Re: Mast Rake and Mast Bend - Part 1 (WAS potter 19 backstay)
Mon, 8 Feb 1999 01:54:08 EST

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West Wight Potter Website at URL
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In a message dated 2/7/99 8:03:28 PM Pacific Standard Time, SR500F writes:

> Judy et al.,
> I'm going to put my engineer hat on for a second to express a couple of
> concerns regarding the use of backstays on the later Potter 19s with the
> lighter section mast.
> As Judy pointed out, the quick way to break your mast is to let it get out
> of column. I am concerned that using a masthead backstay combined with the
> fractional height forestay on the lighter section mast will inevitably lead
> to mast bend (intentional or unintentional) which IS taking your mast out of
> column.
> Here's a little more explanation, for those who are interested, otherwise
> skip to the next message......
> We normally think of our boats sail's and rigging, and the loads on it, in
> terms of what we experience while on the water. That is we think of it as a
> forward - backward and side to side regime. In a boat with an unstayed mast
> like a Laser) where the mast is cantilevered above the hull and deck this is
> exactly the case. When stays and shrouds are added the force diagrams change
> alarmingly.
> The force of the wind in the sails is more or less perpendicular to the
> An ideal stay or shroud would also be perpendicular to the mast, directly
> opposing the force of the wind. The shape of the boat doesn't permit this,
> obviously, and we end up with shrouds and stays that are closer to parallel
> with the mast rather than perpendicular to it. The result is that to
> constrain the sideways force of the wind the stays and shrouds have to pull
> DOWN really hard. All that downward tension (necessary to get the smaller
> sideways force due to the angles) has to be countered by the mast resisting
> it. Thus, the highest forces on the mast are those of compression.
> One shouldn't forget, too, that on a Potter the mast is stepped on the deck
> and any compression loads on the mast will be passed to the boat at that
> point. I've read accounts of racing skippers using hydraulic backstay
> tensioners to such a degree that they have punched their deck stepped masts
> through the cabin top. It's a bit like the hull, the forestay and the
> backstay are the "bow" and the mast is the "arrow" trying to be shot through
> the bottom of the boat!
> Anyway, returning to my original point, the early P-19s had a heavier
> section mast capable of resisting compression over a longer length (without
> bowing out of column) when compared to the smaller section mast of the later
> boats, the rig of which is only compressed over the shorter length from the
> headstay/shroud attachment point down. It may be all that that mast can
> under gusty conditions. This may be a situation where "more" isn't
> necessarily "better"
> I apologize if this sounds too much like a lecture - my real intention is
> that no-one have a bad experience on the water.
> Best regards to all,
> Dave Kautz
> P-15 #1632 "Tilly Lucy"
> Palo Alto, CA

Hi Dave et al,

I understand your concerns, but, being a P15 owner, you're perhaps not that
familiar with the P19, I think you've failed to consider some of the design
features of the P19. Even though I'm going to disagree with you on this
topic, I appreciate your jumping in to the discussion with your concerns.
You're an analytical sort of guy, and I respect that. ;^) Engineers are
great guys to have around, they have good heads for fixing things. Besides, I
consider you a member of the POO society, so all your ideas merit my
respectful consideration ;^)

The P19 was originally designed to have a back stay with the fractional
headstay. That feature disappeared in the early 1990's when the lighter
section mast was adopted, as you pointed out. However, the placement of the
side shrouds was not modified when the backstay was eliminated. On the P19,
the shrouds are directly outboard of the mast step, and provide no resistance
to forward movement of the mast. This is very different from the P14/15
design, where the the shrouds are anchored aft of the mast step. With the
P19's shrouds directly outboard of the maststep, they provide no resistance to
forward rake of the mast when you sail downwind.

On a downwind heading in 20 mph of wind, the P19 mast rakes a full 1 foot
forward, putting very strong torquing forces on the cabin top. The forestay
hangs slack like a clothesline. Not good. You NEED a back stay on the P19 to
keep the mast straight up on a downwind run in a decent wind. A backstay
reduces the chance of damage to your boat and mast when sailing downwind, it
does not increase it. There are many newer P19's with backstays sailing on SF
Bay as evidence that there's been no failures due to the addition of the
backstay. I've never heard of a mast failure attributed to the use of the
backsay, but I'd check with Jerry B, he's been around alot longer than I in
the world of P19s.

