Spreader length

Discussion in 'Sailboats' started by jarcher, Mar 25, 2012.

  1. jarcher
    Joined: May 2011
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    Location: Smithfield, RI

    jarcher Junior Member

    Basically that was all. But as i realize the new upper spreaders will be much narrower than the existing spreaders, although higher, I realize its not a valid comparison.

    It does not matter. Being able to bring the leech in does me no good if the mast is going to fall down.

    Okay, so then I guess its as good as it can be given the CPW. Its still going to be a huge improvement.

    I attached a diagram I drew of the replacement spar. My measurements are from the top down, which I did because I am planning to cut the spar at the bottom.

    With an "I" of 38.8 (466 in) the height of the spreader bar openings is similar to your revised drawing. The upper spreader is 2.75 inches above yours. The lower spreader has a bigger difference, with the bar on mine 5.75 inches higher.

    So yes, the bottom section has 5.75 inches added. I guess its not 10 inches because the upper shroud terminates 13 inches below the top of the mast.

    Adjusting for where my upper shroud attached and where the spreader bar is, the panel length is 123.75 inches. A length for the upper spreader of 25.6 inches creates an angle of 11.78 degrees. Is that reasonable?

    The next panel down has a height of 142 inches, so a spreader of 42 inches creates an angle of 16.67 degrees.

    Finally, the bottom panel length at 182.75 with the CPW/2 of 42 creates an angle of 12.94 degrees. So that's ~0.4 degrees smaller than what you drafted.

    If that is not reasonable I can have the machinist cut new openings int he spar for the spreader bars, but I was worried that additional openings so close to the existing ones would weaken the spar.

    Yeah I thought it was okay. The length between the lower spreader bar and the T Ball opening above it is 142 inches. A 35 inch spreader, plus the width of the mast, makes the spreader come 36.8 inches out from the CL. If the spreader is angled up 2.5 degrees, that makes the angle between the shroud and the mast 14.67 degrees. Since I read in the book I needed at least 12 degrees, I thought I was okay with that.

    Thanks for the correction.

    Thanks again Paul. It seems like these numbers are close, but I realize that in the wrong place, an inch or two makes all the difference.
     

    Attached Files:

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  2. Paul B

    Paul B Previous Member

    I've sketched your rig to the left of what I had come up with.

    The numbers at the far left show the differences in the spreder heights. You lower panel is 10.25" longer than mine.

    Your upper spreader might be a bit short. The nice thing about starting out with a spreader that is a bit too long is it can be trimmed. Not so easy to go the other way.

    There are a lot of different ideas about rig layout. Some people work one way, others another way. Mostly the rigs are similar and don't fall down.

    You might want to talk to your sailmaker about the leech profiles he likes and have him compare to what you plan to do.
     

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  3. jarcher
    Joined: May 2011
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    jarcher Junior Member

    Hi Paul, thank you again, I'll study this carefully.

    At first look, your angles look different than what I came up with, so I'll figure out what I did. I agree, I should start with longer spreaders since I can always shorten them.

    I'll have to use my existing lower 33 inch spreaders as the uppers by cutting them, and order new lowers that are 41.2 inches (43 - 1.8in for 1/2 mast width).

    I called RigRite and they can make the 41.2 inch spreader from SP3 stock with a multi shroud tip. Untapered, its about $270 each. Tapered, they are about $525 or so. I wonder, is tapering worth doubling the cost? I realize there is a weight reduction, but it seems small. Seems like $1,050 is quite a bit to pay for a set of spreaders :-(


     
  4. Paul B

    Paul B Previous Member

    My sketches are just that. I've rounded things off, made some guesstimates, and done all the measurements to the CL. There would be some differences if you compared my numbers to exact measurements of your tangs, etc.
     
  5. jarcher
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    jarcher Junior Member

    Okay, but I am having trouble replicating even what's on the drawing. For the bottom two panels I can replicate the math exactly, but not for the top.

    On the right hand side of the drawing, top panel, I see the length is 131 inches, with a spreader of 29 inches, and the 88 degree angle. The math on that seems to indicate a shroud to mast angle of 12.57 degrees, which is very different than 11.82 degrees. If I omit the mast side of the triangle and plug in 29 inches, 88 degrees and 11.82 degrees, then the math says that the panel height is 139.5 inches, and I can't see where that would come from.

    If I plug in 131 for the height, 88 degrees and 11.82 degrees, it says the spreader is 27.23 inches. Is there any chance your software took out the width of the mast on that panel? But that still would not be quite right though.

    I'm sorry to be a pest, I'm just trying to make sure I understand how these numbers work.
     
  6. luff tension
    Joined: Aug 2010
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    luff tension Junior Member

    This should cover everything for you
     

    Attached Files:

  7. Paul B

    Paul B Previous Member

    If you use the 131 inch panel length you are using a dimension based on the mast wall, not the CL. So you need to use the spreader length based on the mast wall, not the CL. So the spreader length you would use is 29 - 1.8 = 27.2.

