Mast Rake vs. Mast Bend
A few weeks ago, I overheard a couple of sailors talking
about how they set up their boats. One
sailor was explaining that if he raked the mast aft five degrees or so from
vertical that the mainsail would flatten out and allow for better
pointing. At the time I didn’t say
anything. But, he had it pretty much
backward.
Raking the mast aft doesn’t flatten the mainsail. But it does change how the boat sails. Here’s why.
First some technical info: if we find the balance point of the
underwater profile of the boat, we’ll find the CLP (center of lateral
plane). This is the point around which
the boat pivots when you are tacking; and is usually found at about the middle
of the keel. If we find the CE (center
of effort) for the sails, we find the point where the pull of the sails (or
vector) can be estimated. For most of
us, we’ll want the CE to be forward of the CLP so that there is a slight
weather helm in six knots of breeze or more.
This means that the boat will tend to turn up into the wind if you let
go of the tiller. If the CE is too far
forward, the boat will have lee helm and want to turn downwind if you let go of
the tiller. Lee helm is pretty
uncomfortable and potentially dangerous.
So if your boat has lee helm, you might want to rake the mast aft some
to give you the weather helm balance that you like. On Snickerdoodle,
I have the mast raked aft about three inches.
I set up this mast rake when I tune the standing
rigging. By securing a wrench or winch
handle to the main halyard I can see how much rake there is. I adjust the rake by increasing or decreasing
the length of the forestay turnbuckle and correspondingly adjusting the
backstay turnbuckle.
One interesting by product of this CLP/CE analysis is that
we can pretty quickly see that if only the mainsail is hoisted, the CE is way
aft of the CLP. This will cause the boat
to act like a wind vane in all but the lightest breezes. Likewise, sailing with only the jib hoisted
will move the CE way forward and the boat will tend to fall off the wind
continuously.
So, mast rake affects the location of the CE and how the
boat steers. Mast bend is something
completely different.
Mast bend is when we deliberately bend the middle of the
mast forward. Bending the middle of the
mast forward can be done either by tightening forward lower shrouds (or a baby
stay); or by tightening the backstay (most easily done with a fractionally
rigged boat (the forestay is secured to the mast 15% to 25% down from the mast
cap. Bending the middle of the mast
forward pulls the mainsail’s fabric forward and this does flatten the
sail. Why would you want to do
this? Because this allows you to change
the sail from a power setting to punch through waves; to a speed setting if you
have clear air and relatively flat water.
Flattening the sail also depowers the sail; which might be a good first
step if your boat is heeling excessively.
For most of us, if we want to use the backstay method, we will need to
rig some sort of adjustor so that we can increase and/or decrease the backstay
tension easily. For smaller boats, a
split backstay and block ‘n tackle is the preferred rig. On bigger boats you might want a hydraulic
system.
barnaclebillholcomb@gmail.com
