Rudder design/rebuild question

Guys

Wanted to get your wisdom on a project I'm considering.
I have a relatively older design cat with the hull blanketing the rudder.
Was looking to rebuild the rudders for a few reasons, reduce weight, rudder seems to be taking water, change the design for the steering system.
I'm not asking about the feasibility of this project just about the turning aspect:
She turns like a truck with trailer, which can get fun in a tight marina with the wind blowing, want to improve this

What rudder design is the best for the current configuration? they are currently just flat planks but are right behind the hull, would a foil make an improvement considering the location? What design foil? would changes to the shape and size help.... or is there not much that can be done given their location?
See pictures for reference.

Appreciate your responses!
Happy sailing.
Ryan

 

and here's a pic that shows how the hull is wider than the rudder at the attachment

 

Can we say that Google is the best invention after wheel? It has allowed me to identify this cat as Heavenly Twins 27, please correct me if that's wrong.

There is a better picture of the stern here: http://images.boats.com/photos/ovation/7893_102_ex1.jpg and it indeed shows that the rudder is shaded by the skeg and by the stern post, which is probably part of the problem. The other part of the problem is that you have no propellers in front of rudders, the role of which is generally fundamental in creating the fast flow of water over the rudder blade at low speeds. A good water flow is necessary for the creation of hydrodynamic forces.
Skeg rudders have some good aspects, like structural strength, good course-keeping capabilities in a seaway and are difficult to make them stall. On the bad side, the steering speed they can give is limited and they are not good performers at low speed maneuvering, and I understand that it is the problem you have encountered too.

Changing the blade profile won't result in a spectacular improvement, imo, but some improvement can be expected anyways. What is needed is:
1) a nice rounding of the edges of the stern post
2) creation of a slot between the stern post and the rudder blade
3) a rounding of the rudder leading edge.
If done properly, it will reduce the turbulence behind the stern post and will smooth the flow over the rudder blade. As the end result you can expect an increase of the side force due to the slot effect (think of airplane slot flaps) and a decrease of the drag due to a reduction of skeg-induced turbulence.

I am thinking about something like this: View attachment Rudder.pdf

Cheers

 

daiquiri, great reply thanks much. That was exactly the type of info I was looking for.
Yes it is a HT27! you got that right.
I'll look into what this would mean in terms of work.
Do you have any idea what the ratio of the gap should be? I'll post a sketch of my design soon.
And do you think that making the rudder foil shaped would help?

 

Hard to suggest the size of the gap at this point, with no dimensional data of your rudder and of the stern post width. Can you make a couple of measurements and put them here?
Regarding the further streamlining of the rudder, I don't think it will enhance the performance so much that it is worth the effort, since the wide stern post commands here, imo. But if you like woodworking, you can also narrow the trailing edge. Perhaps it won't improve things too much, but it surely will not make them any worse either...
Cheers

 

Quite a burdensome hull form on those hulls, basically two Catalina 24's with a huge a bridge deck, between them.

The plan form of the skeg and to a lesser degree the rudders could stand some improvement, though those hulls will still be what they are.



I'd lose most of the skegs, straightening up the leading edge and possibly shortening it's depth a bit. This would allow a small bit of balance below the skeg, on a reshaped blade, which would ease helm effort a tad.

At the speeds this puppy sails, foil sections wouldn't offer much, but would offer more then a plate, particularly down low in the balanced area. For ease of building, I'd use a "slab sided" foil section, rather than a NACA 00 series (typical). End plates would be an option to fool the rudder into thinking it's higher aspect too. If these sectional shapes are incorporated in concert with the skeg, you'll do well.

I do think things can be improved, but the results will necessitate some experimentation and building willingness. If all of the ideas/modifications are employed, no doubt, you'll see some improvement, but as to how much, well this is where the guess work will begin.

This boat didn't have the best reputation in it's early forms and the molds received several model run changes to address issues, particularly in the keel stubs and nacelle. She's heavy, especially by multi hull standards, carrying a 250 D/L, her SA/D shows good light air potential, but the hulls hold her back to less then an optimum displacement speed. My reports show 7 -8 MPH on a reach, which is pretty sad for a cat with this much area. Essentially a fat cruiser, with lots of room, so sluggish performance should be expected.

