Tuning a rig with a split backstay. Loaded or unloaded? - SailNet Community
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post #1 of 17 Old 07-21-2018 Thread Starter
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Tuning a rig with a split backstay. Loaded or unloaded?

So I'm retuning the rig on my Mirage 33 after working on it the other day. I am happy to report that there are 3 different sized wire on it (split backstay, baby stay, and shrouds) and my Loos gauge has EXACTLY those 3 sizes on it's scale. Lucky!

So my issue is with the backstay. It's a split stay system, with a block and tackle that slides down the two wires to tension them. I was cranking down both aft turnbuckles evenly to hit my 15% on the Loos when I realized I will not have enough turnbuckle to crank it to the (iirc) 28 on the gauge, I got maybe 23~. Then I thought to myself, well this is an adjustable backstay, MAYBE it's not supposed to "start" at the 15%. I did tighten up the adjuster a bit and indeed climbed the gauge up closer to the 28, but then stopped and left it because I felt like I didn't know what I was doing and didn't want to break something.

I set my baby stay to 15%, along with my shrouds, everything looks centered up, and I can see the rake in my mast aft, which I believe is normal for this boat. My last boat ran a dead straight spar, no adjusting, no rake, so this is all new to me.

So... when setting the Loos on the split backstay, do you get it close to tension then sinch it the rest of the way with the adjuster? In my case that still leaves alot of room for that adjuster to crank down.

Bonus question: I noticed under some 20kt~ gusts yesterday my rig pumped a little bit, just enough for me to notice. No sails flying, just sitting at the dock taking the wind hard on the beam. This is the first I've noticed this, and don't know if it's a sign the rig isn't happy, or if I'm just used to small boats with stiff masts.

*The PO did replace the forestay 2 yrs ago, and there is a chance he doesn't have it tight enough, which may explain the lack of adjustment in the aft turnbuckles, but I don't know for sure. That's my next investigation...

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post #2 of 17 Old 07-22-2018
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

In my experience, it is pretty unusual to have a turnbuckle on an adjustable backstay. Normally all of the backstay tension comes from the adjuster which if I understand why you said, would be the block and tackle. The backstay adjuster should have a range of adjustment that allows the mast to sag slightly forward when reaching, and which can seriously tension the forestay in a breeze. When docked I trypically take most of the tension off the backstay and have it just tight enough that it isn't whipping around.

When you talk about the mast pumping, that usually means that there isn't enough tension on the baby stay and aft lowers which in concert stabilize the middle of the mast, and control pre-bend in the mast. If the whole mast is'pumping' (which technically isn't actually pumping) then you need tension on the backstay-forestay, and maybe on all stats and shrouds.

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post #3 of 17 Old 07-22-2018 Thread Starter
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In my experience, it is pretty unusual to have a turnbuckle on an adjustable backstay. Normally all of the backstay tension comes from the adjuster which if I understand why you said, would be the block and tackle. The backstay adjuster should have a range of adjustment that allows the mast to sag slightly forward when reaching, and which can seriously tension the forestay in a breeze. When docked I trypically take most of the tension off the backstay and have it just tight enough that it isn't whipping around.

When you talk about the mast pumping, that usually means that there isn't enough tension on the baby stay and aft lowers which in concert stabilize the middle of the mast, and control pre-bend in the mast. If the whole mast is'pumping' (which technically isn't actually pumping) then you need tension on the backstay-forestay, and maybe on all stats and shrouds.

Jeff
Thanks Jeff i appreciate that info. I'll have a look next time I get to the boat. I had everything tensioned up to spec according to my loos with the exception of the split backstays which were slightly below the 15%, but maybe I missed something. It didn't look like it was thumping real bad or anything but I did notice some wiggle mid mast when a gust would kick up.

Thanks again, good to know!
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

I must say that tensioning everything to 15% of the wire working strength guarantees that your rig is not properly tuned. Normally, upper shrouds, babystays and forestays have much higher tension than lower shrouds for example. But I just looked at the rig on the Mirage 33 and it appears to have a double spreader rig with in-line spreaders. With in-line spreaders, there is no aft mid mast forces to oppose the forward force of the babystay when the main sail is not raised. As a result masthead rig boats with in-line spreaders tend to be more prone to pumping than pretty much any other rig.

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post #5 of 17 Old 07-22-2018
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

The forestay is not set by tension. it is set by length. its sets the mast rake which is used to adjust the amount of weather helm. back stay tension on a adjustable backstay is used to set the forestay tension for the amount of luff curve you want in the sail per the wind conditions. if you measure the split backstay tension on only one side of the split section, it will be half of the tension of the single section of the backstay.
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

OK guys, I'll do some digging I guess. I was going by all kinds of rigging guides that say for sloops set your 15% load up on the rig to prevent shock loading an under-tensioned rig, or to prevent over stressing it with too much static tension.

There may be a double spreader version of the M33, but mine is definitely a single spreader simple mast head sloop rig. Inner and outer shrouds, slightly swept back, and a small baby stay up front.

