shortening the lower spreaders

Discussion in 'Boat Design' started by Steve W, Oct 7, 2012.

  1. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Is there any structural reason not to shorten the lower spreaders by 1-2" each side on a conventional double spreader rig. The upper spreaders are 57" tip to tip aand the lowers are 83" tip to tip, i would like to shorten the lowers to about 80" tip to tip so i can locate the chainplates in a better structural location.

    Steve.
     
    1 person likes this.
  2. debenriver
    Joined: Sep 2012
    Posts: 3
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Rockland Maine USA and Woodbridge England

    debenriver New Member

    Spreader length controls the angle the upper part of the shrouds make with the mast. So the lower spreaders control the angle the upper part of the intermediate shrouds make with the mast. More modern, and more highly stressed rigs will typically be about 9º - 11º. Older, more traditional rigs might be as much as 14º. The smaller the angle the greater the load in the shroud and the greater the compression load in the mast.

    So by shortening the spreader length you will be reducing the angle and increasing the load in the shroud. The rigging wire, chainplates, rigging screws etc. will have been sized to suit the design angle.

    Having said all of that it is probably not a problem because you are not shortening the spreaders by much. 9º is usually reckoned to be the absolute smallest permissible angle, so you should check out what effect shortening the spreaders has on the angle the shroud makes with the mast.

    Shroud load (tension in the shroud) varies with the sine of the angle ("A"): Shroud load= Force/sinA. You can reckon a (very) rough Force ("F") by assuming the the shrouds are sized so that their breaking load ("BL") is 3.0 times the actual shroud load. So F = BL x sinA/3. Once you have a rough Force, do the sum again with the new angle and see how much it increases the shroud load. Then assess whether this new load is permissible for the shroud size etc. I would stress that this is a very rough sort of backwards calculation, just to get a grip on the effect of shortening the spreaders. To do the calculation properly you need a stability curve (a GZ curve) and displacement so that you know the righting moments.

    The chainplates don't have to be directly under the spreader tips – though to have the chainplate-to-chainplate width narrower than the spreader length would be slightly unusual (though of course this is just what diamond shrouds are).

    Lower spreaders are usually longer than upper spreaders because there is more force on the intermediate shrouds than on the upper shrouds, so it helps to keep the rig lighter if the spreaders are longer (larger angle = less shroud tension = smaller wire = less weight). The compromise is usually the sheeting angle for the genoa (longer spreaders = wider sheeting angle = less close winded).
     
    1 person likes this.
  3. philSweet
    Joined: May 2008
    Posts: 2,244
    Likes: 144, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I'm having a hard time figuring out why you can't move the structure 1.5 inches. Just cantilever it out out there. You don't necessarily have to shorten the spreaders, but you do have to analyze the effects. Got any pictures? The chainplates on my Cal28 were bronze straps about 5 feet long, 3 inches wide, 1/4 inch thick, and bolted on with 20 bolts each side to the main bulkhead. I wouldn't have worried about a 1.5 inch offset. But not all boats are built like that.
     
  4. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    A truly excellent response, i will check the angle of the intermediate shroud to the mast. Trust me Phil, shortening the spreaders is by far the easiest and best solution.

    Steve.
     
  5. philSweet
    Joined: May 2008
    Posts: 2,244
    Likes: 144, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    This doesn't hold water. The panel lengths are usually unequal, so that the spreader length alone does not determine staying angle.

    Some arrangements require the cap shroud to have six times the side pull of the first intermediary. They can still have shorter upper spreaders.
     
  6. debenriver
    Joined: Sep 2012
    Posts: 3
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Rockland Maine USA and Woodbridge England

    debenriver New Member

    Well cap shrouds with six times the load of the intermediate shrouds would be an unusual arrangement and it didn't seem as if the rig of this boat was that unusual. The load in the cap shrouds is resolved into compression in the upper spreaders, which has to be added to the load that the intermediate shrouds have to carry. Added to which, when you do the mast loads for the vessel sailing with a reefed main only (no headsail), generally the loads in the intermediates are at their greatest (and the loads in the cap shrouds are quite small).

