they don't, to
my ignorant eyes, intuitively
provide feedback
as to the rudder position. Does
one need a lot
of leverage to turn the rudders?
Is the big
wheel a tradition? Or is it
awesome in some
way I am missing. I notice Steinar
has two
wheels on his renderings now, so I
must be
missing something.



The more I think on tillers, the
more I like
them. A 48' boat with tillers
is rather odd,
no?






























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