View Full Version : Sprits'l boom
06-15-2007, 08:37 AM
I know this has been covered before, but I don't recall it being a thread of its own.
I'm watching a program on Capt Morgan and the survey team searching for his ship is, I think, in the Caribbean. A boat sailed near and it was about the size of mine--16 feet--sprits'l andjib. I don't use a boom and I've been thinking of doing so. The boat I just saw uses a boom, loose-footed. Aha! Think I, folks who sail it 'for real,' make their living with it, use a boom.
Tells me it's a better rig than my boomless. Right now, running before the wind sucks, and this would help. Also help full the sail in light winds.
My mast and spar are bamboo (work great, BTW) and, were I to use one, the boom would be, too. The only drawback I can think of would be the boom swinging 'cross the deck. It shouldn't be a problem brailing.
06-15-2007, 09:05 AM
Go for it -- hopefully your sail isn't so low that the boom will hit anybody.
What method will you use to attach the boom to the mast -- a snotter and cleat, gaff jaw, or other hardware?
06-15-2007, 09:39 AM
I've been sailing this 14' sharpie lately with and without the boom, I intend to change things up a bit and make adding or removing the boom easy while on the water.
The loose footed sail seems much easier to deal with when it picks up and I want to drop the rig in mixed conditions which is a pretty common event where I sail.
I'm new to the rig and learning as I go. dennyhttp://i56.photobucket.com/albums/g193/olypen/catboat2002.jpg
06-15-2007, 09:46 AM
Nice picture...I whipped up a rig last week for a canoe and took it down to Rend lake last weekend. It ended up too small so I ordered a Polytarp kit to do it larger and right. I think I'll add a boom but I bet it's gonna BOOM me until I get used to it.
06-15-2007, 01:11 PM
This thread may be of some interest:
since it shows some nice pics of a sprits'l boom, and a short discussion of it.
At first I had the main loose footed and boomless. It was a bit hard to set it well. I added a sprit boom, still loose footed and it was much easier to handle.
06-15-2007, 02:56 PM
P.A. if you want to brail it normally, you're most likely going to need to remove the boom first, or at least have some system where you can unhook the outhaul and drop the aft end of the boom. There are folks who have rigged up brailing systems that pull the boom up to vertical in the process and brail the whole works, but you'll need to decide whether that's a practical option.
As a general tip, there really is no reason to ever get hit in the head by the boom as long as you watch your angles and stay out of situations that might cause an unexpected jibe (sailing by the lee). When you go to tack, walt until that instant where the sail luffs, reach up with one hand and simply pass the boom over your head to the new side. It takes about a second to do and works a heck of a lot better than hunkering down in the cockpit and waiting for the boom to wander across on it's own. The only trick is that you can't force it. You need to wait for that short period when the sail is slack and the boom isn't loaded with wind pressure. If you learn to do this your dinghy tacks will most likely be faster, crisper and you will get comfortable with rigging the boom as low as chest-high when needed to reduce heeling in high winds.
06-15-2007, 10:14 PM
You don't really want a boom. What you want is a second sprit rigged out to the clew like this -- with a snotter. This difference is important -- a boom will ride up and you'll lose the sail shape. A sprit, which will be angled downward, will hold the sail shape perfectly once you get the angle and tension on the snotter right.
Also, the sprit rides higher and is less likely to catch someone's head. The chap above has both sprits on the same side of the sail. I always put them on opposite sides, which tends to even out the loss due to having the sprits putting a wrinkle in the sail on one tack or the other.
And yes, you'll get a noticeable improvement on all points of sail.
06-15-2007, 10:45 PM
I did a piece in WB on this. Basically if you cut your spritsail so that the clew is high and the tack is low, you will not get hit in the head with it.
I brail my boomed sprit simply with a line that starts at the throat, down through a bee hole under the boom and up to a ring lashed to the throat then down. When you give it a yank the whole thing folds up on the mast. You need to play with the slack in your downhaul lashing so and it helps to use jaws. The WB article has pictures.
