Mast bending

[DickB]

I have a 7/8ths fractional rig on a 42ft yacht. I noticed that when in a swell the mast flexes slightly with the backstay tensioned. I reckon that the hull is flexing about 1/8 - 3/16 on an inch between the mast foot and the backstay chainplate to cause the corresponding mast movement. Has anyone else noticed this and is it a problem or just a characteristic of GRP yachts?

<hr width=100% size=1>Dick

 

[Robin]

Are you sure the hull movement isn't just in your mind? It is not at all unusual for a mast to flex under load in a swell or head sea especially when the forward momentum of the boat suddenly drops as a wave is hit. If yours is a typical 2 spreader rig it will have no fore/aft support at the 2nd spreaders and the section above the 1st spreaders is free to flex, the only support coming from the mainsail which will be less when the sail is reefed. Race boats and those with additional inner stays for heavy weather sails will (should) have running backstays taken from the 2nd spreaders to give this fore and aft support.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[DickB]

I don't think so and I only noticed it when we were motoring and both sails were furled. But you are right, it is a typical 2 spreader rig with semi swept back spreaders. A pretty simple affair really. I am farily sure that the mast was flexing and frankly not a great deal but with the leverage of the backstay a very small movement of the backstay translates into quite a movement of the masthead. I was just not sure whether the standard AWB does this as a norm or have I been selected for special treatment

<hr width=100% size=1>Dick

 

[Robin]

I think we are both having problems here with our desciptive powers!

What you are seeing is something I believe you will see on all boats, it is nothing to do with the hull but only to do with the flexing of the mast - in this case 'pumping' (that is the usual term) when motoring into a sea/swell. Hopefully someone else will chip in with the correct technical info. This pumping will be greater with no mainsail set or with a deep reefed mainsail. You might though want to check that the cap shrouds and lowers are set up tight enough, the swept back spreaders should be giving some fore/aft support to the mast but they cannot do that I think if the stays are too slack. This comes into the realms of rig tuning and is a huge subject, there are books available and some sailmakers and mastmakers have info in booklets and probably on their websites - worth a google and some study! If your rig has never been set up properly (ie it was just stuck up by the yard on delivery) then I would suggest you do spend some time doing it, it will improve performance and handling and be less stressful on the mast/rigging as well.

<blockquote><font size=1>Quote from previous post:</font><hr>

very small movement of the backstay translates into quite a movement of the masthead.

<hr></blockquote>

By this I suspect you mean by tightening the backstay a small amount, the masthead moves quite a bit? Yes it does and that is entirely normal!

Be a little reassured that the designer and builders will have a fair amount of safety margin built into the rig specification unlike on a flat out race boat where they are pushing the limits on controlabilty and rig weight/windage. That is notwithstanding any rubbish you will hear about Bavarias being built down to a price blah blah blah.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[ashanta]

It appears that you may not have enough tension on your stays. You are probably aware that fractional rigs have a high tension set up and failure to get the setting right can cause the rigging to fail. I would get the set up checked by a competent person who is trained in fractional rigs.

Regards.

Peter.

<hr width=100% size=1>

 

[hairbox]

Hi check the seldon website out hints and tips it explains how to set up your rig http://www.seldenmast.com I found this very helpful by the way is your rig Deck or Keel stepped?

<hr width=100% size=1>

 

[charles_reed]

Speaking as one

who has had a fatigue FM due to that very factor, I would suggest that all you're seeing is the mast pumping forward.

7/8 rigs, with swept spreaders, definitely rely on the mainsail leech to provide support when you're heading into a head-sea.
That's the reason that many motor with main up.

the provision of running backstays on many boats is to prevent pumping when the main is reefed, rather than at all times you're beating.

You don't say whether your mast is deck or keel-stepped, the latter definitely has less pumping.

If you can notice the movement I'd strongly recommend you motor with main up - you'll find you can even get drive out of the main 30degrees off the wind.

Most production boats are well sorted and the hull is stiff enough for rigging loads, even if the large un-reinforced areas of hull can mean internal fitting are unbonded in heavy weather, after all they're designed for recreational sailing not serious seagoing.

<hr width=100% size=1>

 

[snowleopard]

whether or not the mast movement is caused by hull flex you can be sure that flexing does happen. heard of a child pumping up a hydraulic backstay... "daddy, why do the guard rails go slack when i move this lever?"

<hr width=100% size=1>

 

[DickB]

Thanks everyone for the replies. My horse sense tells me that this is not a major issue but the yacht is 6 years old and has not had the rigging checked as far as I am aware. I don't really believe that hull is moving to any significant degree but I was suprised when I looked up the mast to see the top few feet flexing (not a lot though).

All in all I think I will get this checked out this winter (whilst in the water!!!) The mast is deck stepped and cap shrouds sound taught (not scientific I know but a good indicator), whilst the lowers are clearly not as tight. I have'nt checked out the uppers yet! Too cold to climb up there!

I will check out the Seldon web site and see what gives...

Again, thanks for the help

<hr width=100% size=1>Dick

 

[Robin]

Taught is a relative term and you need to study the books etc. Final tuning will need to be done afloat in a F3/F4 upwind and will involve making sure the mast stays straight athwartships (or at least doesn't hook upwind at the top) as well as the lee shrouds not being slack. Support for the upper part of the mast will come from the mainsail and from the swept back spreaders which cannot give any support if the stays are too slack. You will find it is well worth doing as the sails will set better too with less weather helm and better pointing as a carrot if you get it right.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[brianhumber]

Re: Try a Barracuda for a bendy boat

As others have said rig tension is critical and fractional does need a lot of tweaking. When a quarter sea hits Ronhilda you can see and feel the hull deflect. Also the ambient temp affects the rig tension, a difference of 10C means I have to either slacken or tighten the rig, as either the tension gets too much and the hull/deck deforms the keel box and the keel starts jamming or the caps, middles and lowers get too slack when beating.
The mast really flops about if I put the trysail up as you loose the triangle of forces the leech gives on 3rd reef.
If I wind up the geny sheet really tight the deck deforms upwards.

Having said all that Ronhilda is now over 16 years old, has sailed tens of thousands of miles and looks after me if I get stupid and get caught out in 40kts plus. When this happens I just keeping reminding myself of the amount of flex I used to see in tankers and that if the hull is not 'giving' in a sea then you are in real trouble.

<hr width=100% size=1>

 

[DickB]

Re: Try a Barracuda for a bendy boat

I guess I was being a bit paranoid about this but I was curious too as to whether there was an issue here. I know that the Bav's, Ben's, Jen's, Duf's etc are built at a price and I wondered if the Hal's, Naj's, Swed's, Mood's (and of course Sad's and Van's etc) which seem to be around twice the price also exhibited this phenomenon.

All in all, I will get the rig checked but I don't think there is a real problem here. Thanks for your help

<hr width=100% size=1>Dick

 

[William_H]

Re: Try a Barracuda for a bendy boat

Just one additional thought that it might be worth checking the integrity of the mast support structure it being a deck mounted mast. will

<hr width=100% size=1>