Mast Rake and Mast Bend - Part 1 (WAS potter 19 backstay)
Sun, 7 Feb 1999 14:17:26 EST

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West Wight Potter Website at URL
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In a message dated 2/7/99 1:00:49 AM Pacific Standard Time, writes:

> What exactly are the advantages of a backstay? Is it only useful if you
> want to bend the mast?
> Bill Longyard
> Winston-Salem, NC

Hi Bill,

Wow! That's a big question. Oh well, it's pouring cats and dogs today and
it's cold, so I'll stay inside and write instead of working on Red Wing.

Before I start, let me put in my standard disclaimer. Your mileage may vary.
8 ^ ). Everyone should rig his/her boat in whatever way makes HER happy. I'm
re-rigging our 1985 P19, Red Wing, like a blue water boat, but that doesn't
mean it's cool for you to do the same thing. And, now turning to the topic at
hand, I hope I don't make any *really* embarrassing mistakes, but I'm *quite*
sure someone will correct me if I do. <grin>

This is a pretty big topic, so we'll start off with discussing how a backstay
affects mast rake and boat balance. In part 2, we can move on to mast bend.
As you read this, be sure to keep the two topics separate.

A good reference to read is the Annapolis Book of Seamanship, John
Rousmaniere, Second edition, Chap 3: Sail Trim, "How Boats Balance" on page
84, and "How to Measure Weather Helm" on page 87. Chapman covers some, but
not all, of the same info in Chapter 10, in the section entitled "Basic sail

There are many advantages and uses for the backstay. But first, do you really
need one? that all depends on your personal preferences and the conditions in
which you sail. You probably *need* one if you sail in high winds. If you
sail only in light to moderate winds, then you don't *need* one, but you may
want one for better balancing of the helm. A well balanced helm makes your
boat faster and helps it point higher.

(Please note: even if you don't have or want a backstay, most of the following
discussion applys to your boat too. You can adjust the length of the forestay
to balance the helm of your boat. But without a backstay, the balance will
change noticeably as the strength of the wind varies)

The first advantage of a backstay is that it stabilizes the mast on any
downwind heading and keeps it from pumping forward and backward as you sail
over swells. Pumping can get the mast so far out of column that it
conceivably could break in high winds. So if you sail in high winds and rough
seas, you need one, in my opinion.

Here's the second big advantage. It has to do with induced mast rake. On a
broad reach to close reach, the mast gets pushed forward a little. This puts
slack in the forestay. If it's windy, the jib luff will curve, putting more
draft in the jib -- which is exactly what you don't want in heavier winds.
This is especially true if you have a furler/reefer on the headsail because of
the considerable extra weight of the luff extrusion. The manufacturer of some
small furlers recommend installing a backstay on small boats (mostly
daysailors, but the Potter is really similar in size) for that very reason.

With a sagging headstay in heavy winds, the boat will heel more, have more
weather helm and you'll be fighting the tiller to keep the bow from rounding
up. (Note, it's the excessive heeling that produces the weather helm, not
increased drive from the foresail which would tend to reduce weather helm!
Heeling excessively digs the chines into the water and also changes the
relative positions of the CE (the Center of Effort of the combined sails)
compared to the CLR (the Center of Lateral Resistance of the combined hull
appendages suchs as rudder, keel, and chine)

Now the main topic! The third big advantage is that you can finely balance
the helm with a backstay. By adjusting the relative tensions between the
backstay and the forestay with the turnbuckle, you can change the rake of the
mast to balance the helm under average conditions. To get rid of a lee helm,
loosen the forestay and tighten the backstay to rake the mast back. To reduce
weather helm, rake the mast forward.

(When we bought our P19 Red Wing, she had a nasty lee helm, partially due to
the fact that the mainsail had been cut down, but also due to the fact that
the forestay was at least 1" too short! One inch of forestay length change
moves the masthead about 3"! You could see that the mast was leaning forward
just by looking at her while she was floating level in the water)

About three degrees of weather helm is perfect --- the boat will round up by
itself if you get in trouble and it will give you just the right "touch" on
the tiller. Anything less than that will give you an "empty" feel on the
tiller and you'll have a hard time holding a steady course.

More than 3 degrees and you'll find yourself tiring from having to always pull
the tiller to windward. With too much weatherhelm, you're just slowing the
boat down and slipping sideways because the rudder is acting as a brake as you
try to hold a steady course.

If your boat has a tendency to head downwind (with the sails trimmed well and
modest heel), then you have a "lee helm," which can be dangerous if you get in
trouble. There are several causes, the most common of which is that your mast
is raked too far forward. Another common cause is too little weight in the bow
for the boat's design (and a P19 sails best with her bow down in the water).
If you sail with only light cargo in the front of your P19, and you have too
little weatherhelm (or have an actual lee helm), you may want to rake the mast
back to compensate.

You can measure this by taking a large protractor and drawing a mark on the
transom 3 degrees off center on both sides. When you're sailing on a reach in
average conditions with sails smartly trimmed and about 10 degrees of heel
(for a P19,anyway, I don't know about the P14s), the tiller should give you
enough "touch" (slight resistance) with a light "thumb and two finger grip"
for you to hold a steady course by feel alone. The tiller should line up with
the windward 3 degree mark you made. If the tiller is pulled past 3 degrees
to windward (assuming your sails are trimmed well and you're heeling about
10-15 degrees on a P19), you have more weatherhelm than you should.

Heres a quick preview into the topic of mast bend (for all you cerebral types
who must have a zillion "yeah buts" on the tip of your tongues) --------->
read on.

Mast rake is associated (and confused) with mast bend. While the two factors
change interactively, the do different things. Mast rake is important for
balancing the helm by moving the Center of Effort of the sails relative to the
Center of Lateral Resistance produced by the keel, skegs, and rudder.

Bending the mast is primarily used to flatten the shape of the draft in the
middle of the sail and to put a twist in the head of the mainsail to spill
wind. (Do this only if your mast is strong enough to handle it, but that's
another topic with many differing Potter opinions!)

When you tighten an adjustable backstay, you BEND THE MIDDLE OF THE MAST
FORWARD, which changes the shape of the mainsail draft in the middle and top.
By tightening the backstay, you flatten the draft in the middle of the
mainsail and reduce its power. This moves the Center of Effort of the two
combined sails *forward* towards the headsail. (In constrast, tightening the
outhaul flattens the foot, not the middle, of the mainsail)

tightening the adjustable backstay bends the *tip* of the mast backward,
loosening the leech of the mainsail with out increasing the draft. This
permits the head of the mainsail to twist off and spill wind in the gusts.
Bending the mast also tightnes the headstay more, flattening the jib, as
discussed above.

Simply put, the boat point higher with a flatter mainsail and heel
less,especially in the gusts.

There's more to the story, what with cunninghams, vangs, outhalus, mainsheets
and whatnots. But this will have to do for now. ;^)

That's about all I can come up with for the advantages of a fixed backstay off
the top of my head. Jump on into the discussion, guys!

Roulez les bonnes temps,
Judy B.

Judith Blumhorst, DC
HMS18/P19 Fleet Cap'n, Potters Yachters
1985 WW19- #266 "Red Wing"
SF Bay, CA