The other day we did a 30 mile reach down the lake. Winds I think, were around 20 mph, solid whitecaps, but as the breeze was offshore waves were only around 1 ft, with the odd 2 footer.
I had my wife (125lb) & buddy (175 lb) on the wire, while I sat bracing a foot against the traveler casting. Occaisionaly we had a hull fully airborne, but mostly the windward hull was just skimming. We were fully powered & really flying, top speed hit 21 mph. I had the main cleated, & other than a few steering inputs to control gusts we drove it hard in this configuration for 15 gps miles.
About 1/2 way I heard a definite "creak", you know, the kind you hear when a line gets pulled to near break point. I chalked it up to transient strains, & as everything seemed fine we kept the speed up. I changed course slightly to clear the reef at the end of my bay & headed for home, still at full power. It was the fastest I've ever seen this 5.7 move. I heard another of the same "creaks", & they were different than the normal sounds this boat makes.
Upon placing the boat on the lift, I noticed the DS rod at a different angle. It did not bend, but appears the entire front beam rotated, (mast ball went towards the bows) The curved block that bolts inside the beam is 1/4" off the tramp track now, that had to be the noises i heard.
I had all straps at the spec 18 ft lbs, don't know what the internal beam bolts were. I have driven the 5.7 in much rougher water, at times so rough we had trouble staying on the boat, or keeping it upright. This was the most consistent wind I have ever seen, no real gust loads, & fairly flat water. The only thing I can think of was not enough tension in the shrouds, & that allowed the mast to move forward under load, transfer load to DS ball, & rotate the entire beam.
Any thoughts, ideas, recommendations? I'd hate to think that I can't pull that much power, cause man, what a ride!

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Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
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Interesting. I don't think the rig tension would have anything to do with it. The mast ball free rotates for and aft so there isn't a direct connection to the beam. Originally they had the center bar to prevent that and about 1984 they put a casting and internal beam bolts in. Those could have been too loose or could have pulled out due to some weakness. I would take the mast down and check that.

In my boat there is no internal beam bolts (before they started doing that). So the trick is to rivet the beam straps to the beam. Even though you have the internal bolts I think that is still a viable option to prevent rotation.

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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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When my 5.2 had similar inclination to shift it's mast a bit, it just bent the DS rod right over. I've put pics on here somewhere.

When it happened I heard similar creaks, although they must have come from different sources. Interesting.

The 5.2 has stainless rivets through the straps into the main beam and that was enough to hold it still. You should pop the beam caps off and see (if you don't already know -- I don't) if you have the chocks with bolts through them to hold the beam in place. If so, doesn't that mean you did some damage? My 5.5 uses the chocks but I've never looked closely at them, as I haven't broken anything major yet.

I don't know the 5.7s at all, but I'd figure out what happened and fix it (the root cause) before sailing it again. I seriously doubt its rig tension. Actually, the tighter the rig, the more the mast will want to do exactly what it did anyway.

--
Rob
OKC
Pile of Nacra parts..
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Quote

You should pop the beam caps off and see (if you don't already know -- I don't) if you have the chocks with bolts through them to hold the beam in place. If so, doesn't that mean you did some damage?

I took this boat to pieces when I first bought it, & again last winter for storage in the garage. It has the chocks. I placed witness marks inside the beam when I assembled it. The actual beam rotated, inside the straps, & chocks. The hole that is drilled in the beam to accept the chock bolts is quite large, you can rotate the beam probably 15* & still put the bolt in. When I set it up I oriented the DS rod vertical, then tightened things down. May be i needed more torque on the chock bolts. Looking into the beam, from the left side, the beam cannot rotate clockwise, as the chocks would run up against the track for the tramp. You would have to shear off the tramp track to rotate it. Going the other way, it is only friction, & the limits of the hole moving against the bolt that prevent rotation. I'm thinking the hole that accepts the chock bolt is purposely large so the factory never had to worry ab0ut critical tolerances.
When I look at a rigged boat, & analyse the stresses it is force on the lower 1/2 of the sail that transfers to the mast, & the mast tries to push the DS rod forward, especially with the sail let out,(downwind).
Nothing is bent or damaged, so I'm gong to drop the mast, realign & tighten everything again, maybe a little more this time.
I need to drop the mast anyway, as i'm changing the jib line to Syderline, changing out one trap line as there are now 2 broken strands,(going to try hi tech line) & I have to make a minor change to how my roller furling hooks up the drum & forestay.
Tough day today, went kayaking twice, had the girls out on the Invitation for an hour, plus I took it for a run, & sailed the Cat twice...barely time for cold cervezas in the heat.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

ed

The hole that is drilled in the beam to accept the chock bolts is quite large, you can rotate the beam probably 15* & still put the bolt in.

You just condemned me to taking my 5.5 apart before properly breaking anything. The DS is clearly rotated a few degrees the opposite direction as yours.

Quote

May be i needed more torque on the chock bolts. Looking into the beam, from the left side, the beam cannot rotate clockwise, as the chocks would run up against the track for the tramp. You would have to shear off the tramp track to rotate it. Going the other way, it is only friction, & the limits of the hole moving against the bolt that prevent rotation.

Don't overtighten; you'll crush the glass. It might be a good idea to sink an SS rivet into 1 beam strap per side a la the 5.2, although hopefully a Nacra vet will chime in.

--
Rob
OKC
Pile of Nacra parts..
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Quote

The 5.2 has stainless rivets through the straps into the main beam and that was enough to hold it still.

I think that was a previous owners mod, I think thye all came with the internal beam chocks. A chap who competed at high levels told me even on a new boat he riveted the straps, & tossed the chocks...not to prevent rotation, but "saves weight".

Quote

The DS is clearly rotated a few degrees the opposite direction as yours.

If you rotate the beam so the chock is against the tramp track, it will lean the DS the other way. As said the hole the bolt goes through is around 3/4".
The DS is out of focus in this photo, but you can see from the witness marks the movement that occured.

Here you can clearly see that the beam rotated counter clockwise until the edge of the hole came against the chock bolt. I don't know if the rotational force caused the deformation at the edge of the hole, or it was caused by flexing of the entire assembly during the last hour of sailing. The assembly manual makes no mention of the DS rod being exactly vertical, only to "rotate it to line up the hole", this leaves about 15* to play with.




Edited by Edchris177 on Jul 10, 2011 - 09:02 AM.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

Quote

I think thye all came with the internal beam chocks.

Neg.

I'm gonna re-align mine and rivet it. those holes are huge.

--
Rob
OKC
Pile of Nacra parts..
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