Sail Draft

[overtheseas]

Hi All,
I'm looking at getting a complete suit of sails built for my 16 ton 40 foot cruising yacht, Seasalter. See the Facebook page "Sailing Yacht Seasalter 1937". The boat will be rigged as a ketch as originally designed. I have found a sailmaker local to where the boat is being restored and has given a good price. However she has asked for a draft and camber guideline and this has stumped me.
Does anyone have an opinion on what this might be? The cloth I am leaning towards is a 9oz Bainbridge Cruising Dacron. The main and mizzen will both have negative roach (and therefore no battens) and be loose footed. There is a club footed staysail and a flying jib. Although I enjoy sailing free, I also like to be able to go to windward when the need arises or necessity dictates.
I had a racing main built for my Hereshoff 28 (built in 1958) that had maximum draft at 43% and pulls me to weather well enough to keep up with my competitors with laminate sails.
Would welcome any thoughts.

[Nicholson58]

With loose footed sails, you can always move the draft forward if the cut is flat by easing the outhaul. As sails age, the draft naturally turns to krud. I would ask for relatively flat sails with the hope of better upwind capability.

[Kestrahl]

The fact that she is asking you for camber and draft position rings some warning bells. If she is a sailmaker it is her job to know draft position and to decide on camber from 1. looking at boat design and rigging 2. consulting with customer on type of sailing being done.

Flat sails doesn't always mean upwind ability. There is a range of camber depth for upwind sails, only if you go beyond that range then upwind ability will compromised. Some yachts will suit maximum upwind camber for best windward performance. Of course you have to take into account how much you think the cloth being used will stretch over time as well.

[overtheseas]

Originally Posted by Nicholson58

With loose footed sails, you can always move the draft forward if the cut is flat by easing the outhaul. As sails age, the draft naturally turns to krud. I would ask for relatively flat sails with the hope of better upwind capability.

Hi Nicholson58, I figure that if you ease outhaul that the draft actually comes aft. This is desirable in lighter airs where a softer entry is better than a hard knuckle. Boat speed is less and so therefore is apparent wind speed.
But this doesn't really address the question of the draft and chord percentage to which the sail is built in the first place. Of course the sail can be played with to a certain extent by playing with outhaul, cunningham and vang tensions, but overtensioning a sail early in its life shortens that life.
Mention was made of the range of camber- What is that range?

[svTOTEM]

Designing max depth (as % of cord), its location along that cord, and indeed shape leading to/from that point has a number of variables. Yes, of course shape is changeable to greater or lesser extent, according to adjustability of rig and skill of trimmer (not to mention the trimmer’s eagerness to make changes).

Mainsail camber % is generally less at the bottom of the sail than the top. Having leech hollow instead of roach obviously equals shorter cord lengths and will effect relative upper and lower depth %s, but hard to know for sure without knowing overall geometry. So for example, 8% depth at bottom is fairly flat, but in the upper sail would show as near board flat. Near the top, 14% depth is common (generalizing here of course, because you can change depth with outhaul, mast bend, etc.), but would show as really deep at the bottom. Designed draft location for non-performance boats is generally 45% to 50%, with slightly rounder shape forward of that and flatter aft. Sailcloth changes that with use.

Leech hollow isn’t going to help upwind performance at all. Is the boat tender/stiff? What is typical wind velocity where you sail? What size headsails do you use? How much rig control do you have? Your sailmaker should learn/understand these variables for your boat, and then add sailcloth stretch characteristics to create a sail design (geometry and shape). Having designed many sail, I can tell you that the mizzen should be flat, very flat. The main much less so, but unknowable without much more information. Good luck, and if you can provide numbers that your sailmaker requires, then maybe you have a new career designing sails!

[Jim Cate]

I too think there is something wrong here! The reason that you go to a sailmaker rather than Omar the tent maker for sails is to utilize their specialized knowledge of sail design. Any one can sew a sail together ( even me, as I have proven). It takes some training, experience and artistic talent to be a sailmaker IMO. So, why should your sail be designed by a know-nothing (you) rather than a pro? I would be looking for a different supplier... one who knows the answers to the questions that this person is asking the customer.

