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West Wight Potter Mailing List maintainer
dfarrell@ridgecrest.ca.us
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Hi Jim,
See comments imbedded in your message below.
Judy
> -----Original Message-----
> From: Jim Nolan [mailto:panache426@hotmail.com]
> Sent: Monday, March 20, 2000 7:30 AM
> To: wwpotter@tscnet.com
> Subject: Standing rigging tension
>
>
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> West Wight Potter Mailing List maintainer
> dfarrell@ridgecrest.ca.us
> List hosted by www.tscnet.com
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> I'm replacing all the standing rigging on my P-19 with two objectives in
> mind:
> a. Making the rig easily tunable with turnbuckles and a backstay shroud
> lever
> b. Making it easy for one person to raise the mast and lock the forestay
> (backstay shroud lever).
As you've implied, being able to loosen the backstay during mast raising
does make things a little easier, but I have noticed that you can raise the
mast just fine using a turnbuckle that is preset to some level of tension on
the backstay. You will probably need some way of gaining a little
mechanical advantage to pull the mast forward when you attach the forestay,
because it needs to be tight enough to prevent sagging of the jib (making it
more full in the belly, causing more heeling and over-powered-ness)when
you're pointing.
The forestay on my boat is very tight, so there's very little sag in it when
the headsail is powered up. I think that's an important issue for P19's,
since it's very tempting to make the forestay loose so it's easier to
connect it during mast raising.
I've tuned a couple of new P19s, with fixed length forestay and backstay,
and it wasn't too hard to get the forestay attached, using a little
mechanical advantage to pull it forward (such as pulling on the bitter end
of the main halyard, standing 10 feet in front of the boat)
>
> My questions are:
>
> If I set it up so the upper shrouds have 250 pounds of tension
> (I'm using a
> Loos tension gauge)will it be easy for me to raise the mast to the fully
> upright position? Will I have to loosen the turnbuckles and then
> re tension
> them every time I set up the mast?
>
Shroud size and tuning tension are determined by a number of factors, the
primiary one being the necessary strength to withstand the forces generated
when the boat is at it's max heel (RM-max = maximum healing moment.) That
maximum load is shared by however many shrouds you have, and based on the
trigonometry of the rig, you can calculate the load on any shroud. If I do
a quick and dirty "analysis" of the loads on a P19 (using typical
"skantlings" from Brion Toss' book, not actual measurements from the P19,) I
come out with a max load of about 600 pounds on the upper shrouds on a P19.
Using a safety factor of about 2.5 to account for shock loads, the required
strength of the wire is 1500 pounds.
I presume you have 1/8" 1 x 19 SS wire on your P19, and the DM5 (newer, less
heavy mast found used since about 1992 (appox date?)). Depending on whether
you have 302/304 SS or 315 SS, the breaking strength of the wire rope is
1800 to 2100 pounds, which is perfect for our P19's.
Your Loos gauge is probably marked in terms of % of breaking stength, not
pounds of tension. According to Brion Toss' book, and a few other sources
I've read, you want to tune the upper shrouds between 15-20% of breaking
strength, intermediates at 12-15% (if there are any, which there aren't on a
Potter), and lowers at about 10-12%. So 15% would be about 240-315 pounds.
The forestay should be about as tight as the uppers, or perhaps just a
ppercentage point or two looser. The backstay should be a little looser
than the forestay or uppers. In no case should you tighten any shroud to
more than 25% of breaking load; that could cause failure. And typically,
anything looser than 10% is too loose, because the wire is still highly
elastic in that range.
Tuning the P19 fractional rig is a special case of the above guidelines.
Since there are no side shrouds to the top of the mast, it's very important
that rig is tuned properly to prevent excessive lateral flexing as well as
"inversion" of the mast under high loads (bending forward at the masthead
with a convex curve pointing aft in the middle in the middle. You can
virtually prevent mast inversion by "tuning in" a little, teensy bit of
pre-bend in the opposite direction; this greatly stabilizes the mast column
from inverting or "pumping" in even the worse winds and waves.