The factory recently re-instated the backstay as an available option on the
P19 after many years of absence. The reason the backstay was eliminated was
to keep the base price of the boat down, not because it was dangerous.

Also, as I'm sure you know, the P19 has a sturdy compression post spanning
from the mast foot to the V in the hull just forward of the CB trunk. This is
one of the strongest parts of the hull, and was originally engineered to
withstand much greater compressive force that anyone would possible put on it
with the lighter section mast. It was designed to stand up to as much
compressive force as you could put on it with the heavier mast and an
adjustable backstay.

The specification for the P19 compression post and V was not downgraded when
they switched to the lighter mast and will certainly stand up to the
compressive forces you can put on it with a fixed backstay. So that shouldn't
be a worry.

Another point is that the P19 shroud arrangement is typically found only on
boats with a backstay, with both an upper and a lower shroud located directly
outboard of the mastfoot. The purpose of the lower shroud in this design is
to limit the forward bend of the middle section of the mast when you tighten
the backstay. It should be tuned very, very loosely when the mast is at
neutral, and tightens as you bend the mast, providing support to the mid-
section of the mast. There would be no real need for the lower shroud without
a backstay. It's not really necessary for lateral stability.

This is exactly how my lower shroud works on my P19 with the heavy mast and
the adjustable backstay. When I got the boat, the lower shrouds were pretty
tight and I couldn't put any bend in the middle of the mast. After Bruce the
Rigger tuned my shrouds for me, the lower shroud permitted the mast to bend
forward in the middle, giving the mast a nice smooth curve all the way from
the base to the tip. I can flex the middle section of the mast forward about
3-4 inches from its neutral position, and flatten the draft in my mainsail
pretty nicely, all without bending the section above the forestay excessively.

Most people don't understand the function of the lower shroud on the P19 with
a backstay. They tune the lower shrouds much too tight. This doesn't permit
the mid section to move forward. When you tighten the backstay with the lower
shroud too tight, all the bending force is concentrated solely in the top 1/3
of the mast (above the forestay) because the middle and lower section can't
flex. Thats a good way to break your mast.

I am of the opinion that you could use an adjustable backstay on even the
lighter section P19 mast with little danger of breaking the mast, if you know
what you're doing and you tune your shrouds properly. Nobody else dares to
say that (uh-oh, now I'm on thin ice), for fear of being held responsible for
somebody breaking his mast. I don't want to be held responsible for anybody
breaking his mast, so I say do this at your own risk. But if my boat had the
lighter mast, I'd do it. ;^)

As a last point, people often argue against the use of a high-purchase tackle
on the Potter backstay (actually I've heard this argument about a vang on the
Potter boom). I think they argue fallaciously. If a 250-pound man pulls as
hard as he can on a 4:1 backstay, he's going to bend the mast more than I do
with my 12:1 purchase. Mr. Muscleman has to yank on it with his whole body
and can't control the bend very well. Dainty little Skipper Judy can adjust
it finely with just one hand and knows when to back off. <grin> If you're
going to add and adjustable tackle to you backstay, I would vote for giving
yourself plenty of purchase so you can control it easily with one hand from a
seated postion.

That's all for now. Keep those cards and letters coming folks! Thanks Dave
for putting forth an opposite point of view. I'll rely on you all to keep me
on track here if I get my facts (and sometimes my opinions) way off track. If
you disagree with anything I've written, please share your perspective.

Judy B.

Judith Blumhorst, DC
HMS18/P19 Fleet Cap'n, Potters Yachters

1985 WWP19 #266 "Red Wing" -
with the original heavier mast and a new, heavy duty boom with fixed
gooseneck, new mainsail, and tracks soon on the cabin top, new working jib on
order, duct tape on stuff everywhere, and,... oh yeah, I just cut a hole in
the floor so I can see into that weird bilge space.... but that's another post
and I need to get offline and work on my boat before the first PY sail of the
season over Valentines Day weekend.

SF Bay, CA