rotating mast stay attachment

Thats strange Rwatson - I've never found that style of attachment a problem. Perhaps it comes about because on the cats you mention there is no effective boom vang resulting in little forward force on the mast to rotate it? On northies there isn't a problem with getting enough rotation, rather there is sometimes a problem rotating from one side to the other because the rotation is forced in by the boom.

 

From memory my problem was caused by two things.

The padeye in the mast was verticle and half round, and the shackles in the eyes at the ends of the stays had to conform to whatever "lay" they could find in the limited space. Ideally, the shackles should be at right angles to the stay direction to reduce the friction, but they ended up twisted and pressured at whatever angle they found themselves in. The forestay shackle invariably lay horizontally on top of the other ones, which should have acted like a hinge, but it didnt seem to help much.

The other problem was that the "line of rotation" through the mast went from a rotating point about half way along the mast chord at the base, to just in front of the mast where the eye was fastened. The forestay (correctly tensioned and mounted on top of the other fittings) pulled the mast padeye firmly forward like a ring through the nose of a bull, and made it really hard to deviate the mast from straight ahead, mast spanner or no. Likewise, the side stays (correctly tensioned) fought each other to grab the mast eye from either side with the leverage thay had on the bottom of the mast padeye.

The answer would be a more efficient fastening on the mast I am sure, but I havnt nutted that one out yet.

I have a poor picture of a carbon fibre rotating mast off someone elses cat that illustrates the problem that I took to see what I was doing wrong. As far as I can see, they had exactly the same problem made worse by double stays (less room for shackles).

If anyone has a solution to the problem, that would be great. There is obviously a trick to it as there are a lot of boats using the method.

 

Dart Catamaran Hound Fixture

This is one small catamaran that I know of that uses this two-hole hound tang. ...and it is really needed here as the Dart has no boom... to forced any rotation

 

Well, here's how L. Francis Herreshoff did it in 1924 in his radical R boat "Live Yankee."

Cheers,

Earl

 

All of those drawings have me puzzled, Hereshoffs most of all. In the close up of the side stay attachment, the stays appear bolted to their respective sides of the mast, while the forestay hangs out on an arm which increases its leverage. Sure the jib halyard is rotatable, but I just cant see it helping the mast rotate.

The two pictures from the Dart cat, show two different methods. The first has the side stays fastened out in front, while the forestay is sensibly mounted close to the mast. This looks the most promising, but surely when the mast rotates, funny things happen to the side stays ?

The second Dart illustration looks like the system I encountered, with all the wires hung from one shackle. That shackle was mounted with the shacklepin inside the mast tang, so that the tang has to fight the squareness of the shackle pin and the pin hole reinforcing. Instinctively,I would have mounted it so that the round side of the shackle was inside the tang, giving some kind of pivot point.

I can see I am going to have to set up one of the old masts in the backyard and do a few experiments.

 

Confusing Dart Dwgs

The confusing aspect of the Dart dwgs is the second one with 4 wires attached is actually the two side shrouds and two trapeze wires. The forestay is not shown in this dwg, and would attach to that lower empty hole that is closer to the center of rotation of the mast...thus less leverage.

In the early days of the big French multihull building programs there were a few of those big rotating mast held up with rigging attached similar to Hereshoffs as they were concerned with the strength of the materials available at the time to carry the loads while projecting out from the mast.

 

Cant the mast - then, if your mast won't rotate, ease the leeward shroud, mast rotates. On a decent sized chord wing mast, a tight leeward shroud will tighten across the leeward curve of the larger mast, cant the mast by tightening the windward shroud, slacken leeward (otherwise you can't cant, (poetic eh?) - but on a smaller, non canting, tear dropped shape mast, I can't see why you're having problems - except that perhaps you've set your shrouds up too tight. IMO the hounds fitting has to project at least three fingers width from mast, with shrouds and forestay attached by shackle to the hounds.
I remember Allan-Williams designed 2 metre chord wing mast on 60 foot Steinlager, had shrouds attached at the mast's widest beam point - bloody mast wouldn't rotate properly - couldn't even winch it round to over rotated position. Same with Barker's 57 foot Sundreamer when she was first launched.
Setting a rotating gantry-type system at mast top is asking for trouble, bad compression loads plus the swivel? point, a small thing, is taking these point loads and is unable to dissipate them, unlike a hounds fitting with uni directional carbon fibres running up and around the mast over a large area. Also it's weight at the highest point.

 

"bloody mast wouldn't rotate properly"

i think the best idea so far is

normal hound
a shackle, then a chain link
maybe even a shackle afterwards

have to rotate easily with this

mal
.

 

now thats a funny idea...
how do you trim those sails?
they are 3 dimensional with 2 seets of canvas forming an triangle like shape and building an almost proper wing-shape with the mast...
the leeward part will flex but how should the windward part be 'compressed'... it will have a smaller chord... flapping loosely?

@rwatson
this mast will rotate... and since the sails are that strange - there is no need for overrotating it...

 

What forward thinking fellows these Herreshoff guys were

 

You should see the rest of the boat The hull was built like an airplane fuselage, with formers, stringers, and and thin wooden skin. There was no rudder as we know it -- the after part of the fin was flexible and moved like a flipper. I must have stared at the plans for an hour at Mystic and still went home puzzled until I found a New Yorker article that said the rear of the fin flexed.

It was the fastest and most expensive R boat ever built, and killed the class in Marblehead. True to form, the NYYC banned most of its features, describing them (IIRC) as "rotating masts, double-luffed sails, and similar contrivances."

Cheers,

Earl

 

Nat Herreshoff's Catamaran

They banned his father's catamaran as well...

"I can with a good whole-sail breeze, boat to windward faster, by a mile an hour at least, than any other sailing vessel afloat." -Nathanael Herreshoff , the infamous yacht designer speaking of his revolutionary 25' catamaran design, Amaryllis .....the year, 1876

http://www.runningtideyachts.com/archives/amarylis.html


http://www.boatdesign.net/forums/multihulls/herreshoffs-catamaran-reasoning-5843.html

 

Oh yes, with sloppy shrouds, it works a treat.

I dont think 'easing a shroud' is a viable solution. You cant set a mast up with sloppy shrouds, its a formulae for mast and gear failure.

On some big boats, thats true. I am only talking a 28ft trailer sailer, where the tip of the mast is the same size as the rest of the mast. We are only talking about moving any fittings a metre further up, and I figure I can use less shackles with a custom fitting, thereby reducing weight.

I anticipate using cross-trees and mast stays to remove the bend in the centre of a mast, like many cats do.

Should be ok ????

 

It's simple enough RWatson, don't grind you shrouds up until they twing, just to tight, then if you're worrying about headsail luff sag, put some runners on - runners do a better job than uptight shrouds any day. Stop thinking like a fixed rig keelboater.

 

shroud connections for rotating mast

IMO, this is a great way to attach shrouds to a rotating mast. Blocks mounted just forward of the chord, allyacht spars does this for Corsair on the C37 and sells the brackets needed. Connections can be very close to the center of rotation of the mast. The second best, imo, is thru bolted mast brackets (double tangs) on the chord. The 2 methods above will minimize chafe issues at the mast while attaching at the foestay shackle, while a lighter option, oftentimes results in contact with the mast on the shroud.

John Franta, Colligo Marine