Tuning the fractional rig on a P19

Wed, 24 Nov 1999 14:37:59 EST

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West Wight Potter Website at URL
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Hi webgang,

It's been too quiet around here lately. Keel trunk is being fixed, can't go
sailing. Hmmm, what to do?

I'm probably gonna regret opening this can of worms and find my mailbox
stuffed full of questions and disagreements about this, but take a look at
two posts I wrote about tuning the P19 rig on the Trailor Sailor Board.

The text follows:
Post 1.

There are indeed two side shrouds on all Potter 19's. The upper attaches to
the same place as the fractional forestay and the lower attaches
approximately even with the draft of the mainsail. Both attach to a chain
plate a little aft of the maststep.

According to my rigger, the Potter 19 is rigged in a classic fractional
manner. The upper shroud provides primary lateral support of the mast column,
as well as aft tension against headstay loads (in conjunction with the
backstay). According to him, the primary purpose of the lower shroud is to
allow you the option of tuning the rig for static and/or dynamic mast bend.
The lower shrouds don't need to be very tight in order to provide primary
lateral support of the mast column. That's the job of the upper shrouds.

So why are they there? In combination with the headstay and backstay, the two
aft-anchored side shrouds can be used to induce mast bend. If you bend the
mast forward at the middle, you flatten the draft and can depower the sail
far more effectively than you can by just tightening the outhaul.

The upper shrouds on my boat are tuned at about 15-18% of breaking strength.
The lowers are tuned between 10 and 12% of breaking strength. If you hit the
upper shrouds sharply with your hand or a wrench, it makes a low-pitch
musical tone. If you hit the lower shroud, it is too loose to make a musical
note, but not loose enough to be floppy ( A floppy shroud makes floppy makes
a "flup-flup-flup" sound). With enough tension on the forestay, this produces
about 1" of mast pre-bend forward at the middle-lower section of the mast
where the draft of the mainsail is deepest.

When I tighten the backstay (my backstay is adjustable) for pointing or heavy
winds, the middle of the mast moves forward another inch. This tightens the
lower shroud, limiting the amount of mast bend I can induce, and (now that
it's under tension) stabilizing the mast laterally in the mid-column section
during heavy wind. It also flattens the draft in the main about 3 inches,
which significantly reduces heeling and improves performance in high winds or
pointing (same thing, it's now high "apparent wind"). The boat sails flatter
in heavy wind, and points higher in medium to heavy wind too.

So the two shrouds are a positive advantage, giving you some tuning options
that you wouldn't have otherwise.

You don't have to tune the shrouds on a Potter the way I do, if you don't
want to fuss with bending the mast. You can tension the lowers just a little
less than the uppers (producing a slightly lower pitched musical note than
the uppers do) and still get plenty of support for the mast column in heavy
winds. (Note for P19-ers - If you have concerns about the strength of the
masthead fitting, you still get alot of performance improvement by tuning
this way, even if you don't have an adjustable backstay)

(BTW, those plastic shroud covers make it a harder to produce the "musical
note" when you hit them. I cut my plastic covers off short so they cover the
shroud only where it rubs against the gelcoat during trailering. It's better
for the shrouds too, since stainless steel needs exposure to oxygen or else
it corrodes quickly.)

And just to clarify things -- In addition to the upper and lower side
shrouds, most P19's have two dinky "babystays" for mastraising and lowering,
running from somewhere on the lower section of the mast to the cabin top,
directly outboard of the maststep pivot point. All these are designed to do
is keep the mast from swinging sideways during raising and lowering.

Most folks leave all the shrouds attached for trailering except for the
forestay. Only minimal extra work is involved in coiling up the extra shroud
for trailering once the mast is down.

>From my perspective, the two shrouds give the boat a heck of a lot more heavy
air versitility in return for the minimal extra work of coiling them up and
bungee-ing them to the mast for trailering.

Fair winds, Judy B

Post #2

The Potter 19 has one upper and one lower. There is one set of shrouds
attached to the mast at the level of the forestay, and one about half way
down from there. They both attach at the bottom to the same chain plate
slightly aft of the mast, at about a 20-30(?) degree mast-to-shroud angle.
The upper shroud stabilizes the mast columm laterally, while the function of
the lowr shroud is to limit forward mast bend and to stabilize the middle of
the column in high winds.

The rest of the rig: Fractional rig. No spreaders. Beefy mast compared to the
righting moment of the hull (which is what counts when you're engineering the
standing rigging. Backstay to masthead (on all but the stripped down "small
lake" version of the boat for $8000 new). Very efficient type of rig
according to Chapter 18 of Bethwaite's _High Performance Sailing_, because
the rig adapts dynamically to increasing wind by (1) bending the masthead aft
and allowing the top to twist off, (2) tightening the forestay and flattening
the headsail, and (3) bending the middle of the mast forward to flatten the
draft of the mainsail.

A rig like this doesn't require as many frequent adjustments in changing or
gusty winds. It dynamically responds with the correct de-powering response
the "fast sense", keeping the boat balanced and sailing flat and fast.

See my other post in this thread for tips on tuning a rig like this. It has
to be tuned correctly to permit this type of dynamic response.

Fair winds, Judy B