Aftmast rigs???

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

  1. cavalier mk2
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    cavalier mk2 Senior Member

    Off current topic but my screen showed the Chris White boat first and I thought of wishbone Fenger ketch style mains for light wind to get the square top jib effect.
     

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  2. Spiv
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    Spiv Ancient Mariner

    Hi Phil,
    Your mast rake seems to be such that the backstays are going to have too much tension.
    The force produced by the forestay bending in the wind is going to be multiplied several times to the back; I hope you have done all the calculations properly as in my opinion you are going to load those backstays to the point that either the chainplates pull out of the amas or the amas will flex up with all the problems associated with the cross-beams.

    I hope you are right and I am wrong, but I had to write it to you, if for not else to force you to re-assess the loads. It looks really scary to me.


    Also, there is nothing more frustrating that to be in light wind with not enough sail out there, so your last paragraph is correct for some of the time only. What do you do in light winds?
     
  3. sharpii2
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    sharpii2 Senior Member

    I agree with Spiv.

    Some spreaders aft the mast might be a big help.

    They would be located about halfway up the mast, giving you both upper and lower back stays, sort of like upper and lower shrouds on a masthead rig.
     
  4. sean9c
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    sean9c Senior Member

    I'm assuming you've looked at the CE of your sail. It appears pretty far forward and becomes more so when you start rolling it up
     
  5. pbmaise
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    pbmaise Senior Member

    Regarding raking mast forward, and reducing mast compression

    Sean9c
    Regarding CE. During the first set of sea trials that CE became an issue when I noticed the boat didn't want to go down wind. I discovered all I had to do was move the tack forward a few inches. Unlike a Bermuda rig, where the sail head is fixed, this sail has no fixed head position. I can move the head way forward and stand the clew on its nose. This is the optimum downwind setting. For upwind sailing, the fact that the mast is a wing mast should help create some extra drive and provide more weather helm and assist upwind. These things said, again performance isn't my goal. I'm looking more at how to create a safe rig that still has some upwind capability.

    SPIV - Stefano and Sharpii2 - Fanie
    Good to hear from you all. Erase the image of the mast from your mind a moment and picture a very large force pulling forward way above deck level. This force is coming from the forestay tension, weight of the sail, and force of the wind in the sail. There is some force from the weight of the mast, however, compared to these other forces the mast weight is not that consequential. At some point the forward pull must be countered with a back force. The farther I can get the back chain plates to the back of the boat the better.
    Raking the mast forward doesn't move the chain plates, however, it has the same net effect. The angle of pull is better. Notice in the drawing that the mast head is almost perfectly centered between the bow and rear chain plates. This means that for every pound of forward pull, the rear chain plate must respond with an equal and opposite pull.

    Both the forward pull, and the backward pull translate into a compression force on the mast. According to my calcs 1 lbs of forward pull ultimately translates into 1.8 lbs of compression. However, this doesn't include the extra compression caused by the halyard.

    In my design that forestay and the halyard are one in the same line. This is a key factor for me to watch when adjusting forestay tension. For every lb of tension that is added to the forestay to keep the sail in shape, there is more than 1 lb of tension on the halyard. Therefore, overall 1 lb each pound of tension on the forestay translates into about 3 lbs of compression force.

    Therefore, reducing halyard compression forces really goes a long way.

    I'm not the first to ponder how to address this question and I've attached a simple solution which I must give credit to others. The drawing was posted on another blog at:
    http://forums.sailinganarchy.com/index.php?showtopic=101686
    It shows drawing up the head sail on a two to one purchase. I've never thought of this or seen one before today.
    This one simple change may reduce a few thousand pounds of compression forces for the price of some extra line.

    Other people describe magic boxes, and halyard locks. However, I'm nervous about the idea of a lock that is 60 feet off the ground in the middle of a storm, and the ones that are commercially made have no where near the rating I need.

    Regarding adding a back spreader. I will certainly look into all aspects of this design. There will be one or maybe two swept back spreaders. Since there is no mainsail I need not worry about them getting in the way of a sail.

    I am missing a key piece of information someone can help me with?

    The owner gave me the maximum compression forces allowable for this mast however, I want to double confirm. I know it was made about 15 years ago and is labeled Proctor and may have come from Switzerland, however, having problems finding any data. Any ideas?

