Balance Lug Design Ideas.

Hi LP.

I decided to add some bracing wires to my design. (see attachments below)

On the first drawing, I added shrouds that go slightly less than half the way up the mast (0.8 Boom Lengths). These are to help keep the rig from twisting, and to divide the bending moment, due to forward thrust, in two.

I also added cross bracing up top, extending from the top of one mast to the side of the other, 0.4 Boom Lengths down. That is as far down as I dare go, as not to have the crossing of this bracing interfere with the Yard.

Looking at it after I drew it, I was not satisfied.

The problem I saw, was that when the masts started to sag, due to athwart-ship bending, the distance between the attachment points would get shorter. This is because the masts can be expected to bend parallel to each other, with the bend facing to windward, and the tops of each mast sagging off to leeward. In this case, the cross bracing would do nothing to keep the top portions of the two masts from buckling.

What such cross bracing needed was a spreader beneath it.

To add the spreader (see second drawing), I had to shorten the distance, that could be cross braced, considerably. The cross bracing now extends only 0.25 Boom Lengths below the top. But at least it's effective.

I expect it to put considerable reverse bend in the masts, as it attempts to hold the tops of the masts square with the spreaders, much more so than the gussets would.

The price for this is greater Horizontal Shear along the lengths of both masts. Such could be dealt with by making the fore and aft walls of the masts out of plywood, rather than thin timber, and having plywood webbing between the side walls.

Another thought has occurred to me.

Since the Boom is not attached to the mast at all, and is guyed to the deck, instead, only about half the Sail Area is actually pulling the masts to leeward. Ultimately, this may not make much of a difference, as, in a conventional rig of this type, the boom is set so low on the mast, it imparts little bending moment. But it imparts some, and I'll take what ever savings I can get.

Now that I've made most of the decisions about bracing, I can start figuring out (Guessing) the loads, and maybe designing the structure.

 

I've been pondering lug designs again and their good tack, bad tack dynamics. I was considering the use of full length battens in my sails as a method for improving sail shape on the bad tack. I've attached the tentative sailplan for my current design. I'm rethinking that portion of the design because of some reading I've done regarding sail interaction with the mast on a BL design. Others may wish to comment on the following thoughts.

Two main thoughts come into play as I rethink performance perspective on this type of rig. The good tack my not be so good. I had never really considered the fact that on the good tack, the mast is standing proud to the air flow and will be a prominent drag producer. On the bad tack, the sail lays around the mast and may actually improve the aerodynamics of the mast. This though is not enough to over come the detrimental effects of poor sail shape. The sail deformities on the non-lift side are going to be less detrimental to performance than the flow separations on the lift side. This goes a long way towards explaining IMHO why there is such a small performance difference between the good tack and the bad tack.

This being said, I wonder if concentrating on mast shape should be the way to in looking for performance improvements with the BL or any lug sail for that matter. Improvements in aerodynamics of the free air mast could reduce drag on the good tack. On the bad tack, a shape that conformed to the sail, but also improved flow on the leeward side could increase the lift potential. An oval or even rectangular might offer benefits over a round section. An airfoil would be better than either of the previous shapes, but presents possible problems with the mechanic of the mast due to it's asymmetric shape about the lateral axis.

Backing up a bit to my idea of a fully battened (a cleaner shaped) lug makes me wonder I would end up decreasing sail performance in two ways. The first is that the mast would have more area exposed to free flow and thus increase drag over the blanketed mast. The second is the flow disturbance that is created by the mast will be greater and possibly delay flow reattachment causing a significant reduction in lift on the bad tack.

Anyways, just throwing it all out there to see what comes back.

