Mast pre bend

T

tonycooper

New Member
Joined
20 Jan 2004
Messages
12
Visit site
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.

Cheers
 
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. It would have been an idea to put this into the racing part of the forum.
 
tonycooper said:
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.

Cheers
Click to expand...

With dinghies and some yachts the mst prebend is created by the tension in the cap shrouds passing over swept back spreaders.

The First 29 apperas to have spreaders that are not swept back. The prebend iis created by the inner forestay ( babystay ??)

A common set up for yachts is square spreaders and cap shrouds plus forward and aft lower shrouds . The prebend can be created by tensioning the forward lowers.
 
Pre-bend is used as along with backstay adjustment, Cunningham, vang and outhaul in flattening the main as the wind increases as well as reducing forestay sag for better windward trim. The real question (as this is normal) is whether the amount of visible pre-bend was giving you a good mainsail shape? Sailmakers do assume that the rig can be bent as required when they are calculating the shape of the sails.

Rob.
 
If the luff of the mainsail is starved out (creases up the luff running back/down generally in the direction of the clew) then you have too much bend.
 
I don't know the boat you sailed on however...
I would suggest that for a fractional rig with bendy mast that prebend which is only just perceptably middle forward would be right for light backstay tension. ie a set up for light winds. Excessive tension on the cap shrouds across the ends of the aft swept spreaders can give too much bend under the baove conditions. Usually the inner side stays when tensioned will pull the middle of the mast aft to take out prebend so may be too loose. It is this ratio of cap shroud to lower (inner) shroud tension that sets prebend. All this assuming it is a typical smaller boat fractional rig of forestay backstay adjustable shrouds and (2) inners with chain plates aft of abeam the mast.
For mast head rigs or those with square spreaders and either an inner (baby) forestay or a pair of forward inner side stays then as said it is the ratio of tension of the aft pulling inner side stays versus the single inner forestay or the pair of forward inner stays. This rig generally is not easy to bend the mast although we often see adjustable inner baby forestay for stronger winds.
With a very bendy mast the mainsail will have a lot more camber which can be easily taken out with mast bend. A mast head rig on the other hand will probably have much less camber so that with a little or no mast bend it can be flattened by outhaul etc for stronger winds.
I am sure it was not just a case of too much backstay tension.ie he forgot to release it after strong winds. But then again some people don't adjust their back stay much. good luck olewill
 
Pre-bend can be handy when racing to windward, what always strikes me when going past racing boats left on their moorings with masts still tensioned thus though, is, ' lazy, cruel bastards ' !

No need to make the boat and eventually her hull suffer for the laziness of the skipper not having the tension/s let off, pure lack of seamanship.
 
Seajet said:
Pre-bend can be handy when racing to windward, what always strikes me when going past racing boats left on their moorings with masts still tensioned thus though, is, ' lazy, cruel bastards ' !

No need to make the boat and eventually her hull suffer for the laziness of the skipper not having the tension/s let off, pure lack of seamanship.
Click to expand...

I'm not so sure... Years ago I had a Tornado cat, and used the 8:1 mainsheet to tension the rig pre-sailing. A shroud broke when the boat was ashore with the rig slack, and I'm not sure that the constant easing slight tensioning under "relaxed" conditions didn't contribute. Obviously couldn't leave the rig at sailing tension, but maybe I shouldn't have raised the shroud to the top of the plate
 
You must log in or register to reply here.

Share this page

Top