    If you use the 29" length you would need to extrapolate the panel length through the wall all the way to the CL. I've zoomed in on the masthead of my sketch and you can see the rigging going through the point of the sidewall and to the CL at a much higher point. In the case of the side I calculated it is almost at the very top of the mast (this is intentional). So that panel length would be almost 140 inches, which you have calculated as 139.5.

    So both of your calcs agree with the software. You just have to look at things in the correct context.
     

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  8. jarcher
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    jarcher Junior Member

    Ah! Now I get it, thank you very much!
     
  9. jarcher
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    jarcher Junior Member

    Is it desirable to keep that angle < 12 degrees?

    Based upon these comments and also:

    - the idea that a wider angle is more sturdy
    - the idea that my wide CPW makes tight sheeting not possible anyhow
    - the concept that I can always shorten it later
    - the need to keep the length of upper and lower spreaders balanced
    - my top panel is 133 inches based upon the location fo the spreader bar opening

    If I go with a 29 inch spreader with the 133 panel height, the angle is 12.38. That might be a little too wide maybe??

    I am thinking that I should use an upper spreader length of 28 inches from the CL, which gives me a shroud to mast angle of exactly 12 degrees. I spoke with my sailmaker (who is also a friend of mine) and his rig has a CPW of 36 inches. He uses a top spreader length of 27 inches and this does not interfere with his sheeting. That makes me thing that 28 in with my wider CPW will be fine.

    If not, I can probably bring it down to 27.6 inches, which makes the angle 11.8, but I suspect of I go much shorter there will be that balance problem.

    Is this reasonable, or am I just clueless?
     
  10. Paul B

    Paul B Previous Member

    No. I generally like to see the caps to be at around 11 degrees for a normal rig, and 10 degrees for something more "Grand Prix". I mentioned 12 degrees because I thought your original thought was to be at (but no more than) that angle. Considering your wide CPW I would expect the angles to be wider than "normal", and certainly wider than "GP".


    Wider is more stable. Especially due to your use of a tube that is a bit marginal for your application. Your wide CPW is actually a bit of a blessing on this front.


    Using 28" (CL to bearing surface) would give you the 12 degrees and would balance, poke-wise, pretty well with your planned lower spreader length of 42".


    I think it is a real positive step for you to get the opinion of your sailmaker. Nothing exists in a vacuum.

    The fact that he uses 27" spreaders doesn't tell the whole story about whether you can use similar effectively. His spreaders might be (should be) proportionally lower on the spar than yours, so they can be longer.

    We also don't know what your sheeting angle (genoa track location) is compared to the other boat. If your track is at 9 degrees and his is at 11 degrees, well that makes a difference.

    Overall I think you have thought this out quite methodically and have weighed the options with dilligence. What you have is not optimal, but I guess you gotta dance with who ya brung.

    Now be sure you use the right rigging sizes, do a really good static tune, get some pre-bend into the rig, then go out on a day with 8 to 12 knots and flat water and do a good final tune. After sailing a few times re-tune (to take out the initial stretch/set) and be smart when sailing in big breeze and sea.
     
  11. jarcher
    Joined: May 2011
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    jarcher Junior Member

    Actually, when I started this thread, I was under the impression that 12 degrees was a minimum safe angle. If its okay to exceed 12 then perhaps I'll go with 43 on the bottom and 29 on the top. If I need to shorten them, I'll do so evenly. My friend did give me all the dimensions of his rig and I'm going to draw it out. I'll ask him what his sheeting angles are, but realistically, I think it will be hard to predice where each sail will fall relative to the upper spreader. I'll start long and if I need to trim an inch I can do that.

    Thanks Paul and everyone for all your help and comments. I did do the math per the Toss book on the shroud loading, but I am not too confident of it because my rig is taller than normal for this boat. I spoke with a nautical engineer about shroud loading and he suggested that I do this calculation using the method described in Skene's Elements Of Yacht Design Eight Edition, which is based upon sail area. But that's easier, because its well documented. The book is on the way.

    Then I'll do the tune, watch it in heavy weather and hopefully come back in 4 to 6 weeks to tell everyone how great it all came out.

    Thanks again!!!

    Jim
     
  12. jarcher
    Joined: May 2011
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    jarcher Junior Member

  13. Paul B

    Paul B Previous Member

    I had a quick look at the pics. How much pre-bend is in the rig? It almost looks like the rig is inverted. I hope that is just a photo issue.

    The best way to check the prebend is to let all the tension off the backstay and pull the main halyard to the mast at the gooseneck. Looking up the rig. The halyard should be some distance from the back of the mast between the spreaders.

    It doesn't look like you have rigged the checkstays. In the 17 knot breeze did the rig pump at all? Was there a chop, or was it pretty flat?

    How much bend can you get in the rig with the backstay loaded up?

