Rigging Query

I have a query which is at this stage only an academic enquiry, so some of the practical aspects of implementation the concept are not detailed.

I have drawn up a conventional rotating mast rig with backstays this is (Single Spreader Rig.pdf). This is in my experience a pretty common setup on a multihull.

I have made the following changes to this rig (alternative Rig.pdf), to come up with what I see as a cheaper simpler & stronger rig:

1. Eliminated the spreaders and replaced them with additional stays.

2. Moved the forestay to mount off the backstay attachments on each side of the mast. On a rotating rig this would require a loop of rope starting at a backstay attachment point and ending at the other, on this rope would sit a pulley with the conventional forestay attached.

I have not explained item 2 very well but the pictures might make a bit more sense.

Ignoring the issues of overlapping genoas etc, is what I have drawn comparable in structure to a conventional rotating rig?

Thanks in advance for critical appraisal.

Alex

 

Perhaps contact Greg Elliot for comment....I believe he uses your alternative style. Since 90 percent of all the rigs use the diamond, I would think that the spreader diamond removes compression loading from the mast step

 

Attached a link to a rotating rig on an open 60 with deck spreaders / no mast spreaders

http://www.360ouest.com/photo_fs.php?id=641

Diamonds work well, no argument against them for fuction, just on multihulls there is a wide base to support the mast so compression should be less of an issue, and spreaders are complex things. I agree in principle the compression at the mast base would be higher without spreaders, can't quantify it yet though.

However mast compression is made up of many things, I would guess removing spreaders would add a percentage increase in compression from the side loading of the mast over the bottom half of the mast only.

Like many things I'm sure the issue has been investigated many times, I will keep trying to understand.

Alex

 

I just don't have much experience with wide shroud base rotating rigging. Compression load is an issue. Have patience with your query on Boatdesign net and see if you can get the attention of Mr. Eric Sponberg for comment. ...http://www.sponbergyachtdesign.com/


Mr Sponberg is frequently on the Boat Design forum and has biblical experience with free standing and rotating rigs.

If I remember correctly Sponberg designed the free standing carbon rigs for Noah's Ark. Evidently spirally wound carbon on a camel dung mandrel post cured in the Sinai Desert.

Have patience

 

Hi Alex,

I'd have to say that the original spreader rig is going to be better, both for function and cost. The spreader rig has 5 stay and shroud attachments to the deck, your proposal has 8 attachments to the deck. More attachments for a rotating mast reduces functionality--it won't turn as well. To improve the rig, you need to eliminate stay and shroud attachments to the deck. Most multihulls will have 3 attachments--one headstay and two shrouds--the minimum that you can have and still keep the mast up. Also, each stay and shroud requires a chainplate of sorts for attachment to the hull, so you are trading off spreaders for more attachments and more rigging. The increase in hardware is going to be more expensive to build.

There is nothing wrong, engineering wise, with spreaders--they keep the mast in column and allow for smaller mast sections--giving less mast windage and less weight. To keep the width of the spreaders down in a rotating rig, designers sometimes use nested spreaders which have shorter lengths and supported spans that overlap. This increases complication, but allows better set of the headsails. Everything is a compromise.

I hope that helps.

Eric

 

The Greg Elliott rotating mast designs never rotated the correct amount, which is to 80-90 degrees each side, because his side stays attached to the sides of the mast at the hounds ... and so his rigs were limited to around 60 degrees, maximum rotation - which is not enough if you want a decent flow from mast to mainsail, with no wind flow disrupting steps. This is to be expected with monohull designers switching to rotating rigs but there are plenty of multihull designers also making the same mistake. If you want the rig to rotate properly, the forestay and shrouds MUST go to the same point on the leading edge of the mast, to the beak hounds fitting, preferably jutting out 50mm. And the same setup must be used for intermediates or lowers. The runners can go to the mast sides but not the shrouds. However the alternative, if you're determined to use the mast sides for shrouds, is to engineer sliding stays (like a flat smile) at the hounds, more complex and heavier, but once rotated the shrouds can remain at the same tension. With the singe beak point the leeward stay wraps slightly around the leading mast edge. This is usually no problem because the stays are not set up as tightly as a conventional fixed rig.
Eric Sponberg is right: there are too many attachments to the multiple staying setup on your rig. But you could run intermediates to the same point as your deck shroud attachment; that would simplify things.

 

Eric,

Point taken on attachments to the deck adding complexity & cost the initial design I proposed had 7 points on the boat to attach rigging vs 5 for the conventional setup with runners.

Can it be explained the difference between a spreader & wires keeping the mast in column with diamond wire angles (looking for and aft) of say 10deg vs a side stay at 10 deg only connected to the mast at the point where the spreader was?

As I understand it the only difference would be the load through the mast below the lower diamond would carry through to the boat, effectively loading up the mast step.

Don't take this that I am dismissing the use of spreaders, I just want to understand.

Gary,
Under rotating on rotating rigs is a pretty common curse, Farrier is now attaching hounds to the side of the mast and the forestay in conventional position lower & forward. Does not rotate as well but has other significant advantages in supporting sails above hounds.

On the drawing I have done as a concept vs your concept of the sliding stays the differences are that instead of all stays sliding effectively the forestay only slides (via a pulley on a loop of rope) the side stays attach to the sides of the mast.

