I picked up an "83 N5.0 and it has diamond wires. I'm wondering if their stock. The top attachment is on the front of the mast and is a single point attachment. The spreader is a straight bar and looks to be in the correct position. The bottom attachment is single with a turnbuckle. The wire is slack to the point that I can squeeze them together about 20" above the turn buckle.
It seems to me that the mast will invert or bend backward under load. I guess that it may just bend on its small access w the slack diamonds.

Anyway, is this a stock diamond setup for the 5.0?

Also, I have a "76- 5.2 and the wires run from center top to a straight spreader, and to center at the. Ottom attachment with a bottle adjuster at the bottom

You probably have the wires for non straight spreaders that have the newer 5.0s. If so, it might a be better idea to change the spreaders instead of the wires.
Porbably this one. Not adjustable though:
http://www.ebay.com/itm/Prindle-Nac...Gear&hash=item2321013011&vxp=mtr

Did the original 5.0 have wires? this looks like a factory setup bu i cant see how it would help shape the mast in a positive way. seems like the straight spreader would bend forward and collapse. I dont really like the idea of having a great big hole drilled in the mast. If I take the current spreader out it will leave the hole un supported. Gives me the creeps.

The 5.0 was factory equipped with diamond wires. The spreaders were adjustable. The factory recommended ~1.5-1.75 rear rake. I do not know what spreaders are on your boat but I would not sail with them.

I think the 5.0 used the 5.2 extrusion, which is pretty flexible. A good starting point for the 5.0 would be 1.75 aft rake with 200-300 lbs tension. This is something close to how they set the boat when it was new, beyond that it is up to you.

The NACRA Manuals are available for download:

http://nacrasailing.com/index.php?option=com_phocadownload&view=category&id=1:manuals&Itemid=91

Try the Vintage

Good Luck

Older ones did have a straight spreader, I've seen a couple.
I would be more concerned about water coming through that hole when capsized. Unless you really care about fine tuning this old boat. I don't see how the spreader would bend forward and collapse. You still have the cunningham to bend the mast. But even without that, I don't see how the mast or the straight spreader would bend forward

Said otherwise, those spreaders only add rigidity in the soft axis of the mast. They don't help bending it. I don't think it's such a big deal for that boat.
This one has sailed for years:

I read your first post again and I think your concern is about the attachment points being in the front of the mast, therefore bending it the opposite way. I'd say that with this straight spreader you need less tension than with adjustable ones, because you are not using the diamong wires to pre-bend the mast. The pictures above are from a friend's 5.0, I checked other pictures and it seems they are also attached in the front.

As long as I know that it was a factory setup I guess I'll leave It as is for now, according to today's logic, it didn't seem right, and I'm sure it isn't right to induce the correct / wanted bend in the mast. The boat is pretty basic right now, I'll sail it simple for a while and see how it does. I'll be racing a P-16, P-18, 18ht, tiger, G-Cat 18 and 16ht this weekend, p-16 is the only boat that rates higher than the 5.0. No line honors for me this time. It ill kick butt on Portsmouth! Bring it on boys!

Practically every catamaran prior to 1985 had straight spreaders that stuck through a hole in the mast. The hot boats started introducing raked spreaders and masts with prebend setups...it didn't take long for the rest to follow suit. The straight spreader has been around a long time - there's nothing wrong with it structurally...it's just not quite as tunable.

Is there any precident for removing the straight and adding a raked spreader? Or would it require a new mast?

I don't know, I'd do it. And I´d try to seal the hole forever, with the right product, not sure what it is.

I'm sure it's been done, but raked spreaders put considerably more loading on the mast...and you've got a hole right where that load is highest. Where you might have run 10 or 30 lbs of diamond wire tension with the straight spreaders, you're probably looking at upwards of 500 lbs or more with the raked spreaders...I probably wouldn't mess with it.

Yea Jake, I was leaning in that direction. That's a pretty big hole an I don't want to do anything that hasn't been done successfully by other brave souls, thanks all for your input.

The purpose for swept or adjustable spreaders on 80's NACRA's was not pre-bend. The purpose was to stabilize the mast against inversion. The 5.2 extrusion is very flexible by any standards. To stabilize the mast the spreaders were swept aft 1.5 - 2 in and the wires the wires were set "tight". By the early 90's we were using 300-500 lbs on the 5.5 uni, which used the same extrusion.

30 year old memories are fuzzy but some type of swept spreaders was being used on NACRA's prior to 1985. The Hobie 18 had adjustable spreaders in 1977 and I remember a meeting at sailmaker where they talked about the importance of raking the spreaders aft in ~1982.

Please explain

In simple terms, mast inversion is when the mast bends forward instead of aft. When the mast is inverted, you no longer have the expected aft curve that matches the luff curve on your sail. When your mast is inverted, your sail shape is "not fast". This is normally, the worst you see.

When your mast is inverted, you can also get "S" shapes and even more complicated shapes. When the mast starts to move or oscillate in "S" shapes, that is very bad, your mast may fail.

Raking the spreaders aft, with tight diamond wires, puts a force on the mast that try's to bend it the way you want it to and (I am told) stiffens it against motion in other directions.

The problem is worse on the 5.5 uni or NACRA 18sq, which use the same extrusion but, the mast is longer and square top mains make it worse. I have seen 18 sq's rip the top of the sail out or break the mast. This normally happened, downwind in heavy puffy air.

Minor inversions happen on a lot boats and are not that bad. Big inversion or complex shapes that move are very bad. When the mast is oscillating, the wrong load in the wrong direction, cab break the mast.

Tapered, pre-bent masts are the next step. You make the mast easy to bend in the directions you want. at the same time you make it stronger down low and stiffer in directions you don't want it to bend.

Thanks!

does the Nacra 5.5sl have straight spreaders? I was thinking of picking one up for singlehanding and slapping a small spin on it...

The ones I have seen had raked spreaders. One even had a carbon mast.

You're doing a lot of "thinking" lately. Feeling OK?