Easily adjustable downhaul position on a balanced lug?

I'm designing my next sailing dinghy for racing, and after many experiments with various rigs on my previous projects, I want to go with the balanced lug rig again. One of the most important controls in balanced lug is the boom downhaul, which is the most stressed part of the entire rig.



The position of where the downhaul is attached to the boom makes a huge difference to the CE/CLR balance, sail twist and general sail shape shape. I find that adjusting it in various courses and air conditions can really help a lot with the performance, especially on the downwind.



The problem is, it isn't easy to transfer that downhaul to different positions on the boom while underway. On one of my boats, where I initially went with a loose-footed boom, I simply wrapped a line around the mast several times, leaving some slack in it, and attached the downhaul to it. When the downhaul is tight, the wrapped line cannot slide. When the downhaul is temporarily released, the line slackens, and can be pushed forward or backwards on the boom.



However, like I said, this only works if the foot of the sail is not lashed to the boom. Lashing brings a lot of other advantages, but makes it impossible for this solution to work. When I realized I have to lash this sail to the boom, I lost the ability to slide the downhaul.

So for my new boat, where I do want to have a lashed foot again, I am trying to come up with a way to have an adjustable downhaul position on the boom that does not interfere with the lashing. Ideally it would be controlled with a single or a pair of lines to slide that downhaul forward or backward as fast as possible.

The obvious solution is to attach the sail to the boom with a groove (like a track in the mast), and have boom downhaul also in a track on the opposite side of the boom. However this is kind of high-tech, difficult to make reliably (the forces on the downhaul in a balanced lug rig are truly enormous), prone to jamming due to friction, etc.

So I'm trying to come up with a low-tech solution to this issue. One idea I had is to run a line (blue in the following pic), tied to the boom at two points, stretched very taught, and have the downhaul (red) attached to this line via a pulley, so it can slide forward or backward, like a ski lift cabin on a cable.



There would be two additional lines running along the blue line, attached to the downhaul pulley to secure it's fore and aft position.

For this to work, the blue line must be as straight as possible. I'm sure you see the major issue here: due to the cosine angle, as the blue line gets straighter, the tensile force on it approaches infinity, breaking the line or buckling the boom. And if the line is relatively slack, then it will be difficult to adjust the downhaul position, due the the pulley having to travel uphill on that line. So this idea is a no-go, unless I use something like a 10mm Dyneema on a boom made from a solid steel pipe.

The only other thing I can think of is to have multiple fixed attachment points on the boom for that downhaul, and use a carabineer to transfer it from one point to another when needed. However, this is slow, difficult to do while underway, and probably impossible in heavy weather.

Are there any other low-tech solutions you can think of to have this downhaul position quickly adjustable when the sail is lashed to the boom as in the first pic?

 

It sounds like you are using the downhaul to do two things. Control the vertical tension in the sail and control the boom's vertical angle

If so I would try-
-A loose loop of line around both boom and mast that keeps the boom adjacent to the mast but still allows sliding.
-A downhaul at the most common position
-Add a vang to control boom angle. As the vang is applied it will try to move the boom forward on the mast.
-So add another control line from the forward portion of the boom back to the mast.

 

Hi laukejas, I like your solution and think I will copy it on mine, even though I have a loose foot, then the boom's parrel won't limit the movement / adjustment. Currently I have a pulley set at the very front of the boom for up wind, and a second one angling back from nearer the middle for down wind use, and I juggle tensions between the two to swing which ever way I need. It gets complicated working 2 ropes for downhaul. I think the answer to your problem isn't a pulley, but a very simple prusik knot, used in climbing. It looks a bit like your pic 2 boom attachment. It will hold and bind when under pressure, but loosen and be easily slid along to reposition when the pressure is released. An alternative I can see to relieve the angular stress you speak of is to replace the blue cord with a steel bar maybe 10 - 12 mm diameter, or other metal rod, hanging an inch below the boom, allowing less angular tension, but making 2 weak points at boom attachment, and extra weight. Try the prusik knot, it may be enough, if the blue cord is 8 - 10 mm diameter, and the prusik knot 5 - 8 mm max. Or a series of fixed boom loops you can clip into could be an effective and quick solution. Good pics and drawings you used there.
See my recent post : Jack Holt Heron re-imagined Part 5 to see my balanced lug rig, all home made including the sail.

