CLR of Hull VS CE of sail

Ive been looking at various designs lately and am curious about why some sail plans lead the hull plan so much while others seem only 5% or so ahead

seems this lead in effort over resistance is almost artistic in nature

any thoughts

 

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.

 

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.

 

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.

 

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.

 

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

 

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.

 

Lead

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

 

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?

 

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

 

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/sa...ship-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.)

 

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

 

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! )

 

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

 

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.

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. 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.