Design Rules - Lug Rigs

[jerrycashman]

Hi guys,

I'm very new to this wooden boat obsession (have just finished a 'one sheet skiff'), but already I am drawn to making slight modifications to well established designs :-)

My next project will be a 'Summer Breeze' sailing dingy (plans on Duckworks) but I want to add buoyancy and rigidity compartments to the bow and stern similar to those that I have noted in several builds and more importantly; want to lug rig her; so will need to move the mast forward and rake a little...

So what I am looking for is some guidelines for mast positioning for lug rigging (and any gotchas anyone would like to offer :-)

cheers. Jerry.

[john welsford]

Has to do with the center of effort. That is the geometric centre of the sail. It needs to be very close to the c of e on the original rig . That assumes of course that the boat was well balanced under the original rig.
If you were to make an adjustment, my experience with balance lugsails and standing lugsails is that you could allow slightly less lead of c oe over centre of lateral plane that with a normal bermudian sail.

JohnWelsford

[Woxbox]

There are balanced lugs, standing lugs and dipping lugs. And any of these might carry sails of varying proportions. But they don't normally have raked masts.
That's a balanced lug on my avatar up there, which is a 15' Steve Redmond Whisp. The one good thing about the balanced lug it is that you can tie off the spars wherever you like, and experiment until the balance and trim are right. Redmond's plan calls for a leg-of-mutton sail with a sprit and club. I moved the mast back about 10" for the lug, and I think I got it right.

Below is a Summer Breeze shot from the Duckworks site. It carries a standing lug, with the tack tied off on the mast, but there is something curious going on there -- there appears to be a sprit out to the clew.
http://www.duckworksmagazine.com/07/projects/summerb/Picture-1301-s.jpg (http://javascript<b></b>:;)

These sails carry their area lower than a comparably sized triangular sail will and develop good power for their size. Plus the spars will usually all fit inside the boat when not in use. All good points.
What you give up is a bit of windward ability, and dousing the sail in a breeze there's a tendency for the yard to swing around and knock off hats or worse.

[Todd Bradshaw]

There are actually a fair number of lug-rigged boats with raked masts, most of the Iain Oughtred balanced lugs, for example. It can be done for several reasons. In some cases, it may make locating the C.E. properly while using existing structure easier, or getting the mast somewhat more out of the crew space. If the sail is boomed, like a balanced lugsail, raking the mast will cause the tail end of the boom to rise a bit as the sail swings outboard for downwind work. If the boat is heeling, it gives a bit more clearance between the clew corner and the surface of the water. This can be a very good thing if it's windy and you're being overpowered. Once the clew hits the water, you've eased the sail out about as far as it's going to go. Having more clearance will allow you to ease out more in a puff, if needed, before the water stops the boom.

The thing that needs to be understood when designing a lug rig is mostly gravity. The angle of the yard and the location of the halyard tie off is fairly critical. The yard needs to be pretty seriously tail-heavy and you don't generally want the angle of the yard to be too steep. This generates the luff tension that's needed - the peak wants to sag, which, balancing on the halyard tie-off point, pushes the heel of the yard up, which pulls along the luff against the downhaul, which is holding the tack and forward portion of the foot down, which creates luff tension, which keeps the draft where it's supposed to be. There are a whole bunch of forces working in a circular fashion to properly spread the sail area. Move your halyard tie-off too far aft or peak the yard up too high and it creates a problem as the yard wants to stand plumb and you lose tension on the luff. All in all, it's not a particularly complex equation, but there are some limits that you don't want to exceed, and as John said, you want your C.E. to wind-up in a similar place to the original so that you don't screw-up the rig's balance.

[alkorn]

Jim Michalak has some useful instructions on balanced lug rig setup at http://ftp.apci.net/~michalak/1feb05.htm

[Lewisboats]

http://www.simplicityboats.com/TSSrig10mast.jpg Here is the original lug sail for summer breeze along with the dimensions needed to place the mast.

Here is the designers site: http://www.simplicityboats.com/summerbreezeduckworks.html

[David G]

You might also glean a bit of lugsail info from Michael Storer's site:

http://www.storerboatplans.com/Faq/tradrigperformance.html

"Think like a man of action. Act like a man of thought" -- Henri Bergson

[Don Maurer]

I just took a look at the write up for Summer Breeze on Duckworks. She is already lug rigged. The Duckworks site shows her with a loose footed lug, but if you follow the links to the designer's site he also shows details for a sprit boomed lug.

[jerrycashman]

Thanks guys, appreciate the responses.. cheers. Jerry.