    How did your genoa sheeting angle work out with those spreader lengths?
     
  14. jarcher
    Joined: May 2011
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    jarcher Junior Member

    Hi Paul, thanks for the reply! Here are some answers...

    Its probably a combination of things. First, those pictures were taken as soon as the mast was stepped, before any tuning was done. The head stay was too long and the back stay too short. Since then, I have added a few toggles to the back stay and taken one out of the head stay.

    Now the mast stands mostly straight up. Its pretty straight, except at the top. Starting at the base of the upper spreaders, it starts to bend back. This is not great. I think its caused by the center line (left to right) of the mast - at the bast - being forward of the chain plate clevis holes just a bit. This is because its smaller. I lined the front of the spar up with where the old spar was so as not to change my J dimension, but this resulted in the chain plates being just a bit aft of the centerline of the mast.

    Its hard to describe, I hope that makes sense. I may wan to move the mast aft about an inch to align the shrouds with the masts center. Also, moving the main aft a tinge could further reduce weather helm, which was still something of a problem the other day.

    I'm still waiting for a day when the wind is very light and I am down there. So far, each time I tried to check that, the wind has blown the halyard all around. I did send a wire up, and confirmed the mast is straight left to right all the sail track. That also told me the mast is mostly straight, both side to side and front to back, except for that little bend near the top I mentioned.

    I am trying to avoid the need for a baby stay. But if I use a baby stay the prebend would be easy to add. I really don't want to do that, but its a deck stepped mast so prebend is hard to induce.

    Also, my sailmaker said the main is cut with very little prebend.

    I need to rig the check stays. The seas were choppy that day and one of the guys working the foredeck said he saw some minimal pumping, but I didn't get a chance to check for myself yet. I'm not sure he would know what pumping is or looks like, so I really need to see myself. From the cockpit there was no evidence of a pumping issue and I am feeling more confident about the stability of the rig.

    However, the problem comes when I put on back stay. Rather than reduce head stay sag, it seems that the back stay tension mostly bends the rig. The result is big wrinkles in the main sail. I think the check stays would allow me to tension the back stay and then tension the check stays to keep the mast straighter, reducing wrinkles in the main and promoting the reduction in head stay sag.

    I'm sure there is some reduction is head stay sag as it is now, with no check stays, but at some point that stops and the mast bends.

    More than I want I think. I think this is a better question for after the check stays are added. I made a bunch of changes, so I need to run the halyard down the aft side of the mast, and put the back stay on and see how much I get (as you said). I'm guilty of changing too many things at once and not keeping careful enough track.

    I'm not sure yet. We can trim the genoa to the top spreader and the chain plate and it does not seem that there is much twist, which is good of course. I say seem because its hard to measure.

    It seems that I can't point quite as well but I can't quite put my finger on why or even be sure its not my imagination. First, my wind transducer is not aligned perfectly straight, so my instruments were not telling me the truth on both tacks. Also, someone (maybe me by accident) switched the reported wind angles from apparent to true before the last race, so I didn't get data I'm used to. I need to (1) switch back to apparent, since all my references are apparent and (2) adjust the "misalignment" parameter. Its hard to measure exactly how far off it is by looking up. I might send a guy up the mast and have him hold it straight, measure the offset then enter the correction. It looks to be about 2 degrees, maybe 3.

    Once my instruments are telling me useful numbers again, I can see what the difference is. If I'm a few degrees lower than before, I have to figure out why. For example, is it related to a less flat genoa than before? My sailmaker said he thinks a reduction is spreader width of an inch would be helpful, but he was not sure and we agreed that shortening the spreaders is the last thing to consider.

    I'm going down to the boat today and sailing tonight, I'll see if I can check the maximum bend again, and I'll be watching the pointing angles.
     

  15. Paul B

    Paul B Previous Member

    This is not a bad thing. It would help you induce pre-bend. I don't know why you are only seeing bend up high. Maybe your intermediates and lowers are too tight (assuming they are also terminated aft of the center of the rig)?


    It would be best to follow the advice of your sailmaker. If he says the main is cut for 1" of prebend, then try to set that up. I am mentioning pre-bend as a safety factor rather than a sail trim factor. If there is no pre-bend and you get the kite up in a good breeze you might invert the rig.


    I would expect to see something moving in 17 knots true. Rigs move. But you don't want a lot of pump when sailing into lump.


    BINGO! That's the reason for the checks.


    With modern plastic sails you sure don't need as much bend as with the older dacron sails. Those "overbend wrinkles" you are seeing are testament to that. Checks should help you control the rig nicely. If you set them up MORC-style you don't even have to adjust them during tacks.


    Once again, your sailmaker should be your guide on this. I think you ended up with an extra inch on the upper spreader compared to what I had calculated. So maybe trimming it could be OK. But it is prudent to sail with your sailmaker aboard and make sure everything else is accounted for before butchering any aluminum.
     
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