Which year (what model) is this one?

 

Actually if you can reduce the gap size to zero (or near zero), with a gap seal it will improve effectiveness. but that means you should also reshape the skag ahead of the rudder to form more of a foil shape that includes the rudder. the gap acts like a "leak" allowing the flow to bled through from the high pressure side to the low pressure side, reducing its effectiveness (aircraft rudders, ailerons, and other control surfaces also improve effectiveness with gap seals0.

for max effectiveness you can completely cut off the skag and make a much larger rudder, foil shaped in profile. You can increase the effectiveness even more if you move the rudder forward under the aft hull so the bottom of the hull acts like an end plate (a spade type rudder with as small a gap at practical between the bottom of the hull and the top edge of the rudder). This means a major redesign of the hull structure to take a strong rudder shaft though the hull (not sure I would do that personally, unless you had to rebuild the aft part of the hull to repair other rot or damage anyway). And of course the hazard on a cruising boat without a skag is it makes the rudder at risk of damage, or catching weeds or other flotsam.

The simplest thing to do is as Daiquiri suggests, stay with the transom mounted rudder, but use a NACA 00XX series foil shapes, also make it larger in area. I would not use any of the more modern laminar foil sections for this application, you will not be able to maintain laminar flow that far back on the hull and it will perform badly. The old NACA 00XX foils are excellent for this application. I would not go more than about 12 percent thick, but that is more a question of strength of the new rudder material, make it as thick as necessary, but no thicker.

BTW, all catamarans tend to be difficult to turn, the long hulls want to drag badly as it sweeps through the water (a round hull, like a floating pot or basket, would turn the easiest of course). Those long keels do not help either, going to a dagger board high aspect ratio keel might help, and improve performance too (but of course you loose the simplicity and ability to beach the boat with the hull off the ground). So it is just the nature of cats to be sluggish in tight truns. But larger more effective rudders would be a really big help, getting them out in "clear", not turbulent, flow will also help by removing the skags.

You will reduce the drag quite a bit as well, if you allow the trailing edge of the rudder to come to a point. or at least to make the trailing edge "flat" at the back of the rudder as small as possible (1/8" or 1/4" max), but keep those trailing edges sharp corners so you get clean flow separation off the back of the foil.

Also, keep in mind that if the rudder turns more than about 15 to 20 degrees to the flow, you will stall the rudder, and it becomes ineffective and creates a lot of drag, so a careful hand on the tiller will also make steering more effective as well. a good foil cross section should also help reduce the tendancy to stall the rudder (sparated flow and far less effective rudder).

good luck.

 

Gents,
some good advice there, but let's try to keep things as simple as possible while still delivering a result.
Every big (or too detailed) modification means money spilling out of Ryan's pocket, so in my opinion we should try to create as little cash-leakage as possible by giving simple yet effective proposals.
Cheers

 

The rudder/skeg gap is best handled by placing the shaft into the skeg's trailing edge and a complimentary cove to receive the rudder blade. This effectively leaves no gap and clean up the flow nicely too.

The skeg and blade should have a contiguous foil shape, with this arrangement, though I still think at 6 knots, this is of dubious value on a relatively low aspect rudder. In fact, some argument could be made for leaving the trailing edge of the blade quite blunt, which would create a significant set off eddies and this would effectively make the rudder "feel" bigger to the flow. Yeah there's some drag with this approach, but it's simple and the boat is doing less then 1.35 S/L anyway.

 

I suggest doing one step at a time. If it was my boat, thise are the incremental modification steps I would perform, with performance tests between each one of them:
1) Round the edges of the stern post and the leading edge of the rudder, with the biggest radius possible. If anything, it will give less drag than the current configuration.
2) Get rid of the skeg.
3) Play with various amounts of rudder gap, starting from zero gap.

As a last (or first?) resource, and a pretty unconventional one, you can think of deploying a bow-mounted daggerboard for low-speed maneuvering, in order to move the steering pivot more forward.

