I have a 1996 NACRA 5.5 with a stripped nut inside the hull. Need ideas on repair.

Details : the beams are held to the hulls using two straps at either end of beam. There is a 5/16-18 stainless bolt at either end of each strap that go down into hull. Embedded in the hull is a threaded element (~100% sure it is a nut). One of these nuts is stripped, I need to repair in order to use the boat with fear of total carnage.

This is how I did it last time on a 1987 NACRA 5.7. the nut location was rear side of the front beam :
1. Cut 4” diam hole for inspection port.
2. Remove the stripped nut : this entails digging / grinding out some fiberglass, until you can remove the old nut which is stripped.
3. Make a “washer” of some kind to spread load - I shaped a piece of teak based on copying shape from putty until it fit real snug up in the corner.
4. Use a longer bolt to attach through the strap onto the nut.
Note : Unlike all the other beam strap bolts on the boat now, to loosen this bolt now requires open inspection port, use wrench on nut when turning bolt.

Inside most NACRA hulls are bulkheads, typically one is located near the front crossbeam, the other at the rear cross-beam. While repairing the 5.7, and looking inside other NACRAs at our club, I noticed the forward bulkhead is almost always located towards the front side of the forwardd cross-beam. On the 5.7, this allowed my repair of the bolt at aft side of front cross-beam to be clear of all the extra fiberglass needed for the interior bulkhead to hull joint.

Now – here is the problem with the 5.5 repair. The location of the stripped nut is in front of the front crossbeam.
a) I really do not want to cut a 4 inch diam hole for access, in front of front beam.
b) Even if I do cut hole and gain access, I am concerned that the nut is “buried” under a *lot* of other fiberglass which creates the bulkhead to hull joint. I just think the amount of material I need to remove to get the nut out, while working with one arm, in a very small restricted space, is going to be burdensome.

Alternate approach : drill out the stripped nut (5/16-18), and tap to next larger size (prob. 3/8-16). Has anyone done this?
Pro : easy quick & simple.
Cons : torque of drilling and tapping may turn the nut inside the fiberglass, making it useless. Also, the nut relies on its top surface to create holding power against the hull when bolt is tightened but after drill & tap, the top load-bearing surface of the nut is smaller, making it less effective. And, it is the same material as original which failed, so it will fail in the same way again.

Is there any chance this threaded element which is stripped is not actually just a small nut, but a piece of stainless – like a “tab” or tang”, that will not rotate as easily and has more “meat” than a nut?

Does anyone have other ideas, and especially, experience with this repair of stripped nut on front side of forward cross beam?

Jim Casto
Austin, TX

Helicoil insert if it's only the thread that are stripped and the hole is not enlarged. Those are especially made for that. Very easy to install. Another advantage if the threads were in aluminum, the insert make those more resistant to wear.

Google helicoil.

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AB
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How about a helicoil insert?

http://www.amazon.com/Helicoil-5521-5-Coarse-Thread-Repair/dp/B0002SRDUU



Edited by peterk123 on Apr 11, 2015 - 01:27 PM.

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New Prindle 18-2 Owner
Former Prindle 18 Owner
Multiple Hobie 16s
Boylston Massachusetts
Webster Lake Indian Lake Narragnsett Bay in Rhode Island
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I just finished drilling & tapping for the larger 3/8-16 bolt. Bolt seems to be holding fine, same torque as other 5/16-18 bolts.

Update : upon inspection, the bulkhead is centered on the beam location, so the bolts located front and aft of the beam would be clear of the bulkhead, if I cut into boat to fix from inside. However, if bolt inside the beam was stripped, I would be out of luck.

I was aware of Helicoil, but chose to drill & tap for the larger bolt.
With Helicoil you either get smaller bolt afterwards (to make room for helicoil itself), or you have drilled/tapped for the larger bolt anyway. Why pay $19 for the little pieces (I already have tap and drill), I chose just buy a 75c-90c bolt? Plus, are Helicoil made of Stainless?
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Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis

With an helicoil you keep the same size bolt. They provide a special tap that is larger than the bolt to compensate for the coil.

The solution you used will work fine and like you said it's less expensive. But somtime you can't do that because the bolt goes through a part where the hole can't be enlarged.

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AB
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Sounds like you already fixed it Jim.
I wouldn't worry about the lesser area of the drilled "nut", it's pretty big, not actually a "nut".
A few years ago I made an album of the inside of early Nacra hulls;
http://www.thebeachcats.c…pictures?g2_itemId=82844
Look at the first 2 photos, (this is a 5.2 that had the hull rip off just at the front beam), you can see how the bulkhead attaches & holds the "nut". Click on "Full Size", just to the right of the caption.
I think the 5.5 will be the same layup. The "nut" is actually a square of material that was tapped to accept the bolt. IIRC the "nut" was made of brass or bronze, it was not SS. I almost stripped one on my 5.7 & ran a chaser through the threads, it was pretty soft stuff.
On boats that have not been apart in years, these bolts can get pretty stiff. Dip the bolts in grease, or something like Teff Gel if you are in salt water before assembly.
The torque spec is only 18 ft lb on those. Much of that is to prevent crushing the tramp slot, but I think some is due the soft material of the "nut".
I saw one fix where the owner used a hole saw & cut a 1.5" hole on the inside of the hull. He put scotch tape one one side of a box end wrench to hold a regular SS nut. Inserted that into the hole, lined it up & used the original bolt,(they are almost 3/4" longer than needed, lots of thread). After tightening wiggled the wrench off, then filled the hole with a 2" "port". The port has a plug you can screw out. He got the idea from one i have on each rear inside of my 5.7 hulls. You can see it here;


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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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my question is related so, i posted here:

We dismantled a Nacra F18 (pre-infusion) for overseas transport. The bolts were stucked and after some effort, we managed to unscrew.They bolts appear to have been glued. The glue or whatever came off the nut, fills the threads of the bolt. The boat is still in transit, but I would like to prepare already for the likelyhood of stripped threads. Anybody have idea on the appropriate size of helicoil for this one? Thanks in advance

The tech section has a number of different pictures of strap bolt repairs. http://www.thebeachcats.com/pictures?g2_itemId=105802 Definitely worth a look at how different guys did repairs.

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Tom
NACRA 5.7 (1984 Sail 181)
Pennsylvania
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