Hull Balance

I am interested to know if there are any good techniques available to design a balanced yacht hull, which will go straight at all angles of heel and speeds.

I am also interested to know if such a technique were available, would this be of interest to yacht designers to develop well balanced hulls?

By hull balance, I do not mean lead of CE over CLP, nor directional stability nor the moment of the the heeled rig, each of which are easily managed and well known.

Rather, the hull balance I refer to is the intrinsic weather or lee helm generated as the hull is heeled.
ie: take a hull without sails, heel it over and push it along. It should go straight at all angles of heel and all speeds if properly balanced.

The end result is a very big improvement in handling, feel and speed of the boat.

Ian Ward

 

Ian,
How would you take into account the effect of the driving force of the hull? i.e. if you heel a sailing yacht, the driving force becomes offset from the centreline of the boat, creating a turning moment. The hull should at least attempt to counteract this, or the weather helm will be excessive. A hull that tracks straight at all angles of heel will not do this, with a resulting increase in helm as the boat heels.
Thoughts?

Steve

 

The turning moment from the rig has far less effect than the imbalance which can be caused by poor hull shape. The two in combination of course can be disastrous.

I have seen boats hit with a gust, round up on the spot, go past head to wind and involunatrily tack with the rudder firmly held across the boat. This does not happen with balanced hulls.

A well balanced hull will naturally get a very slight weather helm as it heels due to the effect of the rig being to leeward. This is good for safety and also give a little better performance.

This helm should be just sufficient to enable the boat to respond to lifts and knocks. Ideally there is no need to touch the helm upwind in any conditions.

A good Hull Balance design tool will enable you to design in any amount of weather or lee helm into the hull that you require. It is even possible for this helm to change with heel angle and hull speed.

If for instance you had a very tall rig, then it would be possible to design in a little lee helm when heeled to counter the turning moment, if you thought this was necessary.

 

I suspect you underestimate the effect of the turning moment from the sails in a gust.

 

No need to estimate!
I sail a yacht with a balanced hull and I therefore know exactly the effect of the turning moment of the heeled rig.

To the contrary, I think that it is not often appreciated how much poor hull balance contributes to the turning moment and increased resistance when heeled.

The reason for this is that its effect is purposely masked by most designers who generally employ a large balanced rudder as far aft as possible. This gives maximum leverage, least loading and removes the "feel" of the weather helm. But of course does not solve the original problem.

 

What calculation did you do to dismiss the CE of the sails so easily? A rig and hull can both be inbalanced and counteract each other to acheive balance.

 

Firstly, the hull balance analysis shows no net steering moment at all angles of heel, indicating the hull has neutral balance. This is then confirmed with model testing at inclinations from 5 to 35 degrees which indicates straight tracking of the hull with no rig.
Finally the full sized yacht shows excellent balance under all conditions, confirming the first two measurements.

Yes you can balance a yacht which has weather helm, by moving the rig forward, altering the sheeting of the sails, flattening and twisting the mainsail etc. The problem with this is that it results in lee helm when upright and you therefore need to alter the sheets and settings as the breeze changes. With a truly balanced hull, this is not necessary.

I have not said that there is no effect from the moment of the rig, it is simply that with a balanced hull, the slight weather helm created as the boat heels is beneficial and very easily controlled. The boat steers itself and needs minimal correction even in gusty conditions.

If I may give a practical example:
Try sailing an International Dragon and a J24 in a 15-20 knot breeze. The Dragon, which is quite well balanced sails sweetly at all angles of heel with almost no helm, while it is necessary to dump the main and pull lots of strings on the J24 just to stop it rounding up. In fact the Dragon is significantly faster upwind in a breeze, despite the J24 being lighter, longer and having more sail area.
Clearly the Dragon, with its well balanced hull is the better design.

 

Wardi,
Have you looked at the sail area/displacement ratios of your two "candidates"? I think you'll find that the Dragon has a lower number, which may explain why there is more perceived balance in the hull. Also, the Dragon is only shorter on the "static" waterline. but has an overall length of 29' 2" and long overhangs that will pick up LWL as she moves.
I will grant you, however, that the results you specify are true - the dragon sails in a more balanced mode than the J24. ;-)
The fact that the Dragon was designed in 1928 and naturally followed the design thinking of the time (long and skinny) while the J24 was designed to try to plane and therefore has a larger beam (8.9') _may_ have more to do with the results than you think.
The dragon was designed by Anker to sail in typical European conditions, while the J24 was designed for US East Coast conditions, which are considerably lighter wind-wise. Testing in 15-20 knots menas you are testing at the Dragon's design point, and slightly above the J24's "reef now" point.
I'm not trying to deny that a balnced hull may be a good thing, just that the effect of the rig and the hull together is a more useful tool.

