Aftmast rigs???

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

  1. Alik
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    Alik Senior Member

    This is exactly what we found when using this aft mast rig - impossible to provide forestay tension. Personally I was not in favour of that rig, but if customer takes 'aft mast' as his fetish - we had to design it.
    So to my knowlegde out of 3 catamarans built with aft mast, at least 2 were sold on secondary market as power catamarans, without mast ;)
  2. brian eiland
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    brian eiland Senior Member

    Sharpie,... I think you have ignored the fact that I have three 3 backstays on my rig,....those two lower ones that attach at the hounds, and a MAJOR one that attaches at the masthead. In general the two lower ones handle the inner forestay, while the masthead one handles that genoa forestay.

    My masthead backstay pulls back on the masthead to resist the forestay's forward pull. We can calculate that load that depends on the force in the forestay, my example here a 1260 kg needed. That backstay then passes over an aft jumper strut that redirects its force down to the base structure of the vessel that is supporting the mast. That backstay ALSO becomes the forestay for my mizzen sail.

    [IMPORTANT NOTE] This backstay that originates at the masthead DOES NOT reattach to the base of the mast itself, but rather to a structure of the vessel, …and preferably to the structure that accepts the compression loads of the mast to the vessel.]

    The masthead backstay now bends over the outer tip of an aft jumper strut that I’ve placed at the mast hounds location, and pushes in on the mast tube. Just as with most conventional shroud spreader elements the aft jumper strut is set to bisect the angled turn of this backstay. By vector analysis the backstay exerts two equal forces of 360 kg each push on the aft jumper strut (2 forces, one from the upper portion, and one from the lower portion,...divide 720 by 2).

    Aft Jumper Strut Push Load to Mast.................720kg
    Aft Jumper Compression Load in Mast.............negligible,...none

    Now I propose that we offset this entire cross-load pushing by the aft jumper strut with an opposing forward jumper arrangement. In order to accommodate the inner forestay and its sail this fwd jumper fixture will likely assume a ‘V’ configuration that is somewhat conventional in form. BUT, the assembly is also unconventional in form. In the first place it is not set perpendicular to the mast tube, but rather in-line with the push of the aft jumper strut. And the included angle between the two struts might well be 60 degrees rather than the more common 90 degrees. AND it will NOT consist of two individual jumper stays (wire cables), but rather will be fashioned of a continuous loop of ‘cable’ that would wrap around the back-side of the mast at its lower ‘termination’, and might even do so at its upper ‘termination’.

    The actual jumper stay ‘cable’ itself will be constructed from one of the new-age synthetic rigging materials such as Dyneema, Spectra, PBO, LCP, Aramid, C-6 carbon tow, etc. Ideally this stay material will have NO pre-stretch requirements thus no pre-loading. It should be very strong upon immediate application of force, and in a minimal diameter that it can be looped around the mast section in a continuous manner, at least on one end, maybe both. As a continuous loop, ‘both sides’ will always be carrying ½ the total load, rather than one side under load, while the other might be slack. There will also be a minimum of ‘fittings’ required to attach them to the mast (less weight, less failure pts). I imagine a simple ‘block’ of material attached to the mast around which the loop of this jumper stay can not slide any further along the mast….and one end needs to be adjustable

    I call this whole assembly a ‘modernized diamond jumper’. It needs to offset the 720 kg force of the aft jumper with its four 4 cables….thus 180 kg each in their horizontal force component:

    Front Jumper Push to Mast..........................................720 kg
    Divided by 4 Strands..................................................180 kg each
    Vertical Force Each Strand..........................................340 kg each
    Total Compression Load in Mast Tube.........................1360 kg

    Are we in agreement with these calculations thus far??
  3. brian eiland
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    brian eiland Senior Member

    So lets move down to the 'hounds area' where the inner forestay attaches, and those other backstays. (I assume we acknowledge that we have taken care of the masthead and primary forestay loads with that discussion just above ?)