I can back off the tension on the backstay easy enough. I assume the tension I had set for the split sections would be fine as they are a smaller dia than the backstay itself which is 1\4", so the smaller cable 15% wouldn't exceed the backstay above the splitter. I did measure both sides of the split and both read about the same, which makes sense since they meet at the same point it would distribute the load to each side evenly. If one side was tensioned further down than the other at the buckle I assume it would just slightly pull the backstay more to that side of the triangle.

In the end I am not a racer, just a cruiser\weekend warrior. I just don't want my rigging to snap or mast to buckle, so I'm only concerned about rake\bend\tension in the "make it safe, don't care about performance" mentality.

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post #7 of 17 Old 07-22-2018
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

Your resting backstay tension should be based on the amount of pre bend you want in your mast, which in turn is determined by the luff curve cut into your mainsail. You can visualize how much prebend you have by attaching your main halyard to the gooseneck and pulling it tight. The amount of prebend will be the distance between the straight line formed by the halyard and the mast. Typically 3" would be a good starting point. If you do not have enough prebend for your main it will look ugly with wrinkles in the middle of the sail. Once you have your baseline prebend set, pulling on the backstay will bend the mast further which flattens and depowers the main. Because you don't have swept spreaders, mast bend is controlled only by backstay and babystay tension. Pulling on the backstay will also serve to reduce headstay sag which is also a good thing in stronger winds.
With regards to the Loos gauge, I would use it mainly to make sure your shrouds are tensioned evenly. You also want to make sure the mast is straight and in column. The former you can do by using the same tight halyard technique to give you a straight line reference down the mast, adjusting the lowers to remove any s-bend you can see. To make sure the mast is in column you can stretch a halyard down to a chain plate on one side, mark the point where the halyard meets the chainplate, then make sure it meets the opposite chainplate in the exact same spot. (For more precision hoist a long measuring tape)
The Loos gauge is a good reference tool, but the only thing I have ever used them for is for tuning racing rigs for different conditions, or initial setup when we trailed boats to events. The most useful information you will get from it is verifying equal tension on both sides imho.

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Last edited by SchockT; 07-22-2018 at 02:50 PM.
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Quote:
Originally Posted by SchockT View Post
Your resting backstay tension should be based on the amount of pre bend you want in your mast, which in turn is determined by the luff curve cut into your mainsail. You can visualize how much prebend you have by attaching your main halyard to the gooseneck and pulling it tight. The amount of prebend will be the distance between the straight line formed by the halyard and the mast. Typically 3" would be a good starting point. If you do not have enough prebend for your main it will look ugly with wrinkles in the middle of the sail. Once you have your baseline prebend set, pulling on the backstay will bend the mast further which flattens and depowers the main. Because you don't have swept spreaders, mast bend is controlled only by backstay and babystay tension. Pulling on the backstay will also serve to reduce headstay sag which is also a good thing in stronger winds.
With regards to the Loos gauge, I would use it mainly to make sure your shrouds are tensioned evenly. You also want to make sure the mast is straight and in column. The former you can do by using the same tight halyard technique to give you a straight line reference down the mast, adjusting the lowers to remove any s-bend you can see. To make sure the mast is in column you can stretch a halyard down to a chain plate on one side, mark the point where the halyard meets the chainplate, then make sure it meets the opposite chainplate in the exact same spot. (For more precision hoist a long measuring tape)
The Loos gauge is a good reference tool, but the only thing I have ever used them for is for tuning racing rigs for different conditions, or initial setup when we trailed boats to events. The most useful information you will get from it is verifying equal tension on both sides imho.

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Thanks! This is great info. I will check that as soon as I get back to the boat. I've found rig tuning to be very much an art as far as I can tell. Everyone has different ways it seems. The method for checking pre bend sounds easy enough. I remember when I first asked around my marina when I bought my first boat how to tension the rig and got a confused look from all the "sailors" and many replied "does it feel tight? Good enough!".
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

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Originally Posted by Guyfromthenorth View Post
Thanks! This is great info. I will check that as soon as I get back to the boat. I've found rig tuning to be very much an art as far as I can tell. Everyone has different ways it seems. The method for checking pre bend sounds easy enough. I remember when I first asked around my marina when I bought my first boat how to tension the rig and got a confused look from all the "sailors" and many replied "does it feel tight? Good enough!".
Yes, I'm sure there are a lot of boats out there that subscribe to the "good enough" rig tuning principles. How often do we hear "I'm not racing so it doesn't matter"? The fact is even if you are not racing, basic rig tuning can make a big difference to how your boat sails. If your mast isn't straight it will behave differently depending on what tack you are on. If your prebend is wrong your sail may be too flat or too full. If your rake is wrong you could have excessive weather helm, or worse...Lee helm. Often times these little problems are only going to manifest themselves when you are out in windy conditions where they can make your boat very unpleasant to sail.

So good on you for paying attention to the little details!

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post #10 of 17 Old 07-29-2018
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Re: Tuning a rig with a split backstay. Loaded or unloaded?

I would like to join in here. I've heard both ways; Babystay does and doesn't affect mast bend. My Question in the context of this original post would be, do you set the babystay for a certain tension backstay released then let it add to mast bend when tensioning the backstay or will it just not increase in tension as the mast is bent? Still trying to wrap my head around what it even does. I've also heard it just keeps the mast from buckling under spinnaker and/or adds some support for inline upper and lower shrouds.
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