Most spar manufacturers suggest a maximum fore-to-aft bend of no more than
2% of the height of the foretriange; on the P19 mast the maximum would be
about 4", which is MUCH more than you'll find you need. The bend is
measured from the aft face of the mast at the deepest part of the curve to a
straight line drawn between the aft edge of maststep and the forestay
attachment points. My impression is that just barely 0.5 inches of forward
mast bend "tuned in" is enough for the P19, and won't overstress the
masthead fitting. That's based on tuning just two newer P19s recently, as
well as my own.
The P19 has a classic "simple" fractional rig. This means that the lower
shouds not only stabilize the mast laterally, they also act as "check
stays" - meaning they limit forward bending of the middle section of the
mast. So what you want is for them to be at 10-12% tension when the mast is
flexed, not when it's perfectly straight. Of course, you can tune the mast
without any prebend if you wish, and the boat will sail just fine, but I
think a little bit of prebend is advantageous because it prevents mast
inversion when there's a big load on the forestay in heavy winds.
It's not obvious how to achieve a balanced rig with a little pre-bend, so
here's a step-by-step way to tune the shrouds. You may have to go forward
and backward in the steps, repeating some steps as you go, but here's the
basic approach. It takes me about 15-20 minutes to do a "dockside" tune on
a P19 (not counting putting rigging tape on everything). My rigger did it
for me in under 10 minutes the first time, it took me about 45 minutes the
next time I did it, and now that I've done it a few times and have a Loos
gauge, I can do it pretty quickly.
1. Loosen all the shrouds several turns looser than 10% tension. At 10% or
higher, the shrouds will produce a very low pitched musical tone when struck
sharply with a wrench handle. Below 10%, they make a sound like
"fwup-fwup-fwup" as they flap back and forth.
2. Tension the top shrouds to about 15%, making sure the mast head is in
the centerline laterally, using the bitter end of the main halyard as your
guide. Pull the halyard very tightly from one side to the other to the
chain plate, using the length of the halyard to see if it's even. (The
halyard will stretch some. Just do the best you can to eliminate halyard
stretch by keeping a lot of tension on the line when you measure)
3. Tension the lower shrouds to barely 10% or a little less, making sure
that the mast doesn't have any side to side distortions. For a visual
guideline, use the hoisting end of the main-halyard pulled tightly down the
aft face of the mast. Put your cheek and eyeball right up against the aft
side of the mast and "sight" up against the mast to be sure it's straight.
4. Tighten the forestay to about 15%, about the same tension as the upper
shrouds.
5. Tighten the backstay to a little less than the uppers and forestay, and
not looser than the lowers.
6. Put a level on the seat of the cockpit, and adjust the trailer tongue
until the seats are level. Then use the level to check the verticality of
the mast. It should be within a few degrees of vertical, typically with 0-3
degrees of aft rake, from my experience. If it's not vertical in the
fore-and-aft plane, adjust the forestay and backstay tension until it is,
and then, if necessary re-tune the side shrouds. (If you move the masthead
aft significantly, the side shrouds will get a little looser than before.
This is because they are anchored to the hull somewhat aft of the bottom of
the mast)
7. Now, tighten the back stay slowly as you watch the middle of the mast.
The mast should curve forward in the middle section between the boom and the
forestay attachment, and it should move a teensy bit aft right at the
forestay attachment. The uppers will loosen slightly, and the lowers and
forestay should tighten slightly. The curve in the mast should be even and
smooth from the very top to the very bottom, with a constant, barely
noticable curvature. Tighten the backstay until the mast has moved foward
about 0.5 inches. (hint: If the curvature is excessive at the top of the
mast above the forestay attachment, or if only the top 25% of the mast
flexes, you probably need to tighten the forestay a little bit more.)
7. Re-check the tension on the side shrouds and check to see that the mast
is still in column from side to side. Your final "dockside" tune before you
head out on the water should give you about 10-12% on the lower shrouds, and
15-18% on the upper shrouds.