    We need to resist the pulling forward of the inner forestay.

    The inner forestay is approx 75% the length of the primary forestay, so to keep things equally taunt should require about 75% of the load of that primary forestay (maybe even less since the inner foresail is much smaller than the primary genoa).

    Front Forestay Force................................................1000 kg
    Inner Forestay Force.................................................750 kg (selected) vector analysis we get.....
    Inner Forestay Compression Load in Mast...................650 kg
    Inner Forestay Fwd-Pulling Force..............................~350 kg

    I have actually consider 2 different anchoring points for these lower backstays, and 2 different configurations of their make-up.
    1) There was a shallow angle ones for sportfishing design, so they didn't interfere with the fishing cockpit so much. These only made a shallow angle (about 10 degrees) with the mast.
    2) There was the larger angled ones that attached at the very two sterns of the vessel, and these made about a 14.5 degree angle with the mast. I'm just going to consider these larger ones in this posting.

    As noted by folks these relatively shallow backstay(s) are going to be highly loaded. For this reason they are going to be made in a split-into-two manner. The will be two anchor points for the split backstay(s), and both legs will always be loaded, ... as opposed to one leg going slack at certain times. This will lessen the loads applied to their hull attachment points.
    Here is the general arrangement, but regrettably it shows the sportfishing version with more shallow attached backstays.

    and here with that wishbone boom raised up for fishing under sail...

    This lower 'backstay(s)' may join together into a single stay prior to attaching to the mast. This would allow the backstay(s) to follow slight athwartships excursions of the mast.
    The attachment to the mast is another important consideration. It will NOT attach in some traditional tanged manner, but rather will wrap around the front of the mast tube and utilize 'cheek blocks' to secure its position,...cheek blocks like these. Jointing the backstay(s) to the front of the mast tube improves their backward pulling angle, and very importantly avoids placing holes in the mast tube itself. Synthetic rigging also likes cheek blocks.

    Broader angle backstay to sterns of vessel (about 14.5 degrees)
    Backstay Load..........................................................1340 kg
    Compression Load to Mast.........................................1280 kg

    These figures are based upon using the lower backstay(s) to resist the loads of the inner forestay. Not all that bad??
  4. Nick12345
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    Nick12345 New Member

    Hi Brian,

    Let me start by stating that I am a big fan of alternative ways of approaching the design and construction of sailboats. Without innovation we would all still have bundles of reeds tied together and a piece of cow-hide for a sail. I once read an old book on boat building that cast suspicion over the use of newfangled “duralumin” for a mast - who could trust a light-weight metal mast when wooden ones have proven so good over hundreds of years! Sailing is indeed a conservative arena and new ideas no matter how good can take decades or more to become accepted.

    That said I am not a fan of bad engineering. In this case it looks like a small bit of knowledge potentially being a dangerous thing. I haven’t crunched any of the numbers on your vectors but you seem to have a good grasp of the general principles. If you had a single mast with no spreaders and stays that only ran in straight lines to attachment points on deck, this vector only approach would get you very close to some useful engineering.

    The big flaw with what you have presented thus far is the lack of an overall assessment of the structure. Most notably a plausible assessment of the mast structure as a lever arm.

    We need to talk about torque!

    To help explain the problem that I see in your rig calculations I have added some colour to the image you posted so that we can hopefully avoid any confusion.

    The main big, and unfortunately false, assumption you appear to make is that the blue wire is the main component resisting the load on the main forestay and that the red and green wires combine as an additional support for just the inner forestay (maybe I'm wrong here but this very much appears to be how you are looking at this?). At some levels, yes the blue wire does help stop the top of the mast moving forward. But this is most definitely not in the same manner as a backstay would. Even if this wire is attached to a strengthened deck point - as you have previously stated - it is essentially what I would refer to as a diamond-stay and, as such, it’s only practical function is to help keep the mast in column. So while it is counteracting some of the force from the main forestay, this is only the component of this force that is trying to BEND the mast - it will do this bit very well btw! - but does nothing to counteract the force trying to pull the mast over.