 

LP,
I have enjoyed your fine quality build. That said, your sail plan is... well.. pretty. By that I mean that it strikes me as something designed more to look good on paper than to function well on the boat. You went with a lug yawl (giving up some pointing ability, gaining some versatility) then you draw high aspect sails to try and get back the performance but giving up the easy handling. The near vertical yard looks to me like it will twist off uncontrollably and the halyard tension will need to be huge. You would certainly need battens to flatten the luff. Your observations about mast drag are all legit. What to do about it? For the main you are pretty close to a gunter rig anyway so I would just make it a gunter with a boom at least to the back of the cabin. Having the sail attached to the mast cuts its drag greatly and the gunter allows you to go to a stayed rig with a light thin mast (picture yourself standing on the deck of your boat on the trailer trying to hoist a thick mast up and into a precise hole on a breezy day -then think about standing on the ground pulling a halyard to flip a light mast up in its tabernacle). A larger lower aspect main would allow your mizen to be smaller and more realistic along with a small roller furled jib (which would be troublesome with the lug achieving slot control).

If you are dead set on the unstayed mast lug style I would suggest you look at the soft wing/junk rig. See post 27

http://www.boatdesign.net/forums/boat-design/design-soft-wing-sails-cruising-49425-2.html

It's a bit modern and more complicated but I think it would give your boat just what it needs -strong windward capability at low righting moment.

 

Thanks for your comments. I would not deny that there is a certain bit of visual appeal that has been placed in the design of my rig. I am about to start conversations with sail makers with regard to my design and plan to keep an open mind with regard to their thoughts about my sail plan. I plan to utilize some one that knows lugs and I suspect they will be my reality check. I am in no way locked into what I have drawn to this point. I agree with your thoughts about the gunter rig and have considered it in the past and moved away from it. As I get closer to locking in the sail shape, I have to consider the reality of the design and step back from the pretty plan in preference to a workable one. The higher aspect plan may not belong on this rig. It raises the CE, it makes lug longer(too long really) in addition to the comments you presented. I love the simplicity of the rig and the fact that the sails stay bent to the spars. I'm fairly set on the BL and will look very seriously at lowering the A/R of the sails to lower the CE, shorten the lugs and even lower the CG a bit. This is a trailerable craft so the time spent rigging should be kept as minimal as possible. Thanks for the softwing link. I will have to read through it.

 

I have some thoughts on the balanced lug that I omitted to give more weight to the gunter and wing. I am not sure if you have heard of a microcruiser design called Paradox, but it has a square balanced lug that roller furls onto the lower boom. This infinitely variable reefing (from inside the hull no less) assures that the boat always has as much sail area as possible while staying 'in the groove'. I find it intriguing to think about a flat, low aspect sail that is always the exact maximum for the righting moment, being able to match or beat a big high aspect rig that is feathering and twisting off the top, or reefed down too far because it is a pain to adjust. I think the ability to reef easily from the cockpit would be very desirable in your boat.

 

Yes, I am familiar with Paradox and am I guilty of owning a set of plans. I specifically bought them to get to details of Matt's roller furling set up. I think I have looked at almost every possible rig (well maybe not the crab claw) and have eliminated most of them from my design. Matt's roller furling BL is quite intriguing, but I don't know that it would function on my sails without some serious modification. It's primary short coming is the lack of a downhaul. According to everything I've read, a multi-part downhaul is key to getting proper sail shape. With Matt's smaller rig, maybe you can get a good set with a hard tug on the halyard. I fear my larger sail is going to take more than the lift from the halyard.

 

All true except possibly the conclusion. There are serious problems scaling up the roller furling. The first question in my mind is 'how big can it go?' and I am not satisfied until I do the math. The halyard can easily be 2 or 3 to one and high performance fibers are available so I am going to say there is no shortage of force at that end. At the bottom you are limited to pulling on either end of the boom. At the forward end there can be a bearing at the mast with the furling spool forward and the sail aft. Or maybe the spool is eliminated and the boom rotation is controlled at aft end with a lock to the sheet. At the back of the boom there is another bearing to the main sheet and the need for a traveler which falls conveniently at the back of your cabin. Realistically you only need to reef the lower half of the sail so there could be a batten and loop around the mast at that point so you wouldn't have the full length catenary, just half.

I am not sure if I have convinced you it can be done but I have convinced myself. The next question is 'is it better than the alternatives?'. I really like the soft wing/junk rigs but they are more complicated and might not be as durable.