    Phil
     

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    Last edited: Sep 15, 2012
  6. sharpii2
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    sharpii2 Senior Member

    Hate to say this, but raking the mast aft has made your back stay tension problem worse, not better.

    To get a decent idea of the true situation, draw a line at a 90 degree angle from the butt of the mast to the back stay. Measure the length of this line, then divide that number into the length of the back stay. The product of this calculation should be the multiple of compression to head stay tension the mast will be experiencing. If the mast were dead up right, the situation wouldn't be nearly as bleak, but then you would have to haul the clew that much further forward to get it around the mast. A worthy trade off, IMHO.

    Your shroud arrangement should do fine in keeping the mast from buckling athwartship. My worry is that it will now buckle fore and aft. Or push right through the step. Or pull the back stay chain plate right out of the hull. Either way, you are looking at rig failure.

    Having the mast vertical, or even raked aft a bit, will do loads to improve the head stay/back stay compression ratio. Next, a fore and aft spreader, that extends from the mast all the way to just above the chain plate, may well cut such a load in half, at least.

    Another benefit to having this huge spreader and associated rigging aft the mast, is the windage it creates may well help balance the forward moving Center of Area of the jib, as it's reefed. kind of like a mizzen sail that is never struck.

    You should really look at Brian Eiland's work on this sort of rig for pointers and inspiration. You will notice his rig pretty much follows the rules I suggest (I got them from looking at his rig and considering the math involved).

    The multi part jib halyard will certainly help as well, and is a great idea.
     
    Last edited: Sep 16, 2012
  7. pbmaise
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    pbmaise Senior Member

    I'm raking mast forward not aft..

    Aloha Sharpii
    I want to get this right which is why I'm posting the ideas of what I am going to do here.

    And please rain on my parade if I'm doing this wrong.
    http://www.youtube.com/watch?v=G_g3kkGH8Mo

    Sorry couldn't resit that I listen to youtube when here at internet shop in Malaysia. I'm going to review what you wrote in detail.

    For the record I'm raking forward. That 13.83 rake forward was found using goal seek in a spread sheet. I set the angle of the forestay to equal the angle of the backstay. That is exactly that rake angle of 13.83.

    One thing about forestay tension to clarify is my rig has a rather unique forestay. The forestay and halyard are one in the same.

    There is no separate rope or wire. The bolt rope in the sail serves as the forestay when the sail if up. It is a 12,000 kg breaking strength Dynema line sewn right into the sail.

    To give you an idea of the the original rig loading I have drawn the previous sail plan behind the new one. The original mast collapsed after the owner had some repairs done on the roller furler. It is quite likely the person doing the work tensioned the rig too high after the change.

    When racked forward 13.83 degrees every 1000 lbs of downward force translates into 1030 lbs of compression on the mast and YES it does add some load on the back stay. I'm using good old Cosine and Sine to calculate these and checking all the force vectors a second time using my friend Pythagoras.

    Wind pressure in the sail that creates a load on the forestay is converted into X, Y, and Z directional forces. X directional force in line from bow to stern are the biggest concern. Those are the ones that pull the mast head forward, and this in turn must be compensated by an equal and opposite force from the backstay. If I rake the mast backwards or stand it straight up, the pulling angle of the backstay is drastically reduced.

    One may think that intuitively that raking the mast way back would be the best way to withstand the X forward directional pull. They are correct! However, the angle the mast must be raked backwards is very high. It is so high that the mast would have to be way taller in order to support the sail, and I would need a backstay located on a barge I was pulling behind the boat. Now that is a cool idea.

    What I found is compression caused by that forestay is good! It is compression in the Z plane (Up and down). I want the least amount of X component force that pulls the mast forward. As you can see in the graphic I can't move the mast head any farther forward since the sail luff wouldn't fit in the available space.

    This mast by the came from a big cat owner who has given up on the idea of masts completely. He purchased himself a sail-kite from France. Reality around where I sail is zero wind, or giant winds. Both conditions are not conducive to a big Bermuda rig.