    To get a better idea of these forces a simple starting point is to drop the vectors and start looking at torque or lever arms. While it is far from a completely accurate assessment, it is good enough for basic calculations to look at a mast as rotating about its base ie. a stiff beam being pulled in different directions by the different stays and their forces.

    So, If the blue wire does nothing to stop the mast rotating forward, what does and what dictates how much force is on this element or elements?

    We now look at the red wire. While the blue wire (along with its spreader) is doing a good job of keeping the mast straight it is passing all of the rotational load on to the poor red wire that takes the full load of stopping the mast rotating (or falling) forward. If you load up the rest of the rig and then cut the red wire, the whole thing goes bang.

    To roughly calculate how a stay will resist this torque load, draw a line from where the stay attaches to the mast and where it finally attaches to the hull (ignoring any route the wire might take over a spreader). Now draw a line from the centre of rotation (the mast base) perpendicular to the first line. This gives a rough length of the lever arm associated with each stay. On the rig you have indicated the result will be a very short lever arm for the back stay (red and green) counteracting 2 much longer lever arms from the 2 forestays. Using the most basic ‘law of the lever’ calculations every kilo or pound of force placed on the forestays will need several multiples of force on the backstay to resist it.

    If you have any doubts on the above make a scale model, pull the stays tight and then cut the red line. If you make the model to match your drawing I guarantee that the rig will fall forward - a very clear indication that this element must be taking all the load. If you add some spring scales to the different stays you will also see just how much extra load that poor red line has to deal with. No need for complicated maths, just practical, real world testing.

    All of this said, don’t let something like this stop you from developing this rig! Just take it as a new insight and part of the challenge of making it all work. Are there ways you can spread this load out? or rearrange to create new attachment points that increase the lever arm? Or do you just accept a monster over sized back stay as part of the solution? these things might produce some negatives, but if they allow other positives to shine through it might be worth it!

    I spend a lot of time teaching design students and constantly remind them of 3 important things. Firstly, never be too precious about your design - no designer has ever created a design that cannot be improved upon and you are no exception to this rule. Secondly, if you have made a mistake or an assumption that proves to be wrong - keep going, this is just more knowledge that will ultimately make your final outcome better! And finally every design is a compromise - there will always be negatives that you will have to accept them as negatives. The key to convincing other people that your ideas are worthwhile is demonstrating that you have identified and accept that there are negatives, but follow this by demonstrating that they are out weighed by the positives.

    Good luck with it,

    Nick aft mast rig loads.jpg
  5. sharpii2
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    sharpii2 Senior Member

    From the looks of your excellent drawings, only the lower back stays are doing any work when it comes to holding the mast from tipping forward.

    The back stay which goes over the aft facing spreader has almost no purchase at all in holding the mast back. It does, however, do a good job of keeping the mast from bending forward above the lower back stays. I don't how far aft the mast step it attaches to the deck, but it appears to do so less than a meter from it.

    For this reason, I expect almost all the loads are on the lower back stays. And this load must be terrific, even before any sails are set on the fore stays. This is because one needs a certain amount of tension to keep the luff reasonably straight.

    You can get around this somewhat by cutting huge catenaries into the luffs of the two stay sails. But this would not only rob you of substantial sail area, but would make roller furling much difficult to do properly.

    The problem of putting teriffic loads on your lower back stays is not only making them strong enough, but making the deck structure strong enough to bear the enormous compression loads, which are probably in the order of tens of tons. And this deck structure must be able to handle these loads without deflecting too much.

    The biggest flaw I see in this design, is the aft raking mast.

    If the aft end of the aft facing spreader were to be dead vertical to the where the upper-back stay attached to the deck, and the mast was stepped dead vertical, then the upper-back stay would indeed would be tensioning the top fore stay. Also, the two lower back stays would be much further aft the mast step, and would therefore need much less tension to do their job. Even then, the loads would be terrific, when compared to that of a more conventional fractional sloop rig, but maybe more manageability so.

