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  #61  
Old 03-16-2008, 05:26 PM
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MAINSTAY MAINSTAY is offline
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Brian asks:
Quote:
I'm sorry, I do not see enough support (backstay, or aft lead shroud) to keep good tension in the forestay. Am I missing it?
The tension on the forestay is maintained by the mainluff stay, which acts like a backstay. See attached files.

As with a backstay, the relative tensions are directly proportional the the angle of each stay from the mast.

In a sloop rig the angles, and therefore the tensions, are equal when the mast is at 50% of the distance between forestay and backstay.

In this rig, the tensions and angles are equal when the mast is around 55% to 65% of the distance from forestay to mainluff stay, depending on the rake of the mast.
Attached Files
File Type: xls Boat2Tension.xls (19.5 KB, 259 views)
File Type: doc Pix6.doc (207.5 KB, 249 views)
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  #62  
Old 03-20-2008, 09:15 AM
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Quote:
Originally Posted by MAINSTAY View Post

In this rig, the tensions and angles are equal when the mast is around 55% to 65% of the distance from forestay to mainluff stay, depending on the rake of the mast.
What effect does tacking have on the mainluff wire tension?
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  #63  
Old 03-21-2008, 10:08 AM
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Spiv Spiv is offline
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SMG 50plus

Quote:
Originally Posted by brian eiland View Post
Hello Stefano,
Are they still building this vessel you posted photos of?? Isn't it about the size vessel you were seeking?
Hi Brian,
yes the SMG 50plus is very close to the kind of boat I want to go and live on.
They are being built is S.Africa, apparently they have orders. The basic boat is U$925,000, pricelist attached.
Perhaps if I had that kind of money I would consider it, but I am sure I can get a similar or better (for me) boat built in Thailand cheaper. As a matter of fact, I could get it built in Western Australia cheaper than that.
Additionally, I think you and I can develop a better rig than that...
Attached Files
File Type: xls SMG50-2008--Price list and Technical specification.xls (185.5 KB, 472 views)
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Last edited by Spiv : 03-21-2008 at 10:11 AM. Reason: wrong attachment
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  #64  
Old 03-21-2008, 10:54 AM
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Spiv Spiv is offline
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Catenary arch mast.

Pericles said:
Quote:
Rather than an "A" frame mast why not give consideration to a catenary arch mast?
I suppose that is could be a development of the 'Wishbone' mast we have been discussing above.
Made in Carbon fibre and with Spectra or Dyneema connecting the two spars for rigidity, it would make a very light and slim rig.
Hinged aft and raked forward it would cause no interference with the sails.
It could be lowered in seconds, and it will not protrude forward too much.
Just the mast I am dreaming about...
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  #65  
Old 03-23-2008, 06:14 AM
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Pericles Pericles is offline
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Well, as the catenary shaped "A" frame mast, has received some approval, I may as well reveal another secret incorporated into my cruising catamaran design. http://en.wikipedia.org/wiki/Catenary

The catenary shape is also superb if used as the cross section outline for the catamaran hulls. Assume for a moment the vessel is floating correctly to her lines. As the vessel is loaded more and more and further immersed into the water, the wider shape of the arch resists immersion and improves the buoyancy. This happens whichever profile is chosen for the hulls, fat or thin. This shape will deliver the minimum wetted surface, par excellence.

Using the characteristic shape throughout from slim, vertical stem bow to wide, weight carrying stern also increases the internal volumes within the hulls without the ugly bulges that I have seen in some catamarans.

This brings me neatly to the almost ideal method of constructing such hulls, which has nothing to do with me. The credit goes to Kurt Hughes and his Cylinder Mold method.

http://www.multihulldesigns.com/pdf/cm33.pdf

http://www.multihulldesigns.com/

Kurt is very candid about his process. "Every building system has
limitations. CM is no exception.There are some restrictions on the possible hull shapes. To keep building time down, CM hulls should be simple curve
sections.
Flares, steps, and hollows are not easily possible. These hulls must have slenderness ratios 10:1 or thinner at the waterline if they have reasonable amount of rocker in the profile. Within these constraints however, virtually any hull shape is possible."