Steve

 

Hi Steve,
A J24 goes well in light airs because it has light displacement and heaps of sail area. It also surfs downwind in a breeze for the same reasons.

The advent of improved construction materials has enabled us to build lighter hulls and this is the obvoius benefit. This does not mean we should throw out good design principles at the same time.

You have spoken about a J24 being designed for light conditions, but also being designed for planing. Surely then it should be able to handle all wind strengths.

Are you seriously defending the J24 as a good design, inspite of the fact that it rounds up uncontrollably in even moderate winds and cannot even beat a much older, heavier design with less sail area across the full wind range?.

I do not see what SA/D ratio has to do with hull balance. If you take a J24 and heel and push it along without sails up, it will go off course, not so with a Dragon.

Being fat, especially aft is no formula for fast planing. The fastest dinghies for their size today are Moths, which do have a flat planing area in the centre of the hull, but they are very narrow and have almost canoe sterns. Also I do not see too many new maxi ocean racers being built that look like J24's, they are all long and narrow as well...surely there is a message here!

Let us imagine building a light weight Dragon style boat with large sail area, equivalent to that of the J24, but retaining the balanced hull. It would retain its easily handled characteristics across the entire wind range, no weather helm, controllable downwind etc, but also be just as fast as the J24 in light winds. An all round improvement wouldn't you say?

Imagine then taking this one step further and having a finer hull with more efficient keel but also retaining the good balance characteristics, now that would be a real improvement, and much faster than a J24 in all conditions.

We can of course argue the contribution of hull balance, as this is not so easily measured, but if you have ever sailed both boats, then you will know what a blessing it is!

 

"Are you seriously defending the J24 as a good design"
Heh heh! No.

My point about SA/D is that the more sail you have per lb means the more moment you have (proportionally) when the rig heels (we'll assume the boats heels at the same time ;-))
I said (look above) that the J24 was designed to "try" to plane. It will under the right conditions, but not often. The Dragon was designed for heavier air than the J24, however, and it shows.
As far as current offshore maxis being long and skinny, you obviously haven't been watching the Open Class for a few years LOL!
Interestingly, I just checked the published (key word...) SA/Ds for both, and the Dragon is 19.0, while the J24 is 20.0. Interesting, no?

The discussion continues....

 

There is a method which can be used which works quite well. Above was someone mentioned the fact that a poorly balanced hull will have far more dramatic an effect than an unbalanced rig. This is very true. Water is 800x more dense than air, and consequently the turning momnent created from an unbalanced hull can be signifigant. An easy way to ensure balance is to find the LCB whilst the boat is sitting static. Then heal the boat to an acceptible heel angle (say 25 deg) and then measure where the LCB is now that the boat is heeled. The new LOCB location should not have moved more that 1% of the total WL length.

 

Coozeman,
Do you know if this method is widely used by designers?
Is it used as a guideline, or is it used as an accurate indicator of hull balance?

I am aware of the idea of balancing immersed wedges of the hull crossection when the boat is heeled, which is similar to the technique you mention and also the Metacentric shelf analysis developed by Admiral Turner (Skenes elements of yacht design).

Are there any other techniques you are aware of?

 

A friend of mine has worked an Excel spread sheet that calculates the curve of areas, LCB etc. The spread sheet also calculates a curve of the centerpoint of each area when the hull is heeled (also retrim longitudal heel according to change in LCB). This way you can see how the hull shape is "turning" in any direction when heeled.

 

Hello Henrik,
Is it possible to use this to accurately analyse the hull balance?
Or is it simply a graphical representation of the line of heeled centres for qualitative comparison?
Does it include the keel?

 

I dont know what you mean by "accurately analyse" the hull balance, but what you get is a graph of how the center of each section differs from the centerline.
Analysing the hull balance seems to me like something you learn from experience.
The Ljungström design was one attempt i guess, a friend of mine once built a R/C Marblehead that was cigar shaped, hull totally symetric in longitudal direction, I guess that hull was well balanced, but it never scored any regattas.....