06-16-2007, 12:24 AM
The sprit-boom that wox mentions is a popular option and might be a good choice. By nature, it tends to be self-vanging, meaning that the configuration of the sail's foot and the sprit-boom automatically limits the amount that the boom's tail end can rise - unlike a boom hinged behind a mast with a gooseneck or jaws. The major effect that this has on the sail itself is that it prevents twist to leeward in the upper part of the sail, keeping the entire sail working. It essentially swings port and starboard like a barn door. A balanced lug with the boom extending forward of the mast will tend to do the same.
It should be noted though that this isn't always ideal in all conditions. Sail twist can rob you of power, but it can also be beneficial at times. In heavy air being able to let the end of the boom lift a bit, allowing the sail's top to twist off to leeward can be a prime way of depowering the rig to save your bacon. It also can help as a shock-absorber if you're getting a lot of strong gusts. The all-on or all-off nature of balanced lugs and sprit-boomed sails may not be your best friend if you get caught out in a blow. If you're sailing with a sprit-boomed sprit-sail and things start to get a bit hairy, the best thing to do may well be to stow the boom and induce more twist in the sail.
It's also worth knowing that the wind itself is "twisted" a bit at the top of the mast. Interaction with the surface of the water actually slows the wind down a bit. As you get higher above the water, the wind speed increases slightly and at the top of the mast, your aparent wind may be skewed slightly to leeward of the wind at deck or boom level. Modern sail design computer programs model the sails in 3-D and one of the reasons is to account for twist and design it into the cut. It's not unusual to see 8-10 degrees of twist designed and built into a modern, computer-cut mainsail.
A sail that swings like a barn door and can't twist to match the wind may well end up trimmed fine at the boom and over-trimmed up top with no way to fix it. If you've ever sailed a boat with a tallish mainsail and just couldn't ever seem to get both the upper and lower telltales flying properly and together, it's pretty likely that the problem was being caused by an improper adjustment in the amount of sail twist - usually not enough up top.
Luckily, on our typical wooden boat sprit or lugsail with a fairly low aspect ratio and traditional rigging this isn't a particularly big deal. The rig isn't that tall or drum-tight and even a sprit-boom or balanced lug is likely to twist a little bit whether you want it to or not. The inability to depower using twist is a little more important and for that reason I hesitate to give sprit-booms a thumbs-up for being a great choice in all conditions or on all points of sail. You can be pretty sure that during a typical afternoon's outing there will be a certain percentage of the time where the top of your sail may look just fine, but in reality is over-trimmed and not working as hard as it should be. Adding two or three telltales, spread along the mainsail leech is the best way to see what's actually happening.
06-16-2007, 07:36 AM
Excellent info...not much out there on these less favored rigs...thanks. I am looking at making a polytarp of about 60 sq ft...what are the typical range of angles (from horizontal) that a sprit Boom should be set at and what affects does changing the angle have?
06-16-2007, 07:45 AM
I've owned a number of boats with sprits on Leg-O-Mutton Spritsails (triangular) instead of booms, and much prefer them. My Melon Seed Skiff has a pair of sprits, one up to the peake, and one down to the clew.
06-16-2007, 08:53 AM
The only drawback I find with sprit-booms is the need to go forward in a small boat to faff around with the snotter -- not always the best place for most of the boat's ballast, no matter how temporarily.
06-16-2007, 02:47 PM
Steve, I think there are too many variables, boat-to-boat and sail-to-sail to come up with an "ideal" sprit-boom angle, but there are some important aspects to consider and the sea trials are probably best done with the ability to move the sprit and snotter up and down the mast for testing (even if it means duct taping the snotter in place for test purposes). The boom angle can vary from nearly horizontal to being as much as 30-40 degrees with the mast end always at least a little bit higher than the tail end. With all the factors which contribute to the answer, it's actually pretty complex for a silly old wood boat but I'll try not to go off the deep end and bury you with too much techno-B.S.
