CLR of Hull VS CE of sail [Archive]

marshmat

12-22-2008, 12:51 AM

Yup, setting the lead seems to be more a matter of experience and art than any clear scientific method.

I've seen sail plans where the centre of sail area leads the centre of lateral plane by anywhere from -5% of LWL to as much as 20%, although most boats seem to fall in the 5% to 10% of LWL range.

What makes it a real pain in the backside to do this scientifically is that neither of these centres is actually known. We can get a centre of area for the sails and for the lateral plane, but the actual centre of effort is awfully hard to calculate without a few hundred hours at the CFD lab.

PAR

12-22-2008, 03:04 AM

These centers are for comparative purposes only. In reality many things affect these centers and they move around, especially the CE. Certain elements of the hull form, appendage configuration, displacement dispersal and type of rig are some of the things that can affect lead. Some rig types coupled with certain hull and appendage trends can generate considerably different leads among similarly sized and rigged craft. Generally, a designer goes with what they know is "about right" for the configuration employed. Past experience, examples of other similar vessels and gut feelings are the norm, which can be a bit of magic compared to other engineering disciplines.

Omeron

12-22-2008, 05:00 AM

I would like to contribute to this thread by asking an additional question.
With a typical modern rig configuration, i.e. high aspect main and jib, and a fin keeled cruiser racer of average hull shape, it is almost impossible to put CE forward enough to produce a lee helm. This manifests itself when you sail against the wind with the jib/genoa alone, and without the mainsail up.
It takes longer for the boat to get into the track, but once it balances itself, it tracks pretty well most of the time and with some weather helm as well.
since the main is not up, the CE is only produced by the jib alone, which is probably much more forward than 20% max design limit.
I have struggled more than enough to keep a boat away from rounding up and fought with tillers, wheels, but i have yet to sail a boat which bears away by itself when hard on the wind.
So what is the incentive to put CE closer to CLR and risk excessive weather helm than playing safe and putting a bigger lead.
If it is to do with performance, then using excessive rudder or being forced to put a reef into the main earlier than necessary, should also be detrimental.

idkfa

12-22-2008, 09:04 AM

With heeling comes additional weather helm from CE being to leeward of the hull as well as the asymmetrical under water hull shape. If one were to sail in light wind you would surely experience lee helm if CE is ahead of CLR.

Eric Sponberg

12-22-2008, 09:12 AM

From my experience, the incentive is to get an appropriate amount of "feel" in the tiller so that you can control the boat better. I have had designs with a very neutral helm, and they are actually fairly difficult to steer because they balance too well. If you let the tiller go, they just keep right on course. In my fetching up days of yacht design, the rule was to have a slight weather helm so that if you fell overboard, or for some other reason let go of the tiller, the boat would naturally round up into the wind and stop. I still think this is a desirable feature. With a neutral helm or lee helm, this will not happen--if you let go of the tiller, the boat will keep on going on its course. This is not a good feature if you are suddenly in a situation of treading water and seeing your boat sailing off into the sunset without you.

This topic was discussed ad nauseum a few years ago in a very long thread, and you might still find it around here somewhere.

Eric

alan white

12-22-2008, 10:29 AM

A lot of boats are designed with far too much weather helm. One I owned, in particular, a 7 meter S2, really suffered from this malady. The angle of the tiller must have been 10 degrees off center when really pushing to weather.
I added a bowsprit and cutter rig and my maximum consistant speed increased 7/10 of a knot (6.2 to 6.9). This on an 18.5 ft waterline.
The helm shifted now from lee to weather as the wind increased, which was okay because in light winds there was no worry of broaching. I still always had a healthy few degrees of weather helm angle in strong winds.
It is nice to know that letting go of the tiller will round the boat up. Some boats, however, have way too much weather helm. Nobody should put up with this if they could monkey with the rig.
Boats vary not only in lead but in how much the lead changes as the wind increases. Modern designs in particular (wide stern flat-iron shape, short keel) have CLRs that shift a lot. Older long-keeled boats with balanced lines were far easier to steer, having CLRs that shifted less.

