Mast pre bend

[tonycooper]

Hey all.....hope all is well.

A friend has a Beneteau first 29 which we were sailing the other week and i noticed there seemed to be quite a large amount of pre bend in the mast - is this normal?

I come from a dinghy sailing background and am familiar with mast set up but thought it odd as there is no deflection in the spreaders.

Any advice much appreciated.

We are not racing it; I was just concerned about loads so should it just put it bolt upright?

Cheers

 

[savageseadog]

Not quite sure what you mean by no deflection in the spreaders but pre-bend in yacht masts is quite normal, how much pre bend is right is another matter.

 

[bbg]

I don't see how you will get any pre-bend if there is no deflection in the spreaders i.e. they are not angled backwards.

I see that this boat is a masthead rig, and photos seem to show what you say - spreaders are in-line rather than swept back.

Are you sure you are not referring to rake? A mast can be angled backwards, but straight.

The only reasons I can think of that a mast with inline spreaders might appear to have pre-bend is if the mast is actually bent i.e. curved - either by design or by use, or if the backstay was on (which would not be pre-bend).

I assume you are referring to the mast while sitting at the dock, with all running rigging and the backstay slack.

 

[dom]

The only reasons I can think of that a mast with inline spreaders might appear to have pre-bend is if the mast is actually bent i.e. curved - either by design or by use, or if the backstay was on (which would not be pre-bend).

I assume you are referring to the mast while sitting at the dock, with all running rigging and the backstay slack.

As a matter of interest, how would backstay tension bend a masthead in line spreader mast? But I agree that this all sounds a bit odd.

 

[knuterikt]

On masthead rigs it's not unusual to set up the backstay adjuster so that you can't take all tension off the backstay when completely slackened.
The reason for this is to avoid inversion of the mast (mast bending forward instead of aft) as the mast will be weekend if inverted.

On masthead rig with inline spreaders this is the only way to set up a pre bend, The babystay might help keep/set prebend on some rigs but it's mainly there to stop the mast from pumping.

 

[bbg]

If it were keel-stepped or there were a baby stay. Edit - and the First 29 does appear to have a keel-stepped mast. I agree with knuterikt that most masthead rigs will have some backstay tension even when fully eased.

On the other thread someone mentioned a baby stay as a means to impart pre-bend. I am not sure that is right mostly it is there to stop pumping. I can't see how it would impart bend in the absence of something pulling part of the mast aft, but I'm not a rigger.

 

[flaming]

If it were keel-stepped or there were a baby stay. Edit - and the First 29 does appear to have a keel-stepped mast. I agree with knuterikt that most masthead rigs will have some backstay tension even when fully eased.

On the other thread someone mentioned a baby stay as a means to impart pre-bend. I am not sure that is right mostly it is there to stop pumping. I can't see how it would impart bend in the absence of something pulling part of the mast aft, but I'm not a rigger.

Well if you hold the bottom, and a point half way up, then pull back on the top, you can't rake it, so you must bend it right? So presumably if you relax the backstay to "max loose" then wind on the babystay the same would also happen. I.e. fixt top and bottom and pull in the middle will bend it.

All with the caveat that I can't remember the last time I sailed with a masthead rig.

 

[bbg]

flaming - that still pre-supposes some tension on the backstay. As mentioned in a couple of posts above, you are correct that a masthead rig normally has some tension on the backstay, even when it is completely eased.

It has been years since I sailed a masthead rig as well. As I recall, topmast backstay does very little to bend the mast. More to tighten the forestay upwind to reduce forestay sag.

 

[savageseadog]

flaming - that still pre-supposes some tension on the backstay. As mentioned in a couple of posts above, you are correct that a masthead rig normally has some tension on the backstay, even when it is completely eased.

It has been years since I sailed a masthead rig as well. As I recall, topmast backstay does very little to bend the mast. More to tighten the forestay upwind to reduce forestay sag.

If the backstay on a masthead rig is tightened the forestay will indeed tighten, further tightening will compress the mast and induce a bend. The degree of bend and the tension required will obviously depend on mast section and stiffness.

 

[bbg]

If the backstay on a masthead rig is tightened the forestay will indeed tighten, further tightening will compress the mast and induce a bend. The degree of bend and the tension required will obviously depend on mast section and stiffness.

And the stiffness of the boat. At some point adding backstay tension will just bend the ends of the boat up.

I remember an article many, many years ago in YW about the IACC boats. The last big monohulls in the Americas Cup. As I recall, they said it was possible to suspend the boat from the three stay points - forestay and two backstay - then put a load the equivalent of a fully-loaded Boeing 747 on the mast step, and the centre of the boat would deflect down by some ridiculously small amount. I suspect most GRP boats are not as stiff.