Center of Effort vs. Center of Lateral Plane

C.L.R. and C.E.

The real argument is that the true C.L.R. of hull-keel combination is not fixed but varies with respect of the leeway angle. Initially, in order to be able to represent in the drawing board what would probably be a yacht behaviour under sail, it has been assumed that the C.E. of the sails and C.L.R. of the canoe body are coincident with the geometric centres of the profile of the sails (i.e. Main Sail + 100% Fore Triangle) and the Centre of Lateral Area (i.e. Canoe Body + Fin Keel profile) only.
The distance betwen C.E. and C.L.R. is called "Lead" (expressed as a percentage of the Length Water Line) and gives an indication of what it would presumably be the balance of the boat under sailing. The afore mentioned value was not related to the "real" lead value as, in actual fact, it was and still is quite difficult to assess it, as the parameters affecting the true position of C.E. and C.L.R. are subject to continous changes when the boat is under dynamic conditions.
Thus, if we take what it is reported in Larsson and Eliasson book "Principle of Yacht Design"......
.........."the positioning of the sail plan relative to the underwater body is too complex to be handled entirely theoretically. Regardless of which method is used for finding CE and CLR their relative location has to be based on experience, if the yacht is to be as well balanced as possible under all conditions".............
Hence, the only recommendations being made for a modern high aspect ratio fin-keel arrangement are as follws:
..."we thus use only the extended keel and compute the location of the centre of pressure (i.e. C.L.R.) on this, assuming that it lies on the 25% of the chord, at 45% of the draft"...
..."For fin-keel yachts the extended keel method proposed here should be used. The following leads are then recommended:
...Masthead sloops: 5-9 %....
...Sloops with a fractional rig: 3-7 %"...

The other values we usually read in the YD literature are usually referred to long keel yachts where CLR is the geometrical centre of the underwater hull and then related empirically to the sail plan by the following rule of thumb:
..."Masthead sloops: 12-16 %"...
..."Sloops with a fractional rig: 10-14 %"...
..."Ketches: 11 - 15 %"...

 

Furthermore, for the C.E. of a sloop rig, the geometric centre of the fore and main triangles is still emplyed.

 

Yes, the lateral location of CLR has a dramatic effect on balance, as can be demonstrated on windsurfers where there is no rudder to confuse the issue. On the original Windsurfer, you leaned to windward going upwind strong winds to move the CLR laterally to windward to counteract the "weather helm" as much (or more) than any other reason.

However on some classes, developing lift from the rudder and trimming the sails in a way that increases weather helm ends up giving you more speed than you lose by the extra drag of the rudder. The J/24 has been one of these.

As North Sails' site says of the J/24, "The boat needs a small amount of weather helm to be able to always sail as close to the wind as possible....More weather helm means a higher pointing and faster boat."

As Quantum Sails' tuning guide says of J/24 preparation, "the goal of preparation in all four areas is to create weather helm in light to moderate conditions to improve the boat's upwind performance."

My J/24 skipper and trimmer were not exactly fools, as demonstrated by their multiple world titles and an America's Cup win, and they (and the rest of their pro team and the world champs etc) found that the J went best with a considerable amount of weather helm, created by a tight mainsail leach. You don't win multiple worlds and nationals by being a bad helmsman and trimmer - you win them by making the boat go at its best even if that means loading up the rudder.

Sure, some boats perform best without weather helm (Int Canoe, Tasar, many multis, etc) but some perform best with weather helm (J/24, Laser, etc).

 

Thanks.

I put a larger rudder as part of a redesign of my boat, as I didn't want to mess with the keel draft to increase upwind performance - that's an area for you pros! The larger rudder giving lift within its drag bucket works quite dramatically, as you said.

 

Physics applies equally to all sail boats...

Weather-helm is needed for human drivers. A defence contractor grade fly-by-wire boat would have as little(practically none) as possible.

I was already aware of NorthSails, etc. I'm also very sure you can find a thousand guys who "hunt and peck" faster than I touch-type(mono V tri).