Brent Swain

12-24-2008, 03:50 PM

Many make the incredibly naive asumption that if they balance a cutout of the underwater profile of a boat , the balance point is the centre of lateral resistance. This assumes that a well rounded forefoot has the same lateral resistance as a skeg or keel. If this were the case, one could make a perfectly balanced rudder by putting the shaft in the centre of the blade. The actual centre of lateral resistance is a short distance behind the leading edge of the keel. This explains the short lead and resulting excessive weather helm in many boats.
Most offshore and coastal cruisers are usually on autopilot or windane most of the time, so the odds of a boat returning to find you if you fall overboard is a feeble excuse for fighting a weather helm. I prefer a neutral helm. 34 inch high solid lifelines is a more logical response to that fear.
Brent

marshmat

12-24-2008, 04:07 PM

Most offshore and coastal cruisers are usually on autopilot or windane most of the time, so the odds of a boat returning to find you if you fall overboard is a feeble excuse for fighting a weather helm. I prefer a neutral helm. 34 inch high solid lifelines is a more logical response to that fear.
Brent
Good points, Brent.
Also, said autopilot is rumoured to live a much longer, happier life if it is pushing against a neutral or weak weather helm. I've heard of boats that burn through two or three oversized autopilots on a year-long cruise, because the weather helm is so strong they wear out trying to fight it.
As to falling overboard- a boat on autopilot or vane ain't coming back for you, no matter how it balances. But that doesn't matter, because your harness is clipped in to something solid and you have metre-high lifelines all around, right? ;)

Boston

12-24-2008, 04:30 PM

Hey Eric
glad you chimed in
Ild love to read that thread you were referring to
and Ive read back threads to pg 30 or so and will hopefully stumble across it soon
do you remember what it was titled
as I could have missed it

I get the picture that the CE will move forward of the CLR in light airs and back in heaver based on hoisting additional sail area that tends to be forward producing relatively the same amout of weather helm in KG/M2
question is how much weather helm do I want to have to hang on to
Ild like be able to hand the wheel to the girl and not have her end up spinning round and round

but I like your philosophy of maintaining a slight weather helm as a safety feature
even though the idea does not always meet with general approval

slight weather helm being the key term
Ild like to employ a autopilot and I agree that if the balance is off to much your going to burn through motors
if I get it just right I should be able to go the old school approach and tie off the wheel and still maintain a heading

Eric Sponberg

12-24-2008, 06:18 PM

Boston, Here are most of them that I can find, in no particular order.

http://www.boatdesign.net/forums/sailboats/center-lateral-resistance-1898.html

http://www.boatdesign.net/forums/sailboats/understanding-relationship-between-clp-ce-sailboat-design-9676.html

http://www.boatdesign.net/forums/sailboats/center-lateral-reasistance-19959.html

http://www.boatdesign.net/forums/boat-design/hull-balance-2517.html

http://www.boatdesign.net/forums/sailboats/yacht-balance-clr-ce-etc-15868.html

Regarding using an autopilot, sure, if the autopilot or windvane is on, the boat will keep sailing if you fall overboard. But the vast majority of people don't have an autopilot or a windvane. Also, if you have to hold the tiller (or wheel) against a heavy weather helm, so will your autopilot and/or windvane, and as mentioned above, it will wear out really fast.

What a lot of people don't realize is that if you are on a long cruise with the windvane/auotpilot on, you can change balance dramatically by pulling up on the topping lift to lift the aft end of the main boom which causes the top of the mainsail to spill off the excess wind. You can fine tune the balance this way. On our cruise from England to California years ago, we did this all the time. We had a 27' full-length keel boat (Bianca 27) with a keel hung outboard rudder. The boat balanced pretty respectably in most conditions. But in heavier air, with more heel, she could develop a heavy weather helm. So we "topped the main" by raising up on the topping lift to spill the top of the main, and we reduced weather helm to nearly, but not quite, zero. We did not lose any appreciable speed because we usually had more wind than we needed. But the boat sure did become easier to handle.

To my mind and experience, I like a little weather helm, not a neutral helm, and I design accordingly.

Eric

(On vacation until next week.)

