Height of spreaders ?

[VicS]

It may seem a daft question but I have my reasons!

How high (as a fraction of the mast height) should the spreaders of a single spreader mast head rig be?

 

[Poignard]

I don't know but, if it helps, I have the designer's rigging diagram for a Twister here and I could take measurements off it for you.

 

[santeana]

My Rival is close to 50% of mast length. Mast 11.05m; height to spreaders from mast base 5.40m (48.9%)

 

[sarabande]

Is this site known to you ?

http://boatdesign.net/articles/index.shtml


I got waylaid looking for spreaders and ended up on bulbous bows !

 

[Cap'n Creosote]

I'm sure I remember reading something about "the spreaders must ensure that the rigging meets the mast head at an angle of no less than 14 degrees"

This would partly dictate the height of the spreaders.

But of course I could be misremembering!
/forums/images/graemlins/confused.gif

 

[William_H]

The spreaders usually also are the attachment for the intermediate side stays. It follows that side (and fore and aft) support as provided by the intermediates and the spreaders themselves should be midway between the top and bottom support. ie between the cap shrouds meeting the mast (fractional or mast head) and the bottom support for the mast ie deck or keel if it is not supported at the deck level.

 

[savageseadog]

Given the chainplate positions work out what height the spreaders would be for the D1 stays to make a 12 degree angle to the mast. That is the maximum height the spreaders should be.The minimum height would be 50% mast height so somewhere in between.

 

[CPD]

To possibly complicate matters, surely the spreader length must also be taken into account. Aren't the "sticky out" things on the sides of some of the older Vendee boats at deck level essentially spreaders ?.

 

[savageseadog]

[ QUOTE ]
To possibly complicate matters, surely the spreader length must also be taken into account. Aren't the "sticky out" things on the sides of some of the older Vendee boats at deck level essentially spreaders ?.

[/ QUOTE ]The spreader length is normally the width of the chainplates. Yes you could have a very low set of spreaders but they wouldn't supporting the mast/rigging. I was assuming a normal rig.

 

[Billjratt]

And to throw in another.... when the mast is slung with a strop at the spreaders, it should be only slightly heel-heavy (ie one person can manoever to lay flat)

 

[VicS]

Thanks for the responses.

I assumed that the spreaders would be half way up but wondered if that should be half way up the sail luff.

The rig in question may have spreaders shorter than the distance out to the chain plates and I am a bit doubtful if the angle made between the cap shrouds and the mast is as much as 12°. Something to look into.

We regularly lift the larger masts at the boat yard from a point (well) below the spreaders so know the problems there but it's not an issue in this case.

 

[greylag]

55% - Sparkman & Stevens rigging and mast design factors 1970.

 

[Bodach na mara]

When I renewed tha mast on my last boat, the old mast had the spreaders at the 50% position. The representative of the rigging company that supplied the new mast kit and rigging told me to locate the spreaders at 55% of the overall length of the mast from the bottom end. The length of the spreaders was equal to the distance from the foot of the mast to a poimt vertically above the chain plate for the main shroud. The angle of the wire with the mast turned out to be just over 12 degrees. I am assuming in all of this that it is a single spreader rig. If the angle turns out to be a lot under 12 degrees, then it may be time to think of an extra spreader.

 

[Poignard]

Figures for Twister:

Mast [stepped on coachroof], heel to truck: 33'0"
Heel to spreader roots: 17'1"
Spreader length: 3'0"
Beam of boat: 8'1"
Angle between spreaders and horizontal: 5 degrees