Please ask you Skipper why? I(we) would sincerely like to know. From the discussions the answer is complicated, regardless.


ps. "More weather helm means a higher pointing and faster boat." Does not count as an explanation, in fact without any further explanation, it is dead wrong.
-The unbalanced forces cause a boat to turn and it will keep turning(until in irons), weather-helm arrest the turn at some human desired point...... Like if you are a footer or a pintcher, (aostslts)

 

It's what you train. A young laser sailor's helm gets tied to a few inches. The new thinking in tennis is let the young players perfect their poor form?....

Weather-helm makes a boat easier to sail and is a necessary safety thing. I think with the right retraining one can be faster around the cans.

Spithill surely did not rely on "feel" to keep Dogzilla at best VMG, he had a heads-up display.

And I've seen a few good monohull sailors catch their butt on a neutral helm tri. It is unfair to ask you for a good(sci) explanation, but if there is one, for my own understanding.

 

leeboard vs. daggerboard

Is it just me or wouldn't a dagger board equipped boat need a longer lead than a lee board equipped one of the same hull design?

My reasoning is this:

With the dagger board boat, especially once it heels, the drive vector of the sails is far to leeward of the lift of the dagger board, causing the boat to turn to windward.

With the lee board boat (with the lee board always on the downwind side) the board's lift vector is a lot closer to the sail's vector, or, if the boat is sailing quite upright, to the lee of it, causing the boat to want to turn downwind.

My understanding is that the board, when it is creating lift, can cause as much or more drag as the rest of the boat.

Am I correct on this?

 

Correct.

 

It's not hard to sail a craft without weather helm well. Many of my friends and I race each weekend in craft that never have any rudder angle, so we are very well accustomed to sailing without weather helm - yet in other craft we find that weather helm is normally faster because of the extra hydrodynamic lift it can create.

It does seem that you are trying to say that you are a better steerer than (say) a J/24 champ, because the purposely induce weather helm at times and you believe that they are only doing that to make it easier to sail. My former J/24 skipper did NOT like the weather helm of the J/24, because it hurt his back. Nor did he need helm to sail well - he was a dual world champ, so for you to assume that he could only sail well with weather helm is rather strange. He sailed the J to a national title win with weather helm in a breeze, because by doing so he created hydrodynamic rudder loading and a tighter mainsail leach. These factors created speed gains that outweighed the extra rudder drag.

To demonstrate that sailors do NOT need weather helm to sail well, look at the Laser or similar craft. Downwind we heel them to windward to neutralise the helm. Downwind steering in a Laser is critical, and we can do it very well with NO weather helm or any other weight on the helm. Everything from luffing to bearing away by the lee is done with the helm barely touched (your coach makes sure of that!) so there is proof that we do NOT need weight on the tiller to sail well. Similarly, there are classes with minimal weather helm (Int Canoe for example) that can easily be sailed well.

However, in other conditions many boats go faster with moderate weather helm, so then we induce that helm.

The "feel" that good sailors search for is determined by what is fast, not what they would want to feel if speed was not an issue. Good racers don't want some weather helm just for its own sake, they want it because it works.

As an analogy, look at heel. Some boats go fastest heeled to windward (some Skiffs, foilers, some windsurfers, Lasers etc downwind) and fast sailors in those classes learn to like the feel of windward heel. Some craft go fastest flat (many dinghies upwind, etc) and fast sailors in those classes learn to like the feel of sailing flat (even when it makes the boat feel dead in some ways). Some craft go fastest with heel (cats doing the wild thing, leadmines upwind in a breeze) and fast sailors learn to like the feel of those classes when they are heeled.

Many sailors can go out and get the feel to sail a Moth, skiff or windsurfer heeled to windward at winning pace, and then step onto a yacht the next day and sail it fast heeled to leeward. The fact that the same sailors can learn the "fast feel" for rig heel even when that rig heel may vary 65 degrees or so (across the range of different craft in different winds) surely proves that the feel for speed is very adaptable.

If we can learn to sail heeled to leeward on Saturday and heeled to windward on Sunday (or heeled one way upwind and the other way downwind on the same day) it would be odd if we couldn't get used to steering without weather helm IF that was fastest - and in fact, as the Laser and other classes show, we DO get used to steering without weather helm in the conditions that is fastest. So the idea that keelboat sailors (unlike you) need weather helm to cover up for lack of ability does not seem to be warranted.

