Nugland

I’ve been spending some time thinking about all of the sail options for the gaff-rigged ketch Centennial, which was designed by Ted Brewer for the Loy family in the late 1970s.

It’s a traditional rig, so my theorizing may apply to other similarly rigged ketches, but it is a leeboard shallow-draft sharpie, so it may not really compare to the heavier displacement, full-keel boats that typically carry such rigs.

She has three primary sails: (clubbed) forestaysail, gaff main and gaff mizzen, with two reefs available in the gaff sails and one in the staysail. Additionally, there is a large mizzen staysail, a main topsail and a jib topsail.

This table spells out all the individual sail options with their moments:

Sail	CP-Fore/Aft (in feet)	CP-Vertical (in feet)	AREA (in square feet)	Moment (square feet times distance from CR)	Inches from CR (negative numbers are aft of CR)
Forestaysail	11.59	13.67	146	1692	139
Forestaysail Reefed	12.92	12.22	102	1321	155
Jib Topsail	9.24	25.10	55	509	111
Main 1st Reef	1.03	15.37	188	193	12
Main 2nd Reef	1.33	13.65	140	186	15
Maintop	3.16	32.54	50	158	37
Main Full	0.67	17.00	237	159	8
Mizzen 2nd Reef	-12.89	11.30	78	-1008	-155
Mizzen Staysail	-6.29	12.34	177	-1115	-75
Mizzen 1st Reef	-13.01	12.81	109	-1413	-156
Mizzen Full	-13.55	14.19	142	-1926	-162

‘CP’ refers to the center of pressure of the sail, and ‘CR’ to the center of lateral resistance of the hull. The CR is a bit of a guess on my part, but the fact that my numbers show her to be nicely balanced on the three primary sails (see below) makes me think that I was pretty close. The CPs were calculated by tracing the sails (reefed and full) in a CAD program (ViaCAD v. 6 on a Mac, FWIW) and having it calculate them.

(CP – Vertical would only be needed to calculate heeling (‘tipping over’) moment, which I’m not doing right now. Enjoy them numbers anyway.)

Moment, here, is the ‘twisting’ force that the sail will exert on the entire boat. Positive numbers act in front of the CR, and negative ones behind, meaning: Positive moment will make the boat bear away from the wind (‘lee helm’) and negative numbers will encourage her to point up into the wind (‘weather helm’). This isn’t going to be a long explanation of the desirability of weather helm, but most sailors do desire some of it; with a little weather helm, the boat will turn into the wind when the rudder is, er, unattended. This is a pretty safe outcome. Too much weather helm, though, and the boat will require excessive rudder to hold her course (which is both tiring for the helmsman and adds drag). Lee helm, on the other hand, will make the boat want to fall off the wind, which can be quite dangerous.

OK, some initial observations of the numbers themselves:

First, both the mizzen and the staysail can really crank the boat around, but their absolute moments are pretty well matched.

Second, the main, on its own, has a fair bit of lee helm (which actually goes higher as you reef it).

Third, it looks like the mizzen staysail will require quite a bit of canvas forward to balance it.

So, now. With, perhaps, the exception of of the main, the sails will usually be bent in combinations. There’s some math required to calculate what the moments will ‘add up’ to, but that’s what Excell is for. I entered all the likely (and some unlikely) combinations and came up with this:

Rig	Inches from CR (negative numbers are aft of CR – weather helm)	Total moment	Total Sail Area	Sail Area to Displacement Ratio
Main 2nd Reef, Mizzen 2nd Reef	-45	-821	218	8
Staysail, Main Full, Mizzen Staysail, Mizzen Full	-20	-1191	702	27
Staysail, Mizzen Full	-10	-234	288	11
Staysail, Main Full, Jib Topsail, Maintop, Mizzen Staysail, Mizzen Full	-8	-523	807	31
Staysail Reefed, Mizzen 1st Reef	-5	-91	211	8
Staysail, Main Full, Mizzen Full	-2	-76	525	20
Staysail, Main Full, Maintop, Mizzen Full	2	83	575	22
Main Full	8	159	237	9
Staysail, Main Full, Jib Topsail, Maintop, Mizzen Full	11	592	630	24
Main 1st Reef	12	193	188	7
Staysail, Mizzen 1st Reef	13	280	255	10
Main 2nd Reef	16	187	140	5
Staysail Reefed, Mizzen 2nd Reef	21	314	180	7

So… What?

Well, staysail-main-mizzen does turn out to seem pretty nicely balanced, what with a moment of only -76 (right around 2 inches aft of the CR). (Moment here, by the way, is in square feet of canvas times the distance of the total center of effort from the CR in feet.) And with an SA/D of 20, that’s a pretty sporty setup.

Staysail-mizzen also comes out pretty well, which is desirable in any ketch (what as ‘jib and jigger’ (staysail and mizzen) is a popular — no — the famous way to reduce sail in a hurry on a ketch).

The staysail, as noted above, does have quite a down-wind pull. From the numbers, it would appear that the best sequence for reducing sail (short of bending trysails) would be:

• Reef the main, maybe twice (leaving staysail and mizzen up) (SA/D: 18 then 16) (Not listed above, but barely changes the total moment.)
• Drop the main completely (SA/D: 11)
• Reef the staysail (1st reef) and the mizzen (in that order) (SA/D: 8, less than a full mainsail by itself)

I can’t see much point in having the second reef for the mizzen, what with the staysail pulling so hard (even with a reef in it). You’d just be begging for lee helm. And I’d rather switch to trysails before messing around with a second reef in the staysail.

Beyond that, you could fly the reefed mainsail by itself, but that only gets you down to SA/Ds of 7 and 5. And it apparently has a lee helm (that gets worse as you reef it).

Going in the other direction, adding sails seems to mean only one thing: flying everything. The mizzen staysail has so much weather helm that you really need both topsails up front to balance it. That said, she sure looks pretty with over 800 square feet of canvas up, and, with an SA/D of 31, she should scoot along quite nicely in anything above a zephyr. Tacking might be a bit of a chore (the mizzen staysail halyard has to come down each time to allow the main to cross the centerline), but any sort of reach would be a gas.

This entry was posted on Thursday, March 26th, 2009 at 2:38 pm and is filed under Centennial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.