Inducing Mast Pre-Bend (masthead rig)

[gandy]

Hi,

Our mast has virtually no prebend, and I'm struggling the understand what the effects of the different rigging tensions will be, so that I can determine how to increase the bend. Its a masthead rig, with inline spreaders, aft lowers and a babystay.

Superficially only the babystay would appear to directly pull the middle of the mast forwards. But in our case its quite a light wire (5mm compared to everything else at 6mm) and I've already used up all the travel on the rigging screw. So something else is wrong. Possibly the aft lowers too tight? Or something else not tight enough.

We're planning to replace the rigging at the end of this season, so now is the time to find out whether the existing wires are the correct lengths or not.

Thanks in advance.

 

[Guest]

One possibility is to loosen the forestay and tighten the backstay so that the middle of the mast stays where it is because of the baby stay but the top is pulled backwards.

OR if the foot of the mast has different positions/slots, moving it back a slot will have a similar effect.

 

[Victorious]

Why do you want to increase the bend in a griffon mast?

 

[SvenglishTommy]

Hi Gandy Goose,

I have exactly the same rig set up(and wire size), and also end up with the babystay very tight with little pre-bend. I just put it down to rig design, and as long as I have just a touch of pre-bend I'm happy.

When setting up I first tension the babystay to 15% breaking load, then the aft lowers (which are also 5mm) to 10%. This seems to do the trick.

 

[gandy]

[ QUOTE ]
Why do you want to increase the bend in a griffon mast?

[/ QUOTE ]Firstly because the rig setup guides that I have seen all recommend some degree of pre-bend, and our mast currently has virtually none (maybe none at all).

The second reason is that we get a low frequency vibration of the rig in some winds, and I can see the lower part of the mast vibrating for-and-aft when this happens. I have been assuming that prebend would stiffen the mast

Your question is interesting. Do you think a Griffon should be straight?

 

[PeteCooper]

The amount of pre-bend that you requires is determined by the cut of the mainsail, not the type of boat.

 

[charles_reed]

Not really the road down which you want to go with the rig described.

I fail to see what you'll gain, or what you expect the benefit to be.

Why not talk to a competent rigger?

Mast prebend is normal on dinghies and desirable on fractional rig boats, but for masthead rig it's, at best, a posture.

 

[Guest]

Can be useful as a way of flattening an overbaggy mainsail a bit. Of course it's better to go to the sailmaker, but in the meantime, a few twists of the rigging screws will help.

 

[Marsupial]

Then why is it that my mast head rig is designed to have prebend? Certainly I would seek the help of a rigger to get it set up right. The stuff about setting the mast to the sail is what racers do after the rig is properly tensioned - and I ve seen a few boats runined by that approach.

I bet the lower lee shouds flop about when sailing to windward, and the forestay looks like a banana with the jib sheeted in. Just wanging on the backstay is not the answer either.


Get a rigger


Cheers

David

 

[Guest]

The basics of setting up a simple masthead rig is not such a black art that you have to involve a rigger for every little tweak. Obviously you have to make sure the shrouds are the correct tension, that the mast is straight, and that in the end the backstay is tight enough to give you a decently straight forestay. Personally I'd tighten the backstay each time I go out sailing to give a straight genoa luff, then slacken it a bit when leaving the boat to avoid unnecessary stresses on hull etc.

But the question was rather about inducing some prebend in the mast, which is a different issue, and not difficult to do. But it's not a long term solution, better to re-cut the main.

 

[Victorious]

The reason i asked.. Was to try to find out what problem the "prebend" was attempting to fix.
Bend in a mast will flatten the mainsail to some extent... The griffin is not a new boat and an old main will often be somewhat baggy.. and the draught of an old sail tends to be further aft than when new.
This all produces weather helm...Which is the most common problem that people tweak the rig to fix.

 

[gandy]

The problem I'm trying to address is the mast oscillation. I homed in on prebend as that is one area where our rig is definitely not as recommended, and one of the references refered to this as a helping to "stabilise" the mast

Funnily enough, weather helm is not a problem even though the main is old. There's noticeable lee helm in light winds, but overall I'm reasonably happy with the sail balance.

I'm certainly not trying to change the mast to suit our old mainsail. I plan to replace the sail, and when I do I will get the sailmaker to cut it to suit the boat. But by that time I want the rig to be correct.