Rudder-in-a-drum

In this month's "Sail" magazine, there is a review of the new Santa Cruz 37. Of interest to me was the rudder design.

http://sailmagazine.com/boat-reviews/best-boats/best-performance-boat-2009/

This is a very similar design to one done by Eric Sponberg (I can't find the link at moment). Anyway, at first blush this seems to me to be a much better approach than a traditional rudder shaft in dealing with steering forces. I have two questions for this forum.

1. Other than perhaps cost, can anyone see any obvious downside to this design?
2. Can anyone suggest a design for owner/builder construction using "off the shelf" components?

Regards,

Sea Jay

 

Obvious downsides? Well, I can see how it might be easy to screw it up if one were to try to build it without thinking through the design. But fundamentally, I don't think there's anything inherently wrong with the "retractable rudder in a drum" concept. Difficulties may arise in fairing the rotating drum with the underside of the hull. Bearing friction may be an issue; that should be easily resolved though. I'm not sure if that rudder sweeps back somewhat- if it is vertical, then its centre of effort would be somewhat forward of its pivot axis, which could cause some unusual reaction at the wheel.... I'd think you would want the rudder blade angled slightly aft so that, viewed in profile, its overall centre of effort is roughly on or slightly behind the pivot axis.

I see no reason why someone with a bit of engineering know-how and some skill with a lathe couldn't whip up something similar, albeit much simpler, out of aluminum or something....

 

SeaJay,

I have done a couple of rudder-in-drum designs, one for Project Amazon which worked very well, and one for Saint Barbara which you can see at the following link:

http://www.sponbergyachtdesign.com/SaintBarbara.htm

On Project Amazon, the drums were 1 meter diameter and the chord length of the rudder blade 600 mm. This put the drum axis just ahead of the center of lift on the rudder blade. These drums were expensive to build, in aluminum, with carbon fiber blades (the blades were 3.695 M tall, over 12', and they weighed about 66 kgs, 145 lbs.). The bearings top and bottom were Teflon plastic strips 50 mm wide and wrapped around the inside the cylinder and on the outside of the drum, recessed appropriately. The top flange was a locking flange to keep the drum from moving up and down, and the insides of the mating surfaces were also lined with Teflon strips.

The design of the Project Amazon drums were laid out with the idea that, if warranted, the geometry and sweep of the rudder blade could be modified and still fit inside the drum. That never happened, and they worked fine as designed. There were also two completely independent hydraulic steering systems, each connected to their own drum, and the drums also had a tie bar between them so that only one steering system needed to be operational for both rudders. Should the hydraulics go down, the drums could also be connected by cable to the steering wheel and then be steered by cable. It was a really neat double-redundant system.

On Saint Barbara, my thinking distilled to having a drum only slightly larger than the chord length of the rudder blade. There is some cant to the rudder relative to the drum, so the center of lift is behind the turning axis of the drum. The bearings, as originally designed by the owner, are plastic ball bearings. The upper bearing seems to work fine, but the bottom bearing has some drag and roughness to it. The rudder drum and bearings are going to be rebuilt this winter. As for balance, the rudders seem to work fine.

Saint Barbara's rudder does not have a drum per se, merely a form-fitting cassett with top and bottom flanges that end in bearings. This was the owner's design, not mine. As a result, there is always water in the bottom of the opening as it sits below water level. This boat sails in Lake Michigan, so it's fresh water.

I should note that the owner of Saint Barbara is a University of Michigan engineer, and he delights in conceiving of ways to solve problems, build them, see how they work, and refine them. So I am responsible for all the lines on Saint Barbara, and the owner worked out the structures for himself with oversight from me.

On both Project Amazon and Saint Barbara, the hull is essentially dead flat right in way of the rudder so that the bottom surface of the drum does not cause any disturbance to the flow. On Project Amazon, both the drums and rudders are perpendicular to the hull surface. On Saint Barbara, the "drum" (cassett, actually) is perpendicular to the hull, but the rudder blade is swept aft about 7 degrees relative to the drum. On my new design, the Scandinavian Cruiser 40, we will also have a rudder in a drum. Again, the hull is essentially flat at the rudder, and the rudder and drum, right now, are coaxial and set about 6 degrees aft cant relative to the surface of the hull. However, due to the slope of the hull at that point, the rudder still has an aft sweep to the water. I may rethink this somewhat and make the drum of such a shape that I can alter the sweep of the rudder for adjustable balance.

As for off-the-shelf components for DIY construction, there aren't any to my knowledge. You just have to cobble it together from what you an ingeniously apply. Every rudder for every boat is different, so there is little will to standardize designs and parts. You just have to figure it out.

