Sprit Rig Design

Discussion in 'Sailboats' started by LP, Mar 20, 2010.

  1. LP
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    LP Flying Boatman

    I'm working a design for a light displacement coastal cruiser with a dual sprit rig and am looking for a couple of finer point on the design of the rig. Attached is the current sailplan.

    My first question is with regard for the sprit an the sail peak. Is the goal to ideally bisect the angle between the head and the leech of the sail? My current aft sprit is 210" long and seem a bit excessive. It's almost two feet longer than the associated mast. In an attempt to shorten the sprit, I raised the foot of the sprit several feet. See the forward sail in the attached plan. I'm afraid that the geometry of the forward sail will not work as there looks to be too much aft geometry in the sprit.

    My second question is about reefing. I like the ability to scandalize the sails as it is a major reduction in sail area and lower the center of effort tremendously, but can a sprit rig be designed with a more conventional manner before scandalizing? I believe it would be nice to drop a few off of the sails by reefing as scandalizing is such a major reduction in sail area.

    I'm also open to other input if anyone sees any glaring deficiencies in the current rig design.
     

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  2. gonzo
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    gonzo Senior Member

    The higher the peak the longer the sprit. They are traditionally more less square. I had a 34 Wharram ketch with sprits. The spar in the main was almost 40'long. In rough weather it beat me black and blue ( no exageration) more than once. Aside from that , I loved the short masts. For better performance you need vangs to the end of the sprit. You can adjust the shape of the sail by lifting and lowering the attachment point of the snotter.
     
  3. LP
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    LP Flying Boatman

    Thanks for the reply, Gonz.

    A couple of questions. Did you mean "traditionally, the are more (OR) less square"? As far as vanging goes, are you talking about the attach point on the mast? I was considering a cheek block with a line run to the cockpit for adjusting the sprit. Do you think that could be realistically possible. You' have to excuse my lack of knowledge on the rig as few of my books have any specifics on the rig.

    Do you think that short masts would be a possibility?
     
  4. sharpii2
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    sharpii2 Senior Member

    Sprit sails are notoriously difficult to slab reef.

    The reason is that the snotter, or sling that the butt of the sprit sits in, has to be lowered or slacked off. There is only so much room before the butt of the sprit starts hitting the deck.

    An alternative plan would be to reef up, meaning bundling the sail below the sprit and snotter, leaving them both in place. This only accomplishes the objective of reducing the sail area. It does not reduce its Vertical Center of Area. But it does get the foot of the sail, and the flailing block, if it is boomless, further away from the crew.

    Some large sprit rigs had huge top sails bent to the same mast. Reefing was accomplished by striking the topsail.
     
  5. LP
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    LP Flying Boatman

    Thanks Sharpii,

    In the fore rig, I have raised the foot/butt of the sprit to evaluated the geometry of the design. The reasons for the change would be to shorted the shorten the sprit and increase the height of the foot end of the sprit in the event that a slab reef arrangement might might be possible. One of my concerns with the new design is the bending loads imposed by the snotter in it's new, higher position.

    Initially, the slab reef option sounds like a good idea, but the added level of complexity (design wise and implementation wise) starts to deviative from the simplicity I desire in the design.

    I'm beginning to think I need to concentrate making the sprits flexible enough to depower in gusts until wind strength makes scandelizing necessary.
     
  6. messabout
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    messabout Senior Member

    It seems to me that flexible sprits are going in the wrong direction. If the sprit flexes the sail will become fuller. Don't you want to flatten the sail when the wind pipes up? If you want to flatten the sail you can do it with mast bend. The sprit puts a beam load on the mast and tends to bend it. That implies that the sprit needs a good measure of rigidity.

    Have you considered a sprit boom rig as opposed to peak sprit? Use a round unstayed mast and when it is desired to reef, drop the boom off the sail and rotate the mast to roll the sail as much as you want. This will work well enough on the ketch rig because you can maintain balance by rolling one sail more or less than the other. In normal operation with the boom in place you can make the sail fuller or flatter with judicious use of the snotter. Sprits for this kind of sail are shorter than the peak sprit type and they are not as far aloft..
     
  7. sharpii2
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    sharpii2 Senior Member

    You don't want to make your sprits more flexible.

    If they bend, the sail will get fuller and baggier, the opposite of what you want in a windy situation.

    It's the Boom and possibly the Mast that you want to be more flexible.

    If the Boom and Mast bend slightly and the sprit stays stiff, the sail will tend to flatten. Especially if the Boom has an adjustable vang on it. If it has a fixed vang, putting more tension on the snotter will accomplish the same effect. But the boom must have some kind of vang to hold it down for this to work.

    The old school way of shortening sail on rigs like these was to have a third mast step into which only one of the two masts was stepped. The other mast was either taken down or left ashore.

    The theme seems to be consistent. This type of sail was not really meant to be reefed. Scandalizing was meant to deal with sudden and brief gusts, not long term excessive winds.

    If you are willing to go with tandem centerboards, one of the two sails can be completely struck, probably the aft one, and the centerboard nearest to it can be raised to move the Center of Lateral Area (CLA) forward, so the boat will balance with just the remaining sail up.
     