8. Put cotter pins in all the turnbuckles and just a little bit of rigging
tape on them. Now you're ready for the "sailing tune" part of the rig
tuning.
9. Go out on the water in enough wind to heel the boat at least 10-15
degrees when pointing with the headsail you use the most frequently. From a
reach to a close haul, check to see that there is no slack in the leeward
shrouds, and that the mast head isn't falling off excessively to leeward.
If there's slack in the leeward shrouds while you sail, tighten the upper
shrouds a half turn (180 degrees)at a time, until there is no visible slack
in the side shrouds. After tightening the uppers, check the tension in the
lowers to see that they're still about 10-12% when you're not sailing, and
that the lowers aren't noticably slack while you are sailing.
10. Check to see that the headstay doesn't sag appreciably when you've
really got the foresail fully powered up; if it does, tighten the forestay a
half turn at a time until it no longer sags more than an inch or two under
load. If you've got the rig tuned right, when a gust hits, the forestay
shouldn't sag noticably. IN fact, if you've go it tuned really right, the
mast should actually bend forward a little in the middle section (between
the forstay attachment and the boom), which will flatten the draft in the
mainsail during a gust, without you having to touch a single trim control.
11. Check to be sure that you have just 2-4 degrees of weatherhelm. When
you're close hauled, at 10-15 degrees of heel, you should be able to feel a
light tug on the tiller when you hold it with a light grip, and the tiller
should be pulled 2-3 degrees to windward (you can mark the centerline and 5
degrees angle to each side, using some tape on the floor of the cockpit).
If you let go of the tiller on a reach, the boat should turn the bow into
the wind after a few seconds. If the boat doesn't turn into the wind by
itself or tends to turn the bow downwind, you need more weatherhelm. To
increase weather helm, loosen the forestay a turn or two and tighten the
backstay. If you have to pull the tiller too hard to keep it going on a
straight course, then you have too much weatherhelm, and you need to loosen
the backstay and tighten the forestay.
12. Some general comments that affect the tuning: If you sail in heavy
winds, you may want to tune your rig in the tighter range of recommended
values. If you sail in lower winds, tune towards the looser end of the
recommended scale.
If your boat is too heavily loaded in the stern, you will have too little
weatherhelm. Move some heavy stuff forward and sit forward in the cockpit.
She should float with the cockpit seats level.
If you have too much sail up, you will heel excessively (past 12-15
degrees), and you will have too much weatherhelm. Depower your sails with
your trim lines or reduce sail area.
If your rigging is brand new, you'll need to tune it once roughly, go out an
thrash about for several hours in some decent wind to let the wire rope
stretch a little, and then retune it.
13. Once you've got everything tuned up, put rigging tape over all the
cotter pins and rings. If you have the closed body turnbuckles that have
the nuts that lock the studs (rather than cotterpins), order some extra nuts
and put two on each stud of each turnbuckle before you start your tuning.
Tighten the the nuts snugly against each other so they don't move when you
trailer the boat, and put rigging tape around them too. (BTW, half the nuts
are right-hand thread, and half are left-hand thread. Any rigging supply
house can provide them)
> I put tape on the shrouds labelling whether they went in the
> front/rear hole
> of the chainplate. The markings have since dissolved. Does the
> short stay
> go in the front or rear hole?
The lower shroud goes in the forward hole of the chainplate, due to the
geometry of the rig. The chainplate for the shrouds is aft of the maststep
and the lower shroud has a greater forward angle since it attaches lower on
the mast.
>
> What should the rake of the mast be? What is a good range of
> slack/tension
> (in inches or knot-secs) that the backstay adjuster should allow?
I can attempt an answer to your question, but I measure it in terms of "mast
flex" or "mast bend". On my 1985 P19, I have a heavier mast (DM6) and a
beefier masthead fitting than the newer P19's. Mast rake (tilt backwards or
forward) is pretty neutral on my boat, and I have about 1 inch of pre-bend
tuned into the mast.
