Mast rake theory help please

Sorry - I've just read (more carefully!) the post where you say that this sag is at standstill.

I'll go back then to where @Biggles Wader suggests that something else is wrong. Has this just 'happened' or is this just after re-rigging the boat? It sounds as if you're at least 2" short of travel and so I can't help wondering about the extra toggle that I mentioned - or something like that.
 
The two biggest reasons for rig failure are incorrect trim and faulty sail handling. Not setting your rig up properly is the surest way of losing your mast.
The Sigma is a fairly competitive design with a relatively slender and flexible mast and if you do not know what you are doing I would suggest having a professional rigger show you the ropes first time around at least.
In a rig: for every action there is a reaction. As Occasional Yachtsman said: you preset your rake with the headstay , then tension the backstay(s) and finally adjust. Tensioning does not just stretch the cable, it also distorts the hull and it is easy to overtension the headstay with dire consequences. We recently had this discussion in a commentary on a vid showing a sudden mast collapse.
 
Laminar Flow said:
The Sigma is a fairly competitive design with a relatively slender and flexible mast and if you do not know what you are doing I would suggest having a professional rigger show you the ropes first time around at least.
Click to expand...

If the OP can't get a proper result by following the instructions then he needs a skilled eye on the job. And needs to be there to follow every move. It will either throw up an equipment problem or a technique error.
 
anoccasionalyachtsman said:
Sorry - I've just read (more carefully!) the post where you say that this sag is at standstill.

I'll go back then to where @Biggles Wader suggests that something else is wrong. Has this just 'happened' or is this just after re-rigging the boat? It sounds as if you're at least 2" short of travel and so I can't help wondering about the extra toggle that I mentioned - or something like that.
Click to expand...
It will always be that way with the procedure in the manual. The only tension there can ever be on the forestay is the slight weight of the mast at 2 degrees aft.

The tensioning of the rig actually makes this worse as the mast is given prebend losing the mast height.
 
Ok, a bit of guesswork here! You mention a survey - is this a new-to-you boat and you're now getting it sorted? If so then is there any evidence of something having been changed/maybe mast re-stepped after storage? If so then has something been replaced since it last sailed? Alternatively, at this age it's almost certainly not original standing rigging, and I wonder if the previous owner had it re-rigged by someone that set it up with a vertical mast - and so extending the forestay to get the rake could well use up all the adjustment on the backstay.
 
anoccasionalyachtsman said:
Ok, a bit of guesswork here! You mention a survey - is this a new-to-you boat and you're now getting it sorted? If so then is there any evidence of something having been changed/maybe mast re-stepped after storage? If so then has something been replaced since it last sailed? Alternatively, at this age it's almost certainly not original standing rigging, and I wonder if the previous owner had it re-rigged by someone that set it up with a vertical mast - and so extending the forestay to get the rake could well use up all the adjustment on the backstay.
Click to expand...
It's fairly new to us, especially if you write off the last 18 months!
I've now shortened the forestay to minimise the sag and I get 4.5"of rake.

It's possible that the prevous owner (who raced the boat and had it re-rigged) had it rigged to this rake. There was a little too much weather helm at 9".
I'll give it a try and see how it sails.
 
hydrophylic said:
It's fairly new to us, especially if you write off the last 18 months!
I've now shortened the forestay to minimise the sag and I get 4.5"of rake.

It's possible that the prevous owner (who raced the boat and had it re-rigged) had it rigged to this rake. There was a little too much weather helm at 9".
I'll give it a try and see how it sails.
Click to expand...

Ah, ok, that puts a different light on it and I'm still getting my head around what's been happening. The aft set shroud base will contribute to pulling the mast back, and you should have been able to get the slack out with the caps. I wonder how tight you went on them? (I did find the Sigma manual, and their mention tightening but not straining the bottlescrews is the least helpful advice I've read for a while!).

As to weather helm, I've sailed a few hundred miles on 36s, and only a lap or two of the Solent on a 362, but I remember them as being reasonably light, so on the one hand you're right to be trying to improve things but on the other I don't think Sigma would have suggested 9" without input from David Thomas himself as he was always fairly active around his new designs. Now, as a new boat owner you probably won't want to contemplate this, but how good are the sails?!
 
anoccasionalyachtsman said:
Ah, ok, that puts a different light on it and I'm still getting my head around what's been happening. The aft set shroud base will contribute to pulling the mast back, and you should have been able to get the slack out with the caps. I wonder how tight you went on them? (I did find the Sigma manual, and their mention tightening but not straining the bottlescrews is the least helpful advice I've read for a while!).

As to weather helm, I've sailed a few hundred miles on 36s, and only a lap or two of the Solent on a 362, but I remember them as being reasonably light, so on the one hand you're right to be trying to improve things but on the other I don't think Sigma would have suggested 9" without input from David Thomas himself as he was always fairly active around his new designs. Now, as a new boat owner you probably won't want to contemplate this, but how good are the sails?!
Click to expand...
I've taken the mains to 370kg which is about 7% and gives the required prebend.
The aft lowers are 'hand tight' as directed so won't contribute to pulling the mast back.
Genoa is very good and main is new, all kemp like the rig so I guess they knew how to make the sails.
 
hydrophylic said:
I've taken the mains to 370kg which is about 7% and gives the required prebend.
The aft lowers are 'hand tight' as directed so won't contribute to pulling the mast back.
Genoa is very good and main is new, all kemp like the rig so I guess they knew how to make the sails.
Click to expand...
Ok, 7% isn't enough, you should be aiming for 15 on the caps. Rest as per the book, but a bit of backstay isn't forbidden - even at rest. In theory it's only for sailing loads, but getting rid of a floppy backstay also cures a floppy forestay while parked.