  8. LP
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    LP Flying Boatman

    Thanks, guys.

    Fortunately, I don't claim to be an expert so humble pie is not necessarily a bad thing. My assumption was that flex would be good in depowering, but a basic understanding of the rig is most useful too. Point taken on mast flex as opposed to sprit flex. There is lots of commentary on the web about certain designer knowing how to build rigs that depower themselves, but I've little textual information on the designing process,.......so far.

    I like the high peaked sprit for aspect ratio reasons and the shorter associated mast. I'm not eager to go with the sprit boom as I will need to lengthen the masts. I'm working towards lightness in the rig altogether, but also in the individual spars. Admittedly, a sprit boom can be self vanging and can contribute to the simplicity of the design. The addition of vangs and so forth takes away some of the appeal for the peaked sprit rig, for me anyways.

    I like the idea of a third mast step and will look at that prospect.

    I was originally going to go boomless on the foreward sail, but fear now that I will have limited ability to control sail shape. Especially when sailing off of the wind. Comments?
     
  9. sharpii2
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    sharpii2 Senior Member

    Hi, LP.

    Looking more closely at your drawing, I can see you out the board just aft the cabin. My guess is that you did this not spoil the cabin plan.

    So, instead of furling the aft sail to reef, I would furl the fore sail. The fore sail appears to be boomless. This would work well with my new scheme.

    A braille line could be wrapped around the fore sail and left standing. To strike the foresail, all you would have to do is relieve the sheet line and then pull on the braille line. The braille line would pull the sail and sprit against the mast in one more or less neat bundle. Then the board could be raised as much as needed to restore balance sailing under the aft sail only. The windage of the struck foresail bundle would help keep the boat from rounding up into the wind.

    This would effectively give you just one reef point. Either all plain sail or only half. Such is the price of simplicity. But you would be able to reef much quicker with a lot less equipment than is typical.

    If you go with a boom on your foresail, I suggest that you leave the sail loose footed so the out haul on the end of the boom acts as a second sheet line. My braille scheme would wok with that setup as well.
     
  10. gonzo
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    gonzo Senior Member

    When you reef, the peak gets attached lower in the sprit. The vang goes to the peak to control twist. Yes I meant more or less square.
     
  11. LP
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    LP Flying Boatman

    Flexing masts implies an unstayed rig and that may have already been mentioned here. One of my design dilemmas was/is the weight vs. windage concerns. I had chosen to go with the added windage to save on weight aloft. The unstayed mast weighed more that twice as much as the stayed mast. I don't mind going unstayed as it falls on the simplicity side of the scale.

    Would it make any sense to stay the masts at 1/2 to 3/4 span to allow for flex, but also try to maintain a lighter rig? Actually, that is a moot point as I am planning lacing the luff to the masts. Weight, as I see it, is a big issue here as this this is a lightly ballasted boat, 25%, in a shallow draft design.

    Does anyone know of a good resource specific to sprit rig design? I fear that standard mast design criteria are going to leave me with a mast that is too stiff to perform as required for this type of design.
     
  12. sharpii2
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    sharpii2 Senior Member

    I would go with rigid masts, as they can be made hollow and light without too much trouble. I don't know how well a sprit sail would do with a bendy mast. I imagine not well as the snotter has to hang from the mast. Also, a bendy mast is only good if it bends the way you want it to. If it bends to leeward instead of aft, it is defeating the purpose.

    I suppose a reasonable argument could be made for making the boom bendy on the aft sail. Convex curve could be cut in the foot of the aft sail and the foot could be laced to the boom. A vang could be attached to the boom to pull it down, causing the sail to flatten. The problem here is that this will complicate furling the sail tremendously, as the sprit would have to be removed and a halyard would have to be used to bring the throat of the sail down. An out haul on the sprit to pull the peak of the sail up to the top of the sprit is an alternative to having to unship the sprit, with this arrangement, but it will effectively double the compression on the sprit.

    A better way of controlling the fullness of the sail would be to use a rigid boom, a loose foot with a parallel attached to the clew (lower aft corner) of the sail, and an out haul. The out haul could then to pull the clew aft, to flatten the sail, or to ease it forward to make the sail fuller. the parallel keeps the clew attached to the Boom, as it is eased forward. It will need a secondary line to hold the clew to it

    The sheet block should then be attached to the mast behind the clew. This is for two reasons.

    1.) to minimize bending loads on the Boom, allowing it to be of smaller section, and
    2.) To allow the clew parallel to be eased forward all the way to the mast. This way the sail can be left attached to the mast and the sprit could be left standing in its snotter. A Braille line could then bundle the whole thing to the mast as the secondary line holding the clew to the parallel is eased.

    This way sail could be set or struck with almost supernatural quickness.

    I think your sails are peeked too high. Looking at lots of pictures of sprit sails, they all seem to have head angles of 0 to 45 degrees from level. Making the mast slightly taller and the sprit slightly shorter will not change your top hamper much, if at all. And the shorter sprit, which as to be rigid will be much easier to deal with. The sail will most likely set better as well.
     
