rotating mast stay attachment

[bob the builder]

hi all


need thoughts from you all.

want a rotating wing mast on my catamaran.

going to use this section with a single diamond.



20 x 9.5 cm, 3.8kg/m, Ix 1.4, Iy 6 (10^6mm^4)

3 x 8mm wire stays, 50m2 sail area. 13m long, boom attaches 2.1m above the deck

http://www.allyachtspars.com.au/catalogue/A1_Masts.pdf




going to sit the base on a tapered roller bearing in custom housing.

the point is to make a FREELY rotating mast (no mast spanners etc, may even drop the gooseneck and use some sort of vice, so when the boom moves it turns the mast as well.)




the question is the attachment system for the 3 stays

i'd like something that lets the wind turn the mast easily.


don't mind going custom. just whatever works.


ideas anybody?

onya,
mal.

[farjoe]

My rotating 10m mast has a bearing placed at about 1/3 the distance from front to back. In theory the further forward you can place it the easier it will rotate.

The take off for the shrouds is a single point positioned at the front of the mast.

[bob the builder]

thanks farjoe.

whats the single point look like? ball tang?

does it rotate easily? or do you have to do it yourself? how much sail are you carrying?

mal.

[gggGuest]

Originally Posted by bob the builder

the point is to make a FREELY rotating mast (no mast spanners etc, may even drop the gooseneck and use some sort of vice, so when the boom moves it turns the mast as well.

Are youaware that such an arrangement rather defeats the object of the section? The idea of the aerofoil sections is that the flow across the leeward size of the mast is even between spar and sail, so that the mast acts as part of the sail, hence the spanner to keep the angle between mast and sail correct...

If the mast doesn't "over-rotate", as its called, then a conventional round tube is probably better.

[farjoe]

Originally Posted by bob the builder

thanks farjoe.

whats the single point look like? ball tang?

does it rotate easily? or do you have to do it yourself? how much sail are you carrying?

mal.

believe it or not it is an adapted fitting from a Hobie 16 which has been reshaped to fit my mast. The main difference is that the pop rivets have been increased in size to 6mm or whatever is the nearest Imperial size. The single point take off has been beeefed up with 2mm SS plates on either side of the hole. This was done primarily to increase the working surface of the hole which has to take a hefty 10mm shackle. The forestay, 2 shrouds and 2 running backstays all fit into this one shackle. Initially i was wary about this arrangement but it has held up for the past 13 years so it has proved itself. I have about 23 sq m mainsail area.

I don't agree with fitting the boom solidly to the mast for the reasons gggGuest mentioned. On the other hand you will need a system to control rotation but also to stop the mast from rotating unnecessarily while moored.

[Gary Baigent]

Nothing wrong with the boom being attached to mast trailing edge - and if the main sheeting system is aligned a little further forward of the boom/clew position, it helps to push the mast round to an over rotated position - which is what you are going to have to do anyway. There is no point in having a wing section that only rotates to be inline with the airflow, that is only slightly better than an old world fixed mast.

[Munter]

And for convenience, have the mast adjustment controlled via a line to the boom so that it is automatically adjusted when the boom is moved. Have a look at NS14 set-ups for ideas.

[bob the builder]

weird you should say that, i was just looking at them




fruity goodness.

points i've not come across

1/ i was not aware of forced over rotation. was expecting the mast to position itself perfectly in the direction of the apparent wind
2/ at the marina etc
3/ main sheeting system aligned a little further forward of the boom/clew position

thanks guys



"If the mast is rotated to its optimum aerodynamic position then it will be pointing approximately in the direction of the apparent wind"
http://sailns14.org/tech.aspx (Bethwaite disciples)

so, i was hoping that if the bearing is all the way foreward, then the mast would naturally set itself in the perfect position.




options so far;


1/
leave the mast to set its' own direction, leave the boom on a gooseneck, and use the traveler only.




2/
if i use mast spanners, this implies the normal ball and cup

(which i don't want to use because i've read stories about them having to be replaced after only a year or two. i want to go cruising for a decade at least without touching the mast)



3/
main sheeting system aligned a little further forward of the boom/clew position, (so as to over rotate further than the apparent wind direction)

( so, leave the mast to set its' own direction, leave the boom on a gooseneck, and have a double sheet on the main )
(i like this)



4/
use rope and cleats to hold the mast in the best position (so as to over rotate further than the apparent wind direction)



5/
set the bearing position at some percentage, say all the way forwards, so that the mast naturally aligns itself in the best aerodynamic position


6/
NS14 lever arm thingy







Attachment options (please add your thoughts!

1/ use a single 10mm shackle to attach all 3 (?(!)) stays to?


2/ hook and ring (though i haven't seen this for any larger boats, only small guys.

http://www.landenberger-onedesign.co...g/IMG_0011.jpg

http://www.landenberger-onedesign.co...g/IMG_0008.jpg

[Gary Baigent]

You need also to use a ball and cup, plus mast spanner rotation lines - in a seaway, without spanner cleated, the mast will flap, last thing you want. On ball/socket, no problem, made of wood, then epoxied with glue/glass fibre combination coating - will last for decades, wears itself in - but periodically you need to waterproof grease the bearing surfaces.

[Munter]

Unless you're going for a really long chord section I think you can assume that the aerodynamic forces onto the wing mast will be consistently overpowered by the forces applied by the gooseneck and battens meaning that you will need a way to control the mast position rather than just leaving it free to rotate. Playing with the fore/aft sheeting angle of the mainsheet may appear to help but will not act consistently across the full range of mainsail movement. I recommend sticking with a tried and tested set up such as a control spanner.

[bob the builder]

Gary
mast flapping - good call

Munter
consistently overpowered - yeah


you've both changed my thinking. thanks



what about having a lever arm/spanner thing on the bottom of the mast (say 12" long) and attach a small block and cam cleat to either side?


i guess the stay attachment is no longer important, as i was looking for a freely turning attachment. so just get anything?

[brian eiland]

Mast Tang for Rotating Mast

Many smaller cats with rotating spars simply shackled all three (forestay & two shrouds) all together onto one tang.

I preferred to provide two separate attachment holes on the mast tang. The forestay was attached to a point that was closer to the face of the mast while the shrouds attached further out. This promoted a greater leverage by the windward shroud to firmly rotate the mast, since the forestay already had a greater angle and leverage, and would resist the rotation. This two hole arrangement necessitated the forestay hole as the lower of the two holes. (got an old drawing for my Firefly tri if I can find it)

I always found it was easier to put a rotation limiter at the base of the mast, rather than a rotation inducer.
Headstay attachment on rotating mast?


If you did a 'search' on this forum for "rotating mast" you would come up with lots of useful information...ie here is one such discussion:
Rotating Wing Mast – theoretical discussion

[bob the builder]

thanks brian


.

[gonzo]

When I raced Penguins, some had rotating masts. The forestay and shrouds were all attached to the forward face of the mast. The boom had a yoke that allowed it to tilt up and down but would turn the mast which had flats on the sides.

[rwatson]

I have sailed a couple of cats, Tornado and Hobie etc that had the 3 shrouds attached to the one point on the front of the mast.

I found it hampered the mast rotation considerably, making it difficult to trim the mast.

On my next project I am taking all the shrouds/stays to the top, and organising a rotating fixture in line with the bottom pivot, to make things betterer.