BEST TYPE OF RUDDER: SKEG RUDDER? SPADE RUDDER? TWIN RUDDER?

                                                                                                                        Hallberg Rassy 49

Almost 10 years ago I posted about this subject when two rudder setups started to be applied to almost all cruising boats, a time when many still doubted the efficiency of the system. Today the situation is different, most boats have a twin rudder, but curiously we see now an increase of cruising boats with a  single rudder set up.

Swan 115

On the main market mass-production cruisers many use now a single rudder, and in the last article about 40ft main market mass-production cruisers, there were more using a single rudder than a twin rudder. I believe that in this case this has mostly to do with cost-saving, and not with rudder efficiency. A twin rudder setup is more expensive than a single rudder and on these boats, they cut in anything they can. This does not mean that a twin rudder is always more efficient. There are many top racing boats and top cruiser racers with a single rudder.

https://interestingsailboats.blogspot.com/2023/10/brand-new-hanse-410-compared-with-all.html

X4.9

Rob Humphreys describes here the benefits observed through tank testing that was performed on an Oyster 885 model, and that convinced Oyster management regarding the benefits of using twin rudders on their line of yachts:

"David’s naval architecture background (David Tydeman is the CEO of Oyster Group) quickly helped me persuade him that twin rudders were the way to go! It was clear that this was going to be a bit of a sea-change for Oyster and I was pleased that David was keen to push this onwards and also to support this breakthrough with a decent budget for tank testing.

Oyster 495

We both felt it would be helpful to have fairly tangible reference information for those owners trying to understand the shift from a skeg-rudder to the twin rudder form for this exciting new model.

In fact our testing session set out to do more than just this because we also used the opportunity to let the spade rudder have its say, just for some form of completeness. 

Tank testing twin rudders

We have often been asked why Oyster has tended to steer clear of spade rudders and the answer has more to do with potential vulnerability than any disrespect for its potential qualities. 

As any Oyster owner knows a blue-water cruising yacht has to be accomplished in a number of different ways, and one of the lower profile requirements has to be an ability to slide backwards against a Mediterranean harbour wall without necessarily endangering the steering gear.

Oyster 825

In our tank testing we were focusing our attention on a fully-pressed up set of sailing conditions, with the boat heeled over to twenty degrees and sailing at nine knots, with a variety of leeway angles and load conditions. .. 

We tested a lot of other things as well but the rudder testing part was most interesting and was totally supportive of all that we had learnt to be true in the field. ...

Oyster 825

For example, with the twin rudders set to just two degrees to the flow the spade rudder needed to be at over six for an equivalent moment, and the skeg-hung rudder at eight – all for the same yaw moment. Put another way, the leeward twin rudder provided 4 times as much force than a skeg rudder!..

From our perspective twin rudders represent a huge benefit and an Oyster owner will really appreciate it too as soon as he has the wheel at his finger-tips. But what’s also interesting is that the system fares a lot better in terms of potential reliability, especially against the spade rudder. 

Oyster 825

The blades are significantly smaller and more lightly loaded, and the span is considerably shorter, making it almost impossible to damage the steering gear when reversing into a quay. 

And of course, with two rudders rather than one there is an obvious increase in the level of redundancy. Unlike some twin-rudder installations, the arrangement we have for Oyster means that even assuming the worst-case loss of one rudder it would be possible to still sail the boat on the compromised tack, albeit with reduced canvas."

http://www.humphreysdesign.com/63/twin-rudders-for-blue-water-cruising

Beneteau Oceanis 51.1 twin rudder

This pretty much resumes the advantages of twin-rudders, but that quicker response is translated also in a smaller sensitivity. They have more inertia due to more moving parts.

Besides Humphreys was making the comparison between twin rudder and a typical spade rudder used in main market cruising boats, not a comparison with a very deep high-performance spade rudder, typically used in more sportive boats.