First of all, you need to generate reasonable tension along both the sail's leech and foot at the same time. A steeper sprit-boom angle increases tension on the sail's leech and reduces it, to some extent, on the foot. A more horizontal angle increases foot tension and reduces leech tension to some extent. You want both edges working, so the key is to find a boom angle which will keep both edges reasonably firm, but not so tight that you're over-stressing the cloth and seams.
Snotter tension then becomes a major factor as it determines how much draft you're putting into the sail. With a lot of tension, the sail can be pulled flat (or nearly flat, depending upon it's cut) and reduced snotter tension creates a sail with deeper draft. Rule of thumb: In high winds or very light winds - use a flatter sail to keep the laminar flow attached better. Moderate winds - ease the snotter a bit and put some draft in the sail. In a nutshell, moderate winds will "follow" the curve of the sail pretty well if it's trimmed properly and in doing so they generate the lift, power, etc. that you need to pull or push the boat along. High winds, or very light winds, are somewhat less cooperative. If the curve is to dramatic, they tend to follow it to a point, but then may separate and go their own ways. When they leave, they take your lift and power with them. So your snotter tension should be adjusted to match the conditions of that day's sailing.
Now the question becomes "Does my sprit-boom angle still keep decent tension on both the leech and foot edges as I adjust the snotter tension and change sail draft?" Through experimentation, you may find that certain boom angles have a broader range of good performance than others do - or you might find that having some sort of boom-angle adjustment system really makes a performance difference. Obviously, we want to keep the system and it's rigging as simple as possible, but it pays to have a general idea of what forces are at work and what they're potentially doing.
You also can't talk about sprit-booms without mentioning the "bow-and-arrow-effect". There will usually be a certain amount of luff curve cut into the sail's luff edge. It may be drawn as a straight line on the sailplan and attached to a straight mast, but on the actual sail it will be a slightly convex curve with maximum round about 45% of the luff's length above the tack corner. When connected to the straight mast, this excess fabric gets forced back into the body of the sail and forms draft. Now think of the mast as our bow, the sprit-boom as our arrow and the sail's leech and foot edges as the bowstring.... When we pull the arrow back (by tensioning the snotter) the bow (mast) bends, just like it does in archery.
Let's say our sail was cut with a typical amount of draft-producing luff round - maybe 1"-2". If we tension our snotter and the Bow-and-Arrow Effect bends the mast 1"-2" guess what? We just lost the effect of our luff round and our sail draft is gone. In high winds, this might be just what the doctor ordered as a depowering tool and to help keep the flow attached. If, on the other hand, it's happening in medium winds where you want some draft, it may be working against you. So mast stiffness and resistance to excessive bending may be another important factor in getting the rig to perform. Boom angle can contribute here as well as certain angles may tend to bend the mast more than others - or in different places (see, I told you this stuff was pretty complex for something that looks so simple).
Chapter III - A "triangle of tension" is formed along the bottom of the sail. The borders of this triangle are the sprit-boom (from the mast to the clew corner, forming the triangle's top side) the lower luff, between the intersection where the boom crosses the mast and the sail's tack corner (forming the front side) and a line of tension which forms between the tack corner and clew corner of the sail becomes the bottom side of the triangle.
Once everything is rigged and cleated-off, this triangle becomes pretty rigid. It can swing side to side as if hinged to the mast along the luff (barn door) but that's about it. The clew corner can't rise or fall much at all. This is the self-vanging, twist-limiting part of the sprit-boom program. A steeper sprit-boom angle makes for a taller and more rigid triangle that resists clew-lift better than a shallower, more horizontal sprit-boom angle, so even here, some boom angles may be more effective than others. If your seeking to resist sail twist, a steeper boom angle is the best choice. On a high wind day when you could probably benefit from the depowering and shock-absorbing nature of a sail having more twist up top, a lower, more horizontal boom angle might be a better choice. This, along with previously mentioned factors might make a good case for brainstorming some sort of boom-angle adjustability into your system - as long as it can be kept dependable enough and simple enough that it doesn't take all the fun out of having a good, simple rig in the first place.