For the theoretical justification, let's hear from some yacht designers etc;

From (I think) Giles Thomas' yacht design course in naval architecture at the Australian Maritime College;

"Good balance is not zero helm required to steer on a straight course.
* Rudder contributes to hydrodynamic sideforce.
* Larger the weather helm, the larger the rudder sideforce produced.
* There is therefore an optimum rudder angle for tradeoff between sideforce and resistance.
* Optimum usually approx. 5 degrees weather helm."


From the Dr Patrick Couser (Australian Maritime Engineering Co-operative Research Centre) paper to the 14th Cheseapeake Sailing Yacht Symposium;

"Figure 22 indicates that an angle of approximately 5 degrees is required for the centre board to be at its optimum angle of attack, and that the rudder would require around 7.5 degrees. This shows that the centre board produces something in the order of 3 degrees of downwash at the rudder. Thus for both centre board and rudder to be operating at their maximum lift to drag ratios, the dinghy should be sailed at a leeway angle of approximately 5 degrees with approximately 3 degrees of weather helm. The weather helm is to be expected since the the rudder operates in the downwash of the centre board and in order for its to have the optimum effective angle of attack some weather helm must be applied."

http://docs.google.com/viewer?a=v&q...ce leeway&hl=en&gl=au&pid=bl&srcid=ADGEESgOUt

See also Larsson and Eliasson, 1994, page 157 where they indicate 5 degrees of weather helm is optimum.


There's some graphs in the Teeters/Pallard/Muselet paper that show optimum rudder angle (i.e. weather helm) for optimum speeds. The information they provide includes;

p 41; "the minimum drag (of a rudder) at a Lift Area of .3 would be achieved, particularly for the deep rudder, with a relatively low leeway and very high rudder angle" (which we would normally call weather helm)."

In summary, they say "it is clear that there is quite a bit of variation on optimal leeway and rudder angles that, depending on heel angle and boat speed, provide minimum drag."

Available at;

http://docs.google.com/viewer?a=v&q=cache:Q1AUm96q7P8J:www.journalofoceantechnology.com/%3Fpage_id%3D73%26id%3D5%26jot%26jot_download_article%3D46+"rudder+angle"+leeway&hl=en&gl=au&pi


And Tom Speer, in a discussion here, again noted that the downwash from the keel affects the rudder so "that the change in leeway angle at the rudder means that the tiller has to be deflected to weather just to produce the same lift that the rudder would if the keel were not present."

 

Tri Calcs

Hi Guys: is there a simple formula for calculating centre of effort-centre of lateral.
Design is 6.5m Trimaran.
Shipwright.

 

I am trying to figure out where to position the mast and sails with respect to the fin keel so that I have some reasonable amount of weather helm. By all accounts and many comments throughout this thread, the CE should be further aft than the CLR as measured from the bow to have some weather helm. However when I look at sailboats out there, I have yet to see one that has the mast (by the same token the CE) that is behind the CLR. They all have the CE considerably forward of the CLR which should produce lee helm. Well I know that doesn't make sense. Please see the picture of a typical sail and keel positioning. I know I am getting something wrong or missing something. Please help me understand this.

 

You are missing the fact that the graphical CE and CLR are not the points that are the physical center of the forces forces and physical center of the keel and hull forces. Use the emperical formulas.

 

The mast location is not the CE. The combined CE is almost always aft of the mast.

 

Somewhere online there is a diagram of the resolved forces as the boat heels.The heeling tends to align the rig's resultant forces with those of the underwater elements of the hull.Which means that no "rule" is absolute.Then there is the matter of how the keel develops it's lift and the effect of different foil sections and the cleanliness of the hull surface,not to mention the set of the sails.

 

Thank you gentlemen for your comments. I am using NACA 63 series profile for the high aspect ratio keel and 0012 for the spade rudder. I will resolve the combined forces for 5 to 30 degrees of heel. Let's see how that will work out.