View Full Version : dory rig backstay: double-braid rope, or wire?
05-19-2004, 06:25 PM
I've been fussing with my sail rig. Below is a look at the sail plan, from John Gardner's The Dory Book .
As you can see, the plan shows a backstay. I've faked it up using the same 1/4" double-braid line I'm using for the sheets and tiller lines. It seems heavy and overly elastic for the purpose. I'm wondering now if it should rightfully be wire?
05-19-2004, 07:42 PM
Lee you might be interested in Sta-set X ,a very low stretch line developed for race boat running rigging .It's put out by New England Rope and is inelastic enough to use as standing rigging in small boats instead of wire . It coils and knots normally which seems like a big advantage for a rig that will be struck .Wire has an unfriendly feel I think,; even before it develops meat hooks .
[ 05-19-2004, 08:48 PM: Message edited by: Bill Perkins ]
05-19-2004, 09:47 PM
I've always felt that particular rig drawing was a little strange; I'm not convinced the backstay is necessary, plus it looks cumbersome, and it's one more thing to attach and remove every time you step the mast.
If you're looking for ways to tie the mast to the boat so it doesn't lift, you can just run a halyard down through the bow thwart, or run the mainsheet to a stern traveller. If the mast has any weight, it's probably not going anywhere anyway. Plus the weight of the boom has some effect on the mast tension as well. If you haven't already committed to your rig, you could alter it slightly per Gardner's 18' Marblehead Gunning Dory, or at least borrow some ideas from it.
My rig is based on those dimensions, with some small changes. The real Beachcomber/Alphas used a taller rig, but I wanted the compromise to improve stowage in the boat, among other things.
The drawn main sheet for the Marblehead dory is also a bit strange - it may work well with yokes, but it seems like it would get in the way.
I made one change since the picture linked below - the main sheet used to run along the boom, with the bow end running through a couple of blocks (one on top of the centerboard case to serve as a downhaul), and the aft end was doubled to a block clipped to the stern traveller.
But while the doubled line gave great purchase, it had too much resistance - the sail wasn't responsive enough in light wind without feeding the line out by hand. I got rid of the block, so
now it is just a single line running astern; the rig is more responsive, the block doesn't clobber the gunwale every time I tack, it uses less line, and the effort to haul the sheet in is still reasonable.
sailing dory pic (http://tuckershobbies.com/lowellsboatshop/wbs2003/gallery/pages/25_kendory1.htm)
05-19-2004, 11:20 PM
It is a bit curious. A 12' long mast, 3" in diameter isn't going to bend much, especially with a mainsail that small. I assume the backstay was added to help keep the jib luff tight, which is good, but if the step is solid and the mast is built to specs I doubt the backstay is really going to add all that much to preventing mast bend or eliminating jibstay sag. The mast is also too heavy for the backstay to function well as a mast-bending, mainsail-sailshaping gizmo.
If it was a functional, necessary part of the rig you could certainly get by with 3/32" wire (around 1200 lbs. test) and probably even with 1/16" wire (almost 500 lbs. test). This is never going to be a "drum-tight" rig and there is little reason to string it with heavy stuff as if it is. As Bill mentioned above though, wire is often far from user friendly. It's main advantage is low windage. These days you can come awfully close in terms of strength-to-diameter with some of the new high-tech lines. You can get line that's 1/8" or less in diameter with breaking strengths well over 1000 lbs. and though it's not cheap, you don't need much of it.
If the partners and step are solid I would certainly be inclined to try the boat without the backstay and would be amazed if a 65 sq. ft. main and tiny jib could pull down a 3" diameter mast on a fairly narrow-waterlined hull in anything short of a hurricane or make much noticable difference in the shape of either sail or their relationship to each other.
Also, something that has bugged me ever since I first saw this plan is that sailplans are flat. They show the sail and boom as if the sail was dead flat and the boom was centerlined down the keel. In real life, the tail of the boom is almost never as low while sailing as it looks on the plan. The backstay, on the other hand, stays in just about the same place, both on the plan and out on the water. There isn't much clearance between the boom's end and the stay on the plan and there is going to be even less when you actually go sailing. Should you ever happen to screw-up while sailing downwind (not that any of us ever screw-up) and accidentally jibe, the boom's tail is going to lift 15-20 degrees and then come flying across and nail the backstay really hard. Best case scenario is that it will be really exciting, scare the crap out of you and convince you to pay better attention when sailing downwind. Most other possible scenarios are also really exciting, but revolve around something breaking. There may be thousands of these boats out there sailing around without a problem, I don't know, but to me, the backstay looks both like excess that isn't really going to improve anything and a potential problem just waiting to happen.
05-19-2004, 11:25 PM
Food for thought, gents, thanks.
Maybe I'll try sailing first without the backstay at all. Gardner says that the backstay is to "rake the mast aft." This is to tighten the jib, I gather. I don't know if it offers any other real benefits.
05-19-2004, 11:31 PM
Todd, you posted while I was typing.
As you suspect, the clearance between the boom and the backstay is very tight. With no tension on the backstay, the boom won't even swing freely past.
But the mast does bend quite a bit with an increase of tension. I don't know what the benefits of that would be-- maybe something for advanced sailors.
Sounds like I can just leave it off for now.
05-20-2004, 12:52 AM
I'd still try it without the stay, but here are a couple reasons why sailors bend their masts at times.
There are various reasons to bend a mast fore-and-aft. Two of the more important ones could be considered "static bend" and "active bend". Static bend would be induced mast "pre-bend" and it's function is to stiffen the mast. It's normally done during the mast raising/tuning phase (usually on more complex rigs than this one) and not messed with much while sailing.