    I, myself, have come up with questionable design ideas, and have been fooled by my own defective math. I usually discover the truth by giving the design what Joshua Slocum used to call "the blow of the eye."
  6. sharpii2
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    sharpii2 Senior Member

    I can see two ways of fixing this design other than making the mast dead vertical:

    First, would be to add a fore mast, which would go where the inner fore stay is, and follow its path until it connects to the main mast at its top at the height of the lower back stays.

    This aft tilting fore mast would then serve to hold the luff of the would-be stay sail straight. It could be designed to take much of the compression loads previously borne by the main mast. The twin lower back stays (which really should attach directly to the mast on top anyway) would the serve mostly to stabilize the mast in the side to side direction.

    True, the inner stay sail would now become a tiny main sail, and would lose its clean luff. But at least the luff would be straight. And it will stay straight under blowing conditions. Under such conditions, the jib would be completely furled, and would therefore be putting no stress on the rig. And the fore mast would be doing all the work to keep the luff straight. In short, you have massive strength when you needed it the most.

    A second, but more difficult to achieve solution, would be to step the mast at the fore end of the bridge deck, and have it tilt aft instead. The masthead would end up at its old location, and the aft facing spreader would remain. Just as with my first solution, the inner stay sail would become a tiny mainsail. This proposal would not be as strong as the first one, but may be lighter.

    With either of them, the cabin structure would not have to be changed.

    And since the top of the mast matters far more than the bottom, when it comes to wind drag, you would end up keeping most of the advantages of a true mast aft rig.
  7. brian eiland
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    brian eiland Senior Member

    @ Nick, Sharpii, et al,

    Turns out you fellows are correct, I made a mistake about the single masthead backstay being able to counteract my forestay loading. When you presented me with the possibility of my error I rejected it initially, because I had held on to this idea for so long, with almost no one else coming forward to challenge my actual rig force analysis for a number of years. I actually just a few days ago constructed a little pipe and string model to double check the mast restraining figures I had presented. The mast did fall forward as I slid that lower anchoring point of that backstay forward to meet the base of the mast.

    I also want to thank you guys for presenting your opposition in such a respectful manner.

    I got out my boat show model that I had built over in Thailand a number of years ago, and I am playing around with new rigging ideas that might be employed to end up with a very similar 'ketch configuration' to this design of mine, as I still feel there are very worthwhile attributes of a single-masted ketch to be had. I'll present those in a pictorial fashion very soon.

    Cheers, Brian
    Last edited: Jul 1, 2020
  8. brian eiland
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    brian eiland Senior Member

    BACKSTAY Modifications

    One of the most obvious & least disruptive ways to try to salvage this rig design/configuration would be to make modifications to the back-staying arrangements. We need to move the hull connection points as far back as possible to get better staying angles.

    First approximation, those new backstays are the 'white lines' in these 3 photos,...



    If we simply move the attachment point of the lower backstays to the sterns of the vessel we increase their angle with the mast from 10 degrees to 14.5 degrees. That decreases their load to approx twice the force of that inner forestay (1500kg). That's about 1.5 times the force of the forward forestay,...for the pair of them.

    Now we take the masthead backstay(s) that passed over that aft strut and move them to this same attachment point at the sterns of the hulls, we effectively increase their loads to 3X the loading in the main forestay,...for the pair of them. Since the masthead backstay originates at the masthead, and terminates at the stern of the vessel. It might well be the same vector as the ones that goe in a straight line down to the rear as opposed to passing over that aft strut,..the forces in the backstay wire would be the same, they just route there a little differently. This non-strut backstay would make approx a 12 degree angle with respect to the mast tube and needs to be about 3x the load as the main forestay.


    So overall we've had to add 3x + 1.5x = 4.5x that forestay load for the combination of those backstays to contend with. That's not as bad as the 6x & 7x times loading being suggested by some.