I've no experience with Kemp sails, but they have a decent reputation on here I think. (Is there a connection with the sparmakers - I wasn't aware?)
 
anoccasionalyachtsman said:
Ok, 7% isn't enough, you should be aiming for 15 on the caps. Rest as per the book, but a bit of backstay isn't forbidden - even at rest. In theory it's only for sailing loads, but getting rid of a floppy backstay also cures a floppy forestay while parked.

I've no experience with Kemp sails, but they have a decent reputation on here I think. (Is there a connection with the sparmakers - I wasn't aware?)
Click to expand...
Spars are kemp.
I'll crank it up then.
Thanks for your help, won't be back to it for a few weeks.
 
hydrophylic said:
I've taken the mains to 370kg which is about 7% and gives the required prebend.
The aft lowers are 'hand tight' as directed so won't contribute to pulling the mast back.
Genoa is very good and main is new, all kemp like the rig so I guess they knew how to make the sails.
Click to expand...
7% is flapping in the wind. Whilst I do not have a sigma I would have thought one would need much more than that
Surely start at 15% & go from there. Pre bend would be controlled by the lowers so saying 7% gives the required pre bend is not strictly true. It is the lowers that do that & hand tight seems totally wrong.
Next point is that even though my caps are at 25% ( loos gauge) the forestay is still fairly slack. It always will be slacker as it runs off at a wider angle. Obviously one cannot check its tension as it has a furler. If one is only applying 7% to the caps then tension to the forestay is considerably less so it will be floppy. On my boat The backstay really only makes the mast bend as it is 7/8 rig. So the latest jib is cut to suit the sag in the stay. It might be worth trying your jib to see how it sets with different rig tensions.
 
hydrophylic said:
I have a masthead rig. Mast rake is set by adjusting the length of the forestay. The forestay length is then not changed during the adjustment of the rest of the rig. This gives a saggy forestay.

My question is, how is the forestay tension set?

When the rest of the rig is set the forestay still sags.
Click to expand...
I still don't understand. Is the forestay initially tight, but sags as you tension the rest of the rig, or can you never get the forestay tight in the first place?

If it's the first, is the rake correct when the forestay is tight (in which case stop doing whatever slackens it) or when it's slack (in which case adjust inside the furler or shorten the stay.

If it's the second, go straight to shortening or adjusting.
 
Early Sigma 362 OOD fractional rigs had very bad mast pumping and and running back stays were fitted to some. I don't know if there was a redesign of the rigging. On the 362 OOD that I sailed, one of the first, the pumping caused the forestay to sag and snap back when the mast straightened. Running back stays were fitted to that boat which fixed the problem. I know that may not be relevant to your problem as you have a masthead version of the 362 and you should certainly be able to set up the rake and have a tensioned forestay.

Sigma Manual, you can check the lengths of the rigging from the specification sheets.

https://go.openbms.nl/uploads/171/113280/documents/Sigma_Manual.pdf

Dumb question perhaps but is the boat level when measuring rake?
 
hydrophylic said:
Spars are kemp.
I'll crank it up then.
Click to expand...
JumbleDuck said:
I still don't understand. Is the forestay initially tight, but sags as you tension the rest of the rig, or can you never get the forestay tight in the first place?

If it's the first, is the rake correct when the forestay is tight (in which case stop doing whatever slackens it) or when it's slack (in which case adjust inside the furler or shorten the stay.

If it's the second, go straight to shortening or adjusting.
Click to expand...
JumbleDuck said:
I still don't understand. Is the forestay initially tight, but sags as you tension the rest of the rig, or can you never get the forestay tight in the first place?

If it's the first, is the rake correct when the forestay is tight (in which case stop doing whatever slackens it) or when it's slack (in which case adjust inside the furler or shorten the stay.

If it's the second, go straight to shortening or adjusting.
Click to expand...
I still don't understand either. Following the manual, all rigging is de-tensioned ten the forestay length adjusted to set the rake. The forestay is not readjusted then at any stage. With the weight of the wire and the furler foils there is a saggy forestay with no tension. How is a forestay tension of over 500kg ever achieved?
 
hydrophylic said:
I still don't understand either. Following the manual, all rigging is de-tensioned ten the forestay length adjusted to set the rake. The forestay is not readjusted then at any stage. With the weight of the wire and the furler foils there is a saggy forestay with no tension. How is a forestay tension of over 500kg ever achieved?
Click to expand...
Clearly you have to pull the top of the mast backwards, and the only thing that's going to do that to any extent is the backstay. How tight is your backstay?
 
hydrophylic said:
I still don't understand either. Following the manual, all rigging is de-tensioned ten the forestay length adjusted to set the rake. The forestay is not readjusted then at any stage. With the weight of the wire and the furler foils there is a saggy forestay with no tension. How is a forestay tension of over 500kg ever achieved?
Click to expand...
The Selden link in post 8 is very helpful - especially the descriptions of tensioning different types of rig .:)
 
You must log in or register to reply here.

Share this page

Top