Took our I20 mast down yesterday and noticed the main beam has compression damage around the top dolphin striker joint, right at the edge of what looks like a stainless steel reinforcement base plate (I don't think the base plate is stock based on the old Nacra manuals). Specifically a ~1.5'' crack, around which the aluminum is a bit depressed.

Pics:



The top of the dolphin striker hole in the beam also looks quite enlarged. There was some play in the dolphin striker when we pushed it fore/aft by hand, which could be due to the hole being enlarged. Not sure in what shape the compression sleeve is (assuming there is one). Bottom of beam looks fine.

Any thoughts on how bad this is? Have you seen this on other Inters?

How about repair options?
- Bigger base plate
- Weld back and add bigger base plate
- New beam (not sure even how to find one)



Edited by southstars2012 on May 01, 2017 - 12:05 AM.

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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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The movement is a problem and needs to be address. The big loads are on the bottom of the rod and transfers to the bar and their beam attachments. This would not have happened if the DS had some prebend (3/8" upward).

There are beams out there. I know of a few. Also, give Kirk or Charlie at Key Sailing a call. They probably have a couple. If it were me I would replace it.

There is alot of aft forces on the beam coming from the spin pole and forces from jib sheets, etc. to consider also. I have been on a boat when it folded up to catastrophic failure when the beam let go. You don't want to go down that rabbit hole.

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Philip
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In my opinion that damage is the result of not having a compression sleeve in there. Looks like the beam was sandwhiched around the DS. You NEED that sleeve in there. Install one and the beam should never be loaded like that again.

The elongated holes are an issue. That can be repaired a number of ways. A custom machined compression sleeve could probably solve that problem.

That crack needs to be stop drilled.

While the damage is bad, I think the beam can still be safely used with some repairs. Though a replacement beam may be easier if you can find one locally. That is until you break off the SS beam bolts, that would be worse to fix IMO.

You certainly should not keep using it as it is.



Edited by bacho on Apr 30, 2017 - 04:08 PM.

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Greenville SC

Offering sails and other go fast parts for A-class catamarans
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Could someone review the procedure for setting pre-bend in the beam? I have not had this damage but would like to go through my beam tension to avoid any possibility of it.



Edited by tominpa on Apr 30, 2017 - 10:23 PM.

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Tom
NACRA 5.7 (1984 Sail 181)
Pennsylvania
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My I20 (2006) had a 1x1 inch square compression post around the dolphin striker bar. The dolphin striker bar hole was not drilled straight up and down by the manufacturer (Performance Catamarans). Consequently, the dolphin striker bar leaned forward. One corner of the compression post appeared to place more pressure on the bottom inside of the beam than did the others. A crack developed from the dolphin striker hole to just past that corner of the compression post on the underside of the beam. The new boats use something a compression post for the striker bar that looks very similar to the beam compression posts.



Edited by traphappy on Apr 30, 2017 - 11:37 PM.

tominpa

Could someone review the procedure for setting per-bend in the beam? I have not had this damage but would like to go through my beam tension to avoid any possibility of it.

You need to create a reference first, straight edge or a string across the beam. The bottom nut on the DS rod is tightened up into the beam until you start seeing deflection of the beam. In this case your looking for the center of the beam to be deflected .375" in the up direction relative to the ends of the beam. This is to ensure the load goes to the DS system and not letting the beam invert. So on this boat in the thread, the load was placed on the top and bottom of the beam. The dent on the bottom of the beam is from the beam getting pre-bent, but there was no proper sleeve in the beam, leaving all of that load on the skin of the beam. The denting on the top is the same issue.


On a carbon beam, any detectable deflection will work. You may not even get the beam to deflect.

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Greenville SC

Offering sails and other go fast parts for A-class catamarans
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I'm almost positive the external beam shape of he Nacra 20 is the same as the Nacra Infusion. I've seen this damage before, bigger plates are a solution but I recommend replacing the beam if it is at all suspect. I would go for a 2017 beam (if they fit), they have been reinforced to handle full foiling loads and will be much stronger than your current beams. Btw, Nacra recommends 15mm, which is more like 9/16" of prebend, though the 3/8" recommended here is more achievable without stripping the dolphin striker post.

Thanks Bacho.

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Tom
NACRA 5.7 (1984 Sail 181)
Pennsylvania
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I can confirm that the infusion crossbars fit the I20.

Thanks everyone for chiming in - lots of food for thought. I agree from the outside it looks like the compression sleeve inside is missing (only way to explain this much of a crack) -- I'll need to fully disassemble and then take a look inside the beam to confirm.

Regarding pre-bend deflection, this is measured with the beam off the boat correct? (i.e. just with the main beam & DS assembly including DS support beams). Currently the beam is attached to the hulls, so I imagine measuring prebend as currently assembled wouldn't tell me much.

It sounds like we're in for a major disassembly job, which I was hoping to avoid (no idea what shape the SS beam bolts and threads are for example, though I guess this is a good time to replace them / rethread).

Please PM me if you know of any Infusion or I20 beams on the West Coast (closer to San Francisco if possible). I'll also ask folks at MBYC in San Diego. Thank you!

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SL
Nacra Inter 20 (sold)
2017 Race to Alaska "Team Ketch me if u can"
- Facebook: https://www.facebook.com/TeamKetch/
- Race video highlights: https://www.youtube.com/watch?v=YTWp4DP0VcA
Sausalito CA
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It sure does not look like there is a sleeve inside the beam, unless as Trapphappy noted, it is a square tube. With the hole that elongated, the end of the tube should be visible.

Quote

Could someone review the procedure for setting pre-bend in the beam?

Tom, you are looking for 3/8" for your 5.7. Go here, & look at the last photo & explanation. It's very easy.
https://www.thebeachcats.…pictures?g2_itemId=73301

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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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southstars2012

Regarding pre-bend deflection, this is measured with the beam off the boat correct? (i.e. just with the main beam & DS assembly including DS support beams). Currently the beam is attached to the hulls, so I imagine measuring prebend as currently assembled wouldn't tell me much.

No. Loosen the lower nut to remove any prebend, then turn the nut up until it touches the beam, then simply adjust it up another 3/8". Done.

Find yourself a replacement beam . . .

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Philip
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Thanks Ed. Pretty much what I thought.

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Tom
NACRA 5.7 (1984 Sail 181)
Pennsylvania
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