Understay for forestay support

hmmm...it does appear that the AC45's carried the pole to the rear beam like you describe. They don't even have sheet loads to contend with...I'll have to think about what you're building and wrap my head around the benefits of that. My intial thought is that it is primarily just to help transmit the extra headsail loads that come with flying a straight luff head sail but that it doesn't have much of anything to do with carrying mast loads.

No Y structure. your pic is of an AC72, check out the AC45

http://3.bp.blogspot.com/_83C0d9qWNoU/TTSxNJ9cjCI/AAAAAAAAbsk/FsHxcMc4g3k/s1600/AC45NZ1D6_3772.jpg

and GC32

http://2.bp.blogspot.com/-Z5e2BSoHy... /gmIks7OFvFc/s1600/GC32AustriaCup189.jpg

Why do these boats have a central cross member?

you said the AC45 and AC72 used understays to

make the entire platform more rigid. It helped carry the headsail loads

-right, that's what i hope to do. All the headsail and forstay loads will be carried by the understay. My alternative if i wanted to keep my NA jib on a furler with an 11.5 foot beam was to build an 8 foot wide bowfoil, which seemed absurd, and was a very heavy proposition.

Because of the severe angle of the understay, for every 1 lb of downward force, i will have 4.88 lbs of rearward force. that means i will have between 700-800 pounds pushing the pole directly back (this is isolated rearward force not total force) do you think i can have that force just dead end into the center of the front cross beam?

As for synthetic rigging, i wouldn't dream of it. i have my 1x19 cable ready to go <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

oops didnt see your reply!

From a metallurgical point of view, I'm curious as to what grade of aluminum is being used for the cross beams or if that was even considered. Torsional loads and tensile loads increasing with the increased beam width.

Aluminum 4 OD X .125 WALL (3.750 ID) 6061-T6

a good deal thicker than stock tubes, which i measured at just under .08 wall.

I couldn't find confirmation of what grade Nacra uses, but considering the marine application, (corrosive resistance essential) and the fact that the integral kedar rail is welded in (machinability/weldability essential), i have a hard time believing they use a 7XXX series alloy which is stronger than my grade. -Most likely they used a 6-series T6 like i am. of course 6061 is the stronger end of the 6-series spectrum.

-I would've liked to go a couple hundredths thicker but it was .125 or .250, no in between. .25 would have been just stupid.

I did do my deflection rate calcs on the beams and i believe it looks ok. That being said, as a matter of principle, i will not be having 2 guys on the wire, ever, and this whole thing is designed as a singlehander, meaning just my 200 lbs to contend with.

I would be very surprised if the nacra beams use anything better than a 6061.

I think I'm with you now...Is your forestay pinning directly to the pole or will you have bridles?

Is your forestay pinning directly to the pole or will you have bridles?

Neither. It will attach directly to the understay via a piece i am machining. The piece will pass through the center of the pole vertically. pics to follow.

Ok, so instead of letting angled bridles carry the forestay (and translate that into inward and upward forces on the hulls), you are dropping it to this structure under the pole and carrying the majority of that forestay load with what is essentially a truss style lever arm supported between the two beams. I understand and that makes sense.

exactly. Only it will be ALL the forestay loads, not just the majority.

Actually, because of the extreme width, i could run all the side to side cables from the front beam eyes i installed. I have half a mind to not use the forward chainplates at all.

this guy didn't:

http://www.boatdesign.net/forums/at... cra-20-lifting-foils-mini-hydroptere.jpg

Brilliant!

i have a question for the forum. I have a F18 spinnaker ready to go for this boat as well. Do you think i need to add another set of diamond stays like they have on the 6.0 to supports the rig? I heard stories of a 5.8 rig bending severely under spin loads, but that was in heavy air double trapped.

I'm wondering if running this as a singlehander will reduce the mast loads due to lower weight? i really don't want to add the second set of diamonds if i dont have to. thoughts????? (i also dont want to lose a rig tho haha)

Update:

the understay sytem is completed, and very functional. Lifting on the forestay attachment point lifts both hulls in sync. i ended up using dyneema sized for stretch, to contend with the huge loads the angle of the understay will produce.

The pole is supported with adjustable cables (1/8"), the pole spreader is fabricated, and also quite functional for pole end support of the chute.

I have the new tramp from SLO Sail in Canvas on (GREAT experience)

I have the SNU snuffer mocked up, but not in its final mounting place (Dennis of barefoot studios was awesome to deal with as well) The spin. retraction system is ready to go too.

I have my furler mounted, i have the spin halyard system finished. I have a automatic furling system in the works as well. Basically when the spin halyard automatically brings the spin tack to the end of the pole, it will also furl my jib. (to make things easier to singlehand downwind) I have it set up in such a way as to be detachable so im not obligated to run them together. will it work? not positive, but we'll see.

Still need to finish the traveler, the new tillers, the boom and main sheeting system and the traps, and then a thousand little details....

Final numbers on SA are about 46 M^2 with all sails flying. 25 M^2 with main and jib. The squaretop main is being built by Whirlwind now.