    Obviously the shroud angle is a function of the panel length and the spreader length and of course panel lengths are not always equal – but usually they are not so very unequal either.

    Anyway - it doesn't really matter – if you reduce the shroud angle the load in the shroud increases – which is basically what Steve W was asking!
     
  7. Paul B

    Paul B Previous Member

    The short answer, for most boats with conventional aluminum spars, is no, there is no structural reason not to shorten the 40+ inch spreaders 1.5" if you are moving the chainplates in a like amount. You should be OK if you move the chainplates inboard 1.5" and change the spreaders to fit.

    The reason for this is the way most designers come up with the requirements in the first place. Most of the calculations people use have a safety factor on top of a safety factor. I've sailed with rigs that have had both safety factors significantly altered, and the rigs were still rock solid. On top of that the selection of extrusions is limited, so the designer almost always has to use a tube that is already over the required moments.

    Now comes the interesting part. If you decrease the length of the lower spreader the angle that the upper shroud is bisected by it is changed. This means you may also want to change the length of the upper spreader to keep things balanced. Rigs can become tempermental to tune if these sorts of considerations are not taken into account.

    Despite what has been written in this thread, it is most often a good thing to have the lower spreader a bit longer than the CPW/2. You should measure what you have now to see if your setup already has some "poke". If it does you may want to mimic that in the new setup. Even if the current rig does not have this you might want to consider it.

    A lot of the things you might want to think about were part of a discussion here a couple of months ago, where a poster was re-rigging his Scampi 30. You might want to seek out and read that thread.

    As always, take what you read on the internet with a grain of salt. Or two.
     
  8. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    A lot of great info here, thanks guys. I have just returned from measuring the angles where the shrouds meet the mast wall, the caps are 10 degrees, and the intermediates are at 16.5 degrees, when i move the intermediates in 2" i get 15 degrees. The mast is 38ft, the bottom panel is 15ft,middle panel 11ft and top panel 11ft (the attatchment is 1ft down from the masthead). The top sreaders are 4'9" tip to tip, the lowers 6'11" tip to tip. The rig is from a Gemini 3200 and is going on a Gemini 3000 and the reason i want to narrow the staying base is that, while the distance between the door openings is the same on both boats the 3200 has a bit more meat in the inboard corner which allows for the chainplate location, if i move in a couple of inches i will have a better settup with continuity of wood fiber.

    Steve.
     
  9. Paul B

    Paul B Previous Member

    I took a look using the numbers you provided.

    The geometry looks a bit strange. The lower panel length looks about right, but the middle panel looks too short. The upper panel (11' + 1'), if you have the headstay at the top of the mast, is actually longer than the middle panel (11').

    Anyway, cutting 1" off the spreader would change the angle of the ints from 17.6 to 17.2 degrees. So not a big deal. You should be able to move the CPW an additional 0.5" to 1.0" (total 1.5" to 2.0") with no issues.

    There is a bit of a problem with the sketch. When changing the CPW/2 to 3.33 (with lower spreader CL to tip at 3.38) the Lower angle did not adjust. I've fixed that and the angle goes down from 12.99 to 12.50 degrees.

    With the narrow angle I would not cut the upper spreaders.

    Overall I was surprised how narrow this rig is on a catamaran.
     

    Attached Files:

  10. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Thanks for the work Paul, i think i will leave the upper spreaders alone and take 2 " off the lowers, this will get the chainplates where i want them. I agree about the narrow staying base for a cat but these are a narrow cat at 14ft and the rig is more typical of a mono. The original rig was a single spreader with a much wider staying base.

    Steve.
     
  11. Paul B

    Paul B Previous Member

    Be careful. Multis have so much more RM than monos the loads go up pretty fast if you are narrowing the CPW significantly.
     