I have seen this system on other boats and it seems to offer a lot of advantages. If there is any wind on the sails the leeward side stay is loose enough so rotation can occur very easily up o a full 80deg. It’s a much more balanced & free system than mounting on the front as increased load on the sail does not induce rotation loads.

I think I need a photo of this to explain.

Thanks
Alex

 

Alex, I was really recommending all the stays being attached to the protruding hounds beak fitting, (see jpeg) not the complex flat smile set up. Even if your forestay is on a pulley/rope, the leeward AND windward shrouds attached to the mast sides, are still going to bind and lockup with rotation - hence the 50-60 degrees (if you're lucky) rotation. I don't believe you can get to 80 degrees unless all the shrouds are very slack.
I have sailed on a big multihull with side stays mast attachments and used to take the spanner lines to a winch and crank the living crap out of it - horrible thing to do, all the stays locked up at less than 50 degrees, and that was all I could get.
On the point of sail load not contributing to automatic mast rotation; I have never found this problem to occur, sheet in hard, mast rotates to the limit of the spanner setting. Of course that is with the hounds/beak setup not with the mast sides system.

 

You understand the differences correctly. The advantage of the diamond wires is fewer connection points at deck level and more free rotation of the mast. The cap shroud would be much more highly loaded because it does ALL of the work of just keeping the mast up, whereas the diamond wires and spreaders are keeping the mast straight.

All the complication of the wires is why I like free-standing rigs with no wires whatsoever. You trade off the wires for bearings and get practically 360-deg rotation of the mast (that is, nearly 180 deg each side).

Eric

 

experience

I switched from beak hung shrouds to side mounted on my 24' tri last season and encountered exactly the problem that Gary has described. My mast used to "flop" over quickly to the new 70+ degree position, now I have a 3-1 tackle and still can't get it past about 50 degrees. I bought nice expensive titanium Colligo tangs to do it with I guess it is back to the single shackle this year and I will use the tangs for running backs. Don't make the same mistake. B

 

mast bend

I did notice a difference in mast bend, with the forward beak, the diamonds have to be set to provide more bend so the mast doesn't invert under load. It makes tuning a bit more tricky, and requires changing from light to heavy air settings. The side mounted shrouds (installed just forward of center of the mast) really helped keep and add some positive bend, right when you need it. I have a fairly stiff al mast section designed for multihulls, but a real wing mast would be less affected. B

 

Gary,

I don't have a lot of experience on big multi's but getting towards experienced on lots of small ones, does this stuff sum?

The “beak” is a neat solution for rotation, no doubt, most of the small muti’s I've seen have a “jeasus” shackle on a protruding tang, on this is separate shackles for each of the stays, lots of big chunks of metal.

These rotate pretty good at low rig tension, the beak system which is well forward would be better still.

However there are advantages in taking the loads to the sides of the mast which is why so many people try, even at the expense of compromising rotation. Attached a pic of the hounds of the AC cup catamaran Alinghi, there is a lot of stuff going on up there!

I have found a photo of the system I think is really quite a good compromise it was taken from the F-boat forum, it's a mast off an F31 Tri - Cheeky Monkee

Bruce,

Maybe rather than going back entirely to what you had this might be of assistance to you?

Also I'm not surprised that the mast behaved differently after the change, I would think in an ideal world the diamond entry point to the mast would have to change.

Can anyone explain how diamond entry points in masts are determined?
I have some thoughts but I have seen so much variation to lead me to concude the smart guys are not all using the same rule book.

Thanks
Alex

 

The answer is: there is no rule book! All masts are engineered from first engineering principles, really. No one has really written any design rules, with the possible exception of Germanisher Lloyd classification society--I understand they have some. That is because every boat is different, and the mast design and structure depend on the overall rig geometry and the corresponding loads. That explains the variation. The end points of the diamond wires depend on the internal structure of the mast (some masts do have them), how the mast is built, and that overal geometry of the mast, particularly at the head. Is the mast a masthead rig or a fractional rig? That influences a lot of things. So we work out the specific structural and rigging details accordingly. Varied boats = varied designs.

Eric

 

Alex, Cheekee Monkey's solution is okay; you can see the slits for the shrouds to move around in - and the rope strop for the forestay is a good solution too. But a beak is simpler, lighter and aerodynamically cleaner. However there are no rules, as long as it works. Alinghi's setup is very complex but also very adjustable. I'm guessing, like the forestay, the shroud staying system can be tightened or slackened to whatever is required. I mean they had a stinker to quickly make the adjustments. Also it looks like they can ease the shroud strops and take the tension to the masthead, for giant reacher support, I guess. Those big reachers would have put giant loads on the sides of the mast (because the mast would be rotated to 90 degrees). It would be interesting to hear what their rig control and design team have to say about their solutions.

 

The diamonds are going to be stiffer than the shrouds. The diamonds are shorter, so less stretch for the same load. But the big difference is when the spreader is deflected to the side, the tension in the diamond goes up much faster than the tension in the shroud.

The stiffness is critical to preventing buckling because buckling occurs when the rate of increase of the restoring force is insufficient for the rate of increase of the deflection. It's not just a matter of being able to support the side load on the mast.