One last thing... I drew the sail above with some foot round to illustrate a point. A sprit-boom generates some fairly serious tension along that black arrow, in a straight line between the tack and clew corners. But - it generates almost no tension below the black arrow. If you build a sail with a nice-looking round on the bottom, that portion will likely flap all day long as you sail. The tension along the very bottom edge that is supposed to be making it a nice cupped shape has been stolen by the sprit-boom and is now concentrated along the black arrow. As a result, it's best to cut sprit-boomed sails with a straight foot which follows that tack-to-clew line and doesn't have anything hanging below it. If you were to cut it with a round, as shown above, that portion would need to be carefully and fairly severely cupped in order to try to snug up the hemmed edge and keep it from flapping. If you are converting an old sail to a sprit-boom and it has foot round, it may be something that you want to have re-cut and removed.
So those are the factors at play when you rig a sprit-boom - at least the ones that I can think of off the top of my head. Do you need to know all this stuff to get your boat to work? Of course not. Many people sail sprit-boomed rigs very successfully without knowing any of this junk and the approach of just hanging a pole up there and going sailing certainly has it's appeal. But those are some of the issues to keep filed in a spare droor in the back of your mind if you're building a rig and want to do some performance testing in an effort to get the most out of it.
06-16-2007, 07:38 PM
The all-on or all-off nature of balanced lugs and sprit-boomed sails may not be your best friend if you get caught out in a blow.
Yep, and the local lakes can get very gusty on many summer afternoons. But the key is to rig your boat so everything can be adjusted. Move the snotter for the sprit boom lower, and you'll get more twist and a more forgiving rig. It's plenty of fun to mess around with this stuff, as we all know, and in the messing you learn a thing or two. And there's no need to get fancy -- learn a couple of knots and you won't even need duct tape. And you may even find you don't need any hardware, either.
Just wear a life jacket during the expermiments, and don't have anything loose in the boat you wouldn't want to lose, and it'll be all that much more fun.
Todd, I'm always amazed at the time and patience you have explaining everything in detail and tossing in the fancy graphics. But after all is said and done, you just gotta get out there and play with your boat.
06-16-2007, 09:42 PM
Absolutely true. There is no substitute for experience and practice. At the same time, if you're designing, building, testing and evaluating your own sailing rigs, there is equally no substitute for knowledge. Arming yourself with some fairly basic information can save you a lot of frustration and time spent out on the water trying to figure out what's going on and how to solve problems or improve your efficiency, especially if you don't have other, similar boats to sail with and to measure your performance against.
One of the things I like best about working with small boats is the variety of innovative, unconventional and sometimes even somewhat crazy rigging ideas that their sailors come up with. Even so, a boat and it's sails still have to interact with the water and the wind and a fair portion of that interaction is predictable if you have a basic understanding of the forces involved and the characteristics of different types of rigs and sails.
06-17-2007, 06:45 AM
Hi, Todd, I have a couple of questions about setting a spritsail for you if you don't mind:
1. My sail creases (3 wrinkles in a tight fan shape) running from throat to clew. I can't seem to iron them out no matter what I do -- tightening the snotter on the sprit, tightening the luff, tightening the clew outhaul (the sail is set loose footed on a boom). I'm beginning to wonder if the sprit is too short and set too high on the mast, although it conforms to the plan set by the designer. I've thought also that it might be the different tension of the sprit on the sail from one tack to the next, but the crease does not seem to diminish noticably no matter which side of the mast the sail stands. It might be too that the sail (6 years old) has just been stretched out of shape and I'm ready for a new one. Do you have an idea about what might be going on here?
2. When raising the sail, how taught should the luff be, just so, or should it be given as much tension as the halyard can stand?
Any advice here would be appreciated. Thanks.
06-17-2007, 07:10 AM
That's me with my sharpie in Woxbox' photo.
I sailed it with boomless loose footed main for the first year.
The mizzen was rigged as the main in the photo from the start.
I found the main impossible to tame at times without the boom. The sheeting angles were poor as well. With the boom it was like a different boat.
The main boom was my first hollow spar, light and strong.
