Before and after pictures of repaired main beam

As promised, here is the patch-job on the Nacra main beam. I thought I had not much to lose in giving this a try. Looks substantial. Doesn't appear to be a clearance issue with the mast base. Still awaiting the post base piece for the V-Bar.
Questions on Pre-Bend....... How does one determine the correct amount? Measure the post before and after tightening the lower nut? 1/2 inch of pre-bend enough?

Greg

Pre-patch

One more of the patch from above.

The repair looks solid.
A pre-bend of 1/2" is recommended in this article.
http://www.home.zonnet.nl/held20/NA60tune.htm

Exactly.

The only comment that I have (and it's too late to fix now), is that the corners of the patch should have been rounded.

Corners cause stress concentrations. Keep an eye on the welds (especially at the corners) for cracks.

Is welding on crossbeams 'acceptable'?

I have an old Marstrøm rearbeam with some cracks lying in my basement, begging for action on another boat. Should be easy to weld if it's 'acceptable'. The integrated traveller pulled off and some other stress-cracks began to form. I psyched out and replaced it.
I was worrying that the heat would change the properties of the surrounding material and make it a weak point?


Tought I should ask when the topic was raised..

It's been a while since I needed to know this, but I think that some Al alloys can lose a third or more of their strength after welding.

If it were a mast - I might be concerned about it because the margins can be pretty close between failure and success - that and the bend characteristics are pretty specific. However, on the main beam (or especially the rear beam) I wouldn't be too concerned about this type of repair. In this case, the repair area extends into some of the lower stressed areas. I would certainly keep an eye on it but I wouldn't expect any problems to come from this particular repair.

Thanks for answering, it sounds promising.

The cracks are in the middle of the beam, not the area with the highest stress i imagine? I'll take a picture and post later..

Just a thought on setting dolphin striker tension.

Let's say have nothing better to do one day and decide to put your boat in sailing trim on the beach for the purpose of checking the toe of your boards. My boat has pivoting center boards and I sail with them both down for the most part so I need to be sure they don't toe out. Very slow.
Most boats when sheeted hard will pull the bows together
so that shouldn't be a problem. If you have all the slop out of the boards and there are toed still toed out what are you going to do? Tightening the striker bends the beam and the effect is to pull the hulls closer together at the main beam which toes the boards in.

This was very effective on the TheMightyHobie18 with it's curved main beam.

So The proper tension is what ever it takes to get this part of the boat tuned (albeit) fine tuned.

And loosen the rig before trying to make any adjustments.

Something else to play with.

The Nacra's with bowfoils experience the opposite effect when sheeted hard. The bows toe out slightly. The foil eliminates the horizontal component of the load that would normally be exerted on the hulls by bridal wires. The rear beam however, bends considerably when sheeted hard on centerline ~1000 + lbs of tension. My 6.0's all toed out about 3/4" (at the bows) more under load then when relaxed. I believe it is most important to have everything parallel sailing downwind or in light air, so that is where I had it set. Once you have the main sheet on hard, you are flying a hull anyway and parallel is less important. Note: the rudders tow out slightly under load also so you might consider setting them toed in 3/32" when the boat is relaxed.

Sail fast,
Eric Anderson

Thanks to all of you who chimed in with an opinion. I wish I had thought of rounding off the patch prior.......
Gives me confidence though to hear that you guys think I'm back in business. One thing I forgot to mention was that the welder welded a square (all he had - no round aluminum ) aluminum compression tube to the patch (and rounded the ends to match the curvature of the beam) before setting the whole thing in place so I don't have to fish the compression tube in through the end of the beam and try to align the holes. I think not having a compression tube in the first place when I bought the boat led to this disaster. I think everyone who buys a used boat should check early in the show if they, in fact, have a compression tube. If you don't go out of your way to check, you'd never know till it's too late. Thanks to the person who sent the link to the tuning guide. I wish someone would write one for the 5.5 SL. Aside from Patrick Ragen I don't know of anyone else who regularly sails one of these to compare notes with.
Thanks again guys,
Greg
Wicked Wet

Hey Greg,
I am sure you already know this, but just to be certain; Make sure that you put the cradle (for lack of a better term) on the dolphin striker to set the end of the bolt in so that is does not slip off to the side casuing your beam to take the full load again...When I replaced my mast step and ball they actually had me seat the cradle and bolt with caulking (not a lot of strength to be sure but I guess enough to help it from slipping..)
Cheers,
Dave
(very new to boat ownership and admitedly ignorant)

I learned it the hard way...... The boat was missing both the mast post pad and the compression tube. Wish I had a better assembly manual from Nacra to show EXACTLY what should be part of the boat and how to tune it.
I still can't believe the little pad is $ 25 though.
Thanks and good luck with your boat,
Greg

Hey Greg,
Still the same Dave...just changed my log-in name since I now have a boat....
At any rate, I still have the instructions from the repair which talk about how much to tension it. The instructions are presumably from Nacra; and although not extremely detailed, did help somewhat... If you are interested, I can try to scan them in send them to you via e-mail...let me know.

Thanks Dave. I got some great info yesterday from Jake and others that confirms the 1/2 or so prebend. That is all I needed for now. I was speaking generally that there seems to be a dirth of good (hell, any!) tuning information on the Nacra boats. At least for mine (5.5SL)