Revised Rudder Design: Dufour24

The rudder on my Dufour24 is a mid 1970's transom-hung design. The rudder pressure can be quite heavy at times and tracking requires constant attention. Sails are older and suffer a loss of shape. While most issues could be remedied simply with new sails no funds are available. Others report even with new sails, this helm requires constant effort/attention.

I would like to build a new rudder, perhaps 15% balanced fore/85%aft the rudder stock.

Existing rudder:
93cm long(in water)x 32cm width
Sweep-back angle of 7.5deg.

I notice many other 7meter boat have a blade of 40cm and all our straight with no angle.

 

Is this boat getting some bottom work done?

 

This is the first haul-out after 8 years on a multi-layer epoxy barrier coat. Just a bit of swimming under and scrubbing each year. The hull is being sanded to give tooth for several fresh layers of copper/epoxy coating. The hull has held up well considering two gales where blown off mooring. The water is blown away for several kilometers in bay areas, so the boat is on the side in the mud. But this one re-floats itself nicely. Dutch winds and very unstable floating bottoms make mooring very interesting. In both cases other boats upwind lost anker and sweep/dragged me along into shore. It part of the experience when you seek to hang-out in wildlife areas instead of going into harbor each night.

 

if your boat is used in any class racing you should check if changing the rudder will still allow it to compete in local club racing.

Changing the rudder sounds like a good idea. note that it is the hinge line relative to the submerged area of the rudder that determines the tiller forces, not whether area is vertical or swept back. So you can keep the same approx rudder size and shape, but change the way it mounts so the hinge line is swept back or forward, to change tiller forces.

 

Thanks, I'll loft the hinge line, this begins to focus the design. I have No competition or speed concerns, simply holding to a course with wind gusting 5-7Bft is the priority. In Dutch waters, the frequency of waves over shallow ground can throw around a 1300kilo/3000lb boat (Bal./Disp.: 40%). We sail March-November so light air is not an issue.

I would build the new rudder on a 35-40mm shaft w/polymer sleeve bearings. I can compensate for any transom angle in the mounting blocks.

1)My questions are the size of the blade,
2)how much to extend forward (of the existing profile) to balance the tiller forces. To assist in holding course.
3) what is the shape of the blade, is this a example to consider?

 

That looks like a good shape. An elliptical leading edge and some balance. You really wont know until you sail it, but if you added spacers and hung your rudder with less rake (maybe cut the rake in half) it might make an improvement.

 

15% balance will make a big improvement, using the same area, just sliding it forward a bit to accommodate this balance. Make the balance uniform from top to bottom and the leading edge could be more erect.

 

Thank you for the ideas. In the first photo, I have placed a white rod to indicate the hinge point axis. Is the suggestion that I place 15% forward of this axis line, a rectangle section about 93 x 6 cm? Then take that same amount off the trailing edge of the rudder blade.

What is the effect if I alter the aspect ratio of the blade? It is now about 3. Could I add a section 93 x 12 and lower the aspect ratio towards 2, provided the additional rectangle section is fore the axis line?

 

Rudder balance may also be due to the rig being a fraction too far aft. You mention Force 5 to 7 Bft so pretty windy. As your sails are old, the leech stiffness will be gone but in fact new sails may make the helm heavier!. Usually the chord in the jib/genoa moves aft too with age and stretch, which does not help speed or best balance in heavy air.

May well be worth trying putting the mast base forward very slightly, but keep the same rake. There is some data regarding leading edge rudder angles over 10 deg being slower and heavier. Personally I'd probably settle for a leading edge in line with the pintles/transom rake. That is well under 10 deg and will move the C of E of it forward. It does not look like it lacks area from the pictures, but you will know if it does, if it is problem to steer in say a Force 4 at moderate heel.

Most 24' ers I've sailed with transom rudders have been big dinghy type and have steered very easily. Depowering with fractional rigs and backstay has usually helped to keep a sweet balance. I have had experience of a number of charter boats around this size never having been set up right - concerning rig balance and this has led to poor helm response. Both ways too, lee and weather. Best (fine) tuned to a known boat of same design and good performance.

