Mast tuning 361

[bshock]

Hi folks, just launched yesterday here in Sandusky, Ohio. Hurray the season had begun!

I store indoors so I have to tune the mast each spring and I've run into something unusual. I follow the Seldon tuning guide instructions, and it says to first tune the cap shrouds to 15%. Done. Then it says to tune the backstays to 20%. This is where the question comes; the port backstay barely registers on the gauge, whereas the starboard backstay is about about 12%. Both backstays are evenly threaded. The mast is currently in column. There is a 13 knot wind blowing against the back of the boat, hitting the starboard side at about 45 degrees.

Any ideas why I can't get the port backstay to tension the same as starboard?

 

[Gunni]

My rigger laughed at me when I asked if I should keep and use a Loos gauge. He said he has a box full oft them unused, given by customers. I have one from an old racing dinghy, also unused. Assuming the rigging is all in spec and stays are uniform you can expect that the wind is messing with you. Check your starboard and port stays, I expect they will show similar differences.

 

[bshock]

Thanks Gunni, I don't have a problem with the gauge. I find it useful in reaching the appropriate tension. The funny thing is that it is only the one backstay that is behaving this way. My cap shrouded and lowers are all tensioning as expected. I ended up getting both backstays to equal tension, but I had to put four or five extra turns on the port backstay.

 

[Davidasailor26]

Sounds like one wire might be longer than the other. As long as the mast is still straight and plumb side to side when the tensions are equal I wouldn't think it's an issue.

 

[bshock]

Thanks David. I think I might have figured it out. The mast appears to be slightly out of column, contrary to my first thought. I looks as though it curves very slightly to port. So I'll go through the process of re-tuning when I get back to the boat. I have an idea of how to check to confirm the mast is in column by hanging a weight of some sort from the main halyard and seeing where that item hangs in relation to the mast. With the mast rake/bend of the 361, I'm thinking that whatever I hang from the halyard should dangle just aft of the mast. If it hangs slightly to port of the mast, then I'll know it's not in column.

do you think this idea will work?

 

[Gunni]

Check you boat for level too, no list. The halyard plumb-bob is very accurate if the boat is level.

 

[bshock]

Thanks Gunni! I might not have considered that.

 

[Davidasailor26]

Measuring the distance from the masthead to the toe rail on both sides can help check as well, although that might have inaccuracies depending on halyard stretch, wind, etc.

 

[DougM]

Hang a tape measure from the main halyard as previously mentioned, measure to the roerail on each side and that will indicate that the masthead is centered. (Don't be concerned by the relative number of threads taken up by the turnbuckles. The original rigger probably didn't get the shrouds built to exactly the same length anyway.)
If you then use a plumb bob hanging from the masthead, assuming the boat is level you can determine if there is any curvature in the mast that needs to be adjusted out with the lower shrouds.
This usually isn't a one time and done process. The first sail of the season will stretch things out, and you will have to readjust, and probably a couple of times after that.

 

[bshock]

Thanks so much guys. Should I release the tension on the cap shrouds and backstays before I try to center the mast with the lower shrouds?

 

[justsomeguy]

Should I release the tension on the cap shrouds and backstays before I try to center the mast with the lower shrouds?

No. I'd make sure that the masthead is centered first, a la @DougM , then bring the rest into column.

 

[bshock]

Got it, guys, thanks so much!!

 

[onlinelee]

Hi, just to throw my hat in, this is what Beneteau main UK agent told me when I had to lift my mast...

• Mast upright and straight, boom off.
  
• Increase cap shroud tension (1 turn per side, then repeat, DO NOT do more than 1 turn on a side before adding turn to other side)
  
• Once cap shroud tension is hard to increase you should have the mast pre-bending by about 2 feet (maybe more) measured by hanging the main halyard/topping lift straight down from the top and measuring horizontally out from the gooseneck.
  
• Tighten lowers until pre-bend is reduced to around 6-8 inches - again, DO NOT do more than one turn before matching the other side.
  
• Go up the mast and tighten the intermediates until hand tight then add one turn.
  
• Tighten back stays handtight.

 

[Sandy Stone]

2 feet? Man, that sounds like a lot of prebend for a cruising boat. I bet it _is_ hard to increase the shroud tension. And you reduce it to 6-8 inches by adding even more tension on the lowers? I always thought the general rule of thumb for prebend was about half the depth of the mast section, before adding backstay tension, maybe more if you have a Jboat or a B&R rig. Also, the OP didn't say whether he had in-mast furling, but if so I don't think you would want much, if any, prebend.

 

[onlinelee]

2 feet? Man, that sounds like a lot of prebend for a cruising boat.

Mmm, thats what I thought, and I didn't achieve it either. I ended up with just less than a foot to start and ended up with about 6", partly due to a new forestay that came up 10mm short! I followed the instructions given and the result seems about right, with just a little weather helm.

 

[DougM]

Two feet sounds excessive.
So far in this discussion nobody has mentioned forestay tension/backstay tension contribution.
Because the main shrouds are swept back, as soon as you start adjusting backstays, the tension in the shrouds is going to ease. Its kind of a never ending cycle.
This is an annual ritual on my boat. There is no turnbuckle on the forestay, so even in initial setup I have enough mast rake to create more weather helm than I would prefer. Add to that split backstays which are almost impossible to connect without attaching the main halyard somewhere aft and putting tension on the masthead, and it becomes a pain. Then when the backstays are attached, the tension between them has to be equalized.
About the time the season is over, I have everything where I am happy with it.