Directional Stability

I have noticed that modern yacht designs tend to have the keel well forward and in some cases the centres of effort, bouyancy and lateral plane aligned, which seems contrary to traditional design practice. Having the keel forward like this tends to produce directionally unstable boats.

You can notice this when you let the tiller go while under power, these boats tend to veer off course rather easily and then head in a circle of rapidly diminishing radius. It feels rather dangerous as you must concentrate hard at all times and cannot let the tiller go even for short periods. This is a major problem, especially when short sailing handed.

It seems there is a relationship between centre of lateral plane CLP and centre of buoyancy CB which plays a significant role in determining directional stability of a yacht. Moving the keel aft tends to improve directional stability.

Good directional stability means you can let go of the helm to go forward to trim sheets, pull sails up or down etc without going the boat going substantially off course. One upside of poor directional stability of course, is that these boats steer better when going backwards than forwards!!

While I can see a benefit in manouverability for match racing boats, there seems no obvious benefit for most fleet racing boats and especially for day sailing, cruising and charter boats.

Why are so many modern boats designed to be directionally unstable?

 

wrong lead.

 

Carry it a step further, if the center of buoyancy is too far aft, when you heel or roll under sail the bow dips, the stern will roll out and the mast will rotate forward as the boat rolls or heels. This will put the center of effort and the center of lateral resistance in different planes. The boat will have terrible directional stability.

I think the center of buoyancy is migrating aft because of the decrees in boat weight and the increase in crew weight, as a percentage of displacement.

Is the keel moving forward because of the tendency to have larger mains and smaller headsails?
Robert Gainer

 

Autopilots?

But appart from that, I am not sure that it is true if you include the rudder as part of the lateral area. Rudders have got a little bigger, and keels may have moved a little forward to compensate, but not by much. What is more noticeable is that keels on modern performance yachts have got shorter so you really need to use the rudder to maintain directional stability. This points to having some easy to operate device to lock the rudder when you need to leave the helm unattended, or just switch on an autopilot. I think modern yachts with short keels and the rudder well aft have excellent hands off directional stability with the rudder locked.

John

 

Sensitivity

I'm not sure that the boat you described in the condition you described is unstable. The further forward the keel the greater the effect of the sideways force created by the turning prop; seems to me it would be bound to turn if you let go of the tiller..

 

Isn't it because the rudder is now larger (compared to the keel) and more highly loaded (as a leeway reduction device) and forms a much higher percentage of the lateral resistance? Therefore, any alteration in the forces on the rudder (ie a reduction in the forces holding the rudder steady, caused by the helmsman walking away and leaving the tiller floating) will have a much greater effect. Furthermore, any "error" in the rudder forces (ie lashing the tiller off 2 degrees to one side when under sail) will have a much greater effect when you have a much better modern rudder.

I converted my 28'er fin keeler from a big skeg/small rudder, to a very deep spade. The steering characteristics have changed radically. Formerly, I could often walk away from the helm downwind offshore, for maybe 30-45 s. Now, even after carefully trying to judge the angle at which I need to tie off the helm, she will self-steer only 10s or so. Nothing but the rudder/skeg has changed so it can't be the rest of the design, it can logically only be the rudder/skeg. Any tiny error I make lashing it off has a much greater effect on the course, because the rudder is more powerful than the old one.

The boat still self steers nicely upwind (how can a boat not, when well trimmed - actually almost all the time one just has to drop the tiller extension into a corner and she'll steer herself upwind or close reaching) and it's much more enjoyable to sail under that big, effective and well balanced rudder than it used to be under the little old one, where a tack required a big heave. The first long sail I had with the new rudder was a 45 mile solo beat in winter, and I just stayed up driving by hand for hours because the boat was such a joy compared to the way she had been upwind under the old skeg-hung foil.

The boat tacks much faster, points higher, sails faster, is much easier to sail onto moorings and alongside, it's safer because of the superior manoeuvrability. Downwind at speed in waves it's fun because it has always tracked like an arrow and still does, but now you can work the waves more. Motoring under power at high speed is a ***** and I plan to make up a shock cord arrangement to hold the rudder centrally. This, though, is a very cheap retrofitted rudder and it is very close to the prop wash so some problems must be expected IMHO.

Another example is the 5.5s, which went through a period of being notoriously bitchy on the helm. This was about the '60s, when they weren't allowed to have spade rudders. Once they were allowed and boats were modified to spades (instead of rudders on the back of ever-shortening keels) they became lovely to handle again. I believe. Same boat, same rig, same everything but rudder and it transformed their handling.

You could probably design a boat that would be "modern" and not react to someone walking away from the helm, but that would probably mean the rudder is not resisting leeway and therefore it's only drag upwind rather than lift.


So, all in all, IMHO it's very difficult to identify an "unbalanced" hull without trying to isolate the effect of the steering arrangement.

I agree that moving the CLR aft makes for nice-handling boats in a breeze, but what about the problem of lee helm in the light though?

 

Just to clarify a few points:
The directional instability I refer to can occur under power in either direction whenever the boat moves forward, independent of whether the motor/propeller is in operation.

Lashing the helm in order to keep the boat on track indicates that it is directionally unstable. If you need to lash the helm, then there is definitely a problem, and this is exactly what I would like to better understand and solve.

In directionally stable designs you can let the helm go completely (no lashing) for several minutes at a time and the boat will remain on its original course.

Leeway is not involved when the boat is upright under power, so this should not influence the behaviour.

Balance while sailing can be achieved by placing the rig in the right place, ie: lead of CE over CLP. This can be done whether the keel is placed forward or aft and does not relate to directional stability without sails.

Further thoughts appreciated!

 

The directional instability may be due to an improperly balanced rudder, not necessarily the overall balance. Also, no boat is perfectly balanced for all points of sail and angles of heel.

More modern boats, with a penchant for good upwind performance, are trickier to keep on track for a number of reasons. High aspect foils with small amount of lead (lead as in CE-CLR) give good upwind light air performance, but are tougher to keep in the groove.

Some modern boats designed for ocean reaching have good tracking as more lead is designed into the boat.

Almost all single-screw boats (sailboats as well as ships) have a pull to port when under power because of the prop (assuming it is right-handed). This is due to thrust imbalance on the blades as the prop rotates giving a side thrust.

 

Further thoughts? In my case, where only the rudder/skeg has been changed, the boat has changed from one that would go straight under power (with an un-lashed helm) to one that whips around in tight turns.

So only the new spade rudder is the "problem".

 

Many modern design have apalling running handling at sea and the helmsman fights the boat the whole way downwind.

I've always found that a large husky rudder skeg on a fin keeler produces a much safer helm .
When properly designed the rudder produces just as much turning force for a smaller rudder area than a spade and when you run over a sunfish or a log the rudder stock doesn't sheer.

I think it all comes down to modern lightweight high performance displacing sensible safe and strong.

IMHO
Jim