Those shapes highlighted in red are NOT used in catenary shaped hulls. The upmteen panels used to create the molds will be catenary shaped and the flat surface profiles are patterned to ensure the port and starboard halves of each hull mate accurately together along the keel line and open up as the correct shape. There having been much discussion elsewhere on this forum about the benefits of rocker, I submit that the least amount be used, to minimise hobby horsing, whilst retaining fast tacking ability.

Comments welcome please,

Pericles
Attached Thumbnails
Main-less rig-catenary-pm.png  
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  #66  
Old 03-23-2008, 10:15 AM
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Catenary Hull shape

Very, very interesting idea Pericles.
Why don't you start a new thread, I will gladly subscribe to it and we should get a lot of interst and hopefully good ideas will develop.
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  #67  
Old 03-23-2008, 03:51 PM
masalai masalai is offline
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Pericles, Certainly Kurt Hughes produces a sweet and fair underwater hull but topsides - Yeuch... & I am not sure I like the aft sections on the powercats for long distance cruising efficiency... But them's my perception - as with most designs it is a "personal thing" else there would be just one efficient design and no need for other views or architects.... Viva la difference!
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  #68  
Old 03-31-2008, 05:30 AM
sigurd sigurd is offline
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mainless cat

what are the little strakes, or what to call them, in the bows?

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  #69  
Old 03-31-2008, 05:54 AM
masalai masalai is offline
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Sigurd, I am only guessing, to deflect water? You will see different solutions on many very skinny hulled powercats, such as the maincat, Malcolm Tennants designs, Kanga Bartles, Robin Chamberlin & other power cats use a full length "rail"... My model was supposed to have one but lost most during "fairing" and build process by the model-maker... Have a look at my photo galery for the Chamberlin C-10 metre version which I nearly bought but I wanted significantly more (fuel tank) range... If you look at the model video and from bow on you can see the water flowing up the hull and under the bridgedeck to fall off around midships. I am guessing this is the reason - but I could be VERY wrong... as another thought - to house a mechanism to furl the sails remotely... Or to provide the engineering to facilitate electric or power furling... Looking again at the image I am inclined to figure on the latter as the hulls are VERY slender. . . That is John Hitche's design & build called "X-IT"
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  #70  
Old 03-31-2008, 06:58 PM
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Quote:
Originally Posted by masalai View Post
Sigurd, I am only guessing, to deflect water?
That's it
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  #71  
Old 04-01-2008, 06:30 AM
sigurd sigurd is offline
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Apparently John Hitch is the correct spelling.

You know whether it works well? I thought it looks like it will throw some spray because of the steep angle.
Is it ment to give any appreciable lift, or just break up green water coming back towards the bridgedeck.

I think it is nice with those skinny hulls and also the faired shape of the bridgedeck seems nice.
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  #72  
Old 04-01-2008, 04:56 PM
masalai masalai is offline
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Main-less rig and post 21 and also at http://www.boatdesign.net/forums/sho...146#post185146 otherwise, unless someone on land knows X-IT very well, you may just have to find him out cruising somewhere...
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  #73  
Old 04-04-2008, 11:22 PM
Triroo Triroo is offline
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Here are a couple of photos of X-IT going to windward in light airs.
Attached Thumbnails
Main-less rig-p1010078_7.jpg  Main-less rig-p1010079_7.jpg  Main-less rig-p1010076_9.jpg  

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  #74  
Old 04-05-2008, 07:29 AM
sigurd sigurd is offline
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It looks like the bows should have been pointier, or what do you think.
I'm thinking about the small, short spray "bow-wave" or whatever you'd call it.
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  #75  
Old 04-07-2008, 10:09 PM
masalai masalai is offline
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Triroo, Thank you very much. I admire his work, If you know how to contact him and John Hitch is amenable, please PM me
Brian
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