 

That is an interesting idea, so there's basically three running lines if I understand correctly. What you are describing sounds a lot like the dual vang system often found on GIS, with a bit different geometry, plus that one additional line.

What you described and what I saw in that Jack Holt Heron re-imagined Part 5 post, also looks like a dual vang system. It does seem like the simplest solution to my problem, but I wonder if there is some smart way to rig these two vangs so that one vang only controls the downhaul force, and the other only controls the boom angle, rather than both lines doing a bit of both?

From the pic above, the bleeter and vanghaul system kind of does that, but I imagine the rang of movement is very limited there before the vanghaul angle becomes too steep to get proper downwards force component...

The steel pipe idea is doable, but that pipe would have to be ridiculously stiff (and thus heavy) to endure the forces without yielding...

 

Hi again laukejas, this morning, (sunday), I rigged an under slung rope on the boom in your design, using part of an old trailer sailer sheet of about 12 mm diameter, stretched from the boom's front to 2/3 the way back, or a bit more, nearly 3/4 back, and made a secure knot to hold it tight, then a short length of 5-6 mm cordage was double looped in a prusik knot over it, then tied to dynema going to a 2:1 pulley system, and a simple horn cleat on the mast. I took the photos at home before tidying up the old lines, and running 2:1. I rolled and lashed my usual down haul systems up out of the way so no tangles, and had a sail in light winds. It worked very well, and was fairly easily adjusted. The prusik knot worked so well that I took the extra loop out of it, and then it still gripped well in all positions, and was more easily adjusted. Down haul was tight and effective, all the way forward for pointing, and nearly in the middle for down wind, and any position in between. The under slung rope didn't go slack much, and even on full sail there was enough down haul length and tension without bottoming out and limiting tension. My boom is strong and not very flexible, so nothing there went out of shape. The boom just needs a notch filed in it as an anchor point for the rear knot on the under slung rope, to ensure it doesn't slip in strong or blustery winds. I was so impressed, that when I got home I removed my old systems and will just use your system, with some fat rope knot tension improvements. None were dynema or slippery new ropes, just old worn stuff I had as backups lying about. I say well done on your invention, it works well, holds well in all positions, adjusts easily, and is simple and effective, for me anyway. I commend it, and recommend it. Much easier than juggling 2 down hauls, and less ropes and pulleys.

Photos with sail at 1st reef:
1. modified clove hitch; 2. set for down wind; 3. doubled prusik knot; 4. up haul and down haul; 5. set for up wind - pointing high

 

Well hot damn!! I didn't expect that this would work at all, thank you so much for trying it out and proving me wrong lol! I would have expected this arrangement to fail due to the slung rope stretching or breaking, but if you say this is working, then maybe it is a valid solution after all. I suspect the angle of that rope (how much sag is left in it) is critical, and perhaps with such a moderate sag it is actually doable.

I suspect that in order to move that prusik knot, you have to release the tension in the downhaul, correct? It holds too tight to slide while under tension? Also, is sliding a one-hand or two hand operation?

In case it's a two hand operation, I have one more improvement in mind, which might be an unnecessary complication... Not sure.



Basically attach two small pulleys to where the slung rope is tied to the boom, attach the ends of that rope to the prusik knot, and let it hang loose. Pulling that rope one way or another should slide the prusik forwards or backwards. It isn't very useful as shown in this sketch, but that line could be run further back the boom, or go through one or more blocks on the mast partner, or basically anywhere, to make this adjustment accessible without having to walk forward to the mast.

The question is whether it is possible to attach that line to the prusik so it doesn't skew it or otherwise jam it. Perhaps the end of that rope could go onto the opposite end of the prusik, and form a small (but loose) loop around the slung rope, so when it's pulled, it basically "pushes" the prusik, rather than pulling on it. The loop would have to be small enough so it doesn't go over the prusik, but large enough so that it doesn't jam on the slung rope.




Something like this. The size of the loops is exaggerated here.

Maybe you'll want to try this... Or not. I'm still not sure if this is worth the hassle.

