Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. sharpii2
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    sharpii2 Senior Member

    Brian.

    Due to the darkness of the background in the photo, I cannot actually see just where the stays and shrouds actually attach to the mast.

    My guess, and it's only a guess is they attach to the center of the mast.

    As for twisting, the fore stay tensions are countered with greater back stay ones, so the more tension you put on the fore stay (or more accurately the jib luffs), the more the back stays are going to hold the mast against twisting.

    I see in the photo that there is a mast crane, facing forward, which has a fore stay attached to it. Perhaps its purpose is to resist mast twisting.

    I stand by what I said about the aft facing spreader. I still believe its purpose is equalize the support of the top jib luff to that of the lower one.

    Looking at the first photo, which has a more angled shot, I can see there is only one aft spreader. Looking at the second photo I can see that, indeed, the back shrouds come down at different angles. They may or may not attach at the same points on the hulls, but they clearly attach near each other.

    The consequence of this is the top one comes down at a shallower angle than the bottom one. That and it being longer will probably incline it to stretch more. It looks to me like this aft facing spreader was carefully calculated to eliminate this difference by putting a proportionately greater load on lower one (the back shroud which attaches behind the lower jib luff, the one I called the "lower back shroud").

    Yes, the channel was about two boat lengths wide.
     
  2. brian eiland
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    brian eiland Senior Member

    I don't think we are yet on the same wavelength here :):confused:

    Imagine looking down on the mast tube section, ..likely it is some elliptical
    section. The forestay probably attaches to the center front skin of the mast tube, and it pulls forward, AND to leeward on that tube, inducing a twist force in the mast tube.

    The backstays from the masthead are likely lead out to either beams of the transom. And likely (like most leeward shrouds on vessels), only the windward one is really loaded up (certainly more heavily than the leeward one). These backstays are also attached to the outer skin of the mast tube. This puts additional twisting forces into the mast tube itself. And it just so happens that the twist induced by the forestay and the backstay are acting in the same direction.


    I don't think crane in the photo is supporting any mast staying rigging,...rather it appears to have some sort of halyard or something like that?
     

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  3. brian eiland
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    brian eiland Senior Member

    You might have a look at this PDF file that was posted in another forum discussion....

    http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-3.html#post34686
     

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  4. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Rules Designing Our Boats, or Mother Ocean

    Somewhere back in this subject thread I posted something expressing this general thought that sometimes our vessels get 'designed by the rules (handicaps)' rather than what might be the alternatives.

    This morning I was looking back to find an old weblink for a rig designer that was no longer working correctly for me. I found I had to update his link:

    Applied Engineering Services of New Zealand
    ...under the 'Info' subheading I found this...
     
  5. brian eiland
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    brian eiland Senior Member

    Homemade Carbon Fiber Mast

    In that previous link for AES, I also saw this rather interesting bit
    http://www.aes.net.nz/home/info/
     
  6. brian eiland
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    brian eiland Senior Member

    Hi Sharpii,
    Here is another interesting observation on that twisting force we discussed.

    http://www.aes.net.nz/home/info/
     
  7. markstrimaran
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    markstrimaran Senior Member

    mast bend above the hounds. how much?

    Hi
    The trimaran in the picture is my boat. The mast is from an old Hobie cat that is worth it's scrap value. I have had it out in 20 mph winds with the small Genoa on the front stay and noticed the top 5 feet curled leeward about 6 inches. I was sailing along a tree sheltered area and came into an area of open clean air with 30 mph gusts. I quickly dropped the sail and switch to my smallest innerstay sail.
    The inner stay is attached to the mast about a foot above the hounds. The two side stays and the baby back stay all fasten at the same location. I am calling this point the hounds. I had no noticeable mast bend flying the 90 sq/ft small sail on the inner stay.
    The mast rebounded fine and is still straight when untensioned.
    I am considering a second set of side stays to the mast head. Spreaders are a last resort.
    I have read that masts can be bent some, but how much is too much? The mast of a typical sail boat is loaded over the entire leghth of the bolt rope slot. The sail load is consentrated at the mast head on a aft mast rig.
    As rigged I have been only using the front stay on light wind days.
    Thanks
     

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  8. sharpii2
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    sharpii2 Senior Member

    A second set of "side stays" (shrouds), to the top of the mast will certainly help.

