One
of the more outrageous risks we took with the OZ PD Racer was as a
response to the amount of sail we wanted to put on this little 8ft
(2400mm) boat.
(By more outrageous - it was the innovation I was
most worried about - there was a very big sigh of relief when the boats behaved and performed so well)
We figured that around 80 square feet would be a sensible step up from the 45 to 70 square feet of most of the existing boats.
In
the end our mainsails were a true measured area of 86 square feet (8
square metres) but mounted on simple, but bendy, wooden masts the
boats proved both fast and easy to handle.
The
combination of the short hull giving a very low wetted surface and the
generous sailplan resulted in startling light weather performance - the
boats move really well with even a tiny breeze as can be seen on the
videos.
A
problem with the large sail area are that with the mast as forward as
possible there are only two choices - the sail has to go extend up or
extend back.
Extending
up means bigger heeling loads making the boat harder to hold up in
stronger winds not a good choice for good all-round performance
Going
back means the centreboard has to move back too. The drawing
right shows the extra sail area compared to many of the other boats and
the amount of movement of sail centre and centreboard required to keep
the boat in balance.
Normally
centreboard cases are somewhere between one third and one half way back
from the bow. In our boats the centreboard case is in the back
part of the boat. It actually looks all wrong - but it worked
well in practice.
Robert suggested that the centreboard would be
too far back - whereas I was fairly sure it was in the right place to balance
the sail area. He was grappling with the same problem I had when I moved the board back that far!
Hi Robert,
You are a good observer!!!
The
board is a bit behind the normal visual position of the centreboard in
most boats - but it is the right place to balance against the sail
centre of effort.
That's all that defines whether a boat is
"hard mouthed" or the opposite - at least in small boats. BTW,
providing that the centreboard and sail are in about the correct
position (ie within about 4 inches on the puddleduck) the biggest
determinant of "hard mouthedness" is whether the boat is being sailed
heeled or not. Sail it flat and it will balance if everything
else is approximately correct.
The biggest difference between
good small boat sailors and poor ones is how much they heel the
boat. You can see this in any racing
fleet. The guys at the front are flat, the ones
at the back are heeling.
In light winds SOME heel - 15 degrees
is nice to reduce wetted surface and to allow gravity to calm the sails
down - but by the time the wind is filling the sail reliably the boat
should be being sailed dead flat.
Note
that I was quite wrong about the heel in light winds. The PD
Racers just dont like being heeled much at all. Heeled more than
a few degrees the corner of both the bow and stern transoms disturbs
the water badly - you can let them heel a bit but you have to watch that the corners don't dig in at all. _______________________________
But back to the design side of things ...
In
the last few years in out and out racing yachts there has been a
tendancy (healthy for once) toward big mainsails with big roaches and
small jibs with little overlap (genoas that overlap the mast by much
have finally been recognised as the inefficient sail area they really
are).
The end result is that keels in these boats have been moving further and further aft to keep in line with the sail centres.
Biting
Midge (my partner in crime who bankrolled the OZ PD Racer project) is
about to tell you all that these ideas were first developed by Aussie
big multi designer Lock Crowther in the late '70s and early '80s.
Monohull designers are only just catching on to what has been now a
long established trend in multis.
In
the PDR it allows us to carry a big mainsail with significant transom
overhang and still keep the centres in line - if we had got it wrong
the boats just wouldn't move as well as they do in the pic right.
Two
sails is just too much complication to our minds - and single sails
are generally more efficient - they can be trimmed moment by moment by
the available single crewperson for a start.
I have kept the
rudder a bit bigger than I thought necessary (length is about what I
thought right but it is quite a bit wider (which also enabled us to
make the accurate template shaped centreboard and rudder in one piece
to make the shaping easier and more accurate and cut them apart after
shaping. This is to provide more steering force as the rudder,
being much closer to the centreboard (the pivot point when turning) has
less leverage. Its like a lever - if it is short (less distance
from fulcrum) then the input force has to be greater.
It's all about interrelationship and making decisions that have a positive effect for the boat and it's building in total.
But
you are right - we are running a little risk in bringing it further aft
- but it is a calculated one - I think there will be little problem,
but when the boats go in the water we will know.
For these two
prototypes we have made fairly elaborate facilities for altering the
rake of the mast - so we can get the balance perfect.
Once we have
worked it out the partner and step will be drawn up to be much simpler
than the current one (even though our boats are lighter than almost everyone
elses the more elaborate partner and step is just toooo much extra work).
But, if there are problems we can move the cases - about half a days work for the two boats.
In
the PDR it allows us to carry a big mainsail with significant transom
overhang and still keep the centres in line - if we had got it wrong
the boats just wouldn't move as well as they do in the pic right.