Eric

 

Eric,

Thanks for weighing in on this subject and for including the link to your work. You have provided a lot of good info for me to digest. I thought that ball bearings would likely be used, but I hadn’t thought about Teflon strips that sound less troublesome. It is also helpful to know that there are no off-the-shelf components.

I am curious as to the motivation for use of this concept. I understand the thinking on a dual rudder boat such as Project Amazon, as well as for trailer-ability as on the Santa Cruz 37, but now you are incorporating this into a third boat. Do you think we will see more of these in the future? Other than the ability to lift the rudder, do you see other advantages over conventional shaft designs? As always, thanks for your contributions to these forums.

Best Regards,

Doug

 

We have this system on my sons boat and have been very happy with it.The boat is a 24ft foam and glass uldb which i designed and built in 1987,i originally designed it to use a soling rig to take advantage of the many lightly used sails available from olympic campaigns.The boat has always been something of a lab rat for trying out different ideas.I gave the boat to my son a decade ago and since then we have shortened the chord and lengthened the retractable bulb keel,added a flicker and a lot of roach to the main,added a masthead assym and a 5ft articulating carbon sprit and the rudder in a drum.
The reason for the rudder mod was that when i first built the boat it had a plumb transom but the hull skins extended a foot or so further which necessitated a vee shaped notch to accomodate the transom hung retractable cassette rudder.The only problem with this arrangement was the outboard being off center was difficult to keep in the water because of the hull extensions.we couldnt get it deep enough so we decided to mount the motor in the center and move the rudder inboard but it still needed to retract for trailering hence the drum rudder solution.The way we did it was to use a piece of 12"dia pvc pipe as a mandrel to lay up a glass inner drum,we used sheet wax on the outside to give clearance and also to create races in the outer drum for bearings top and bottom,the fiberglass outer drum was laid up over this.Large holes were cut thru the hull and cockpit sole and the outer drum installed and glassed in.We built the hollow carbon constant chord blade in a folded aluminum mold,the blade has a chord of only 10" but it is quite deep.We again used sheet wax for clearance and layed up its trunk around this.We used the piece of hull which we saved from installing the outer drum on the bottom of the inner drum and then installed the trunk for the blade.We use roller bearings made out of 1/2"ertalite plastic rod in the top and bottom races,there are 72 at the top and 72 at the bottom.After the first test sail we realised we needed to kick the blade aft to get some balance,this was easy to do by just cutting an angle off the leading edge of the blade inside the trunk so that when we push the blade down we then kick the top foreward which angles the bottom aft,probably about the same as Eric did.It was a fun father and son project and has been a great success the boat steers well even with the huge 666 sq ft Melges spinnaker.
Steve.

 

On the SC 40, the motivation is to make the boat trailerable and able to fit into a container. The rudder has to retract, and you don't want to go through the hassle of physically withdrawing it from underneath the boat.

Structurally, however, in the general case, the motivation is to get away from the very high stresses that necessarily occur in a conventional rudder stock. A rudder stock is asked to perform a number of functions all at the same time: hold onto the rudder blade, resist bending stress, resist torsional stress, don't bend and twist too much, and still be able to turn freely. Rudder stocks are confined in their tasks by their geometry which prevent them from doing these jobs efficiently--that is, they have to fit inside the rudder blade which necessarily is a very narrow shape. Small and narrow geometry means small moments of inertia and section modulus with which to resist deflection and bending/torsion stresses. So, thinking outside the box (or, in this case, the blade), get rid of the rudder stock and replace it with something else. Go outside the blade, and you come up with the rudder in a drum concept. If you do that, you free up the geometry and in the same stroke you greatly reduce the stresses and deflections. You pay for this by a larger geometry that takes up more space in the boat, but it can be built fairly easily by less expensive materials.

Will we see more of these? Probably; I don't think they are going to go away. But I can't say how popular they are going to be. If I had such a crystal ball, I'd likely be a very rich man.

Eric

 

It's intereting but I fail to see some of the advantages claimed.

If the entire drum rotates then the bearing diameter is much greater than for a more usual arrangement. I don't see how that "takes almost all the friction out of the steering system" as claimed in the review. Surely the reverse is true.

The seal on the underside must be normal to the bearing axis which imposes design constraints on the hull shape in this location, since it must be either flat or part of a sphere in order to rotate. The cassette also ties up valuable volume which may be needed for prop shaft, exhaust or access for plain maintenance and repair.

I like the removable rudder feature but is there any reason not to make the cassette non-rotating and removable and much narrower - just wide enough to accommodate the rudder, which can then rotate within the cassette on a regular stock and bearing? It would require a disconnectable link between it and the tiller but not a big deal.

