Steering & handling

I have experience with two conventional 36’ yachts of similar proportions having quite different steering and handling behaviour, which I would like to understand and improve. One has a keel hung rudder and the other a balanced spade rudder.

The boat with keel hung rudder tracks straight even when the helm is released for significant periods. It requires some load to initiate a turn and then turns slowly in a large arc. The helm is relatively heavy and must be held with significant pressure to maintain the turn, but when released, the rudder self-centers and she sets off happily, tracking straight on the new course.

The helm of the boat with a spade rudder requires constant attention and cannot be released for extended periods. It is easy to initiate a rapid, tight turn and once turning, it will not stop. In fact, it will keep turning in a tighter radius, requiring significant effort to arrest the turn in order to establish the new course.

One feels easy to manoeuvre but a little uncontrolled, while the other tracks much better and is easier to sail but is less easy to turn. Ideally, I would like the best of both worlds. The boat should hold its course and track straight, then turn easily in a tight turn, but straighten on the new course whenever the helm is released an hold this course hands free.

What is controlling this behaviour and is it possible to improve both boats?

 

Welcome to the Forum hazza;
I assume both the boats you are referring to are sailboats?

 

Yes, both boats are conventional masthead sloop rigged sailing yachts around 36' overall, 28' waterline, 6 tons displacement with a moderate length keel.

 

So the first has a full keel with a keel hung rudder and the second has fin keel with a (obviously) poorly balanced spade rudder?

 

Yes, the first boat has a moderate length keel with an unbalanced rudder attached conventionally.
The second has a spade rudder well aft, away from the keel. Whether it is 'poorly' balanced all depends on what you mean!
It has around 15% of the area ahead of the axis of the rudder stock. So it is somewhat balanced, whether this is bad or not, or if it is solely responsible for causing the very significant difference in handling is what I am trying to understand.
I can easily reduce the amount of balance, but would like to know what difference this would make before making the change.

 

Regarding the boat with the spade rudder, what is she like when she is sailing close-hauled?
Is it possible to balance her so that she will sail herself, perhaps with the helm lashed with a bungee (or similar)?
And if yes, how long can you leave the helm for, unlashed, assuming that the sails are trimmed well?

Do you have some side profile photos that you can post of the two yachts out of the water, showing their keel and rudder configurations please?

 

What is the angle to the vertical of both rudder stocks/shafts? FWIW, the long keel is what causes the exceptional tracking of the first vessel and most likely the vessel was designed that way, i.e. to track well. What year or who designed the the first boat? The fin keeler, on the other hand, has a poorly balanced rudder for what ever reason. The rudder should never be so unstable that it wants to rotate off center. That is a dangerous condition for all sorts of reasons, least of which is crew fatigue.

For the first boat all you can do is to totally reshape the underbody. The second vessel will, at least, need a new rudder with better balance.

 

Both boats have been designed with fully balanced hulls, so there is no weather helm generated at all as they heel, even when gunwale down in a fresh breeze.
They also have the CLP aft of the LCG and both are normally quite directionally stable when sailing, but the yacht with balanced spade rudder can veer off course under power which is when I notice the need to constantly tend the helm.
They both sail perfectly balanced in most conditions upwind which is really good. I can let the helm go completely and they sail themselves, responding naturally to gusts, lifts and knocks without intervention.
There is no need to lash or hold the helm on centre at all. I do of course try to anticipate lifts with some manual intervention when racing, but otherwise they sail very well on their own.
They will sail happily like this for several minutes, until a large change in wind direction or ocean waves upset things. In steady conditions I can often sail 15-20 minutes without intervention, even on reaches as well as upwind.
I do adjust the sail trim a little between light and heavy air just to maintain rig balance, but from 10-25kts there is no need to make any changes. I do not normally need to ease the mainsail, even in heavy gusts, she just heels a bit more and gradually responds to the gust by pointing higher into the wind until she settles on a new course at full speed.

The major and striking difference between the two boats is when turning to tack or under power. The keel hung rudder requires a reasonable load to turn the boat, but then self-centres when I let it go. It feels safe, but turns slowly.
The balanced spade rudder is completely different, it is easy to turn and just keeps turning by itself. When I let it go, it locks into the turn, requiring quite some force to bring it back on centre to arrest the turn.

Here are some photos of the rudder of the two boats.
This is the balanced spade rudder


This is the keel hung rudder

 

Both rudders could be better balanced.

The keel hung is harder and introduces snagging issues if altered. Extend the rudder downward and forward under the existing keel.

The spade rudder needs to effectively be moved forward on its shaft. Add more blade to its rear and or remove some of its forward edge. Or as already stated replace

 

What is happening is pretty simple, the long keel boat has no propwash at all, the rudder works like a trimtab on the keel. The other boat has propwash, the boat is responsive, but beeing balanced it pushes over fast and stays there.
There is not much you can do, the long keel boat you can only cure by fitting an extra spade rudder behind the prop, or with thrusters. The short keeled one could be improved with a non balanced rudder, but this will affect steering force under sail.
If the rudders are properly balanced under sail as you say, fit thrusters to one boat to make it more maneuverable and put a hydraulic helm on the other.

 

The more I look at those two photos, the more I think something is off. It's like they are not quite finished in design, or some other issue. Almost like sister hulls of a 1960's-1970's sailing yacht deliberately built to test something. And the CPP's location really add to that feeling.

 

You are somewhat on the right track.
The first boat is a cold moulded triple skin oregon/epoxy hull built in 1975 with the rudder on the back of the keel.
The prop does not direct flow over the rudder, so she is harder to steer under power, but very nice and well behaved when sailing.

The second is a subsequent development of the first, built around 1990 with a shorter chord keel and separate spade rudder for improved steering.
It is foam/glass construction with a full duplex stainless keel. Around 55% ballast ratio. Performs extremely well for her size, especially upwind in a breeze.
Flow of the prop over the rudder gives good steering under power.
Both boats have feathering props, recently fitted.

 

So these are two custom sisters who were homebuilt? Who was the designer? There are definitely issues with shape and arrangements. It almost looks like someone took an old full keel design and for the 1975 hull just clipped the deadwood and for the 1990 hull tried to convert the lines to a fin keeler. Either way, they were unable to successfully manage the rudder arrangement, mostly because they left the existing prop shaft log arrangement. As I stated, the 1975 design will take significant underbody changes to improve handling; the 1990 sister will need a new rudder (perhaps a more reasonable balanced horn design I'm thinking) because there are significant operational/structural issues with what is shown.
And finally, for completeness, it looks like these are canoe sterns? Or just pinched with a tiny transom?

 

Just as an aside to the excellent observations above, I am wondering where the engine is on the long keel boat - is the propeller shaft at a slight angle to the centreline, to allow it to go past the rudder stock (but then would involve a very long propeller shaft), or does it have a vee drive arrangement?

Re the spade rudder, do you know what the diameter of the stock is? It looks rather small, in relation to the side profile area. But I guess it must be strong enough, as it has been there for 30 odd years now.

 

On the long keel boat, the head of rudder stock is below the prop shaft. Both are on centreline.
I was thinking I could add a small spade rudder further aft at the end of the waterline, behind the prop, to improve turning and perhaps also link it to the existing rudder.
Perhaps I can get the best of both worlds!

On the shorter keeled boat, I am planning to reduce the amount of balance on the rudder as suggested, as it seems this may be what may be causing the helm to lock over into a turn and not self-centre when released.
I was thinking around 5% balance ahead of the axis of the shaft should do the trick.