Lugsails-spar bend-still in the dark

[bluedog225]

I know lugsail questions have been the subject of a lot of posts but one point is still eluding me.

I've been reading up on balanced lugsails for a while now. I am going to get a set ordered soon. About 100 sq ft for the main and 60 for the mizzen.

I plan to build my own masts and spars. One thing that is holding me back is understanding the function of flexibility in the spars.

I've read that lugs benefit from a stiff mast so that you can crank down on the 3 part downhaul and get proper luff tension. I understand this and why it's important.

I understand how one can adjust the downhaul tension and flatten the sail thereby reducing power.

I also understand that a properly set balanced lug is self vanging to a degree but that it can twist to spill a gust. The amount of twist available depends, in part, on where the downhaul is set on the boom. I've got some experience with this and understand how it works.

Here's where I get lost. I've read that a spar should have some bend to it so that it can bend in a gust and thereby depower a sail. I'm picturing a full and properly set sail on a beam reach at 10 knots. A 25 knot gust hits the sail. Is it as simple as the spars bend to dump air off the leach? Or is something else at work?

The other thing I can't figure is how to accommodate the sail cut when building the spars. I've read that the sail needs to be cut to accommodate the spar bend induced when you set the downhaul. But the sailmaker I talked to is very reluctant to give any advice on spar construction. So which comes first? Do I order the sails and then trial and error the spar stiffness. There must be a better way.

I have the original sail plan for the boat but no corresponding spar tapers/materials. I think they are lost to history.

Any thoughts appreciated.

Many thanks.

Tom

[Woxbox]

Well, the resident expert will be along any minute to straighten this all out, but my short answer is this: you're over thinking the whole thing. My experience with the rig is that the sail doesn't twist much at all, and the harder you set it up, while the sail will get flatter, it will twist and spill wind even less in a gust. And I have very light and flexy spars.

On my whisp, I tie in the reef early and often, because the only way she shrugs off gusts is by heeling quickly and alarmingly. I haven't gotten dunked yet, but it's been close. :D

What hull are these sails going on?

[bluedog225]

Sea Pearl 21 (based on Herreshoff's Carpenter)...wood upstairs but not down. Well, that's not true; the sails are plastic, as are the lines. But...balsa core, teak rails and wooden masts...so I suppose....:D She's a wood boat to about the same degree a jelly filled, glazed donut is rye bread. But I do my best to only ask about the wooden parts.

[Todd Bradshaw]

Yard - 1:64-1:60 (length vs. diameter) Taper - heel 82%, peak 62%
Boom - 1:56, Taper - heel 80%-85%, peak 70%
Mast - Diameter 1:52 of the length above partners (down to 1:55 for big boats and 1:50 for small boats). Taper for lugsail masts 70% at masthead, 95% at half-height.

These are Iain Oughtred's figures and seem to work quite well with the sails that most sailmakers seem to be building. The question(s) however, are extremely complex to answer if you really want to get down to the bone on them. More to follow.

[Michael Beckman]

I used spruce trees that were about 20 years old thinned from a stand that was a bit too thick. Stripped the bark, coated with linseed oil a few dozen times, then sealed the ends with beeswax. They seem to work well. No cracks after about a year and a half.

[john welsford]

The balance lugsail is a rig that I like and use in quite a few boats, here are some observations.
Spars, I use a very powerful downhaul on the boom. Its taken to a point as close to the mast as I can get and direcly under the boom when the boom is centered.
Heaving on the downhaul pulls some bend into the boom flattening the lower part of the sail, the section of the spar dictating how much effect this has. I prefer the boom to flex about midships.
Same happens at the head of the sail, down haul tension flexes the yard and flattens the sail. Where the taper is and how much dictates where that occurs, and I prefer it to happen from about midway along the yard to aft with the after end of the yard quite fine and flexible in relation to the rest of it.
Other tuning tips, the further forward along the boom the mast is, the tighter the luff relative to the leach. The further forward along the yard the tighter the luff and the more twist you get in the sail. So by moving the sail fore and aft relative to the mast you can change the whole characteristics of the rig, the rule being move the sail and spars forward relative to the mast in light weather and aft in heavy weather.
Its an incredibly tuneable rig, and repays careful experimentation.

John Welsford


I know lugsail questions have been the subject of a lot of posts but one point is still eluding me.

I've been reading up on balanced lugsails for a while now. I am going to get a set ordered soon. About 100 sq ft for the main and 60 for the mizzen.

I plan to build my own masts and spars. One thing that is holding me back is understanding the function of flexibility in the spars.