With this approach, space is released in this important location, the cassette bottom can be faired into whatever shape best befits the hull for its purpose, the rudder axis is essentially independant of hull shape, the stock can be as heavy as you wish and the bearing is accessible for servicing with the boat in the water. Much the same concept as the keel in fact.

Please be gentle with your rebuttals as I am not a NA.

 

Fascinating idea. I have a 46' sailing cat that might benefit from this approach. The boat does not have keels so the rudders are the lowest appendage which makes me nervous in thin water and precludes beaching. Retractable, high aspect daggers in a cassette/drum would allow beaching as well as providing more clearance for the props. I would expect a performance improvment as well since the current rudders have an aspect ratio of 1:1.

Thanks to all for starting the wheels turning.

Mike

 

Ancient K,
There is no reason why your idea of a rudder in a cassette would not work. All it requires is working out the design and engineering for the various parts. It would be a more complicated system, of course, than a simple conventional rudder installation, maybe a little less complicated than a full drum. You are still left with a rudder stock that has to fit inside the rudder blade, and that necessarily entails designing for high stress.

Eric

 

Ancient Kayaker,the cassette rudder system you describe is very commonly used in New Zealand and Australia on cruising catamarans,quite often the cassette is located in an open backed trunk in the sugar scoop allowing it to be hinged in case the rudder were to strike a submerged object.The first cassette rudder i am aware of was used on the Davidson designed daggerboard IOR quarter tonner,FUN which did very well at the worlds in Corpus Christie probably 25 years ago.
There are several advantages of the drum system,one is that as the blade is complettly within the drum it can be operated partially retracted which would be usefull on boats which can retract their keels,such as catamarans.Another advantage would be a reduction in turbulance where the blade meets the hull.
On our boat we rarely ever lose rudder control even though at only a 10" chord but deep it has less area than the previous rudder.It should be noted that the old rudder was a transom hung retractable cassette rudder so had no balance.
Steve.

 

Interesting information.

I wasn't too enamoured of the drum scheme because of the size and potential for bearing friction, and frankly even the alternative cassette scheme I outlined seems complicated compared with a transom-hung pivoting rudder. However, the cassette and drum concepts put the rudder closer to the keel which should improve agility when tacking.

On another tack, I am looking at a scheme that would involve rotating the keel for changing tack, with a simple skeg instead of a rudder. Have you heard of this idea being tried?

 

We dont find the rudder drum intrusive at all,at least no more so than the space taken up by any conventional inboard rudder with the quadrant for wheel steering,cables etc,in our case it is 12" diameter and mounted just in from the transom,we use ertalite roller bearings top and bottom,the bottom bearings are water lubricated while the top ones are dry,they do make some noise but the whole thing rotates very freely under any loading,the only down side is the limitation of chord length dictated by the drum size and the need for flat hull sections,ours is not quite perfectly flat,but its insignificant over the distance it rotates.
On your rotating keel idea, it has been done but i think only a few degrees each side more to reduce leeway.The boat im thinking of was Wai Aniwa,a Dick Carter designed one tonner built in aluminum in New Zealand for Chris Bouzaid.I have no idea how succsesful it was as it was pretty soon locked in the middle.It may have violated the rule or it may not have worked as planned.I think this was around 1970.
Steve.

 

Come to think of it, large diameter bearings with hevy stresses and zero maintenance have been working for years in our washing machine! Thanks fot the info on rotating keels. I was thinking of about 30 to 45 deg either side, but it would not be practical in a large boat due to power requirements and is not useful in a boat with good upwind performance and agility. I am looking at it for my sailing kayak; slow and turns like a city bus.

 

A couple of questions. Since my cat has hydraulic steering, would I still need to angle the rudder blade back a bit for balance or would this not be an issue with hydraulics?

If I do need to angle the rudder blade, could I just angle the whole drum/rudder complex so that they are coaxial but set at a 6-7 degree angle in relation to the water surface, or does the rudder have to have a different axis than the drum?

Thanks,
Mike

 

that would work, but some advantages of the drum over this are:

With a drum,
1)the rudder can be immediately pulled up no matter what the rudder setting
2)the rudder can be still be operated no matter what the depth.
3)the rudder is structurally well supported throughout these changes

your simple lift:

the advantage of your proposed system is that the hull bottom shape could be virtually anything within reason and the rudder could still retract. Shape the bottom of the retracted rudder to match the hullshape. Maybe a good compromise would be a 2step or 2part horizontal split where the lower part is just large enough to some (but very diminished) effect, but is hidden behind some hull protection. Latch release/fix would be part of the retract/deploy procedure/mech.

.