To illustrate the concept, take a soda straw and stand it on end. Now put one finger on it's top and gently press down. You'll notice that as it starts to bend, it gets stiffer. Mast pre-bend uses the stays to do just that to a mast and lock the mast in that position. It allows racers to get adequate stiffness out of an extrusion that by itself, isn't stiff enough to do the job. This allows the use of lighter, smaller diameter masts and less weight aloft. It can also help keep a flexible mast from "pumping" (bending back and forth) due to waves, wind puffs, changes in sheet tension and other rig-shaking things that disrupt the airflow over the sails. Obviously, with both straws and masts, there is a point at which too much pressure has negative results and the column breaks, so like the wicked witch said "These things must be done delicately."
"Active bend" would be using mast bend to change the shape of the sails. The Starboat, with it's 30+'-long mast which is smaller in diameter than your dory mast is one of the best examples. It has no stern-mounted backstay, but has port and starboard upper and lower running backstays that move fore-and-aft on tracks and a big mast-bender lever under the deck to work the bottom part of the stick. The boat has a big main and small jib. Cranking the upper backstays, easing the lower backstays and tightening the lever bend the mast into a very visible curve. The sail has no reefs, but when you bend the mast enough, you can form the mast into the same curve as the sail's luff. This essentially defeats the draft-creating action of having a curved sail luff in the first place and the result is a very flat sail, noticably de-powered for heavy air. If the wind goes down, you straighten the mast out by easing the stays and bender, the luff curve starts doing it's thing again, creating draft, and the sail starts generating power again.
At the same time that bending the mast is affecting the main, it's tightening the jibstay, flattening the jib and moving it's draft forward - a better shape for upwind sailing or high-wind-speed sailing.
Static pre-bend can also affect sailshape in much the same way, though it would usually be something that's set at the beginning of the day to match the conditions and to power-up or de-power the sails (depending on how closely your bent mast matches the luff shape). It would generally be left that way for the entire outing. Star sailors, on the other hand, un-do and then re-set the runners and mast bend on every tack.
Since you actually can noticably bend your dory mast with the backstay (which does surprise me with that stick) it should be possible to pre-bend and/or actively bend the mast to achieve some of the same results, though they probably won't be as dramatic as they are on a boat with a noodle for a mast, like a Star. It's worth messing with as you get some time sailing the boat under your belt, just to see how much difference you can make in how the rig works in different conditions.
The boom overlapping the backstay in normal use is a cause for concern though. At best, it's a pain in the butt. At worst, it can be dangerous and coming up with an easy to use, practical solution can be a real brain-teaser. If the boat turns out to really benefit from backstay tension, it might actually be easier to install running backstays about 3/4 of the way back and within easy reach, switching them over from tack to tack than trying to wiggle the boom's end past a stern-mounted stay that it hangs-up on with every tack or jibe. Let's hope it sails well without a backstay at all, which is still quite possible.
05-20-2004, 10:41 AM
Lee I think the backstay might pay off as an aid you just set up when closehauled ; the idea being to help the jib set effectively .It might make a surpriseing difference in performance on that point of sail by helping to keep the jibs luff straight when it breezes up . This would be an easy theory to test at least .There must be a reason the stay was shown in that experienced dory guy's book . Most of the time it could spend tied off to a cleat at the base of the mast ( if you use line , not wire ) , out of the way when jibeing .
05-20-2004, 12:38 PM
Guys, here's an almost visible look at the rig with the backstay on. Taken as a lightning storm was brewing, just before I scurried back into the shop!
You can see the bend in the mast. In that position, the boom just barely passes under the stay. I was thinking of three fixes for that.
First, to lower the entire main sail a couple inches.
Second, to run the backstay over the top of the mast, and run it to an eye-bolt as high as possible on the transom. (Here it is run through the rope beckett.)
Finally, the boom could be trimmed down an inch or two.
As long as you're looking at this picture, tell me if you think the head of the jib should be tied up higher or lower on the mast. There are no dimensions on the sail plan, obviously.
05-20-2004, 12:51 PM
Ken, your description of the original rigging of your mainsheet sounds like what I'm trying to do, making it serve as a downhaul, as per an illustration posted by Todd. In this picture, you have to imagine the sheet running through a block on the mast and back to the hand, rather than simply tied off at the partners. And aft, it will be secured to a ring on a rope traveler over the stern seat:
Does that look familiar?
This picture also shows the distance between the boom and the partners. As I mentioned above, I'm considering lowering the whole rig a bit. What say you?
05-20-2004, 08:10 PM
Measuring jibstay height from an enlarged version of the plan it would seem that the stay intersects the mast about 53" down from the masthead or around 87" above the middle of the gooseneck as drawn. Looking at the photos, I suspect your stay needs to attach higher up on the mast. This will also open the leech of the jib up a bit and may help prevent the jib back-winding the main (air coming off the jib blowing a dent in the mainsail shape when close-hauled). It generates more jibstay tension, too.
I don't think lowering the boom should present any horrible problems, though I'd probably try things as-is first and see what happens. Be certain that the sail is fully and snugly stretched-out along the foot (with a few inches to spare) before thinking seriously about shortening the boom. Being a cross-cut mainsail, the cloth weave of the sail panels intersects the foot on a fairly steep bias and may stretch a bit in use and over time.
This boat is dying for a couple of simple running backstays instead of that transom-mounted one. Most people find them to be a bit of a pain, but I've had enough boats equipped with them that I guess I'm used to tending them as just another part of the plan.
[ 05-20-2004, 09:11 PM: Message edited by: Todd Bradshaw ]
05-21-2004, 07:05 AM
When you're on the wind you may find that the force of the main sheet trasferred through the main's luff does all the head stay tighetning you need.
If not, I do think runners would be much safer and simpler, especially when you go to gybe.
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