Can anyone direct me to a reference on mast spreader length, and position?
I am replacing the rotted wood masts on my Cheoy Lee 31 ketch with salvaged aluminum masts. I will be cutting down a 27' mast for the 22'6" mizzen mast. The spreaders on the aluminum mast are mounted to welded brackets. I have enough length above and below the spreaders to cut the aluminum mast, and duplicate the position of the spreaders on the wood mast. However; the spreaders on the aluminum mast are a couple inches shorter than the spreaders on the wood mast. Should I change the position of the spreaders on the aluminum mast relative to the position of the wood spreaders because of the length difference? The upward tilt angle of the aluminum spreaders is about the same as the wood ones. I would like to be able to use most of the same shrouds and stays for the new masts, so I will be keeping the same overall length and position of the attachment points whenever possible.
The problem with using the shorter spreaders with the old shrouds is that the shrouds will be too long. Shorter spreaders also may need to be positioned differently, probably higher, to keep the same angle to the tangs on the masthead fitting, and moving them will probably change the angle they have to bisect on the shrouds.
To calculate the rig, you might look through Brion Toss's Complete Rigger's Apprentice (http://www.briontoss.com/catalog/books.html). You will want to, as near as possible, arrange the spreader to bisect the angle of the shroud, which will mean a higher lift if the spreader is shorter, depending on the spread at the base. Keep in mind the spreader will be in compression: a toothpick is pretty strong in cmpression, but crushes easily if you push on its side.
However, I would strongly encourage you to create new standing rigging to suit.
"However, I would strongly encourage you to create new standing rigging to suit. "
The rigging is virtually new, except that it stood unused for 2 years. All the fittings are mechanical and in perfect shape. I'll have to replace some wire because I'll need longer than the original.
The mizzen spreaders are 2" shorter than the originals, but can be adjusted to "tilt" up a little more. But, it seems to me, that shorter spreaders would need to be lower to bisect at the same angle as longer ones?
I'll experiment with cord before I move anything.
Lengthening wire: there are a few options if you only need a couple inches longer. For example, adding a toggle, or finding a longer bodied turnbuckle.
Shortening a spreader will (generally) make the angle more oblique, usually requiring the tip to rise slightly to split the new angle. In most cases the change will be very small, and within the "adjustable" range of the fittings. We're talking up or down maybe an inch at the tip. It's fairly easy to calculate the angles geometrically, just takes time sitting down with the measurements and a calculator. Of course it's a lot harder to measure the angles on the rig, or to tweak them precisely. I used strict measurements to place my spreader tips, and they still look 'off' to me.
I found a copy of "The Complete Riggers Apprentice" by Brion Toss. It expains spreader angles and position. I'll be shortening both masts a little more than 10%, and using one size over design wire and turnbuckles. I won't be doing any racing, but might cross an ocean, so I want plenty of strength. I plan to replace my 14 chainplates with slightly beefier ones also.
One rigging item that bothers me, is the stay that connects the mizzen to the main mast. I don't like the idea of losing both masts if the main mast breaks. There is no room to run forward stays on the mizzen, because the main boom comes back almost to the mizzen mast. Is there such a thing as running fore stays?
Quote from: okawbow on August 17, 2009, 09:25:47 AM
Is there such a thing as running fore stays?
Why not?! And it makes a dandy place to hank up your mizzen stays'l, too.
The triatic stay is to avoid *either* mast going down. It's also a possible place to hoist additional sail area up high where it's most effective. Masts which are independently stayed are no more and no less likely to fail, however rigs which depend on running stays are somewhat more likely to fail simply due to human error - forgetting to cast it off or to set it up at an inopportune moment.
That said, you will have some room for a pair of forward-swept shrouds, even if the boom comes right back to the mizzen. Give enough tension these shrouds could allow you to dispense with the triatic stay for the mizzen. They will not let you dispense with the triatic for the main. The main will still need a powerful backstay arrangement of some form. It may be possible to run a split backstay, with chainplates somewhere aft of the mizzen step. This would certainly require the attentions of a professional rigger, in my opinion.
"The main will still need a powerful backstay arrangement of some form. It may be possible to run a split backstay, with chainplates somewhere aft of the mizzen step. This would certainly require the attentions of a professional rigger, in my opinion."
There actually are split backstays on the main. They connect to chainplates aft of the mizzen. So, the main mast is well supported.
Sounds, then, like the triatic might be dispensed with. But, again, I think I'd talk with a rigger. If you have insurance, you might need to have the boat surveyed after any such alteration to the rig.
You might be able to take a copy of the sail plan/rig plan down to a rigger's shop (and a dozen donuts + pot of coffee) and just talk about things without having to pay for a rig survey or anything formal. You can get advice as to what kind of service you'd need to determine whether that stay can be removed, as well as what else it's doing for your rig. It's really pretty important to develop a relationship with your local rigger, anyway, so you can get the favors necessary to keep your boat in good order inexpensively - special parts ordering, picking up just one of whatever, rig surveys, and of course the occasional gratis advisement. They also know where to bring stuff for stainless welding.