I've read that lugs benefit from a stiff mast so that you can crank down on the 3 part downhaul and get proper luff tension. I understand this and why it's important.

I understand how one can adjust the downhaul tension and flatten the sail thereby reducing power.

I also understand that a properly set balanced lug is self vanging to a degree but that it can twist to spill a gust. The amount of twist available depends, in part, on where the downhaul is set on the boom. I've got some experience with this and understand how it works.

Here's where I get lost. I've read that a spar should have some bend to it so that it can bend in a gust and thereby depower a sail. I'm picturing a full and properly set sail on a beam reach at 10 knots. A 25 knot gust hits the sail. Is it as simple as the spars bend to dump air off the leach? Or is something else at work?

The other thing I can't figure is how to accommodate the sail cut when building the spars. I've read that the sail needs to be cut to accommodate the spar bend induced when you set the downhaul. But the sailmaker I talked to is very reluctant to give any advice on spar construction. So which comes first? Do I order the sails and then trial and error the spar stiffness. There must be a better way.

I have the original sail plan for the boat but no corresponding spar tapers/materials. I think they are lost to history.

Any thoughts appreciated.

Many thanks.

Tom

[Todd Bradshaw]

"Here's where I get lost. I've read that a spar should have some bend to it so that it can bend in a gust and thereby depower a sail. I'm picturing a full and properly set sail on a beam reach at 10 knots. A 25 knot gust hits the sail. Is it as simple as the spars bend to dump air off the leach? Or is something else at work?"

The reason that spar bend tends to depower the sail is that bend caused by increased wind pressure on the sail and increased sheet tension eats up the sail's draft near the spar. The head, for example is cut with a certain amount of round. When attached to a straight spar, the fabric in that round is forced toward the middle of the sail (its position adjusted by broadseaming) and it creates a predictable amount of draft. If we then bend the spar, the draft will be reduced (less round being forced toward the middle) until we get to a point where the spars bent shape matches the round shape along the head and at that point, we no longer are creating draft with our round (we have a flat sail, or a large flat portion of it near the spar). In most of the conditions when you would be exerting high sheet tension on the sail (like sailing close-hauled to weather or sailing in high winds) this reduction in the effect of the head and foot rounds and the subsequent flattening effect on the sail are desirable.

Continue to bend the spar even farther and the sail usually starts to distort, becoming too tight across the middle and slack at either end of the spar - which is generally very bad sailshape. So there are limits to how much bend is helpful, and keeping undistorted sailshape through the broadest possible range of conditions by limiting bend is generally going to be far more important than gaining a bit more shock absorbency from spars flexing in a gust. In general with typical wooden spars, the stiffer you can build them, the better. They will still flex in use, but building a sail that fits them through a range of conditions will be less of a guessing game.

We can't usually completely eliminate the possibility of sail distortion, but we can delay it. We build-in some extra round when cutting the sail. Essentially, we are assuming that most of the time, we will have some spar bend going on (because light wooden spars bend fairly easily). We want the spars to be able to bend a bit in "normal" conditions without sucking all the draft out of the sail and this is what the extra round, in the form of added bend allowance does. In practice, the amount added to make draft doesn't need to be much. On a 75 sq. ft. lugsail, the head round for draft is probably only going to be 1"-1.5" or so. However, in order to maintain that draft in normal conditions with typical wooden spars, we will probably need to add a couple inches of extra round as a spar bend allowance - so our total head round on that 75 sq. ft. lug may be 3" or better.

The same thing is going on along the boom if the sail has a laced-on foot. Boom bend sucks the lower draft out of the sail, so extra foot round may be added to make up for what will be lost to bending. On a loose-footed sail this is less of a problem since if anything, boom bend tends to move the tack and clew corners closer together, creating slightly more draft, rather than eating it up. This is usually not a huge difference though and seldom needs to be allowed for, plus, loose feet are more quickly and easily adjusted if needed.

So we allow for some in-use bend and cut the sail to work on bent spars because we know that they aren't likely to be straight in normal conditions. The exact amount allowed for bend is pretty much up to the sailmaker's judgement. On the other side of the coin... we really don't want to build in a bend allowance that is way too big. The reason for this is light air. If you are sailing in light air and the spars aren't bending as much as they normally do, then any excess bend allowance works like any other bit of edge round and makes more draft. You want a fairly flat sail in light air as the flow over the sail won't follow a deep draft curve. On some sails with long yards and booms, it might be possible to justify building in 6"-8" or more of bend allowance for normal-to-heavy-air conditions - because those long spars really can bend that much. However, take that same sail out in light air when the spars aren't bending much and suddenly you have a big baggy sail with too much round and super-deep draft. The result is lousy performance, so in order to get decent all-round performance we may have to compromise on our final bend allowance figures.