On the two new P19's I've tuned recently (that didn't have adjustable
backstays), I think we put about 2-3 degrees of aft rake in the mast. We
tuned a minimal aount of pre-bend into the mast, probably 0.5 inches or
less, to be on the cautious side of things. ( I have a $10 level that marked
in degrees that I got at the hardware store)
The older big Potters like mine came with an optional backstay adjuster. I
use my backstay adjuster to put an additional 1.0 -2.0 inches of forward
bend/flex (not rake) in the mast in very strong wind. The "rake" of the
mast doesn't change significantly, but the additional forward bend in the
middle of the mast "sucks" out nearly half the depth of the draft in the
mainsail, which really depowers it. Tightening my backstay reduces
weatherhelm in strong wind because flexing the mast flattens the mailsail
all the way from foot to head; raking the mast aft would increase
weatherhelm by moving the CE of the sailplan aft.
However, with the newer, lighter DM5 masts and mastheads, I am concerned
about how strongly the masthead fitting is attached to the mast (and perhaps
the lower backstay attachment fittings). I don't have any practical
experience with the DM5 rig, so I'd urge some caution when using an
adjustable tensioner with the backstay.
So *A WORD OF CAUTION* is needed on this topic. My caution is based on a
lack of practical experience with the DM5 masthead fittings and backstay
attachments, not on any real world experience of rigging failure.
If you tension the backstay, using the adjuster, and the masthead fitting
starts to distort or pull out of the mast, you're endangering your rig --
Stop immediately! If you can tension the backstay without unduely stressing
the masthead fitting fasteners, you're probably okay. Observe the masthead
and the lower backstay attachment very carefully when you tension the
backstay under a fully powered up sail. Keep an eye on it during tensioning,
and inspect the rivets and other components carefully afterwards for signs
of failure.
The backstay primarily pulls DOWN on the masthead fitting, putting it in
compression against the mast tube, so I think you'll be okay. I think the
fittings can handle it, but I can't be sure. So, Jim, you'll need to check
to make sure!!! Please experiment cautiously and observe the results very
carefully. And *do* let us all know what you observe.
As for the structuraly limitations of the mast itself, it would be difficult
to damage the mast unless you were grossly heavy-handed with the backstay
adjustor. Just don't flex the mast more than maximum 4" specified by most
manufacturers (the 2% of foretriangle height that I referred to above, as
measured from a straight line between the masthead and the mast step). If
you need a good "eyeball" guideline, don't flex the mast more than the
fore-to-aft dimension of the mast (which is 3.25 inches on the DM5 mast)--
that should keep you well within the safety zone.
>
> What is the thing on the top of the mast called? It's the thing
> that has two
> pulleys and the backstay attaches to it (also keep the mast doughnut from
> coming off the top of the mast). The pulleys are cracked and I'd like to
> replace them. Do I have to replace the whole thing? The pulleys
> have riveted
> axles.
The fitting at the top of the mast is called a "masthead". The part that
sticks out for the backstay is sometimes referred to as a "crane." I'm not
sure of the correct answer on how to repair the cracked sheaves (pullies),
but on my boat's masthead, the sheaves were replaced by drilling out the
rivets and replacing them with bolts (be sure to use the correct length
bolts, without threads where the pulley is, and nylon locknuts.) You can
contact Dwyer Mast company to see what they recommend www.dwyermast.com.
>
> Thanks for the help,
>
> Jim Nolan P-19 Panache #426 Colorado
*********IMPORTANT:************ One last caution, I've re-read this twice,
and don't find any typos -- but that doesn't mean they're not there. And
perhaps I've overlooked some important fact.
I genuinely urge that those of you who have some expertise in this area will
comment, correct, or improve what I've written here. I'm counting on y'all
to keep me from giving bad advise to anybody :^)
Fair winds,
Judy B
1985 WWP-19 #266 Redwing
SF Bay, CA
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