Curved mast.

[Clive P]

I've known since age 12 how the Swallows made a new mast!
I know what tools Lee Valley have for making birds nest spars!
I know from WBF how I might taper that mast,
But how would I, or the rest of you, go about building this curved mast?
No, I won't be making one, but just interested.
I guess the picture is between 1915 & 1925, Swedish, of course!
What the Dragon should have had,
Clive

http://farm3.static.flickr.com/2781/4467334050_5b208bc0a7_o.jpg
Windspiel XXll

[Tom Robb]

Are you sure that the back stay hasn't cranked that curve into the mast?

[gibetheridge]

With modern materials I would laminate it, but I expect that they steamed it in before stepping the mast.

[DGentry]

Yes, in my uninformed opinon, I think Tom has it, and that those are straight masts with a bend induced by a combination of backstay tension and the extremely shallow angle of those forestays. Note the second boat, X2, with a seemingly longer forestay spreader (never conceived of that!), and it's less radical mast bend.

Marconi rigs were kind of in their infancy back then, and there was still lots of experimentation going on to see what worked best. I'm guessing this was an experiment, which worked for one boat, was copied by others, then was later discarded for even better solutions. A forestay spreader - cool!

Of course, I'm not familiar with this class, and that could still be the standard. Seems unlikely, though.

In any case, running rigging systems, in combination with the standing rigging, can bend the heck out of masts, so having a permanent bend is both unnecessary and would make for less adjustability.

Dave Gentry

[Thorne]

Do a search here for 'Xena' and you'll find lots of info on curved masts and how to build 'em. Also IM/message the builder kenjamin -
http://ford.physics.fsu.edu/CastilloDeSanMarcos.jpg

http://www.woodenboat.com/forum/showthread.php?t=109234&highlight=xena

[JimConlin]

Even before asking "How?", you should ask "Why?".
A curved mast greatly complicates the sailmaker's job. It is difficult and therefore expensive to get sails to set well on such spars.

[andrewe]

Under the 'Why' bit, those boats were extremely competative, so the had their reasons. The Dutch curved their (short) gaffs..?. And Iain Oughtred likes a curved gunter on his Grey Seal.
Aircraft wings also are better thus (Spitfire) even if the costs are higher.
A

Oh, and the Finn had a curved (by the sail tension) mast.

[Clive P]

Talking of Spitfires......Did Mitchel see this? The first Spit flew in 36 (I think) Clivehttp://farm3.static.flickr.com/2765/4467308821_68c90a8346_o.jpg

[Jay Greer]

Bird's mouth masts are easy to build in a curved form. Some guys end up with them even without trying. But, the method is to, build a spar bench out of two x four cross spalls the bench is built to the desired curve. Then the mast is laid up with the after stave face down on the bench The staves are then held with rubber banding material and clamped down to the curve. Herreshoff used a complicated jig but this way works fine. Be sure to make a dry try first.
Jay

[mizzenman]

In the early incarnations of the scerry cruiser rule the sail area of the mainsail was measured as the area of the triangle made by the three corners of the sail. A bent mast would thus give free unrated area.

The curved leach in the sail would make the sail 'sacky' when a reef was put in.

The bend was made during the lamination. Later when the real area of the sail was measured the bend fell out of fashion.

[JimConlin]

Under the 'Why' bit, those boats were extremely competative, so the had their reasons. The Dutch curved their (short) gaffs..?. And Iain Oughtred likes a curved gunter on his Grey Seal.
Aircraft wings also are better thus (Spitfire) even if the costs are higher.
A

Oh, and the Finn had a curved (by the sail tension) mast.

I have no worry about straight masts that bend. Damfino has a 44' unstayed carbon mainmast. And I have no quarrel with the largely theoretical benefits of an elliptical planform. My point is that a curved mast adds needless complication and/or ambarassment for the sailmaker.