    I've about another 3 weeks before sailing for the Philippines to work rework, and triple work every thought.
    Phil
     

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  8. brian eiland
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    brian eiland Senior Member

    Dear Phil (pbmaise),
    I would ask you to go back to my posting #345, and review just the first portion of my vector analysis.
    Rigging Force Review for Aft-Mast or Mast-Aft

    If you were to do the same thing with your configuration: say take a one inch long vector down the direction of the forestay, ...then break this vector down into its two component parts, one down the mast, and one perpendicular to it, pulling the masthead forward. That component that is perpendicular must be offset by an equal force pulling back on the masthead. Draw that somewhat horizontal vector in. Now look how your backstay might produce that backward force (that horizontal vector is one of the two component parts that make up your total backstay vector (load).

    I think you will find it is a VERY LARGE load because of your 'shallow angled' backstay(s). That is the reason I incorporated the aft-jumper strut to get a better angle for my masthead backstay to resist the forestay loads. Its a game of angles.

    And unlike your quest, I was looking to get the same overall sail area as the sloop rigged boats on a substantially shorter mast.

    Please do that little vector analysis I suggested above and tell us what you think then.
     

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  9. brian eiland
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    brian eiland Senior Member

    ...like upper and lower shrouds on a conventional single spreader rig
     
  10. sharpii2
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    sharpii2 Senior Member

    Hi, Pb.

    You're right. I did say "raking the mast aft", I really meant raking the mast forward. All i can say to that is Boooone head. Me, not you.

    I also made another mistake.

    I said "draw a line at the butt of the mast, that is at a 90 degree angle to the mast, to the back stay. Measure it then divide that into your back stay length."

    What I should have said is: "draw a line at the butt of the mast, that is at a 90 degree angle to the mast, to the back stay. Measure it then divide that into your mast length..."

    This change should alter the results significantly, but will not erase the fact that, with a forward raked mast, the back stay angle is so small that you will end up with a back stay tension that is many times the head stay tension. This back stay tension translates directly to mast compression loads.

    With this set up, a sudden gust could take the mast out before you even realized what was happening.

    To rake the mast aft, you move it's butt forward, so it is raked aft the same amount you have it raked forward now, leaving the head where it is.

    You will end up with the same Center of Area placement and the same head stay length. But you will have to give up the boom and go with two sheets instead.

    The head stay will increase the compression load on the mast, for sure, but the back stay will not only have way less tension, more than making up for the higher mast compression loads due to that, but will be shorter as well, and have less stretch.

    Even putting a large spreader aft the mast, at it's present rake, may not save you from trouble, as it will push forward against the mast, perhaps causing it to buckle.

    To resist that, you need a lower shroud that is raked aft enough to resist any forward movement of the point on the mast where the spreader connects. With the forward rake of the mast, you have in the drawing, that will be difficult to arrange.
     
  11. sean9c
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    sean9c Senior Member

    The first objective, for the rig, on your list is improved upwind light air performance. The new rig has appx 1/3 the SA of the old rig. How does this meet the objective? Seems to me if you found the old rig to have appx. the correct amount of sail area then the boat with the new rig will be woefully underpowered.
    Also, as I mentioned before, it appears that compared to the old rig you are moving the CE of the sailplan quite far forward and more so when you reef. This has to negatively effect performance.
     
  12. brian eiland
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    brian eiland Senior Member

    After you do that little vector analysis I suggested above, then I have 3 alternatives I will present to you. But first I want you convinced that what you have in mind at present is not valid.
     
  13. pbmaise
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    pbmaise Senior Member

    Vectors and upwind analays

    Thanks Brian I am still doing the vector analysis and one big spread sheet.

    Essentially I'm dividing loads from each line into X, Y and Z components and then calculating mast compression for each force. I'm finding I understand it best if I work through the equations and I then stop for a day and check everything.

    I've discovered some new information that you already know based on that post 345. Loads have to be brought down to the deck and not to the base of the mast. I happen to have done that right when I brought the halyard down to a turn block that was mounted on the deck and then over to a deck mounted winch.

    In the original Bermuda rig there were no less than 3 winches mounted on that big 79 foot mast. Each of those winches was pulling up sails or pulling on a reef or out haul and adding compression loading.