Boston

12-24-2008, 08:30 PM

hey Eric thanks
seing as how I love a challenge
Ill try and work up a formula that will estimate in KG/m2 of effort placed on the rudder
mechanical effort placed on the helm is going to vary with each design
but I think if I consider actual lateral resistance to be actual CLP and the actual center of effort to be the center of area then the formula need only have consideration for the angles of attack involved with each and the wind speed
I think
if it were that easy though someone would have dreamed it up already
but who knows

I'll definitely check out those threads and see what I can see
thanks again
B

rrrg
I see lead, wind speed, and healing arm will all have to be related in the formula

Omeron

12-25-2008, 06:18 AM

One (or two) healing arm should definately be included in the equation, especially after a long passage with a heavy weather helm, as well as a heeling arm! :-))

Boston

12-25-2008, 04:58 PM

why do I suspect I spelled something wrong again
and no worries
Im used to it
best
B

messabout

12-31-2008, 02:56 PM

Boston

12-31-2008, 03:25 PM

fortunately I wont be contending with that problem as Im bying plans from a well known and very accomplished source

the torture is choosing the boat for the most likely conditions

Im experienced in one small area of the world
Cape Cod area
and with plank on frame only
I dont know jack about other construction methods

thus the questions about stuff that seems to be more intuition than science

so far I have three serious contenders for the coaster I want to build
all 39~46" and all three different configurations

and Im still up in the air

marshmat

01-01-2009, 02:56 PM

And one more variable attached to the CE. When you are on an offwind leg the CE of the main swings forward. It may move forward a considerable amount depending on the aspect ratio of the main. If the layout is configured to avoid lee helm when reaching, then a squatty main may very well set up some serious weather helm when close hauled. When reaching or running the presumed center of pressure is well outboard which will amount to a moment arm.
A valid point. And I would think this would be more of a problem with high-performance sloops and such, than with a multi-masted type.... perhaps part of the reason we see so many ketch rigs on long-term cruisers? Or am I totally off base here?
If the boat is a planing type, or even a semi planing type, then the CLA is someplace other than where it was when operating in full displacement mode. Thus the helm may change its mood. Way too many indeterminant variables it would seem.
That's why the good NAs cost big bucks..... not sure I want to know what Farr or Juan K. charge for all the analysis on a one-off racer!
Eric, Par, Matt and other clever guys use some science and ,methinks, a whole lot of art when laying out sailplans. In my own silly efforts, I just rebuild the damned boat as many times as it takes to get it right. My method not recommended for professional builders.
I think Eric has the science down pretty well, Par seems to me to be the type who treats it as an art derived from lots of experience. Don't lump me in with those two.... my sailing experience is very limited and I have yet to build one of my own designs.

PAR

01-01-2009, 07:34 PM

I have no issue (okay, I'm lying) with the science of it, but it makes ugly boats Matt. Initially, we all start out with what we believe is a "sound" base in the fundamental concepts, principles, theory, etc., then reality bites us in the butt. We design a narrow focus craft (for example), with certain expectations and damn if it doesn't exceed or not quite measure up in some or multiple regards. This is a perplexing phenomenon and often the case, other wise we wouldn't bother with trials, we'd know precisely how things are going to "act", trust the math and "go with it".

Reality shoves experience down our throat and eventually, if you've been paying attentions, patterns and similarities appear. We tuck a line in here, increase deadrise there, etc. all because we've learned from a previous model, not necessarily the science, rather literally our own butt.

This may be age induced cynicism or it may just be a learned response to a set of variables. Figuring out which it is, possibly is the art aspect. An example is a little sloop I'm working on. It'll be fast, in fact faster then the previous model it's based on, because it's lighter and slightly narrower. Will it be faster then the model I've targeted to whip up on, well I don't know, I hope so. Maybe this is the "art" of it.

Brent Swain

01-02-2009, 05:50 PM

I once met a French couple in an aluminium centreboarder,who had daggerboards at the stern angled outward, like twin rudders. They said that with both boards down they couldn't get the boat to round up or broach, regardless of how hard they drove her. The had to pull the boards up to even be able to manually steer anywhere but dead downwind. Maybe a good idea on ocean cruisers designed for good downwind control in strong following winds.
Brent
Brent

Boston

01-02-2009, 06:45 PM

so why is it on the basic friendship sloop type the CLR is so far behind the CE
were as on the basic Bermuda rig it seems closer

PAR

01-02-2009, 07:56 PM

There are many, most quite huge differences between a typical Bermudian rig/hull form combination and a Friendship. The CE is actually closer to the CLP (less lead) on the Friendship, then a usually proportioned Bermudian sloop, for many reasons. The 26 foot "in the sprit of" Friendship I'm working on carries about 7% lead. She's a fairly burdened hull form and drags along a 266 D/L, with an 18.6 SA/D, using a Cp of .56. A 26' modern sloop would be half her weight (and D/L) have possibly a higher SA/D, particularly down wind a higher Cp and the lead would be in the 15% to 18%+ range, especially if carrying lofty, big roach sails.

