Schooner sail plan- centers of effort

I'm working out the sail plan now for my schooner (in building stage). I have one I like (.jpg file attached to this post) and now of course I may need to tweak it for balance. When finding the combined centers of effort of the entire rig, how do I account for the overlap of the forestaysail, jib and flying jib? It makes sense that I simply treat them as centers and not as triangles, but then I also feel like some account needs to be made for the overlap. Anybody?
Boat specs are:

Beam: 17.9'
L.W.L.:63.1'
Depth of hull (not draft):10.35'
L.O.D.: 74'

I've got 3 tons of lead ballast hung below her very substantial deadwood keel. The wood part of the keel is also 3 tons. The entire boat thus far is made from a hardwood called Takien Tong. Very rot resistant and dimensionally stable, moderately easy to work.

That's Tigger. Any help on the center of effort thing will be great.
Scott

 

Treat them as centres. The overall centre of area should be 0-17% of LWL forward of the centre of lateral area. That gives you a "ball park" of over 10 feet to play with. I'd tend to aim at 10-15% of LWL to start. If the boat has lee helm, just add mainsail area.

 

I suspect the CLP on that underbelly profile will move aft as she heels. The relatively shallow forward areas will lose their "bite" (decreased dead rise) and the lead should be less then what has been offered (no offence intended Randy). I would try 5 to 10%, but would like to know more about the hull shape to help define the issue better.

 

None taken. Lead is something that everyone has an opinion on:

Good balance can be achieved by placing the centre of gravity of the sail plan directly over the centre of lateral resistance, or perhaps an inch or two ahead of it. John F Sutton

For balance, The lead of the centre of effort over the centre of lateral resistance should be 12-14 per cent of the waterline length in the case of a shallow hull, fin keel or centreboard craft; about 10 per cent for deeper, more traditional yachts; and about 8 per cent for cruising yachts of classic form. John Teale

Balance: for racing machines of the scow type the lead of CE over CLR should be 5 to 15 per cent of the waterline length; for shoal, full-ended centreboarders, the lead lies between 7 and 11 per cent. For full-ended keel boats the lead is generally a little less; for cruising boats of normal form it is about 6 per cent. Norman L Skene

To achieve balance, the correct lead of CE over CLP (=CLR?) as a percentage of LWL is 7-12 per cent for a schooner; 11-14 per cent for a ketch; 12-15 per cent for a yawl; and 13-17 per cent for a sloop or cutter. He seems to be talking about conventional designs in conventional sizes (say over 10 feet LOA).Dave Gerr (The Nature of Boats, p301)

I suggested starting on the high side as It is much easier to move the CE aft than to move it forward on a schooner. All it takes is a longer boom and gaff.

To move the centre forward might require more major surgery.

I'll agree that 15% is a bit high, your 10% would be a better starting point.

I would also try to design the sail plan such that with all normal sail (no topsails, triatic, or flying jib) there is a bit more lead to counter weather helm when heeled. If the CE moves aft a bit with the flying sails set, the boat will pick up some weather helm that it may need when not heeled very much. Another sail to consider is a staysail from the main top to the deck about as far forward as the base of the foremast. If done right, the CE of this sail will be at (or very near) the CE of the whole sail plan adding lots of sail area without changing the balance.

 

Wow, you guys are great. That's lots of good info which reinforces some of the same things I've come to understand from some of the same reputable texts which were quoted. But, hearing it first-hand sure encourages me that it's the right way.
Randy, excellent point on hedging toward a more forward initial placement of the center of effort, as lengthening the main boom/gaff beats lengthening the sprit (if possible) any day of the week. Any opinions on boom-kins to facilitate this?
A wise designer once said "On a one-off boat, don't do anything on the rig that can't be undone". Oh wait, that was me! Maybe not so wise, but it's my tack on this thing anyway.
While I have your rapt attention, what do you guys think about one piece vs. 2 piece masts on a gaff rig like mine? I'm building up all of my masts/spars to be hollow, about 1/2 diameter being hollow on the masts. So (theoretically) I can make a one piece mast as long as I like (currently the mainmast length is 85' keel step to tippy top). Given equal everything, what are the relative benefits/downfalls of going one piece vs. lower and top masts?
Also, for PAR, attached to this (I hope) are two jpg's that describe her form a little better. Your input is immeasurably appreciated. I'm happy to share any other info on the hull and rig (under construction both) if it'll clarify my questions (I still have a lot more, promise).
thanks heaps,
scott

 

Lead can be misleading because the requirements change with boat type, hull-shape, rig etc. and the fact that CEs are taken from Geometric centres for simplicity. It used to be the case that designers could not do much better than an approximation, but I would dare to say that with modern computers we can do quite a lot better.

The first problem is that the pressure distribution over the sails is unknown, but it is not hard to find with either a Vortex-Lattice-Method or 3D CFD method (depending how accurate you want the answer).

Secondly, there is the same question for the hull. The hull is more complex due to the air/water interface, but none-the-less it is soluble with a reasonable degree of accuracy.

If you know the forces and moments on the hull and sails, then you can fairly accurately simulate the boat's performance and the weather/lee helm problems you are likely to encounter.

Tim B.

 

With those bilges, I'd can confidently suggest that 8 to 10% would be fine. I do agree that the sail plan should be balanced under her working sails only. The topsails and flying stuff will be light air power brokers and the helm will likely want a little help with a higher and farther forward CE, which those sails will provide.

A pole mast or Bermudian if your prefer, would be a much better solution that a fitted topmast. It's lighter, cheaper, easier to build, has less windage and makes hoisting arrangements simpler.

