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Old 26-09-2020, 17:02   #1
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Rig tuning question

We have an Oday 39 with Isomat spars, and I have been trying to get my rig tuned properly prior in preparation for ordering new sails. All of my wire rigging is 5/16" on a double spreader masthead rig with straight spreaders. I have cap shrouds at the masthead and intermediates at the second spreader, both caps and intermediates terminate on deck inline with the mast, then I have forward and aft lowers at the first spreader. I have about 10" of rake on the mast, and maybe 2 - 3 inches of bend as I have been told these rigs are meant to be pretty straight. Using a Loos gauge I currently have the backstay at about 20% breaking strength in order to keep the headstay (with Harken roller furler) from sagging off more than 10 - 12" when on the wind in 12 - 14 knots. When I started with the backstay at 15% I got close to 18" of sag to leward in the headstay when on the wind which seems excessive(?) I have the cap shrouds at around 12%, and I have run the intermediates between 6 and 10%, but the problem I have is the top of the mast from the second spreaders up falls off to leward when we are on the wind. The Selden tuning guide says to loosen the windward intermediate in this situation so that the mast falls off in column. At anything below about 8%, the leward intermediate flops around like a noodle and even at 10% it is pretty loose - loose enough I can still adjust the turnbuckle by hand. I have the forward and aft lowers just tight enough that the lewards don't flop around and to induce the very slight forward bend. Unfortunately we don't have any riggers in our area and I am not sure what to do keep the top of the mast in column.
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Old 26-09-2020, 18:27   #2
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Re: Rig tuning question

If any part of the mast is falling off to leeward in 12-14 kts your shrouds are too loose. Crank 'em up until the mast stays where it should be (and in column). Start from the bottom and work your way up. Maybe leave your Loos at home that day, it sounds like it's causing you more problems than it solves.
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Old 26-09-2020, 18:45   #3
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Re: Rig tuning question

I worked for OíDay. You didnít say at what wind speed you have this issue nor sea state. Why do you feel there is an issue..does it look wrong or are you concerned about it not being in column. Itís not pumping is it?
I guess I canít see the problem clearly. Any photos.
Have you contacted anyone with a similar boat.
Lastly, please be careful cranking things up. Iíve seen a mast bend permanently from over tension. New boat too.
Happy trails to you.
Mark and his relaxed manatees
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Old 26-09-2020, 19:21   #4
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Re: Rig tuning question

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Originally Posted by Manateeman View Post
I worked for OíDay. You didnít say at what wind speed you have this issue nor sea state. Why do you feel there is an issue..does it look wrong or are you concerned about it not being in column. Itís not pumping is it?
The example was in 12-14 (>18 apparent) knots, light chop, but the mast conditions are similar in similar wind with no chop. The mast is not, pumping just the top (above second spreaders) of the mast falling off to leward when hard on the wind. Perhaps I need to take the tension on the intermediates up to or beyond the tension of the uppers, but that is contrary to what I thought I knew, and the Selden tuning guide for example.
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Old 28-09-2020, 19:34   #5
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Re: Rig tuning question

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Originally Posted by Scott Fuller View Post
The example was in 12-14 (>18 apparent) knots, light chop, but the mast conditions are similar in similar wind with no chop. The mast is not, pumping just the top (above second spreaders) of the mast falling off to leward when hard on the wind. Perhaps I need to take the tension on the intermediates up to or beyond the tension of the uppers, but that is contrary to what I thought I knew, and the Selden tuning guide for example.

If it's the top of the mast that is falling off, then it's the cap shrouds that need tightening, right? Go sailing and adjust adjust adjust until it looks right to you. Leave the Loos at home. The gauge is only useful to get numbers after you have a setting you like (to allow you to quickly return to that setting).


No two sails, rigs, and boats are identical and different settings will work best in different conditions. Experiment! Have fun!! Deliberately set it up wrong to see why it doesnít work. Figure out ten ways to almost have it right. You will be a tuning wizard for your own boat in no time. Seriously, the only way to mess this up is to try and set it to some number that someone else told you was "right".
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Old 29-09-2020, 04:24   #6
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Re: Rig tuning question

Are the cap shrouds clamped at the upper spreader ends? Sometimes the upper spreaders need to be angled up slightly.
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Old 29-09-2020, 06:24   #7
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Re: Rig tuning question

My understanding is the spreaders will always be pointing up to some degree. You want the angle between the spreader and the shroud (above and below the spreader) to be the same. Since the angle is almost always greater above the spreader you will have to angle the spreader up to equal these angles.

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Old 29-09-2020, 06:40   #8
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Re: Rig tuning question

Be careful using straight breaking strength percentages because you could hurt the boat especially if it's older by over tensioning.
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