Cheers

 

All of the proffered mods will help rudder effectiveness to a small degree but, on this particular boat, I doubt you will see the difference. Looking at the amount of immersed lateral plane and how it is distributed ,as well as the high displacement, it will take considerable steering moment to turn this boat easily and that does not appear to be available. Most suggested mods mainly reduce drag. The total drag is now so high that any improvement in rudder drag will not improve things enough to be worth it.

Low speed maneuvering of cats is always a problem and with this boat, only more so. Is a bow thruster out of the question?

I recently helped a friend rebuild the rudders on a 36' heavy cruising cat with a central IO pod. These rudders are fairly deep, have good foil shape and are relatively free of forward interference. The boat turns and steers pretty well but it does not have many of the problems you face.

Good luck with it.

 

Tom,

I disagree, improving the drag on the rudder has double benefit. right now with so much turbulance and poor flow around the rudders means they are less affective. the more attached flow on the rudder surface the more effective it is. Just guessing, right now perhaps 2/3erds of the rudder is not creating any turning moment because of the poor flow around it and over its surface, everything you can do to improve that will improve rudder authority. the other is if both rudders are create a lot of drag, it makes it more difficult to turn (imagine having a drog off each hull, sluggish performance and each one will prevent it from turning). make it cleaner and it will turn easier too.

The foil shape is not just for drag reduction, but to allow the fluid to follow the surface better, stay attached, and deflect the flow better. The rudder, no matter how large, operating in a large separation bubble, will not be effective because it can not deflect the flow to create the lateral force to turn. cleaning up the flow around the rudder will make it turn better. the more flow that stays attached to the rudder surface, the more effective the rudder will be.

so cleaning it up may not improve overall speeds much, but should considerably help with rudder control.

 

Petros,

No doubt the rudder can be improved. The real question is, will any detected improvement be worth the time and cost of the changes? Considering the overall shape and specs of the hull, I doubt that. Making the rudder deeper would be the easiest and most effective change but would make it vulnerable to grounding damage. Anything else still leaves the improved rudder in the wake of hull turbulence. Obtaining laminar flow in this wake seems to be a questionable. Not the first person I have disagreed with on the forum and all this advice, including mine, is free and only worth what someone can make out of it, which may be, not much

With all their credited brilliance, the ancient Chinese never developed a laminar foil rudder behind their big junks and relied on a big honking barn door to get turning force in the wake of all that hull.

 

To Ryan and you other guys:
I have just found this picture of that cat: http://www.sailblogs.com/member/distantdrummer/images/p1010442_scale.jpg which has made me re-consider my previous suggestions. The rudder stock is not at all that thick as the previous pics and descriptions had suggested, so the turbulence and rudder shading is a minor problem here. The boat is sluggish essentially because it is a relatively heavy cruising catamaran with deep hulls, long keels and a shallow rudder.

Tom, I think that you have given a good hint on what could be tried as an improvement of the existing rudder. I am thinking of 2 possible solutions. Will make a quick drawing and will post it here in a couple of hours.

 

Tom,

you made a classic error I suspect, you have confused my statement about "attached flow" over the surface of the rudder with laminar flow vs. turbulent flow. It is a beginner error. Turbulent boundary layers can be attached and detached, (it has nothing to do with it being laminar or not). That really crappy rudder design results in large amounts of detached flow making the rudder ineffective.

There is NO WAY there will be laminar flow that far back on that hull, but by creating a better shape just ahead of the rudder, it can, and should be, still an 'attached' turbulent boundary layer. A turbulent boundary layer that is attached to the surface will actually have good rudder authority, in fact better than laminar flow over the rudder surface, even if it could be achieved.

You WILL improve rudder control by doing the following, there is no debate about it:

1. Make the rudder larger. either longer chord, and/or taller/deeper.

2. Improve the flow upstream of the rudder by either reshaping or removing the useless skeg.

3. Improve flow over the rudder by improving its shape to a foil cross section to keep the flow attached.

the more fluid you can curve (or accelerate) around the rudder, the more effective it will be, this is acheived with better flow, larger surface, better shaped surface.

I know your advise is well meaning, but I think you misunderstood what I was describing, and not contributing anything helpful. it is not an issue of laminar or turbulent flow, it will be turbulent. It is a matter of keeping the flow attached over the surface of the rudder, make it bigger and improve the flow field a head of the rudder better.