I'll bet she has some sweet waterlines and buttocks. That hull shape was reasonably common in use, as appendages became more and more centralized while the ends were being honed down. They like high aspect rigs, because the hull will want to fall off, while the high aspect rig will want to head up, partly negating this conflict. If she has a particularly fine entry, this will help her in this regard. Her CE will not move around much (compared to a single sticker or cat boat for example) because of the divided rig, so placement should be carefully thought out.

 

PAR - Can you tell me a little bit more about your view that 8%-10% lead would be fine given the bilge shape? What leads you to this opinion? I'm unclear on this issue, as the boats center of lateral resistance has already "accounted" for the profile of the hull, but the deadrise effect is a bit foggy for me, so your statement makes me wonder if I did a good job in the bilge design or if I overlooked something.

 

At low angles of heel, the fore foot area and aft to about the foremast step, possibly farther aft, remain "engaged", offering effective lateral plane. After about 15 degrees of heel, those areas no longer have sufficient dead rise to be effective as lateral plane. They have become near horizontal in sectional shape at 20 degrees (leeward side), which causes the CLP to move aft. This tied with the seemly hollow, nicely drawn out entry and long fine run will affect your lead placement.

Clearly you've tried to maximize the profile, for a CE that will be farther aft then typical in single stick craft or a ketch. There are a number of "qualifiers" I use for lead. The expected force of the lee surge wave, the effects of hull depth, draft, centerboard slot and other eddy makers, shape of the hull ends, rig type, aspect ratios, rudder placement and type, etc. would be a short list of qualifiers.

Generally, I employ 3 - 5% for centerboard uni rigs, 5 - 7% on a centerboard sloop, 5 - 8% on a keel boat, 16 - 18% on a masthead sloop with detached rudder, 12 - 16% on fractional sloop with detached rudder, 12 - 17% on mast head sloop with long keel, 12 - 16% on ketches, 8 - 12% on schooners.

 

Oh great !, I was thinking yep, yep, 10% untill your last post Par, now I'm just confused. [more corectly, Thinking 10 - 7% now.]
I hear you guys, every boat is different ! .
How accomadating are you fellow members ?. There are many a designer amongst you all, I, am not !.
I'm trying to build my own yacht as I can't afford or find what suits our needs/wants , but before I can go much further I need to make decisions as to placement and size of Daggerboards [ in my case] affecting the CLR ,and rig size / placement.
I know there are recognized ratios, sail area / displacement [ don't know those yet ] , I can work out my centre of effort , [but is my sail plan big enough ? ] and CLR , but the placement in % of waterline leangth ??

I , like many others no doubt, joined this fantastic forum because of it's wealth of knowledge [ the members ] not to mention the whit & sarcasim - reads Humor - and have learnt so much and am gratefull for all advice to date
If I were to post some line drawings, dimentions,displacment etc, am I asking too much from profecional people for help advice via this forum ?.
I'm thinking that many of you designers enjoy the "shop talk" , and am not cynical enough to think that you visit here to hook clients.
I think on the whole, generally speaking, the members are a clever and experienced lot , more so in their paticular field .
Enough said, [ by me] I wait.

 

Hi Scott,
I think things are looking pretty good with your sailplan. I think you've gotten some pretty good advice here on the lead. But like everyone says it is an educated guess.
I will bring up one minor point. The peak of your staysail looks like it may be a little close to the mast to allow for hoisting things like blocks, etc. It also doesn't need to be that close for any aerodynamic reason either.
I am also curious about your topsails. Can you describe them for us?
Also I am curios to know if this vessel will normally be sailed shorthanded or with several crew members. This looks like a very interesting boat.

Cheers!

 

Sorry Scott, looking at my post from last night in the light of day, I think I was a bit rude , make that unthoughtfull, in posting on " your" thread.
I didn't mean to hijack it, just didn't think. Came in from working on the boat with questions of CLR's & COE's going around in my head & found your thread regarding same. I guess I should post my questions in a new thread huh .

 

It seems that PAR has given some very useful information, and I would agree with his estimates for the sailplans listed (if on the generous side), with the caveat that they refer to bermudan rigs. As he has quite rightly given a list of 'qualifiers', employing a gaff or any other type of rig would be one of them.

And whilst there have been good reasons given for using a pole mast, a major advantage of the gaff rig is using a fidded topmast. Notwithstanding the fact that you have to send someone up to retrieve the fid, the ability to strike the topmast when weather dictates is an extremely seamanlike feature. Also in its favour are the structural considerations; that the mainmast can be stepped and stayed very securely, with lighter scantlings and rigging for the upper spar. There's more redundancy naturally built into the system - if you lose the bowsprit for whatever reason, it won't threaten the mainmast. In a multiple rig setup, if there were a rigging or spar failure high up, it wouldn't threaten the lower masts. More work, yes, but greater overall security.

 

When considering geometric centres for the sail plan and profile of the hull to set lead, what difference is there between a leg o' mutton Bermudian rig and any other rig? Area is area. We are not considering aerodynamic/hyrodynamic centres of effort. As "square top" mains see wider usage I am not aware that new rules are needed/used to get balance right.

 

It must be remembered that these are not scientific calculations - they are instead largely empirical guidelines. What we are actually trying to do is line up the dynamic centres of pressures for the hull and the sail plan, whilst heeled and upright, with full sail and reefed. As PAR stated, issues such as detailed hull design influence this dynamic balance as much as the sail shapes and locations.

So, 'what difference is there between a leg o' mutton Bermudian rig and any other rig?' Well, as the centres of areas are used with a certain amount of lead to give an acceptable approximation of real-life centres of pressure, as the shape, aspect ratio, efficiency, etc. of the airfoil changes so does the movement of the centre of pressure in relation to the geometric centre.