It may have been the way I had it rigged, but there was no way to depower by letting the sail twist, instead the sail took over.
I couldn't get the angle to flatten it and sheet it in.
Reefing would help, but was necessary a lot sooner than with the boom.
I'm not saying it couldn't have been made to work better without the boom, just that the boom worked for me.
The main then sheeted to a block/camcleat at the aft end of the centerboard case, I think it's three to one. This solved the problem of how to sheet around the mizzen too.
06-17-2007, 01:47 PM
Here is the sprit rig I designed for a Banks Dory, a Schooner Dory!
[IMG]http://farm1.static.flickr.com/84/413064031_0882e1611a_m.jpg (http://www.flickr.com/photos/dansdories/413064031/in/set-72157594574329966/)http://farm1.static.flickr.com/167/413099656_aabb40feb3_m.jpg (http://www.flickr.com/photos/dansdories/413099656/in/set-72157594574303785/)http://farm1.static.flickr.com/162/413058554_4de0e98210_m.jpg (http://www.flickr.com/photos/dansdories/413058554/in/set-72157594574329966/)http://farm1.static.flickr.com/187/413064526_b2c6822746_m.jpg (http://www.flickr.com/photos/dansdories/413064526/in/set-72157594574329966/)
06-17-2007, 06:58 PM
Sharpie, it's hard to get very specific long distance and without seeing the sail. Since sails aren't flat and we hang them up by their corners and edges, somewhere in there between the spars there is some extra fabric waiting to be distributed. Big diagonal wrinkles usually indicate that there is more tension along the wrinkle line than there is on the surrounding fabric. The extra fabric is being pulled into a wrinkle by the tension and there aren't enough forces pulling in opposing directions to spread it back out and remove the wrinkle. Since the sprit is our only real means of generating some tension to oppose the wrinkle, it's probably the place to start and a longer sprit that is anchored lower on the mast should be able to exert more opposing tension. A hunk of closet pole and some jury-rigging in the back yard might give you an idea whether it will help or not.
On four-sided sails, the throat are is a bit touchy and generally won't tolerate an awful lot of 3-D shape. Any panel-shaping and/or broadseaming that might lie in that area should be pretty subtle and fabric thread-line orientation of the corner patches should be aligned as closely as possible to the base fabric making up the panels. Failure to do either of both of these things can generate fan-shaped wrinkles in the throat area, though they rarely extend all the way across the sail. Usually it's pretty localized and simply makes for a lumpy or wrinkled throat area.
As a general guideline, start with your halyard tension snug enough to remove any wrinkles along the luff (you still in some cases will have a few small ones in the luff tape reinforcing the edge, but it's the panels themselves that matter). The same would be true for any edge attached to a spar (boom, gaff, yard). Increasing tension beyond this point begins to gather excess fabric along the spar, reducing sail draft and in the case of the luff, generally moving the sail's draft forward. This could be helpful in heavy air or when sailing close-hauled, but isn't something you want happening all the time.
So my best suggestion would be to check your halyard tension and then check that the luff-to-mast attachment (lacing or whatever you have) is as even as possible all the way up and down the luff and then start experimenting with the sprit length, tension and angle. You may even want to try setting the boat up with the snotter tight enough to put in a slight wrinkle from the peak down to the mast, along the sprit and then see if wind pressure even things out when you get out on the water.
06-17-2007, 08:38 PM
Thanks, Todd. After writing this morning, I thought about the luff ties. I switched to an arrangement like J. Dillon's where I have a toggle holding the robands and sail to the mast. I gave them some extra slack so that I could raise and lower the sail without too much trouble and this means that the luff stands about 1.5 inches or so off the mast near the halyard sheave whereas before I used no halyard and simply lashed the throat to the masthead and had luff ties that held the luff close to the mast. The extra play in toggled robands might be some of the cause. I will simply have to fiddle some more. Thanks for the observations, I am grateful for them.