    I learned from this that even though there may be wind locations on this mast that I should still run halyards to the deck and across to the deck winches. I also discovered this concept of using 2 to 1 purchase to reduce halyard tension. Overall both changes reduce mast compression caused by the halyard by 75% versus a mast mounted winch run over one block.

    Looking around the marina I discovered that some of the other rigs bring the halyards down to the mast step and then turn to a rig near the cockpit. However, they don't all do it correctly. I found this drawing on Selden, that shows that even though it may look like load is transferred to the deck, it is transferred back onto the mast from below.

    Putting a spar way up there as in your design makes me nervous as it is a wing mast. However, thanks I'm running calcs now and will be very open to ideas. That spare up near the top by the way in your design is similar to one I'm sure you know about at this link:
    http://www.barefootsworld.net/windwalker/aftmastsailingcalc.html#measurements

    One thing that I worked on today were iterations between forestay and backstay loads. What I mean by this is a forestay load creates a Compression and residual force countered by backstay. The backstay in turn when countering this residual force creates compression and a secondary residual force.

    Regarding making upwind progress with so little sail area, its magic. Rather, there is no desire to run the boat at any high speeds and I didn't mention what angle into the wind. The previous owner couldn't make any upwind progress with that big 79 foot rig. They told me they ran on the third reef at all times and so, they too were running a small area. Further they had the boat way weighted for cruising. My hope is that one sail flying well with some lift effect and removal of a few thousand pounds will help me get maybe a little into the wind. 25 degrees, 35, 45? Not likely. Maybe 60?

    Phil
     

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  14. brian eiland
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    brian eiland Senior Member

    For just a rough analysis we don't have to consider the Z direction so prominately. Of course with the wide staying base of the multihull it will help reduce the loads.

    And I was NOT looking for an in-depth analysis, but rather just a realization that it would be extremely difficult if not impossible to rake the mast that far forward and utilize straight backstays as you've drawn. With that in mind we might move on to some other ideas.

    Halyard loads are pretty straight forward really. Just going to a 2 to 1 purchase doesn't really change the loading. It just makes it a little easier to raise the sail. Remember the load it takes to hold that sail up is the load, period. Now if you choose to divide that load up by using two strands of halyard rather than one, then you have to realize that the loading being seen by that sheave at the masthead is still the same. And its the same on both sides of the sheave, so the compression load to the mast is double the haylard load in either side. If one of those sides has two strands rather than one, then it has half the load, BUT its twice (two strands). The only way to get less compression loading by the halyards is to 'cut off one side',....provide a halyard lock at the top.



    i am familiar with that project as it was brought to my attention some while back. I made the comment to the gentleman that I did NOT feel that he properly addressed the forward loading that his aft-strut was going to exert on his mast (no resistive elements), ...in addition to the loadings his 3 forestays were going to have....ill conceved.

    Not only that but this is just the static loadings,...how about the more dynamic ones under sail.

    There is very little magic (if any) that will make up for good old sail area to displacement for moving a sailing vessel. And as Tom Speers and others have pointed out the drag of the sailng rig verses the power it can develop is an important element upwind performance.
     

  15. brian eiland
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    brian eiland Senior Member

    First of 3 Ideas for Phil's big Tri

    Here is the first of 3 alternatives I thought you might look at. Go to my posting #348
    Aft Mast Alternative on Big Trimaran

    1) considerable less rake
    2) all 3 sails furl
    3) the lower backstays I show going to the main rear beam might even be anchored out further

    4) ....and convertable back to a more standard rig....

    "There was one other nagging question he had, that needed to be taken into account....what if the fwd leaning mast idea would not work?? This persisted to be such a big question in his mind that I had to give considerations as to how my mast aft design could be converted back to a more standard rig configuration without a great deal of expense, and/or trouble to him.

    I still chose an 'all-3 sails-furling' arrangement....my single-masted ketch concept. But I made the mast rake almost half (6 degrees) of the original design. And the mizzen sail was made a bit larger in proportion. Thus this rig could be converted to a straight standing cutter rig with the mizzen becoming a more traditional mainsail attached to the aft edge of the mast. Or a new larger mainsail could be constructed for the mast that could be extended upward (taller), but still stepped in same location. The cutter jibs would then both be fractional, but would not require modification."
     
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