  13. LP
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    LP Flying Boatman

    Thanks Sharpii,

    All very informative.
    You'll have to forgive my ineptness. I'm trying to visualize what dynamics would be taking place if the mast was to bend off aftward or to leeward. My thought was that if the rig did flex to leeward or aft, the sprit would fall off the leeward and depower. Are you saying that the sail will camber up and power up because of the flex? What if the flex was limited to the upper portions of the mast (above the snotter so that sprit tension is conserved)and it was stayed in a manner that allows for aft flex. Question though. If the mast was bendy, could snotter tension be used to flex the middle section of the mast forward (or the upper portion aft) to pull fullness out of the sail to depower? I just reread an earlier post and you and messabout both mention mast flex as a means to flatten the sail. I'm thinking that a mast that is only stayed laterally, but free to flex longitudinally might work towards flattening the sail in addition clew tensioning as you also mention. Do you thinking I'm over doing it? I could restay the mast if I felt it was too bendy.


    I'm not sure I fully understand your unshipping the sprit and needing a halyard to bring the throat down. Perhaps, you are talking about a standard halyard arrangement for raising and lowering the throat. I guess with a brailing system, you never really lower the sail. The rigid boom makes more sense than the flexible one. Sheeting would have to be from the end or forward to a block (just like you said) without intermediate attachments on the boom with your parallel system. I have never used a brailing system, but I like the idea of not having a bundle of sails over my head when the sails drop.


    The head angle is only a few degrees above 45. I suppose you are right about the sail setting better. I was trying to push the envelope and gain a little bit in aspect ratio.

    Thanks.

    I drew up the outhaul and sheet arrangement. Outhaul is grayish. The parallel is yellowish. I drew a strop to hold the sheet block in greenish and the sheet is blueish. Sheeish. :D

    I suspect there will be a braille line attached to the outhaul also. I may persue more discussion of the braille in another post.
     

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  14. sharpii2
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    sharpii2 Senior Member

    It's an interesting idea. A possible alternative to roller furling. A mini voyager I'm designing ended up with a braille furling system on its tiny mizzen, so the sail could be simplified, set and struck without having to stand on the tiny deck. The jib on the tiny fore deck has a down haul so it too can be set and struck remotely with relatively crude equipment. Only the main, in the middle of the boat is dealt with conventionally.
     

  15. LP
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    LP Flying Boatman

    Google...Google....Google....

    Bouncing around the web can sometimes be fruitfull. Here is a link to a design that had a peaked sprit. It has the higher peaked sprit similar to what I have drawn up and it has slab reef capababilities!

    http://www.vivierboats.com/Img/elorn_en.pdf

    I've also located a tid bit of history about the basis of my design.

    http://www.goole-on-the-web.org.uk/main.php?key=672

    And one more link that was my original inspiration for the design.

    http://www.duckworksbbs.com/plans/gavin/bluestone/index.htm

    They also have the high peaked sprit so I'm feeling comforable in that aspect of my design. I do feel that the foot of my sprit was located too low and so have moved it up to midspan on the mast. This also puts it on a more opportune location to utilize snotter tension to induce mast bend. I ran across this elsewhere in the forum.

    "I don't know how much time you have spent racing on boats with sprit sails (by which I am assuming that you mean a leg of mutton sail with a sprit, rather than a quadralateral sail with a sprit) but the snotter takes the place of the traveler or boom vang in controlling twist. If you are racing a boat with a sprit sail, (especially if the boat is as big as 8.5 meters) you adjust the snotter almost as often as you would have adjusted the traveler only it is a much harder adjustment to make and it also changes mast bend adversely (pulling bend out of the mast just when you need more mast bend to depower). Also as you ease the sail onto a reach the snotter needs to be loosened because the snotter tightens as it twists around the mast, flattening the sail when you would normally like to power it up.

    There are other issues with sprits such as the chafe on the sails, and the ability to sail faster on one tack vs the other which adds an artificial tactical decision to the race. Reefing is more difficult. All and all sprits make sense for small non-performance oriented boats or low performance trainers like an Opti but are not very suitable for bigger race boats."


    Though not specific to the peak sprit it does offer insight into using mast bend with a sprit to depower your rig. Tightening the snotter will induce a bow in the mast, if it is dimensioned properly, flatten the sail and depower it. I think the the sail will set ok. It looks to me that in a higher peaked sail, You would need less aft tension and more upward tension to set the sail, where as, a lower A/R would need more aftward tension at the head to spead the rig properly. The inherent angle of the sprit in each case appears to promote the most desired tensioning in each respective case. Additionally, there is less bend moment put on the mast in the high peaked sprit as the sprit is more vertical. Because of the shorter chord length, I could see less bend would be needed, but I could also see a greater effect of power reduction on what bend did take place.

    It was also good to see the slab reef option in the Vivier design. Another benefit of shortening he sprit to move it to mid-span. I allows more room for slab reefing and still keep the foot of the sprit clear of the deck.
     
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