Cap Shroud Layout

[Phil Alderson]

I am currently working on a new rig and am not sure how best to set up the cap shrouds.

The Options I am thinking about are;

49er style: with the caps coming from the mast tip through the top spreaders and then down to the shroud points.

Diamonds: the caps come from the mast tip through the top spreaders and down to the base of the mast. they might go through the lower spreaders but would not be deflected much by them.(you could also run them through the tips of the lower spreaders but this works best with a narrow shroud base.)


49er style is I think more robust for holding the kite loads as the wire pulls back to shroud points it is easy to adjust for the day when rigging. As the wires go to a wide point it holds the top mast in column till it has quite a lot of bend which is good for power.
But; It puts more rig load through the boat, main shroud tension and cap tension are interdependent so can be tricky to setup.


With Diamonds, static cap shroud load is kept in the mast, it is robust enough to cope with kite loads. But unless carefully set up the diamonds can move from side to side slightly as the mast bends sideways, It is difficult to setup the tension repeatable if you have a lashing and adjustment is heavy. Is there an effect in leaving the mast bent when not in use?

I think I prefer 49er style but most boats seem to have diamonds, what am I missing how do they control the rig dynamic better?

[Will_Lee]

I originally drew Atum's rig with the uppers going to the shroudpoints, but Alex Vallings said that there's not much difference in performance, but it is easier to rig so I decided to do it that way initially... and never changed it!

I imagine that it is easier to make adjustable uppers when they go back to the mast Aqua style.

Truly i don't think there is much to choose between them.

[craig_guthrie]

Hi Phil,

Interesting thoughts.

I think I prefer 49er style but most boats seem to have diamonds, what am I missing how do they control the rig dynamic better?

I think spreader angle (sweep) and length of spreader has something to do with this. Another thing that comes to mind for me is the distance between the spreaders relative to compression on the tube between these two points...which leads me to think about the shroud base width...narrower...more tension required...aka Open 60's with wide spreaders and shroud base vs...no shrouds and an extra wide shroud base with deck spreaders...there has got to be something in between.

Caps as diamonds through the top spreaders on a turnbuckle at the mast base seems like the best way to influence the side bend issue. I like the thought of travelling through the lower spreaders as well with slight deflection FORWARD of the straight line of the cap shroud as well as OUTBOARD from the straight line of load.  Forward redirection/management of the load by going through the lower spreaders would help with side bend as well.

Mast dynamics of sidebend is three fold with managing kite load, crew on the wire both up and down hill and then the side bend of the mast for depowering in gust response/ opening the distance between the jib leech and main luff. As I think more about this, I also am thinking about the angle which the crew will be hanging from the mast in relation to the mast movement...Rudder foils bringing us back futher from rig...putting more aft pressure, influencing bend from almost shear load instead of compression...Makes me like the idea of travelling through the lower spreaders as well with slight deflection FORWARD of the straight line of the cap shroud as well as OUTBOARD from the straight line of load.  Forward redirection/management of the load by going through the lower spreaders...as I mention above.

Of to do chores and dream of the build project which will start at hopefully the end of August!!!
Craig

[kevin_ellway]

Hi all

This is a common question. My take on it is this.

1) The caps perform 3 basic functions
a) To prebend the top mast
b) To inhibit side layoff of the mast tip
c) To prevent mast inversion.

2) The 49er layout does a) and b), but not c). This is one of the reasons for the 49er mast breakages.

3) The Vallings layout is a better engineering solution. You have D1 to the gooseneck, D2s to the lower spreader shrouds and caps which run from the base of the mast (or gooseneck), around the ends of both sets of spreaders to the mast tip.
Here, the caps predbend the tip and inhibit side bend, until the cunnigham load overbends the prebend. The caps around the lower spreaders try to push the mast forward, but this is constrained by the D2s. The mast is effectively locked solid at the location of the lower spreader set and the thrust from the caps prevents inversion.

The net result of this set up is that the buckling load limit for the mast is increased because it is fully supported at gooseneck level, lower spreader level and at the hounds. This means you can use a lighter section or more rig load. The mast will never invert. The mast is really simple to set up. Just make the section below the hounds straight and add about 5% prebend in the tip.

4) As an example of the above, we were sailing the SK4 with Dylan Fletcher's 49er. We both had to flap the kites to get around an obstacle (a keelboat). The 49er mast flexed horribly and flapped around. The SK4 mast, hardly moved.

Most importantly of all is that the mainsail matches the mast. Fyfe sails match Vallings rigs. Standard Hydes, for example, have more low down luff round and less upper round, so do not suit this type of mast. With this style of sail, more low down bend may be required, so the 49er style rigging may be better. So how you stay your mast will depend upon the mast type you have and the sails you have.