    I went back to my old monohull version of this aftmast rig and found I had rigged this vessel somewhat similar,....a masthead backstay anchored all the way at the stern, and the lower backstays was well.
  9. brian eiland
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    brian eiland Senior Member

    Double Backstays (or Split-backstay)

    I've long been aware that double backstays don't always do what we ask of them. Well known rigging expert Brion Toss wrote, Why Double Backstays Are Evil. I posted that over here on another subject thread,
    Hounds, Cheek Blocks, etc

    In the past I've suggested that the upper attachment of my split backstays become a 'single pendent line' so that it might follow small atwartships movements of the mast tube while still maintaining loads in both legs of the split backstay going down to its hull attachments,...just as shown in Nick's enhancement of my illustration.

    If you look at this next illustration you might realize that the upper portion of the masthead backstay(s) could also be acting like that 'single pendent line', ....that then splits off into a pair (double) of backstays (green lines).

    I really like the idea of dividing of these backstay loads in half when they reach the hulls.

    I would be interested in taking this idea one step further. How could this split-backstay arrangement be combined with anchoring the upper ends via some sort of cheek-block arrangment??
    Hounds, Cheek Bocks, Etc
  10. sharpii2
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    sharpii2 Senior Member

    You have made vast improvements in your rig design. The split upper back stays relieve the lower back stays of the responsibility of supporting the forward fore stay to a large extent.

    I think the lower back stays should go all the way up to the mast tangs. I see no advantage to having them join together and attach to a single back stay which then goes up to the mast tang. The disadvantage is that the lower back stays will tend to pull the two sterns together. They would do the same even if they went all the way up to the mast tangs, but by a considerably lesser degree.

    If it were my design, I would have three cross-beam structures. The first would be all the way aft, even with the transom. The second would where the mast steps. And the third would be where the inner fore stay connects. I would also have a spreader beam at the bow. These three main cross-beams could all be sturdy bulkheads which would have to be part of the cockpit or cabin structure. And all of this to work well will have to be rigid, in not only fore and aft and a thwart ships bending, but also in diagonal torsion.

    This will almost certainly prove difficult if not impossible to achieve in the catamaran format.

    If it were tried with the trimaran format, the main hull could be built rigid enough to take all the fore and aft tension and compression loads, leaving only the athwart ship loads to the cross beams. This would be decidedly easier to engineer.

    As it is, I think you have at least designed a rig that won't fall down, and will probably work at least reasonably well. You will still probably have to cut a considerable catenary into at least the jib, if not the staysail as well, to get it/them to set well with questionable stay tensions.

    To make the two sets of back stays less evil, it may be possible to make each a single stay which which runs through a turning block attached to a tang. This way, the tension will always be almost equal on both sides, even as the hull and deck structure flexes in a sea way.

    IMHO, the three best sail rigs for a catamaran, with its inherent relative lack of rigidity are:
    a single sail behind a mast, a jibless ketch or jibless schooner rig, and of course, a three-quarter sloop rig, which is most typically seen on them.

    Anyway, I wish you the best of luck on your design.
  11. brian eiland
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    brian eiland Senior Member

    Perhaps have a look at these major bulkheads in my design,...tied into that central nacelle running down the length of the vessel,....that trapazoital shaped mast support column that is attached to the main bulkhead, ...the skins of the top deck and floor, ...and the central nacelle.

    I had written in the past,..
    Last edited: Jul 2, 2020
  12. brian eiland
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    brian eiland Senior Member

    As pointed out by Brion Toss, you don't want 2 'individual backstays' as one will always be going slack depending on sideways movements of the mast .

    Perhaps as you suggested it could be a single continuous backstay that runs through a turning block attached to the mast,...maybe via a loop of line around the mast tube rather than a tang. That's part of the reason I brought up the mast cheek subject.

