RE: sail trim and cdi questions

JBlumhorst@aol.com
Mon, 29 Mar 1999 15:43:08 EST


- - - - - - - - - - - - - - - - - - - - - - - - - - -
West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
- - - - - - - - - - - - - - - - - - - - - - - - - - -
In a message dated 3/28/99 7:39:27 AM Pacific Standard Time, Gka525@aol.com
writes:

<<
Subj: sail trim and cdi questions
Date: 3/28/99 7:39:27 AM Pacific Standard Time
From: Gka525@aol.com
To: wwpotter@tscnet.com

hello fellow potterers,
Im reading the manuel for cdi and it says when
sailing reefed move the jib leads foward or you will get poor sail shape. it
also could cause the furler to rise off the bearing . with my previous working
jib setup the jib lead is not adjustable the sail i plan to use is an IM
lapper do i need
to add jib leads with adjustable tracks? [snipped] >>

Hi George and Webgang,

Here's my $0.02. It's long, and it's hard to proofread my own stuff, so I'll
hope y'all will forgive me if there are any mistakes. But please correct
anything that I got wrong.

Now, before I launch into one of my long, rambling posts and before I get any
flame mail from the folks who prefer to "keep it simple, stupid," let me
insert my usual disclaimer: Your mileage may vary -- this is just one of the
many ways to approach rigging a Potter -- not the only way, or even
necessarily the best way. I'll try to present both sides of the issue, not
just my own preferred solution.

Warning: All folks who prefer to be "nontechnical" sailors should click their
delete button right now <grin>!

The answer to your question about whether you have to move the jib lead
forward when roller reefing is "yes and no." It all depends on whether you
want to get the most out of your boat and, to a lesser degree, the cut of your
headsail and the angle of your existing jibsheets from the clew to the leads.

The position of the jiblead determines how well the jibsheet pulls down and
aft on the clew of jib. If the lead is too far forward for your headsail, it
pulls the leech of the sail too tight (causing turbulence) and the foot of the
sail will be too loose with huge curve. If the lead is too far aft, the foot
will be too tight (bottom of sail will be too flat) and leech will be too
loose (spill too much wind) and the air flow over your mainsail won't power up
the main.

Ideally, on a sloop rig, you want the curve in the leech of the jib to match
the shape of the forward part of the mainsail (trimming the sails according to
the "slot"). That's so the increased airflow from the jib over the main
increases the lift produced by the mainsail. Even with the fractional rig,
this is still true. And the high clews on the factory sails by Odessey are a
feature that increases this effect.

So what happens when you furl the headsail on a CDI unit? Typically, the
jibsheet pulls the foot of the sail too tight, and the leech gets too loose.
It's the same effect you would get if you left the headsail alone and moved
the jiblead aft a few feet. Therefore, when you furl the jib, you usually
want to move the jiblead forward to keep the tension on the leech of the jib
so that the sail still has a good shape.

If the jib gets a funny shape when you roll it up, you mess up the "slot," and
the wind over the main is very turbulent. The main could loose power or even
get backwinded by the turbulence, making the boat very unbalanced. It's hard
to predict exactly what would happen on *your* boat, but you could find out
that you couldn't point very well and your helm would be unbalanced (you'd be
wrestling the tiller). That's an important thing to be able to do if you're
trying to get to shore to windward, especially in rough conditions.

(You can read about this topic in Maloney's "Chapman Piloting," 62nd edition,
page 228, 235. Also, the topic is more extensively discussed in Rousmaniere's
"Annapolis book of Seamanship," 2nd edition, page 70. Both authors say that
you need adjustable cars and tracks with a jib roller furler. Furthermore,
Rousmaniere goes into some detail about the sheeting angles, recommending no
closer than 8 degrees off centerline for a non-overlapping jib, and no closer
than about 10 degrees for an overlapping jib.)

One other advantage to having tracks has to do with the fact that the shape of
the draft in the jib changes significantly when you roll up the sail. With
tracks and cars, you can compensate somewhat for this. However, in general,
your headsail will not have a very good shape if you roll it up so that it's
smaller than 60 or 70% of its original area. (A flatter headsail generally
rolls up better than a really full draft, something to consider when you take
your existing lapper to the sailmaker for converting it to use on the CDI
unit.) However, in a really big blow, you can still roll the sail up to
handkerchief size for safety's sake, even if it doesn't perform optimally

If you do decide to install tracks and cars, I recommend you consider putting
the tracks on the cabin top at the outside edge. As many folks on the Potter
list probably already know, that's where I put the tracks on my P19 "Redwing,"
on the advice of my rigger and the owner of UK Sailmakers who made Redwing's
new suit of sails, and after experimentation on the water under sail. A few
other P19 owners have done the same thing and they've written to say that they
love having the tracks there.