  12. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Im not too concerned Paul, because the 3200 (which the rig is off) is the same boat as the 3000, the only major difference is that they installed this, taller double spreader rig, 2ft further foreward on the bulkhead, where it should have been in the first place. (in fact this was the location of the rig when it was the Aristocat 30 in the UK, before it became the Gemini in the US) Balance should not change as the centerboards are in exactly the same position and the center of effort is essentially unchanged. The only change i am making (apart from major beefing up due to poor construction) is moving the chainplates inboard a couple of inches, i suppose i could do this without shortening the spreaders but the caps would not be paralell to the mast and may look a little strange, or it may not even be noticeable. Which way would you go?

    Steve.
     
  13. Paul B

    Paul B Previous Member

    I have not been in the business for many years, but the spar designs done for rigs of this type generally had the lower spreader a bit longer than the CPW. I have the calculation sheets for a couple of the top spar builders back then, a couple of the biggest name designers, and one of the top sailmakers. All showed a bit of "poke" on the lower spreaders.

    I personally know of thousands of rigs built like this. Standing at the dock you probably would not notice a couple of degrees on the V1s, but it is there.

    I know there are books that show the V1s going the other way, with CPW wider than the lower spreaders. I don't know why they do this. The length of the lower spreader is never the limiter in sheeting angle. I also don't know if the people writing these books have ever had an actual boat built.

    If it was me I would not cut the lower spreaders 2" and move the CPl so it was directly under the spreader. I'm attachig a mod of the original layout of yours. I think it represents cutting the spreader 1" and moving the CPl 1.5". You can see the "poke" of the upper spreader and the lower spreader are very balanced (0.62 and 0.60).

    If you cut the spreader 2" and have the CPl directly below you would see the "poke" of the lower spreader much less than the upper. If the rig is not set up very firm and you get some sag you might end up with no "poke" at all.
     

    Attached Files:

  14. debenriver
    Joined: Sep 2012
    Posts: 3
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Rockland Maine USA and Woodbridge England

    debenriver New Member

    The length of the lower spreaders limits the genoa sheeting angle by virtue of the wires coming down to the deck, not because the genoa chafes on the spreader ends - that is usually the upper spreaders on a double spreader rig, which is one reason why the upper spreaders are generally as short as possible – the sheets tend to foul the V1, V2 and D1 wires. So if the rig base can be narrower you get a more convenient sheeting arrangement, specially for a deck-sweeper or other low-footed genoa.

    Cutting 1" off the spreaders and moving the shroudplates in 1.5" to 2" seems an eminently sensible solution to your problem. The load in D2 (for a 1" shortening) will increase by about 2.3%.

    Do you have single or double lower shrouds (D1)?. If single, and they will move in as well then the load in D1 (for a 2" move) will increase as the angle narrows from 12.7º to 12.1º – the increase in load will be about 4.6%.

    Designers do try to produce the lightest rig that will hold the spar up safely and I don't agree that they generally apply safety factor on safety factor. Safety factors for a double spreader rig are generally 3 for the masthead shrouds (D3 - V2), 2.3 for the intermediates (D2) and either 2.5 or 2.8 for the lowers (D1) depending on whether there are double or single lowers. Also if V1 is discontinuous (i.e. V2 does not come down to the deck) then the safety factor in V1 is usually increased to 3.2.

    Of course we are limited by the materials available so we can only go from, say, 8mm diameter wire rope to 9mm – there is nothing much in between and so we have to choose the next largest size unless the smaller size is very close.

    Same with the mast tube, though here there is a much bigger variety of sections/wall thicknesses available.

    What I am saying is that good designers generally don't over-engineer rigs. One very good reason for this is because the one thing they don't want is unnecessary weight aloft.
     

  15. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    I have double lower spreaders with the forward ones being in line with the mast rather than running forward and the aft ones running aft. Im now thinking, after reading the last couple of posts that i may just leave the spreaders alone and move the chainplates in an inch or two.

    Steve.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.