J45 single deep rudder, as in all J/boats

Regarding more advantages of a spade rudder over a twin setup, if you want to make a sharp turn or turn around, the single rudder can do it much faster, and in a much tighter circle, and that is important in regards to manoeuvering at the marina or port, especially if you don't have a bow thruster, and also very useful while racing around marks.

A single rudder is especially adequate for a not very beamy boat.

Dehler 46 single rudder, as in all the Dehlers (except the 30)

On modern ultra-beamy sailboats having a solo rudder implies a deep rudder and that increases the problems regarding med mooring and also the needed boat draft.

Also, unless the rudder is extremely deep (and for that it needs a deep draft) at high angles of heel, when the boat is heeled by a gust or is over-canvassed, the twin rudder allows the boat to be kept under control. All the rudder is in the water and the water flow is centered with the rudder, while on a single rudder, it will be partially out of the water, and the water flow will be passing sideways to the single rudder. In these circumstances, this gives a higher efficiency to a twin rudder setup.

Wally 93

Only sportive cruiser racers or race boats have really deep spade single rudders. Very beamy main market cruisers have relatively deep single rudders but much less deep than on sportive boats, which normally are not so beamy. And when they are very beamy, like Pogo, they use twin rudders.

Main market beamy cruisers are designed to sail with relatively small angles of heel, and in those situations, their single rudders work perfectly well, and can even be more agreeable to steer than twin rudders, but in extreme situations, they will not offer the same boat control as a twin rudder.

Arcona 465

If the boat has a single rudder and is to be sailed in the Med I advise against buying the shallow draft option because the keel can be shorter (with more ballast), but not the rudder, and you will end up with both keel and rudder with almost the same draft.

As the depth near the quay is always less than some meters away you will end up risking hitting the bottom with the rudder while backing instead of hitting it with the keel. The keel is resistant to small impacts while the rudder has a much smaller resistance. If you need a boat with a small draft buy one with a twin rudder.

Rudder in a full keel boat

Looking back at the rudder design, the first sailboats had a full keel and the rudders were on the continuation of the keel, offering the best rudder protection. But besides the full keel being responsible for a lot of drag, they had an efficiency problem and had to be big: the rudder to work well has to be in the way of the water flow, and the further away from the keel the better they work. The rudder on a full keel is just immediately after the keel, and that does not allow them to be very efficient.

A Moody with a skeg rudder

The next step in the evolution of yacht design towards a more efficient and better sailboat was to separate the rudder from the keel, using a modified fin and bringing the rudder aft. Almost all the rudders of that era used a skeg, most of the time an integral one that protected all the length of the rudder. That improved the rudder efficiency over the previous solution but also revealed some problems regarding using skegs. 

But they found out the skeg could not be very strong. If it is very strong the skeg would not break with a very strong impact or grounding, but the force transmitted to the hull, multiplied by the long arm due to the depth of the rudder, would risk breaking the hull at the insertion point, what is far worse than a broken rudder.

Skeg rudder was destroyed by an Orca attack

Skegs become weaker, and, like the rudders, sacrificial, to preserve hull integrity in the event of a truly violent shock. The sad situation with Orca attacks on sailboats, mainly in Portuguese, Spanish, and Moroccan waters, showed that all types of rudders could be damaged and that the 4 sunk sailboats went down because the rudder breakage led to a loss of hull integrity, and that, to water ingress.

It is relevant the case of a Moody 66, one of the last Moody to be built, a strong boat with a skeg rudder that was attacked by orcas and that did not sink almost by miracle, due to the help of a powerful water pump delivered by helicopter. The skeg did not break and as a consequence, the hull broke. The damage was quite impressive, making for a costly repair.

Moody 66 skeg rudder and the breached hull

One more step brought boat design to modern spade rudders, designed for maximum efficiency, far away from the keel, designed with a sacrificial function: they should be strong enough to sustain all the abuse strong sea conditions could have on them, resist small shocks but towards big shocks, they should break or bend before the hull breaks, to protect hull integrity. Not an easy compromise designing a rudder that responds to these two opposed constraints.