    You would do well to avoid the temptation of having your new set of shrouds attach to the hulls in front of the mast step. What this will do is pull the mast head forward when a side load is applied. Once the mast head moves forward, this shroud is effectively lengthened, allowing the top of the mast to bend again. If they attach to the hulls slightly aft the mast step, that should be OK. It would be much better than being attached in front of the mast step.

    Your new shrouds should run as parallel to the mast as possible, from the side view, if they are to have the effect that is desired. You might consider replacing the existing pair of shrouds with a thicker pair that is say 1.5 times thicker than the original diameter. This is just for the top pair. Leave the lower pair as thick as they already are. The goal is to get the top pair to stretch less than the bottom pair, and thus keep the mast from bending.

    While your at it, you might increase the thickness of the stays holding the derrick (the boom like thing in the back) down. I presume its aft end is held down with a pair of stays which connect to the transoms of the two outer hulls.

    The biggest problem with a mast aft rig is getting enough support aft, as the mast is so far back.

    I like to have the fore and aft stay base aft the mast to be roughly one half of that that is before the mast. For example, if I had say a 20 ft of fore and aft staying base, I would put my mast no further aft than 13 ft, 8 inches. This would probably not do for a jib only rig. If I wanted one, I'd rake the mast aft and put up with boomless jibs. or I'd put the mast 15 ft aft and have at least two aft facing spreaders and have three sets of shrouds, one to each spreader height plus one to the mast head. The amount of staying cable gets dizzying.

    From your post it is clear that your rig, as it is, works reasonably well. The fact that it is on a multihull complicates thing, I am sure, so there is probably not much practical room for improvement
     
  9. markstrimaran
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    markstrimaran Senior Member

    Mast bend above hounds

    Thanks for the reply.
    This is my 3rd mast rigging set up. The cross member that holds the mast step and shrouds is very rigid 1.5" x 3" 14 Guage steel tube. As are the outrigger arms. Which are bolted together.
    The black "derrick," "mizzen spar" is an 8 feet section of mast. It is rivoted, and epoxied to 12 inch long steel insert. That was bent, welded ,and hammer forged too match the ID of the aluminum mast. This steel plug is welded to a 4 bolt flange making it removable. As is the mast and cabin frame.
    It's mating flange is "indistructable" seriously overkilled. So it is a free standing spar, as their is not any suitable shroud anchor locations.
    I have not measured how much tension I can get on my fore stays. From what I have read a relaxed jib stay puts more camber in the sail providing more low end power.
    It's purposed for trolling in any wind wave conditions, weekend camping, and river trips that have low bridge clearance. On inland saltless water.
    I am building this boat as a half scale model of a trimaran that I would like too build some day. Something to make a few mistakes with that does not cost much. I have read most of this thread and want to thank you all for some great information.
    My thoughts on any disadvantages due too drag and non aerodynamic are greatly countered by the fact that, the fish can't hear me coming. Seriously the silent trolling ability clarifies my Panther Martin's ultrasonic spinners, so much. That the fish seem to follow me like a flock of hungry seagulls.
     
  10. markstrimaran
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    markstrimaran Senior Member

    A new picture. Aft mast 2 sails up.

    Added mast head stays. The boat is 30% faster with both sails. Verse the single head sail.
     

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  11. brian eiland
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    brian eiland Senior Member

    Interesting Mark, keep more photos coming. Perhaps you could get a friend to photo or short video of you under sail?
     
  12. markstrimaran
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    markstrimaran Senior Member

    First trial.

    Here are some sailing pictures.
    Wind was 5 mph. Average GPS was 3.2 mph. Unsure of my tact angle just yet. Have not looked at my chart plotter. 8 hrs of sailing mostly down wind in 2mph winds. I need more wind.
    I am still figuring out my video editor.
     

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  13. brian eiland
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    brian eiland Senior Member

    Silence or Noise to Attract Fish

    I was just looking back thru some of your postings, and I got a real 'kick' :p out of this portion. I'm just going to love referencing that over on a few other forums where fishing guys have touted their engine noses as a real attractant for fish ;)
     
  14. markstrimaran
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    markstrimaran Senior Member

    Last edited: Aug 2, 2015

  15. gggGuest
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    gggGuest ...

    Good way to end up with a treetrunk that weighs and costs more than it needs to... Unless you already have a known good layup schedule I don't believe there's any real substitute for doing the maths and getting it right.
     
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