There's very little in common between the two, except they both sail and float.

ancient kayaker

01-02-2009, 08:50 PM

I hesitate to rush in where NA's fear to tread, but all the discussions I have seen on this topic so far seem to assume it can be illustrated in 2 dimensions in the profile view. Looking at the thrust vector in the plan view it is immediately clear that it crosses the centerline aft of the assumed CE, especially on a broad reach.

marshmat

01-02-2009, 09:22 PM

Exactly, kayaker.
I think folks like to look at the lead of sail CE over hull CLP in 2D plan view simply because it is simple. It's been done that way for long enough that it's possible to compare new designs to known ones.
As you point out, the actual lift vector in 3D space looks very different. The sketch in post #22 illustrates rather nicely why, even though the CE of the sails is ahead of the CLP, the boat will want to round up into the wind... there is a net moment, in this case counterclockwise, if you release the helm.

Boston

01-02-2009, 10:29 PM

why I asked
they seem to be so completely different
yet both have there benifits for what I want a boat to do
coast in the pacific northwest

given the rear sweeping aspect of the friendship hull design
I would have never guessed it had a closer balance than the Bermuda style rig on say a fin and skeg hull

PAR

01-03-2009, 05:28 AM

Much of the differences are rig related, though the hull and appendages do play a role. The Friendship's much lower aspect ratio, linked to the gaff's trait to not move it's CE as far outboard, combine to keep the lead lower. There are smaller or shorter couples created in comparison to a higher aspect Bermudian sloop.

You have much to learn grasshopper . . .

Boston

01-03-2009, 05:55 AM

Lmao

ok
I prefer to think of myself as a small fish in a big pond
but for now
grasshopper it is

my take on boat design is its one of those things that takes a life time to learn
and yet its not something you really learn its more of an art than that
I have a short period of time before I start sawing and pounding something together
about enough time to maybe make an decent decision as to what will suet my needs best
certainly not enough to design anything

its called delegating the responsibilities
someone designs
someone builds
someone sails
few dew all well
me
I drink beer and steer particularly well
I got a pretty good grip on who to invite to the party
and have a knack for vacations

its like a job
otherwise who would buy all those designs
eh

cheers mate
B

ancient kayaker

01-03-2009, 10:17 AM

For me boat design is one of those "find out what works then keep on doing it until you're bored" things. I get bored easily so I have to look for a new way of making a fool of myself on almost every boat. Thus design is the brief interval between figuring out what went wrong on the last one and the joyous anticipation of the next and perfect boat.

I used to use grasshoppers to catch small fish in the town pond when I was a kid, then I'd take the small fish over to the river and try to catch a biggger one. That part never worked though. Think it's called business development now; I never did get the hang of it.

PAR

01-03-2009, 02:10 PM

You can learn enough about design in a reasonably short time to fully develop a design, but not without instruction. I think you could do the WestLawn course in a year, which will provide enough education to successfully pen up a boat.

Boston

01-03-2009, 03:29 PM

Im workin on modern yacht design and a few others that have been suggested
but I think you guys are selling yourselves short
the intricacies of a truly exceptional design are not the result of a few years study

PAR

01-03-2009, 09:44 PM

Your quite correct Boston, but most designers tend to be modest (except for me of course as I'll "flash" just about anyone), particularly in the shadow of the many greats of recent past and a few still working. It's not unlike a carpenter of 30 years experience. Fresh out of trade school, they had a fair idea of what they were doing, but in comparison to a wood grain weary veteran, they're mostly thumbs. You have to start some where. Now is a good time my friend.

It's truly a sin we couldn't all be like Olin Stevens. His grace, poise, understated expertise and particularly his love of life was a joy to all who met him and honestly infectious. Even those who only met him briefly, like me. In these shadows, it's pretty easy to be humble.

messabout

01-04-2009, 03:48 PM

Your craftsmanship is apparently well beyond the ordinary. Gorgeous stuff in the pix. You can build the boat, without question. The sloop with the ungainly boom is not for the faint of heart. I shudder when I imagine jibing that thing. A fine looking boat but the rig ...........? Give it a ketch or yawl rig and live longer, less strenuously, and far more easily balanced for all weather.

As for the cost of sails, it will be helpful if you can manage to win the lottery.