I loosened the wires for the winter, took note of the numbers, but I really would like to know for sure what the number is. I've looked it up and it depends on sorce. new boat manual states "38" or 280 lbs. the old manual for these boats is more generic stating "Medium tension would be when you can depress both wires against the mast simultaneously 12" up from the point of attachment"

That seems loose for one, and second. I want a number, I can borrow a gauge at the club.

also if anyone know's about the crossbar mast ball bar tension (dolphin striker) , feel free to let me know. thanks

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1988 Nacra 5.2
YouTube link to see boat in action:
https://www.youtube.com/playlist?list=PLddinE6IorSrPpZrlvcwAjRzSIQsKYPG1
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The diamond wires should be loose like the description of being able to have them touch to the mast 12" above the tang. While there are many possible configurations I have never heard of anyone running them actually tight. The idea is to let the mast bend to get the sail shape you want when the mast is fully rotated. If the diamond wires are tight and tensioned the mast can't bend to flatten the sail and there is little need for mast rotation.

So try the 12" setting, rotate the mast so it is nearly 90° to the sail, pull in the main sheet hard and look at the sail shape. If you want a flatter sail you can loosen the diamonds up some more. For sure there are limits here, but you are a long way from them at 12".

As to the DS rod you want to tighten the bottom nut until you just start to see the main beam beginning to bend. This preloads the main beam upward so that the DS can better distribute the downward loads out to the beam ends. Then remember to snug the top nut down, but it does not need much in the way of torque. You don't want to crush either the main beam or the bushing tube that is inside the beam around the DS rod.

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dg
NACRA 5.2 #400
This End Up
Original owner since 1975
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If I put my diamonds to where you can touch them on the mast 12" up it literally measures a 0 on the gauge. That's on a H18. That's why you hear the old school measurements noted at where you can touch them to the mast. It's too loose to even use a gauge.

32 is a good number for tension if you want to try the tight diamond thing.

We are talking about very different boats here. I thought we were talking about a NACRA 5.2 not a Hobie. The NACRA has a very bendy mast that you use to control the flatness of the sail. I have no experience with the diamonds on the much stiffer and heavier Hobie masts.

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dg
NACRA 5.2 #400
This End Up
Original owner since 1975
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Whether your stick bends easy or not if your diamonds are loose enough to touch to the mast 12" up it's gonna be a 0 on the gauge. All of us have bendy masts we use to control the flatness of the sail.

The books/manuals I've seen for the boats with "flexy" masts that run tight diamonds use around 32 on the wires to start and like 40-50mm of rake. None of that is from experience on an h18. It's all going to depend on your goals and if you need depowering or not. If you don't have an adjustable downhaul tight diamonds will make it hard to depower. There's not really a perfect number to try. Just s ballpark to start and play around with. I'd just do max rake and tighten them up to close to 30 and take her out and go from there.

I emailed nacra awhile back and they said all their aluminum masts are the same extrusion across the boats up until they lost the tools and made new ones with the intention of the new extrusion having the same characteristics as the old one. So just about any tension settings you see for a 5.5, 5.7 etc will still get you a ballpark.

thanks a lot for the replies, I am going to have to loosen a little bit more, and get some advice at the club when I'm adjusting it. I really apologize for missing a bit element here. I have the triangle style spreader, not the single rod style. In the manual it states "diamond wire tension is the single most important tuning device" Thanks again for the feedback.

So the 5" large main front BEAM I called it crossbar should just begin to bend upward when tightening? I'll start with WOW. The manual stated no more than 1" of play on the DS spreader. I tightened it a bit from where it was and I didn't really change much. I'll need to look into this a bit more. thanks again

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1988 Nacra 5.2
YouTube link to see boat in action:
https://www.youtube.com/playlist?list=PLddinE6IorSrPpZrlvcwAjRzSIQsKYPG1
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I ran the 12" rule on mine, the spreaders on the 5.2 are just spreaders, and keep the mast from snapping under load unless you have a newer mast that has the spreaders angled aft, the tension plays more part on those style of masts as they preload aft mast bend. Once the mast rotates and there is air in the main, you'll feel tension and you can tell that it is doing it's job. Hobie 18 spreaders are quite a bit different. Actually bought some to put on the 5.2 mast but then sold the boat.

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Cesar (Cez) S.
Hobie 16 (had a few)
Nacra 5.2 "Hull Yeah"
Vectorworks XJ - A class (not named yet)
West Michigan (Grand Rapids/Holland Area)
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Think of your main beam like a prestressed concrete girder. By tensioning the rods inside the girder it makes that girder (beam) bow upward, but the point of doing that is to spread the loads that occur at the center outward to the supports (the hull). The boats work fine if this beam isn't prestressed, but the beam is a stronger structure more consistent structure with that slight upward preload. Without the preload the main beam is actually bending downward under high load until the strength of the DS structure comes into play and that changes your shroud tensions and mast rake.

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dg
NACRA 5.2 #400
This End Up
Original owner since 1975
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Quote

So the 5" large main front BEAM I called it crossbar should just begin to bend upward when tightening?

You want the beam "bent" upwards 3/8". The "play" that NACRA refers to is grabbing the V brace beam strap & trying to move it up/down. If it is sloppy, your main beam strength is limited, as it is not acting like a truss.
Here's how to do it...the last few photos. Don't forget to loosen the TOP NUT BEFORE you start cranking on the bottom one, seems obvious but...
https://www.thebeachcats.…344c4bd40c3282f4295fced6

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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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