So, we can only build the sail to one shape and it has to be a compromise. We design it with the desired draft, add what we consider to be a reasonable bend allowance for average conditions and average spars (adjusted a bit by boat type, expected sailing conditions, etc.) and sew it up. Even with fancy computer-designed sails, the bend allowance is no more than somebody's best guess. Overly bendy spars just make the job less precise and usually reduce all-round performance. As mentioned, you will still have quite a bit of adjustability and you will notice when you start using the sail that it really is a big hunk of fabric being supported by just a few strings. You can make some fairly substantial changes in the sail and how it performs by experimenting with the tensions on those strings.

[obscured by clouds]

Yard - 1:64-1:60 (length vs. diameter) Taper - heel 82%, peak 62%
Boom - 1:56, Taper - heel 80%-85%, peak 70%
Mast - Diameter 1:52 of the length above partners (down to 1:55 for big boats and 1:50 for small boats). Taper for lugsail masts 70% at masthead, 95% at half-height.



Thanks for that Todd. I had been struggling trying to refine the figures for Sibrwd's mast- originally 21' it's now going to be between 19 and 20', mainly due to the pieces of DF I got being a bit over 19' long. The unsupported section of the mast is 17'6" above the deck partners.

Are the above figures for mast diameter referring only to solid wooden masts or can they be utilised for a hollow birdsmouth build?- maybe with the usual caveats of over-building by 10%

Applying the 1:50 ratio I end up with what seems like an overly skinny mast - so I wonder if I'm applying the sums properly

For what it's worth I have been aiming for a 5.5" diameter hollow mast at the partners [following all the recommendations in dimensioning and over specing] with a parabolic taper down to 3" at the masthead with a nearly constant wall thickness all the way - heavier up top but stiffer too.

[bluedog225]

Yard - 1:64-1:60 (length vs. diameter) Taper - heel 82%, peak 62%
Boom - 1:56, Taper - heel 80%-85%, peak 70%
Mast - Diameter 1:52 of the length above partners (down to 1:55 for big boats and 1:50 for small boats). Taper for lugsail masts 70% at masthead, 95% at half-height.


Thanks for this and the detailed response. I hate to be the dunce. I've got a fair amount exposure to math but your notations elude me.

Take the 1:64 (length vs. diameter). I would read that to mean 1 unit of length to 64 units of diameter (which clearly seems off). Is it the other way around? An inch of diamter for 64 inches of length? That seems closer.

Or am I off on both counts?

Also, does "above the partner" mean above the point where the spar meets the mast?

Finally, do I start the taper from the middle or from the point the booms contact the mast?

If there is a book that explains it in simple enough terms, I will step up and try and figure out some of this on my own but your help is greatly appreciated.

Thanks again.

Tom

[Todd Bradshaw]

"Above the partners" is the distance from the deck, mast thwart or whatever the mast comes out of at deck level to the masthead - the exposed, unsupported section sticking up above the hull. Below the deck level, the mast can taper pretty substantially as it nears the mast step on the keel or bottom. If, for example, we have a 14' long mast with two feet buried below deck, the above partners length would be 12' (144"). If we want our diameter figures in the 1:50-1:55 range, 144" divided by 50 would yield a diameter of 2.88" and 144" divided by 55 would yield a diameter of 2.61". The slightly larger diameter for big boat masts (1:50) is because they tend to have more righting moment and usually less ability to heel over easily to spill pressure, taking strain off the mast.

For taper, our "half-height" is located halfway between the partners or deck and masthead (6' below the masthead for our 14' example with 2' buried and 12' sticking out). We would only want a slight amount of taper at that point for a lug, with the diameter there maintaining 95% of its maximum. This is to help keep the mast stiff down low for good luff tension. Other types of sails (like spritsails for example, which need less downhaul tension) might taper more and sooner. Above that mid-point, our lug mast could taper more, ending up with about 70% of max diameter at the masthead itself.

These are just guidelines, but from what I've seen, they seem to work pretty well for most small-to-medium-sized boats when specific designer's dimensions aren't available. I'm not up to speed on up-sizing specifics for birdsmouth construction so I can't tell you, but I'm sure we have some folks who can. Most of the boats that I deal with use solid spars, so the question rarely comes up. Those customers who have built birdsmouth masts did really seem to enjoy that part of the build though, watching the thing go together.

[bluedog225]

Thanks. That's just what I needed.

Tom

[James McMullen]

I think John Welsford's post is right on the money. The lug has become my hands-down favorite small boat rig.