If you decide to stay with this slung rope system (with or without that new extra line), please let me know when you get a chance to sail in really strong winds... I wonder if it will hold up then and stay tight enough to make it possible to flatten the sail as usual. Also wondering about the gust response. Please be careful though! Have a backup downhaul (set slack) just in case this breaks. It is not a fully proven solution after all

 

It's a two handed job in that one hand to hold the prusik knot and the other to hold the boom or thick rope to re-position it. My boat is 10 feet long so I only have to lean forward, undo from the horn cleat, hold the boom, push or pull the knot to new position, and tighten the down haul, and cleat it. I have bungee cord - octopus straps stretched from the boat's sides and over the tiller, so the boat self steers for the few seconds needed. Pushing the knot all the way forward is the most difficult part; but still easy.
Something else I noticed: I have both up haul and down haul cleats on the mast, so all the loading is self contained in the mast, and the foot just takes the combined weight, not the added line pressure. Your boat, and many others, has the down haul going to the deck, thus loading that pressure into the mast foot, and deck. Maybe not a big deal on small boats, but something to keep an eye on; you don't want to pop your deck fittings, or grind away your mast foot - keep that pocket clean of grit and sand. Looks like you have room for cleats on the mast above deck if you wanted to go that way. Just a thought.
Your double pulley traveler system could work, but you need to have that loop very tight, with twice as much cord to stretch, and also find a way of locking it in your desired position; and then it's getting complicated again. Remember KISS - keep it simple stupid. Sort out a tiller lock or self steering system, and then moving the down haul prusiuk along the fat rope will be a breeze. 5 seconds to un cleat, 5 seconds to re position, 5 seconds to re cleat. It takes me much longer to put reefs in the sail, and I do that while Mr bungee steers for me.
I had a nice little sail today, messing about with boats, but it's back to work for me for another week, so enjoy your invention, it works, and keep thinking, see ya.

 

Very good points, thank you. Yeah, on that boat I showed, I do have the fittings on the deck, the idea was that since the deck is strong anyway, might as well attach stuff to it rather than the mast, idea being that this also keeps the mast stepped, safe from falling out and breaking the partner during capsizes. But on my new boat I plan to have a bridge partner:


So yeah, probably it would be best to secure these lines to the mast itself, like you suggested. I plan to have a carbon fiber mast, so I need to think thoroughly on how to attach these cleats so I don't weaken the mast in the most critical part. And then probably have another short line to keep the mast from jumping out of the step during capsizes.

As for that loop system, the idea is that you don't need to have that loop tight, it can be left loose, and there is no need to secure it, because the prusik knot holds it's position through friction. The idea is that when pulled, this line provides the remainder of the lateral force that is required to overcome the friction of the prusik knot to make it slide. Like you wrote, the amount of loops in that prusik determines how much lateral resistance it has, so it can be tuned until it never slides on it's own during normal operation, but can still be slid when pulling on that looped rope. Maybe there is a sweet spot for that, not sure. But yeah, this is most likely not needed for cruising. I'm thinking from racing perspective, trying to come up with ways to make these adjustments super fast, without having to leaver the tiller

Let me know how it works in stronger winds whenever you have a chance to sail again!

 

I had another sail on the weekend, in slightly stronger winds, but still less than moderate. Your original system worked fine, again. The prusik knot never slipped. The only change needed for racing I can see, is a method to hold the boom right backward for up wind pointing. With the prusik knot all the way forward on the fat rope, jammed behind the forward knot, and down haul pulled tight 2:1, the boom was still a few inches forward of the mast, which may or may not be the best position for up wind pointing. I don't know. If my down haul went to the centreboard case behind the mast, instead of to the deck alongside the mast, that problem would be solved I think. Something simple could be used to hold the boom end back closer to the mast, like a cord loop on the boom end hooking onto a knob on the side of the mast, easily disengaged; or a vang pulling the boom rearward, and down, but that involves another cord and cleat to adjust. Something else I thought of: a spring loaded hand clamp (like a strong car battery jump start lead clamp) could replace the prusik knot if it gripped well enough under down haul loads, giving fast one handed adjustment to any position, but could be a bit clunky, and cause wear to the fat rope.
Try your simple system, it works. See if you are faster than your competition with it. More complicated means less time with full attention on racing. Good luck racing.

 

Another sail, windy, 12 - 15 knots, gusty, changeable, chop with some whitecaps, and streaks, sail reefed second from top, again the prusik knot never slipped. I tried a cord at the boom front tied to a knob at the mast, but the knob fouled the boom when uphauling and reef changes, so abandoning that idea. I think the couple inches sail forward isn't detrimental to pointing high. It's supposed to be 'balanced' anyway. Easily moved knot for a down wind run, and back to near front for pointing again. I like it. Simple. Do it.