Mast bend

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rbeange

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Hi, I had to unstep the mast this winter and having had it put back up I'm trying to get the tensions correct. Its a fractional rig with single swept back spreaders. Now, I'm no where near the 20% breaking strain I've been recommended - I guess it's 16% on the cap shrouds and 10% on the back stay. But there is already significant bend in the mast - if you can see the picture. Is the bend too much and if so, why is the bend so much given the tensions are low? Thanks vm for any advice!image.jpg
 
My mast was rather like that when newly set up, but my rig is quite different. The riggers fiddled with ours and the bend is now further down, but mine is double spreader. People are often surprised at the amount of bend and clearly some rigs will take it.
 
I've always put bend in the mast to suit the cut of the main. Looks like too much tension on the backstay. It's pulling against the forestay, but that is lower. If you need lots of bend, but spread further down the mast you need your caps shrouds tighter, but I still think it looks too much on the backstay. Just my opinion.
 
Why 20%? Why is 16% "low"? Is 10% on the backstay how you intend to leave the boat normally? (it looks like the kind of rig you should be adjusting the backstay to suit sails and conditions - what does it look like with the backstay looser?). Are you racing or cruising? How tight are the lowers?

These are some of the questions related to setting up a mast - is there a class association where you can ask as well?
 
rbeange said:
Hi, I had to unstep the mast this winter and having had it put back up I'm trying to get the tensions correct. Its a fractional rig with single swept back spreaders. Now, I'm no where near the 20% breaking strain I've been recommended - I guess it's 16% on the cap shrouds and 10% on the back stay. But there is already significant bend in the mast - if you can see the picture. Is the bend too much and if so, why is the bend so much given the tensions are low? Thanks vm for any advice!View attachment 41627
Click to expand...

Download the Selden guide, worth it's weight in gold!
 
IMHO the backstay on this kind of rig is not standing riggijng but more like running rigging. I think this kind of rig should be set up with nil backstay tension. The mast then should be straight or with just the smallest amount of middle forward bend. I would not go to so much static tension but if you want to, go for that tension on the cap shrouds. This will depending on the aft sweep of the spreaders push the middle of the mast forward. You then apply tension to the lower shrouds to couteract this and get the mast straight. Do not try to set the lowers with tension gauge. It is the shape of the mast that matters. The cap shrouds will also determine the tension on the forestay. Any adjustment of the forestay is only to set mast rake. ie slightly top aft is usual.
You should then be able to crank on the backstay in stronger winds to induce mast bend so allowing the top of the mainsail to fall away and take out some of the camber in the middle of the mast. I trust you have an adjustable backstay.
One modern racing 8 metre (Bakewell White 8) here has a box on the floor of the cockpit. Out of this box comes controls for main sheet, boom vang and backstay tension. All adjusted for gusts virtually continually. Even for cruising crank on the backstay with strong wind and have it off for light wind. good luck olewill
 
+1 for the Selden guide - works a treat for my single spreader fractional rig. I'm not afraid to wind in the full compliment of tension though. A good tip (as mentioned above) is to lay out your mainsail and have a look at the amount of pre-bend that's cut into it - assuming you have a good sailmaker, that should give you an idea how much bend should be in your mast and where it should be located.
 
William_H said:
IMHO the backstay on this kind of rig is not standing riggijng but more like running rigging. I think this kind of rig should be set up with nil backstay tension. The mast then should be straight or with just the smallest amount of middle forward bend. I would not go to so much static tension but if you want to, go for that tension on the cap shrouds. This will depending on the aft sweep of the spreaders push the middle of the mast forward. You then apply tension to the lower shrouds to couteract this and get the mast straight. Do not try to set the lowers with tension gauge. It is the shape of the mast that matters. The cap shrouds will also determine the tension on the forestay. Any adjustment of the forestay is only to set mast rake. ie slightly top aft is usual.
You should then be able to crank on the backstay in stronger winds to induce mast bend so allowing the top of the mainsail to fall away and take out some of the camber in the middle of the mast. I trust you have an adjustable backstay.
.....l
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Exactly that.
It's almost a big dinghy rig with a backstay.
The backstay is used to bend the mast as it's not practical to use the kicker quite as you would in a dinghy.
It also takes a lot of load when the spinnaker is working hard.
Upwind, until you are overpowered, you don't need much backstay tension.
 
rbeange said:
Hi, I had to unstep the mast this winter and having had it put back up I'm trying to get the tensions correct. Its a fractional rig with single swept back spreaders. Now, I'm no where near the 20% breaking strain I've been recommended - I guess it's 16% on the cap shrouds and 10% on the back stay. But there is already significant bend in the mast - if you can see the picture. Is the bend too much and if so, why is the bend so much given the tensions are low? Thanks vm for any advice!
Click to expand...

You need to tighten the lower diagonals to straighten the mast
 
lw395 and William_h get my vote.

Set rake with Forestay.
Tension cap shrouds.
Remove bend with aft lowers to around 3" at spreader height.
Set forward lowers to lock lower fore and aft in place.

Backstay is a tuning control not standing rigging.

Caveat - Some of the lightweight French pop out boats don't like 20% as you'll find that doors below will no longer close or lockers get really stiff.
On a polish 32' the other week the door overlapped by 1/2 inch after we tensioned the rig!
 
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