[Jay Greer]

I have no worry about straight masts that bend. Damfino has a 44' unstayed carbon mainmast. And I have no quarrel with the largely theoretical benefits of an elliptical planform. My point is that a curved mast adds needless complication and/or ambarassment for the sailmaker.
You are absolutely right on the mark!
My own work with curved masts involved a broken S Boat spar.
Jay

[Clive P]

Interesting replies!
Knowing my luck & skill, If I try to make a bent mast on a jig, it will
come out straight!!
The Dragon was designed with a straight mast to keep costs down.
In the 1950s people started raking their masts.
To avoid weather helm, the mast base was moved forward about 15 inches.
My own mast rakes back about 4 feet.
Clivehttp://farm3.static.flickr.com/2783/4467472111_e02f7a875b_b.jpg

[Dr.Spoke]

The more extreme curves on the square-meters are "built" into the mast, with a certain amount of "extra" curve coming from the backstay. How you do it? No idea, haven't had to do it yet!

[andrewe]

I have no worry about straight masts that bend. Damfino has a 44' unstayed carbon mainmast. And I have no quarrel with the largely theoretical benefits of an elliptical planform. My point is that a curved mast adds needless complication and/or ambarassment for the sailmaker.
I did say the 'Why' bit. No argument about the grief for the sailmaker. Of course, if pre-bent, it depends how stiff it is.
A

[James McMullen]

I owned an Oughtred Fulmar that had that rigid, curved gunter yard. The sail was really hard to adjust in getting the proper tension along the head--and in fact it never really set right at all. There was no setting of lashings or outhaul tensions that would work well in any change of windspeed. There would always be a new bag or wrinkle that would appear in a different area inside that curve the minute the wind speed changed up or down. When I wrote to Iain with questions about this problem when I was thinking of building a similar rig for a Grey Seal, he admitted that it was pretty tricky to get this sail to set just right, and that he fully endorsed the principle of changing over to a straight gunter yard rather than the curved one for Grey Seal builders who were unhappy with the issue. His more recent Wee Seal design uses the straight yard instead from the get-go.

I think that the problem is that the tension adjustment between the throat and the peak outhaul is a straight vector, rather than moving along the curve, so all of the shape above that line is really controlled only by the cut of the sail cloths, with no easy adjustability.

Note, this is completely and utterly different from the dynamics involved in a straight spar that is bent into a curve, rather than a rigidly, pre-bent curve. All of the really successful bendy sparred rigs such as Lazers and Windsurfer sails use the dynamic tension of the straight spar that is being bent to allow the differing amounts of bend forced in through sheeting, backstays, or outhauls to adjust and change the sail shape on the fly. A pre-curved rigid spar doesn't allow for any of that beneficial adjustment.

[bamamick]

As I understand it there is a builder in Hungary making the curved spars for square metre yachts out of carbon now. No telling how much such a thing would cost. I think that a fully-rigged mast for a Dragon from Nordic or Petticrows will cost you around $10-11K US without shipping. We have had a few members have wooden masts built in the last couple of years (I believe that Elk Spars has done some) but I don't know the costs there, either.

Mickey Lake
AIDA National Secretary

[Clive P]

I wonder if the same thing that made cotton sails so delicate to stretch when new, might have helped them to fit to a curved mast.
What did they use for glue in the pre-war days? I remember in carpentry shop in the 40s, thick brown Hide & Hoof, also known as fish glue, boiling on the stove!
I remember it as smelling worse than epoxy!!
Finally,and I am serious, was/is there an element of Asthetics?
Any curved spar catches the eye!
This curved flagstaff on Jan Koppell's Vigrid in 1942 shoes what I mean! Clivehttp://farm3.static.flickr.com/2778/4469984421_ebe29aa94a_o.jpg

[Noah]

In the early incarnations of the scerry cruiser rule the sail area of the mainsail was measured as the area of the triangle made by the three corners of the sail. A bent mast would thus give free unrated area.

The curved leach in the sail would make the sail 'sacky' when a reef was put in.

The bend was made during the lamination. Later when the real area of the sail was measured the bend fell out of fashion.

Mizzenman summed it up very well, but perhaps more emphasis should be put on the fact that you can't really reef the main sail.

I'm sure there is some benefit to the shape of the top of the sail as well - tip vortexes and such. It may be small though! It is also an early way to add "roach" to the sail.