Hope this is of some use.

[craig_guthrie]

I think the 49er issue stems from using two different materials in the rig.
Matching sails with rigs is key as Kevin suggests. I hope to go with a combo like C-Tech and Fyfe.

Questions:
What material is being used for the D1's and D2's? Diform wire? Vectran or Spectra and what size?
Are these led to adjustable shroud plates?
Considering that we are adding to increased loads passed to the boat, do you suggest an increase in the laminate in the shroud base, mast post and deck frames area?
Craig

[neal_gibson]

my caps are adjustable rs800 style

with diamonds and a 4:1 adjuster at the base of the mast the caps go through both spreeaders also
all i feel mine needs is a sea sure loop mounted on the mast to keep the caps central and it'll be perfect

[ghislain_devouthon]

My caps are 49er stylish.

I'm not really happy whith them.

I don't really now weither it is due to an over soft c-tech topmast or to the caps themselves.

It's hard to rig as the mast top bend is really depending on the forestay tension which is far less the case with a diamond scheme.

The side bend seems also to be far too pronounced which means I'm often lacking some power in medium. NBS will be probably reriged before the national.

[Will_Lee]

Atum's Caps are 2mm 1x19 wire

The other wires are 2.5mm 1x19  I think.

Antidote's caps, lowers and D2s are 2mm PBO and the shrouds and forestay are 2.5mm 1x19. 

[Phil Alderson]

Thinking about it the D2's are key to c-tech-fyfe setup together with stiff strong spreaders.

I will be using a gnav and will have the lowers going to the top gooseneck, so closer to the lower spreaders than with std lowers but not D2's.

I think I need to put the sail on the mast and see how it looks to work out the bend plan. I can always spec the length for the caps to keep my options open.

[Will_Lee]

Something that has appealed to me for a while has been D2's to the top of the gnav, and no lowers: Good foredeck ergonomics, good underboom ergonomics and a pair of wires saved.

[phil_kirk]

Pocket Rocket has no D2's but the lowers go upto the  top of the GNAV and hence are nearer the D2 position.

From an engineering point of view it is best to lock the mast with D2's at the lower spreader root.

SLippery's mast has 3mm dyform main shrouds and 2.5mm 1x19mm caps, D2's and lowers.  The D2's and lowers are both lashed to the mast with dynema. Tough as old boots.

I would support replacing all but the main shrouds and forestay with dynema or vectran/v12.

While the rig may be infinitely strong with all this support the hull has got to take all these loads.

[kevin_ellway]

Gnavs introduce another type of problem. If you put the caps through the lower spreaders then the mast is pushed forwards at some distance above the position of the D2s to the higher gooseneck. At the same time, when you tighten the gnav, the boom pulls backwards at the lower gooseneck. You now have a bending couple and it is easy to get an S bend about the upper gooseneck.

Like anything else in sailing, it's all about where you want to compromise. Anything other than the Valling's set up, which is essentially what has been used on yachts for years,  will mean less mast stiffness  since Euler buckling load is proportional to length cubed (I think)), Missing off some wire (i.e. like the RS800 set up) or adding a gnav can create a bit more space in the boat. Wires, especially pbo, spectra etc, don't weigh much though and the windage is unlikely to be significant. So it's a question of space versus engineering in my book.

[tim_unerman]

When I lengthened the mast on Pocket Rocket I thought about adding more support at either the spreaders of at boom height.
 In the end I decided it did not need any more support due to the main lowers being quite close to the spreaders and thus had significantly reduced the unsupported length.
Also during the extending process I stiffened up the lower mast quite a bit, up to boom level which also helps notably in increasing the critical compressive force.

I think if I was starting from scratch I might go down the Shiny Beast method of having a high angle boom.

[ross_burkin]

High angle booms are horrible in light airs! Crewing Shiney at Weston was...uncomfortable. Being able to sail the boat comfortably will make you go a hell of a lot faster than deciding wether you have/don't have some wires.

[Phil Alderson]

Having a quick read on euler buckling.

http://en.wikipedia.org/wiki/Buckling

The interesting thing is not just the effect of the length on buckling but also the effect of the end support. i.e. fixed or able to rotate.

if the longest un-supported section of the mast is from the gooseneck to the lower spreaders/D2's. The top of this section is effectively fixed (held in column by the rest of the mast above spreaders)  and with a deck stepped mast the bottom is also fixed( held in column by the lowers and the mast below the lowers, however with a mast sitting on a stump to the gooseneck the base of the mast is effectively pinned and able to rotate so the unsupported length is multiplied by a factor of 0.707 rather than 0.5 before being squared.