Cheers,

Jim

[overtheseas]

svTotem- yup that is exactly the conversation I was hoping for. As much as the info is useful to the sailmaker (or tent maker), it is also of interest to me. Simply to be able to think about the numbers involved. I think of myself as a reasonable sailor and have moved boats all around the world but the knowledge is intrinsic and by feel rather than by comparison to another in a quantative way. I have never sailed Seasalter. I bought her as a wreck. She came from the board of a very well known (early) designer who was renowned for his adherence to the metacentric theory. Although designed and built as a ketch she was converted to cutter after WWII and then raced the 1949 Sydney Hobart coming 6th. From this I take that she was a stiff though robust boat capable of withstanding fairly heavy weather. But she is no lightweight flyer (16 tons on a 35 foot waterline) Within the spars there is no facility for altering the various tensions. As with older style boats all rigging is fixed- no retensioning of backstays or baby stays! I favour negative roach as the chafe can be enormous with battens.
The winds I hope to encounter will always be less than what will most likely occur! Isn't it always thus? But having met the obligations of the shore life I am returning to the wider horizon once again. Cruising areas? Well I have sailed the standard cruising circuit of the Indian, SE Asia, Red Sea, Med Black, Baltic, North, Atlantic, East US, Caribbean, the Pacific and round Aus until I'm dizzy. Would like to visit Faulklands via the Horn and up the east coast of South America and would love to sail the North Pacific loop. I think this boat is the perfect platform.

[Stu Jackson]

I'm very familiar with Maurice, having read (almost) all of his books. This one might help:

Little Ships and Shoal Waters: Designing, Building and Sailing Shoal Draught Cruising Yachts--With a Cruise or Two in Both Blue and Sandy Waters: Maurice Griffiths: 9780851770567: Amazon.com: Books

Swatchways were stories before the "Hitler War" as he called it. The First of the Tide has the last chapter devoted to yacht design.

The linked book may have more information about sails than Tide has which discusses boats themselves for thin water sailing.

He also wrote another one whose name I can't recall about yacht design.

IIRC, in most of the books he has mainsail roller reefing around the boom, so mainsail draft wasn't one of his "specialties!"

ITWMB, I'd do what you're doing by going to internet forums and asking. I'd try to find forums that dealt with older boats with your rig.

Just yesterday we had the annual "Master Mariners" race here on SF Bay, with a large contingent of older boats with rigs like yours. Many of them still had gaff rigs, which Maurice discusses in Tide, and the advantages of being able to scandalize the main quickly. Don't know if you want to go there.

You might also want to find some locals with rigs like yours, even if not the hulls, for guidance and assistance. You might also want to enlarge your contacts with different sailmakers unless you've done that already.

Good luck with Omar!

[svTOTEM]

Overtheseas – it’s always great to see a sailor passionate about a boat’s restoration, with intentions of then taking her places. We are in Langkawi (leaving tomorrow for Straits of Malacca), and I was just taking to a neighbor on his 1936 Colin Archer gaff rigged cutter. He recently sailed it here from Kenya; had a spot of rough weather near Maldives but all came out fine. Once upon a time I design/built sails for a number of classic yacht, notably Gleam (1936 S&S 12 meter) and Neith (1907 Herreshoff 54’ sloop) – fun projects. Good luck with your and maybe we’ll see you out here!

[overtheseas]

Hi Stu- Seasalter is actually from the board of Dr. T. Harrison Butler the English Ophthalmologist who was a designer from the 1920s through the 1940s. He was keen on the Metacentric analysis which had as its aim to produce a hull form that maintained its directional stability through all angles of heel; that is no increasing weather helm as a boat heeled.
Attached is a photo of her shortly after her launch in 1937 at Port Adelaide, South Australia.