[kenjamin]

First, I'd like to say that the argument that one should not pursue a curved mast design because it makes the sailmaker's job too difficult, to me, is just ridiculous. Why should sailmakers be exempt from having their creative talents being pressed beyond their normal day to day designs? I like to imagine that there is a talented sailmaker out there somewhere who would welcome the challenge and maybe even savor a chance to succeed at something out of the ordinary. Jeff Frank at Sailrite Enterprises did a pretty darn good job of designing the sail panels for my curved mast. My late wife and I sewed the panels and it was our first attempt at sewing sails. The job for both sail designer and sail builders was further complicated by the fact that I had insisted on full battens for the mainsail. Considering the fact that it was a first attempt for the designer and builders, it turned out pretty well.

Secondly, no matter what the current state of the art is concerning curved masts, sailors will always be left with the fact that when going to windward, there's no avoiding that the wind resistance of the mast must be subtracted from the lift vector in the forward direction of the sail. If one can minimize the aerodynamic drag of the mast and the rigging, then there's a good chance that it will help with the forward progress of the boat. As many point out, however, the design of the cut of the sail for a radically curved mast, is no picnic in the park for the sailmaker and the construction of the curved mast itself tends to be difficult and expensive. Still, if the shape of the mast is given design priority then that could (notice I said could, not would) be the ultimate path to ultimate efficiency in going to windward – especially if the other elements of the rig can be designed to work efficiently with the given ideal mast shape.

Thirdly, what I'm finding out about my particular curved mast design is that it is very strong especially in the forward/aft direction because that is the orientation of the glue lines used in the lamination. This seems to enable it to effectively set headsails without the use of stays. Also the shape of the mast itself helps in this function. Because of the unique shape of my mast, much more of the upper part of the mast is actually in compression rather than bend due to the force of head sail tension.

And fourth, the idea that a curved mast must be made too stiff to have it's curvature adjusted during its use is false. My mast is very stiff and is not a good candidate for adjustability underway but to say that a different curved mast could not be built to have adjustability underway shows lack of imagination. The fact is that a rotating, free standing, radically curved mast will still have an centralized axis of rotation. Where this axis of rotation emerges high up top from the leading edge of the the mast, the mast can be controlled and shaped without necessarily effecting the free rotation of the mast. (See the dashed purple line in the drawing below). Stays can be fastened near enough to this axis of rotation so that they do not necessarily effect the free rotation of the mast.

There is much work to be done before a radically curved mast design can ever reach its full potential but to say "it's just too difficult" is just plain wimpy. I really don't know if all the problems with my own birdwing mast design will ever be worked out, but I feel strongly that it's a design direction that's worth pursuing. After all, making our boats sail better is what we do for fun, eh?

http://ford.physics.fsu.edu/BirdwingDrawing.jpg

[bamamick]

A buddy of mine has made three different Finn sails for me and we all know how bendy a Finn mast is. His comment is not that the luff is difficult to do, but that the bungee in the luff makes it difficult to do. As far as him doing a nice entry and a good looking leech it hasn't been a problem, but the bungee requires a good bit of time to put in and get right (I guess. It just seems as if it would require more time that the average sail and cut the profit that the sailmaker gets from the sail. Maybe after you do a few it's not such a problem?).

Mickey Lake

[JimConlin]

Another issue that occurs to me is how, if a mast is curved only at the top, as in the square meter boats or S boats, what happens when the sail is reefed and the curved section of sail luff slides down onto the straight section of the mast?

On further thought, I suspect that what made the idea appear to work better on the old boats was the elastic nature of cotton sailcloth. I can't imaging anyone tolerating the demands of cotton sails today.

[kenjamin]

The other day I received my Iain Oughtred Shearwater plans and at the last page of them there was the familiar quote from the designer, "Have fun!" That is the same message he left with me when I wrote to him about experimenting with a curved mast design on his Caledonia Yawl hull:

http://ford.physics.fsu.edu/letter-sm.jpg

People can concentrate on the problems with my curved mast. They can point out the wrinkles in the main. They can complain about the ugly oar-shaped mizzen (that I'm about to replace). They can say I butchered the sheer of my boat (yeah, I did but I'm going to address that soon too). And they can point out the hardened drips of epoxy on my composite-reinforced bamboo booms. But those people will be hard pressed to find anyone having more fun with their boat:

http://ford.physics.fsu.edu/XenaSails.jpg