More efficient keels, with a smaller foil (increasing the distance to the rudder) better-designed hulls, and lighter sailboats, gave cruising boats a better sailing performance while the development of solo racing boats, to be sailed mostly on the trade winds, showed that very light beamy boats, with the beam brought aft, had a performance advantage in regards to easiness, being more stable, and sailing with less heel, making them easier to plane downwind, even under autopilot, without losing too much performance upwind. Overall that easiness makes them, out of very weak winds, or upwind sailing, the faster boats.

Moddy 66

But they had a problem with rudders: being extremely beamy they would need a hugely deep rudder that would not be very efficient upwind with the boat heeled due to an extremely asymmetric water flow, meaning that, while heeled the wet surface would be a diagonal, the center of it passing far away from the single rudder.

This led to the use of twin rudders located aft, at the center of one of each asymmetrical narrow water plans (wet surface) that were formed when the boat sailed upwind heeled, to port or starboard. These rudders have the additional advantage of being much smaller, less deep, and therefore generating less effort than a single rudder, making them more resistant.

IMOCA racer. The only thing in common with contemporary very
 beamy main market boats, is being both extremely beamy, but
 these boats are very light and can compensate for a less good
performance upwind with an easier beam reaching and
downwind planning speed.

It took some decades for the big brands to opt for extremely beamy hulls, with beams brought aft, as the typical hull design to be used in main market cruising boats, mainly because they had to wait for the unusual shape to be accepted by the typical conservative cruiser, as a nice one. 

The first production cruiser to be designed this way, decades ahead, was the Levrier des Mers/Cigale by Finot/Conq. They had designed previously several of those solo racers (IMOCA) and were actively involved in their development and security. 

The Cigale/Levrier des Mers, in their first versions, appeared almost 30 years ago. The difference between the two denominations had only to do with the interior layout.

Cigale 15 with a two-rudder set-up, below,
Hanse 460 with a single rudder.

The 2004 Cigale, a 46ft boat with a 4.20m beam looks today moderately beamy, if compared with today's main market cruisers of about the same size, for instance, the new Hanse 460 (45.5ft), has a beam of 4.79m or the Dufour 470 (45.9ft) with 4.74m beam, and they are not the only ones, being that the contemporary design tendency for this type.

Below the Solaris 50. The new Solaris 50 is beamier than the previous 50
 and while the first one had a single rudder, the new model has a twin rudder.
The Solaris is a lot less beamy than the Hanse 460. The Solaris is much
 bigger (48.5ft to 45.5ft) and even so it has less beam (4.78 to 4.79m)

It should be said that the reasons for designing the Cigale this way had to do with providing an excellent sailing performance in the trade winds, maintaining good upwind performance, and allowing for a boat easy to sail at speed.

The Hanse design has to do with sailing with little heel, not needing much ballast (relying mostly on hull form stability) and most of all offering a huge interior volume. 

Both the sail performance and safety stability have nothing to do with the Cigale's and they are incomparably worse.

The 2003 Cigale displaced 10000 kg, the slightly smaller Hanse 12600Kg. Both have about the same draft (2.20/2,25m) but the Cigale has a 33%B/D and the Hanse only 26.7% and I am not sure if this is not only true for the version with a 1.75m keel (being the B/D smaller on the version with 2.10 keel).  The Cigale had also water ballast. 

Note that on the shallow draft version Hanse 460 (dashed) the
single rudder is less deep, offering less control with the boat
 heeled. Below, XP50, with a deeper rudder, offers more control.

They have in common a single rudder and the beam pulled aft. Only on the more recent versions of Cigale, the 16, by Marc Lombard (2017), and the 15, again by Finot/Conq (2024), that are beamier, do they have twin rudders.

The new version comes with a swing keel, with all ballast on the keel, and has a considerably bigger length than the Hanse 460 (45.5ft to 47.6), however, it has a less beamy hull (4.70 to 4.79m).

