Let's remind ourselves of the reason that Blondie put the junk rig on Jester: to race to windward across the North Atlantic. That puts a premium on windward performance and hence high AR. So if we are to hypothetically re-rig Jester, we ought to be increasing the AR by maximising the hoist and bringing the foot of the sail as close to the deck as it will go (racing sailors know that closing the gap between sail and deck is an effective performance booster - think of deck sweeping genoas).

Scaling the drawing of Jester in PJR, I find that the distance from head to foot of the sail, along the mast line, can easily be as much as 8m, without fouling the pramhood (there are no lifelines to foul). If I scale up my drawing of Weaverbird's sail to this dimension, I find that I can get an area of 32 sq m. The chord/DWL is 0.666 and the AR is 2.3 so this would be an easily handled, well-performing sail, with enough area for racing across an ocean, but not so much as to become unmanageable in an ocean gale. Adding more sail area by increasing the chord would not increase the windward performance, so I wouldn't want to go up to a chord/DWL ratio of 0.8, even if the rudder is able to control such a sail.

But the chief need is to get more sail area onto Jester. Her sail area is only 22 sq m, the same as Weaverbird's on a boat with twice the displacement.

Iain Grigor wrote:

But is that a job for a naval architect, or can it be done on an educated=guess, suck-it-and-see basis?  Just how big a job is it?  And just how does one go about it?

John Letcher's book: Self-steering for Sailing Craft, chapter on Controls (ch. 8, pg 108) gives a quick and dirty method to evaluate your rudder/hull's coupled turning moment. Fortunately you don't have to find the book (printed in 1974). Mr. Letcher has given his permission to download the book:

http://www.jesterinfo.org/selfsteeringforsailingcraft.html

I'll throw out some numbers for boats I've evaluated using Letcher's method, without explanation....for details check out his book.

Letcher recommendation is 40-60%. My boat, an IOR inspired 70's Roberts 34 comes in at 55%...off the wind I feel my rudder could be larger even though it is within range. Now compared to two older full keel designs: a folkboat evaluates to 100% and Laurin Koster 32 evaluates to 82%. The numbers support the notion that the rudder-hull couple on Arne's folkboat has twice the turning moment as the rudder-hull couple on my IOR type....a statement which I believe to be true.   

Ian,

the matter of rudder and how to make a vessel stay on course and tack, is a big one.

My guess is that the vessels in the East and West were developed in two different directions. Ships in the West early got some keel (long shallow, like on Viking ships) to reduce leeway. These ships relied on the longship trim and sails to balance. In addition, since western ships used square-sails, they did not get increased weather helm when falling off to a broad reach and run, so they were easy to steer. However, they never did that well when close-hauled.

The Chinese ships appear to not have had keels. For leeway resistance they often used centreboards or leeboards, fitted rather far forward. As a second centreboard, they used the rudder. For this reason, the rudders grew a lot bigger than we ever saw in the West. This combined with their broad fore-and-aft junksails gave excellent upwind performance, better than just about any working sailing vessel in the west. Their sometimes super-broad sails no doubt added a lot of weather helm on a reach, but with that huge, and sometimes balanced rudders, they coped, in particular if they could raise the centre-board or leeboard. I haven’t seen any signs on photos showing the Chinese mainsails canted forward for reaching.

The combination of forward-set cb. and big rudder gave the Chinese a big freedom. In case a sail was disabled, they no doubt stood a better chance to stay under control and limp home than our traditional windjammers did. These had to do repairs to re-balance the rig before they could continue.

Yes, Paul J Thompson, did a fine job of improving his La Chica schooner, by building a new rudder. (Paul, btw, also is a great help when I run aground with some CAD problem, thanks Paul!)

As for getting the lead (i.e distance between CE and CLR) right on our boats, the thing is that it is not so difficult to get the boat balance for up-wind sailing. However, junk (and gaff) sails are so broad that the sail centre moves out over the side when we fall off onto a reach. That is why I always preach the gospel of big rudders. Canting the junksail works as well, as we know, but that is in my view a plan B, when there is not room for a big rudder. Just an armchair opinion.

Arne

 

Shifting the sail back and forth no doubt works  -  it has proved so in practice. Still, the need for doing so depends on the vessel and the size and position of the rudder. I certainly will not need it on Ingeborg. My hunch is that if we could bring old Chinese junkmen to the West and let them have a look at a line of laid up yachts, they would point at them and exclaim “ toy rudders!”

BTW, since I have been laid up, myself for a while, I have spent some time and finally learned how to digitize a jpeg diagram of a hull profile and make an all-digital sail plan on it  -  without using scissors and tape. Looks neater, me thinks.

Cheers, Arne

 

David Tyler wrote:

Iain, written out in full, that would be: "much longer boom parrel and batten parrels".

Have a look at this photo of Tystie. And this one, too. There are single and becket blocks at the mid-point and at the forward end of the boom. A line runs from the becket of one block, around the mast, around the sheave of that block, down to a deck block at the foot of the mast, back to a clutch near the companionway, back through another clutch, through another deck block, around the sheave of the second block, around the mast, and finally to the becket of the second block.

It really is very easy to cant the sail across the mast given that the bottom portion of a junk sail wants to fall forward anyway because the sail is slung from midway along the yard, and our sails are always set up in such a way that the boom is being forced aftward to prevent this happening. So as David described the lines above, to cant the sail you release the line to the forward end of the boom, and haul in on the line which pulls the boom forward, and very little force is needed for this. To bring the sail back to its normal on the wind position a little more effort is needed to pull on the line which forces the boom back.

The only complication is that any luff hauling parrels need to be adjustable so this will not work with standing luff parrels. So for me the process of canting the sail forward is to ease the sheet, (but often not needed), release the luff hauling parrel, release the line that forces the boom aft and pull on the line that pulls the boom forward, cleat everything off, and then set up the luff hauling parrel again. It only takes half a minute to do all this. The only complication is that, on Footprints at least, I cannot gybe with the sail canted forward because of course the aft end of the boom droops down quite low so if I gybe the boom and sheets will catch on just about everything.