    PS: Regrettably Brion Toss passed away a bout a week ago, and won't be able to contribute his deep knowledge of sailboat rigging to the subject. But a colleague of his has asked what my question to him might have been.
    Last edited: Jul 2, 2020
  13. sharpii2
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    sharpii2 Senior Member

    If the back stays are to be as one, the must go through a turning block. If they don't they will chafe through rather quickly. They may even have to be 7 x 49 wire rather than the usual 1 x 19 stuff. This is because they would flex quite a bit at the top. You're pendant idea may not be so bad if the pendant were made much shorter (maybe about 1/10th the total length). But if you use one for the lowers, you had better use one for the uppers as well, or you will be putting a lot of extra stresses on the rig. Each part of these split back stays had better be strong enough to do the job of two of them alone. This is incase one should fail.

    Now, it must be clear that the split back stays are doing nothing when it comes to athwartships loads. For this reason, you are going to need very sturdy shrouds to do this work. With your model, I see only one on each side. I believe that you need at least two, one upper and one lower. And these need to be massively strong. If any one of these parts, you could lose the whole rig.

    I also believe that you need to forget about the scoop sterns. This is because I think you need transom there to take the very high loads of the back stays. As you have it now they attach too what amounts to an open hull section. What could happen is that the top of this open hull section could bend inward by as much as an inch or so. And this would make it impossible to maintain proper fore stay tensions. Adding more tension would only cause it to bend in further until it breaks. It will be difficult enough to keep the twin sterns from bending in word, even with the transoms. This is why I suggested having a cross beam there.

    I firmly believe that if you want to keep the mast aft rig, the scoop sterns have to go.
  14. Will Gilmore
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    Will Gilmore Senior Member

    You have probably already discussed this. This is a long thread and the individual posts are quite in-depth as well, so I haven't been through most of it. But, I have to ask,
    Why the need to lean the mast forward? If you step the mast farther forward and straighten it up, you can set it up like a B&R rig while keeping the aft staysail anchored to the same point on the deck as currently planned. This would actually allow that stay to add to the backstay resistance against the forestays. The forestays would be a shallower angle to the mast, thus contributing less force to pulling the mast forward than currently positioned, yet the staysails could maintain their original angles. The foot of the jib would have to either be shortened to allow clearance of the mast, if you were looking for self tacking, but it shouldn't be a problem to allow the overlap, either.

    The only issue, I can see, would be in attaching the wishbone, but there are other options, such as a deck mounted boom or even a stay mounting.

    Wouldn't making the mast more plumb simplify and reduce the forces all around?

    If you've already beat this question into the grave, no worries, just say so and I'll spend more time on the previous pages and keep my thoughts to myself until I know more of what's going on.

    By the way, I am very intrigued by the mast aft rig. I think I read somewhere that they referred to it as an 'A' rig or something like that.

    Good looking boat, by the way.

    -Will (Dragonfly)

  15. CT249
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    CT249 Senior Member

    At the risk of derailing a thread, one can argue that the belief that sailing is conservative is untrue, and ironically only accepted because it's the conventional wisdom.

    Just to mention a few innovations, carbon hulls arrived before carbon chassis were used in F1, before all-carbon bicycle frames, etc. Leading aircraft designers like Sopwith, Fairey, Ted Wells etc were involved in sailing and didn't generally blaze a trail despite their background. The Topper dinghy was for some time the world's largest blowmoulded poly structure. The modern plastic "sit on top" largely followed the lead of the early poly Windsurfers. SOME sailors in SOME areas are conservative but such labels should not be associated with the entire sport.

    Sorry to distract from the thread but it's also not good to allow what seems to be unfounded critical comment to go by. It is also relevant because when one digs into the true reason why "innovations" often lie fallow in sailing, it turns out that they often had major faults or just didn't address the needs of most sailors. That can itself be a valuable angle to which to look at other innovations, because many of them are not ignored because of conservatism but because they actually do not work as well as the existing methods.
    Will Gilmore likes this.
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