Deep rudders, some almost the depth of the keel, are more vulnerable, not only due to the bigger efforts that a deeper rudder generates, but are also more exposed to breakage due to groundings or eventual shocks with submerged objects. 

The better protection that the keel offers to a single rudder while sailing is no match for the greater reliability of having two rudders, instead of one. When a rudder breaks due to a shock with debris, if you have two, you still have steerage on the boat, and only upwind with some heel will you lose all steerage. 

Above, Bavaria C45, below, Oceanis 46.1.  The Oceanis twin rudders
 are much less deep than the Bavaria C45 single rudder.

We can also talk about the superior directional stability a twin rudder provides, making the work of an autopilot easier and that's one of the reasons why almost all offshore long-distance racers have twin rudders.

So, the single spade rudder is as dead as the skeg rudder? No, both types, the spade rudder and the twin-rudder have advantages and disadvantages, depending on the design criteria and type of hull, one can be a better solution than the other.

I continue to see designs of racing boats and of top performance cruiser racers, designed with CFD and VPP extensive use, that continue to use a deep very narrow spade rudder. Rating formulas can have to do with that. Still, I am more inclined to think that for absolute maximum performance, less drag, and maximum rudder sensibility, those rudders can offer advantages, especially if the racer is not very beamy.

But we are talking about boats designed to offer maximum performance, with somebody full-time at the steering wheel, and sailing on the limit, with a busy crew controlling the sails.

Solo racers: above, Imoca, below, class 40. All the boats on
 these classes have two rudders

Solo racers have for many years adopted twin rudders as the better solution for sailing with a short crew on very beamy boats, and that has probably not to do with absolute performance, but with the advantages in speed that a more easily controllable boat can offer, and at the superior speed an autopilot can be used while racing.

Regarding cruising, if the boat is not used also for racing (cruiser-racer), and if it does not have a moderate or small beam, the advantages of a single rudder over a twin rudder are less than the disadvantages, especially if the beam is huge, like in many contemporary main market cruisers.

The main advantage and the main reason why it is used in less expensive and very beamy cruising boats is to be cheaper to build, allowing to cut costs.  Also, it can offer a slightly better sensibility and mainly, it gives better maneuverability in the marina, but most of the owners that buy these boats have them with bow thrusters, and that makes this last advantage negligible. 

However, if you want a sportive boat and don't want a bow thruster, it can make a huge difference and a single rudder can make sense.

72ft class Cannonbal. Almost all Maxi 72 racers have a single rudder,
like almost all TP 52 racers.

Today marina "corridors" are narrow and the easiest, and sometimes the only way to put a 40ft boat (or bigger) on a berth without a bow thruster, or help from a marina dinghy, is to sail backward and turn sharply over the berth. 

A boat with a very deep spade rudder will be able to turn almost over its keel and with practice you can do this without trouble. With a twin setup, the turning angle is much bigger and it will be much more difficult or even impossible to do that. 

Grand Soleil 44, which won the ORC world championship on
the last 3 years, has, like all cruiser racers and racers built
by Grand Soleil, a single rudder, as well as all x-yachts and
all Arcona Yachts.

On a not hugely beamy cruiser-racer with a very deep rudder, you will not have less control at big angles of heel, you will enjoy the extra sensibility a single rudder can offer, will be able to put the boat where you want, and choose your way in between the waves, with a precision a  twin rudder set-up will not be able to offer. 

You will have more precise and nervous steering and you will have more fun at the wheel. But on autopilot you will miss the superior directional stability of a twin rudder, which will be able to steer the boat in conditions where, with a spade rudder, you will need to hand steer, needing to reef the boat sooner and go slower to use the autopilot.

Of course, these are generic considerations that are valid but do not take into consideration the way each boat is balanced and designed. The rudder does not work alone but in conjunction with a hull, and hull design has an influence on the rudder's effectiveness and feeling.