 

My suggestion is to try the simplest thing first. I can see the pintle angle from that rod in your photo. If you remove the gudgeons from the transom you can engineer and install backing blocks that reduce the aft rake of the leading edge of the rudder. The overall original shape of the rudder looks high aspect. It's not bleeding edge modern technology, but it looks pretty good.

Anyone with a kick up style rudder can tell you that changing the rake makes a bid difference with neutralizing the helm.

In addition, take this opportunity to fine tune the rudder ( also disassemble and inspect.) The leading edge must need some repair and the whole thing could probably use some careful filling and fairing (with a template.)

After that, it might be logical to refocus attention onto the rig.

 

There's a lot of things I'd do, before cutting up the rudder. The sails are likely causing most of your difficulties, coupled with rig trim, I'd bet all of the issues you're experiencing will go away with new sails. The first thing I'd do is check the mast rake against the sail plan. It probably needs to rake forward a bit. Next, there's no compensating for stretched out sails, so have them recut, borrow some from someone, buy good used or new ones, as there's just no way around this issue.

If you really want to make a rudder, first consider there's ways to improve what you have slightly, with the addition of a fence or two. This will keep the top of the blade working over a longer range, where is now, it's just bleeding off pressure and sucking down air. An end plate at the tip will do similar and effectively make the blade "think" it's aspect is longer. The same is true with "standing 'er up" a bit.

If this isn't enough, close up the gap between the transom and the blade, by recessing the pintles into the leading edge. Of course, you'll need to insure the blade can still swing through a full range of motion.

Lastly, you can simply add a piece to the leading edge of the rudder and try it. Yeah, the shape will be all wrong, but in modest wind strengths, not really a big deal and it'll tell you if the amount of balance added is going to do much for you.

In reality, it'll ease helm pressure, but if rudder deflection is still high, no gain will be noticed, in fact a very slight lose, from the extra area on the blade. As the sails bag out, you have to put in more helm to compensate for the camber and CE moving to places it really shouldn't be. This creates a larger "couple" over the boat's "centers" and you have to crank down on the helm.

In fact, when folks complain of a heavy helm, aside from the usual questions about sail condition, mast rake, etc., the first thing I want to know is, what is the angle of incidence of the rudder, relative to the boat's centerline, when sailing close hauled in 10 knots of wind. Your boat should be 3 - 4 degrees and if you're cranking in 5 or more, it's not a rudder problem and making changes to the rudder isn't going to help much, except make you think you've done something.

 

A higher aspect ratio rudder, while theoretically more efficient (less drag for the amount of lateral force or lift generated), it also means it will be easier to stall. in rough conditions over controlling the tiller can result in a stalled rudder, and loss of control, increase in drag. I do not think I would go that way unless you are willing to do that to get slightly less drag.

It is fairly common on recreational sail boats to intentionally use low aspect ratio rudders to make them more forgiving for boat owners.

However, I think that often rudders are too small, so if you are going to make a new one, making it larger, and using a foil profile, will make it more responsive, and require less tiller input to control the boat.

As far as balance goes, consider that if you use a NACA 00XX foil (say a 0010), it will have its dynamic center at about 25 percent of the average cord. So if you put the hinge line at 25 percent it will be "neutral", meaning it can swing around with little tiller effort, even under high lateral load. So the 15 percent of average cord will give you reduced tiller forces, without the losing tiller feel (that can lead to unexpected tiller commands when not paying attention). Putting it at ten percent would give you higher tiller forces, and perhaps better feel if that is what you want.

Thought extra effort to make, putting a plate at the water surface (perpendicular to the main rudder surface) will also make it more effective, as PAR points out. It will also make it more difficult to store since it will not lay flat. I do not think I would put an end plate on it, not only does it make it act like a higher aspect ratio rudder, it will be more prone to damage and picking up weeds and debris.

I like the idea of temporarily tacking on extra area to go test it out, that way you can get a feel for it before you make a whole new one. Using foam carved to shape, and held on with duck tape should work long enough to see how it works.