Nacra 5.0 Crossbar Attachment

The setup you describe is correct. One set, on main beam, not rear. Keeps beam from rotating.

My 84 Nacra does not have beam straps and has the two threaded studs that go through the beam (4 each beam). They then have the casting on each threaded stud with a nut to secure them.

Thanks Kevin. My boat is an '83 and that is what my second photo is of above, looking in from the end of the front crossbar. I bought an '84 parts boat that was as you describe, 4 studs with 4 castings per crossbar and that's what prompted my inquiry.

Yeah they only did the 4 casting thing for a short time. Most of them were straps and 1 casting or only straps and a center beam.

leeboweffect wrote: In preparation for installing my new tramp it appears as though the front crossbar must be unbolted to thread it. Looking things over, I noticed that the crossbar attachment arrangement employs a an internal casting to distribute the load:

It seemed odd to me that the casting was present only on the two outer bolts (both front and rear crossbeams) and not the two inner bolts:

Is this the OEM setup or should there be castings on both the inner and outer bolts?

Thanks in advance.

My old hulls have only one stud per side, in the front and back. Later boats didn't have studs at all - they had a threaded hole (threaded brass chip imbedded in the fiberglass) and loose bolt that was installed. My studs were too long for factory castings (my studs did not look factory installed either), so I just used large fender washers, pre bent into a curve the same as the beam tube. Jack at Nacra, and others here, have said the castings are not necessary. Get the hulls aligned and tighten beam straps, then pop a couple SS rivets through each beam strap, into the beam. And note that there is very little, if any rotation force on the front beam (the mast sits on a smooth ball after all). It's mostly the rear beam that wants to rotate because of the tension of the main sheet pulling at an angle.

Thanks Tom. The forces acting on the beams that could cause rotation had not occurred to me. Good info..

And note that there is very little, if any rotation force on the front beam (the mast sits on a smooth ball after all). It's mostly the rear beam that wants to rotate because of the tension of the main sheet pulling at an angle.

That is not correct. the forces on the sail want to rotate the front beam. The mast ball is not flush with the beam, it sits up, creating a lever arm.
The mast is free to rotate on the ball,(vertical plane) in fact it often unscrews & rescrews the ball slightly, but in the horizontal plane it definitely wants to rotate that beam.
Prevention of beam rotation is THE reason the older 5.2's came with that centre post that went from front to rear beam. to save weight racers often tossed the beam chocks & put a couple of rivets in the front straps.
As for the rear beam, the sheet should not pull at an angle. On the boomless boats the beam is installed with the traveler track slightly forward, aligned with the sails clewplate. Boomed rigs angle slightly rearward. This means there is little rotational force.
Here is a photo of my 5.7 beam rotating
http://www.thebeachcats.com/pictures?g2_itemId=80922
http://www.thebeachcats.com/pictures?g2_itemId=80925

Good point Ed concerning the front crossbar and rotation from the forces generated by the sails. If you use an internal beam casting, with a centering boss, there would be no rotation:

Leeboweffect does not have the hulls with the straps therefore he cannot tighten the straps and place stainless rivets into the straps to prevent rotation. His boat has just the internal beam castings with one casting containing the centering boss on each side of the beams.

I do not have personal experience with the setups Lee has, both mine use straps. I think Kevin is right, in your case you need 1 centering chock on each side.
I hope you can find them...I only have the regular ones.
Get that boat in the water!
Given a choice, I prefer the 5.7, because of the versatility, but the 5.0 is a soloist dream.

On my N5.2, I want to remove the center post, I have straps holding the crossbeams, the question is:- will rivets hold the beam in place? I'm thinking that the loads placed on the beam could shear the rivets and had planned to use 1/4" SS carriage bolts through the strap and into the beam, 2 bolts per strap - or just go with the internal beam casting.

thoughts?

will rivets hold the beam in place?

Yes, lots of people have done it that way, two SS rivets/strap

Thanks Ed. I agree, one internal beam casting with centering boss per side of both beams (outermost position). Then one "regular" no boss internal beam casting per side of both beams (innermost position) should be the best arrangement. This solves all hull alignment and crossbar rotation concerns.