06-17-2007, 08:59 PM
Who's building all these spritsails with the fabric panels pointing funny directions? You can cross-cut them (panels perpendicular to the leech), vertically-cut them (panels parallel to the leech) mitre-cut them (panels perpendicular to both leech and foot) or even scotch-mitre them with panels parallel to both leech and foot, but panels striking the leech on diagonals are notorious for prematurely stretching out and flapping.
06-18-2007, 08:17 AM
The only drawback I find with sprit-booms is the need to go forward in a small boat to faff around with the snotter -- not always the best place for most of the boat's ballast, no matter how temporarily.
I agree with the folly of going forward on a small boat. I rigged the snotter on my sprit sail to run back to the dagger board trunk. I also added a block and made it 2 fall for better adjustability. I find 3/8" difference on the tension makes a world of difference in the sail trim. As the wind conditions vary so does the required tension. Sprits are great fun to sail as there is always an adjustment to mess with on basically a one string sail.
06-18-2007, 12:09 PM
Hi Todd +all
Very good points about sail construction.
The funny fabric pannels would be me (think the main sail is bad take a look at the fore sail). The Rowing Club had a dory and a bugget of just under 100$ to recondition the dory, build two masts, booms, sprits, partners, a reinforceing bulkhead, rudder and hardware to hang it, running rigging, and sails! the sails are cut from the mouse infested jib of a 50+ ft cutter, the leeches were the sewn edge of the original sail, you can see an old set of grometts in the main along the leech! The entire rig was very loose, our rigging lines were potwarp we scavenged, but the sails set and drew wonderfully, pulling us from Salisbury to Gloucster Massachusetts (with out a centerboard/keel/leeboard etc, just the dory and a rudder)...Then we sailed alongside the Bluenose II, what a weekend. (:
we will be making the trip again this Labor day weekend.
while working on the project I had to ask myself, If something is worth doing right, and doing it right means not doing it at all, is it worth doing any way?
06-18-2007, 02:13 PM
Well for future reference, always remember that the leech is the biggest unsupported portion of any sail (and one of the more important ones at that). Any time you're dealing with materials which are woven or which contain fibers (including laminates and even the scrims inside polytarp), do whatever it takes to square-up the weave with the leech. Failure to do so will generally drastically reduce the sail's life span and can make it nearly impossible to really get it to set properly and hold it's designed shape due to lack of control over bias-stretch.
06-19-2007, 06:45 PM
WOW! Holy smokes, guys! What great feedback!
I wasn't ignoring you all; I was cross-state at St. Augustine.
Okay, I think I now know "all" there is to know about this. Todd, I may even publish you! Dang, man, what a font of knowledge!
Thanks to you all for your input. I'll get me a nice piece of bamboo from the bamboo farm where my mast and sprit came from and get me a boom. I've got a feeling it'll tack better and etc., and getting hit isn't much of an issue since it's only a small diameter bamboo. My sharpie had a big cedar boom--thank goodness for "soft" wood!
06-19-2007, 06:53 PM
Oh...to answer a question...I'll figure out how to attach the boom so I can brail it all up like I do now. Of course, the sail will stay loose-footed, with an easily adjusted outhaul of some sort. The clew is just over the forward of the stern sheet. Maybe the boom will cut down on my getting whacked (WHACK!!!!) by the tackle at the clew when I tack. Dang that smarts....
06-19-2007, 07:39 PM
I think getting hit with a block on a boomless sail is probably worse most of the time than getting boomed. I had the outhaul fitting pull out of a loose-footed iceboat sail's boom once at about 40 m.p.h. All of the sudden the sheet was slack, the boom was down on my shoulder and the block (still attached to the flapping clew corner) was furiously beating me in the head. It was about five seconds of high-speed chaos while I tried to figure out what on earth was going on.
06-19-2007, 07:57 PM
Todd, I have to agree! This is one of those small, bronze blocks. The first time, it hit me behind the ear and actually knocked me down; I saw stars! Good thing the sail was luffing and little could go wrong. The second time it just hurt like hell. The boom will keep it off my ear!
Thanks again for your copious and helpful input. Let me know if you need a copy editor for a future project.
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