Re: Shrouds
Wed, 19 May 1999 13:27:46 EDT

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West Wight Potter Website at URL
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In a message dated 5/19/99 8:29:56 AM Pacific Daylight Time, writes:

> any way on to the
> reason for this post, does any one know why the shrouds on a boat with a
> back stay are set back to the rear of the mast? Other than messing up the
> sail shape on a run? Seems to me that if they were even with the mast you
> would not hit them with the sail as soon. The back stay must be strong
> enough to support the mast without the shrouds help.
> Dan Rickert First mate "THALES" wwp p-19 212 skipper Vagabond 14 dinghy #
> 316

Hi Dan et al,

Here's some stuff I learned from reading and from my rigger. I'm not an
expert, but this is my simple-minded understanding of the situation.

It's not just a queston of holding up the mast at the tip with the backstay.
It's a question of stabilizing the mast along it's whole column length to
prevent buckling.

The back stay stabilizes the upper third of the mast column against a forward
force. The two side shrouds stabilize the middle and bottom third of the
mast column against forward forces. You'd need a much heavier, stonger mast
or swept back spreaders if they weren't located somewhat aft of the maststep
A stronger mast would have a thicker section on the wall, larger diameter
fore and aft and weigh a ton (and it's not good to have more weight aloft).
Warning! Technical Discussion coming. : ^ )

Com'on guys be tolerant. Please don't flame me for being technical. Go
sailing. Be happy.

Sometimes I Potter lazily (when winds are light to mderate)
Sometimes I Potter like a technical cruiser (for comfort and safety when
winds are higher)
Sometimes I Potter like a technical racer (when the beers are at stake!!!
Or when sheer exuberance takes over !!! : ^ )

Y' all should sail whichever style you enjoy. Be happy, Go sailing.
Y'all should make your own decisions about rigging, and don't do anything
that makes you nervous.
Y'all can spend a ton of money customizing your boat, you can customize it
frugally, or you can sail it the way it comes right out of the box.

Advice to a new owners: wait til you know more about your boat, do some
research before you change any rigging. Be happy, go sailing... (now if I
could just learn to listen to my own advice!!!)

Disclaimer : Consult your rigger for advice on YOUR boat. Don't take my word
for it.
Tuning Shrouds on the P19 (if you have a feeling of deja view, check your
archives circa February)

I learned about this mostly from my rigger, since I'd never sailed a
fractional rig before. ("When I was young" I learned my basic sailing skills
on masthead sloops with the usual full complement of trim adjusters -- like
adjustable backstays, vangs, cunninghams, outhauls, etc.... and I still sail
a one-wo/man dinghy with superb sailtrim controls. )

The Potter has the shrouds arranged like a classic fractional sloop rig,
headstay partway up the mast. The upper and lower shrouds are positioned so
you can tune a curve into the mast safely if you want to. (Most fractional
sloops have adjustable backstays, I'm assuming there's one in this
discussion) When you curve the mast, the draft of the sail gets flatter and
the top of the leach gets looser and can twist off in a gust. (With a vang,
the draft gets flatter, but the leach and head get tighter and cant twist off
to spill wind in a gust)

Flattening the mainsail when pointing point higher, and sail faster on any
point of sail higher than a 90 degree reach. You release the backstay for
any point of sail lower than a reach. Flatter sails cause less heel when
close hauled. Flattening the sail in high winds gives you an additional
windrange (of maybe 5-8 knots) before you need to reef. Flatter sails cause
less heel when a big gust tries to slap you down.

The classic way to tune a fractional rig is to have a snug upper shroud, with
the lower shroud a little less snug when the mast is standing up perfectly
straight in neutral (no wind) with normal light tension on the backstay.
Then you tighten up the backstay just a little, putting a curve into the
mast. The lower 2/3 of the mast bends forward when you tighten the backstay.
Tuning the upper shroud tighter than the lower shroud gives you a smooth
curve with no sudden changes in curvature and gets the forward-most curve at
the middle of the draft in the mainsail.

If you have an adjustable backstay, the side shrouds can be tuned to
facilitate bending the mid section of the mast forward to to flatten the sail
in high winds. Properly tuned, the lower stay limits the amount of forward
travel in the lower 2/3 of the mast to a safe amount so you don't overstress
the mast along the lower 2/3. To avoid overstressing the mast on the top
1/3, you have to use common sense and self-control.

If your lower shroud is too tight compared to the top shroud, tightening the
backstay bends only the top third of the mast, focusing the stress all in one
place. That's dangerous. What you want is a gentle, smooth curve the whole
length of the mast, with the most forward part right at the level of the
mainsail draft.

Self control isn't hard. *IF* your shrouds are tuned correctly, you only
need to move the middle of the mast forward about 2 inches (and the top back
even less) to really flatten the mainsail and give yourself another 5-10
knots of wind range before you reef. Also good for pointing higher while
reducing heeling at the same time. With a 12:1 purchase on the backstay, you
can move the mast a half inch at a time.

I'm not afraid of permanently breaking the mast with the adjustable backstay.
Did you ever watch a mast sway under load? I bend the mast less than that
to get all the advantages of an adjustable sail shape.

I have the older, much heavier DM6 mast that Herb Stewart originally
specified for the HMS18/P19. The original Herb Stewart rig design also
included an adjustable backstay. The new P19's have a lighter version, the
DM5 mast.

I spoke to Mr Dwyer about the mast. (see my website
<A HREF="">P19 Mast
</A> ) and he thought the lighter mast was plenty strong enough for the P19.
You can make your own decision or ask a local rigger to look at your boat.
Please make sure that the rest of your rig is well maintained and up to par.

Judy B.

Judith Blumhorst, DC
HMS18/P19 Fleet Cap'n, Potters Yachters
1985 WWP19 #266 Redwing
Sailing on SF Bay, CA
(5-35 knot winds, 2-4' chop, 2-6' swells, and currents up to 6 knots)
Visit <A HREF="">Judy B's
West Wight Potter Pages
and <A HREF="">The Official Web Site of
the Potter Yachters