There are several advantages to putting the tracks there. First, they're out
of the way and aren't taking up space on the side deck, which is very limited
to start with. Secondly, they will be about 12-14 degrees off the centerline,
which will improve your upwind performance without backwinding the main.
Third, you can reach them easily from the cockpit or companionway to adjust
the car. Fourth, the cars slide really smoothly, much better than when
they're on the vertical wall of the cabin. Last but not least, the outboard
edge of the cabintop is very strong where it turns the corner to the vertical
wall, and you won't compromise the integrity of your boat. All in all, IIMHO,
it's the most sensible place to locate the tracks.

One word of caution -- if you don't have the factory/Odessey working jib or
lapper, you need to check whether the clew on the jib is high enough. I have
read a few posts on the Potter list about custom lappers that sound to me like
the clew is much lower. (I've seen at least one custom-made lapper for a P19,
not made by Odessey, that wouldn't work with this set up. It had the same
area as an Odessey lapper, but it had such a low clew that it was a deck
sweeper. I think in this particular case, the sailmaker wasn't informed that
he was making the sail for a fractional rig. He designed it more like it was
going to be used for a masthead rig. It's probably a great sail downwind, but
kind of weak upwind.)

Even if you do have the factory standard sails, you will probably have to trim
the lapper and the genoa when you convert them to use on the CDI, because the
luff will be too long for the CDI luff extrusion. I talked about that with
Dennis, the sailmaker at Odessey, just recently to make sure I had the correct
information. I had a little problem with my lapper when I had it converted
for the CDI. I'm planning on having my Odessey genoa converted very soon and
I wanted Dennis's advice on how to do it correctly the first time. The main
point -- make sure you tell the sailmaker how long the luff extrusion on the
CDI is when you take your sails in to have the luff tape installed.

(BTW, one extra thing I'm having the sailmaker do on my jib -- he's putting
vertical cloth markers on the foot of the sail it to mark a 25% and a 50% [or
whatever percentage he recommends] reduction in sail area when it's rolled up.
That way I can reef to the same place everytime and know exactly where to put
the adjustable cars.)

When Odessey makes a lapper for the CDI, they cut a little off the luff in
order to make the luff shorter. This also reduces the depth of the draft --
it's better to have a flatter sail when you roll it up or else it gets really
baggy and wrinkled. When Odyssey makes a genoa for the CDI, they trim off the
luff to reduce the luff length and they trim the foot of the genoa as well to
make it a little flatter.

There's one special set of circumstances in which tracks don't make a lot of
difference -- that's the case when the angle of the jibsheet makes a perfect
right angle to the headstay. In this special case, the jibsheet pulls at the
same angle on the clew, no matter how much you roll up the sail. So the foot
and leech still would have relatively similar tension. However, when you roll
up the sail a lot, the draft position would change and you won't be able to
compensate for that without tracks and adjustable cars.

Please keep in mind that you don't have to install tracks just because the
books say you should and because some P19 owners have. It all depends on the
details of where your existing jibleads are, the height of your clew, the cut
of your sail, the conditions in which you sail, and your own particular
preferences and sailing style. It may be that the jibleads are positioned
very well by the factory and tracks aren't a big deal, even with the CDI. And
maybe it's not that important to you to get the best performance out of you
P19. Before you go to all the trouble of installing tracks, you could try
installing the CDI and testing it out in moderate conditions to see if you
really need or want tracks.

If you want to see how I installed the tracks on Redwing's cabin top, go to my
website and look for the article on "Installing Genoa Tracks on the Cabintop
of a P19." It has a lot of detailed instructions and pictures. I used
expensive (about $300) , high performance hardware, but you could probably do
it much more cheaply with less fancy hardware for less than $150.

Hope this answers at least some of your questions, George. If you have any
questions, please don't hesitate to email me.

Now, a last word to the Potter web gang, I know some of you (you know who you
are!) prefer to keep your Pottering simple, so please don't take it personally
that I'm suggesting a "complicated" solution for a P19. Gimme a break <grin>,
and don't flame me -- I'm the first to admit that I'm a "high-performance"
junkie! It improves the performance of a P19 (with or without a CDI) if you
have genoa tracks on it, but most P19's don't have tracks and their owners
love sailing them just the way they are!

Regards,
Judy B.

Judith Blumhorst, DC
HMS18/P19 Fleet Cap'n, Potters Yachters
1985 WWP19 #266 Redwing
(Rigged so a petite woman can solo)
Sailing on SF Bay, CA
(5-35 knot winds, 2-4' chop, 2-6' swells, and currents up to 6 knots)
Visit <A HREF="http://members.aol.com/jblumhorst/HomePage/index.htm">Judy B's
West Wight Potter Pages</A>
and <A HREF="http://songbird.com/potter_yachter/">The Official Web Site of the
Potter Yachters</A>