Catamaran Evolution

JCD, my compliments on your progess, and o, its "fighter" i was reading "finger" stick steering
after that drum ruffle, common, let us know if its hydrolic or what? on generator? i like the idea tho.

without trying to generalise or classify rig types or even naming a wingmast (strong section and well possible to selfbuild) or wingsails
not mentioning other rig types, for a RC/CR cat a good choise could be a schrouded rotating mast with spreaders
and diamonds beeing stronger/faster than a freestanding witch again can be good for cruising.

 

Hello Rick,

Dammm...can't keep up with you. Okay, I looked at what you generated, but have more questions than answers. Straighten me out if something throws up a red flag. After some calculations...I'm convinced.

Outboard rudders are nice for maitenance and as you pointed out need to be bigger to get the same lift due to ventilation. Inboard rudders can be smaller due to plate effect, but are bad for maintenance. Either are easily enough protected by different methods. I intend to use the inboard rudders.

The optimized rudder below the waterline is .66 foot chord by 2 foot span. That puts 1.33 sq. ft. below the waterline. My first question is whether that is enough area below the waterline for steering in close quarters while at harbor at lower speeds. I suspect the section will be difficult to stall at top end for that chord and find it to be beefy at 1.6 inches.

My understanding differs by different opinions and I'll point them out.
Those figures gives an aspect ratio of: Span^2/Area = AR = 3. That is on the money and it is my understanding that the ideal aspect ratios would be between 2 and 3.5.

Rudder area is estimated by two methods, sail area and lateral area, 1% and 5% respectively...or therabouts. Sail area, which is the method I prefer to use, is approximately 500 sq. ft. for the Green Lantern so, there should be about 5 sq. ft. of rudder area. Divide that by 2 rudders and we get 2.5 sq. ft. per rudder. Considering the 1/3 you suggest, that pretty much nails it on the head at 1.66 sq. ft. which more than likely means that the optimized hull is without a doubt proven accurate.

Everything looks superb, but another concern for me is that the optimized rudder puts only .6 feet of span into clear water with my current draft of 1.4 feet. That translates into .4 sq. ft. of area in clear water.

Visualizing it, I would tend to think that is very low as they will lift at higher speeds but will probably have very little "required" drag at lower speeds probably making them ineffective for slower speed steering. Yes, required drag...unbelievable but true. I am not in any way contradicting the computer model and chances are that it is correct and I am wrong, I just need clarity on what the results mean... in layman terms if possible, and what your thoughts are on such small area in clear water.

Also, in my mind, area below the waterline is the same whether vertically or horizontally and only restricted by perpendicular force deflecting the rudder so if a choice was given, I would prefer the deeper rudder.

There is another method I remember reading which takes draft and LWL into account but I remember little about it and results just didn't appear adequate for multihulls, so I discounted it right away.

That is my thinking. I am definitely looking forward to having you further explain the results of the optimized rudder. Hopefully, I can suggest a span, a chord and AR that produces my thinking on the required area and you can optimize it to compare with the current rudder. In fact, if you don't mind, try to optimize a rudder that puts 1.1 feet of span into clear water and let me know how it compares. That should translate into 2.5 foot span, 1.04 ft chord, 2.6 sq. ft. area and aspect ratio of 2.34.

In the meantime, I will try to look at these attachments closer and try to understand the results.

Thanks

J

 

Hello Masalai...

Thanks. Use whatever idea you need and if I can help with your design let me know.

WTF? Come to think about it, I haven't seen your thread or design. Where is it? I would like to take a looksie and see if I can find anything I can use in there! Point me to it.

Yeah right...yours is probably twice as good and I'll get a kick in the head when I go look at it.

J

 

Look in my gallery, in the comments on the video is the discussion thread site. A search using "masalai's model movies" should get both if I have spelled right?

 

Hello Yipster...

Thanks. I worked hard on her this time around.
It's Fighter Stick Steering. For the mechanicals, I'm thinking the simplest is going to be steering cables...as I mentioned in the thread. The fighter stick handle will actually be a fighter stick like a fighter plane cougar, but instead of using toggles for weapons etc, I might design the wiring to the stick so that I can toggle between different commands to different components. Those might include:

Navigation lighting, motoring lights, deck lighting, spreader lighting, am/fm radio preset stations, vhf radio, volume, gas gages, navigation gages, radar, gps, motor ignitions, motor gages, bilge pumps, raising or lowering the engines, raising or lowering the daggerboards, etc... and maybe I'll throw in a screen on the dash and actually use the Flight Stick to fly a combat game while underway or docked...well, maybe the grandkids might enjoy that. The cougar has something like over 100 programmable options so it will all be there, readily available. Yeah...these kinds of ideas just keep coming into my head, so it may not be viewed as conventional ideas, but I air them anyway.

I think I was able to decipher your code...without you generalizing or classifying or naming any wingmasts or wingsails or not mentioning any rig types whatsoever, I have concluded that you meant to say, that the current rig I have in mind, a shrouded mast with one set of diamond spreaders is best suited to the design. Don't worry, I won't give away the key to the code.

I'm attaching the mast and boom sections called out for your perusal and comment.

Thanks
J

 

JCD
You should open the JavaFoil applet and have a play. It is very easy to use. I can help you if you want. Understanding foils is a valuable tool in anything to do with boating - particularly sail boats. The numbers from JavaFoil correlate well to foil test data and the software is much easier to use than Xfoil.

Area is not the sole determinant of foil power. The other important factors are thickness and aspect ratio. With AR, the higher the more efficient. With thickness, somewhere around 20% is most efficient. I have not done a full analysis on the exact thickness for optimum but I read somewhere it is 20 to 25% and I have just stuck with 20%. I have always found it to be better than thinner sections in the cases I have considered. If you spent a couple of hours with JavaFoil you could get the optimum for your particular conditions.

You really only have to contend with boudary layer. The rudder is not shielded by the hull as there will be streamline flow over most of it. You can determine the thickness of the boundary layer at the stern but I doubt it would be more than 100mm and the actual severly disturd area will be much less. (Have a look at:
http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/22903
see the rudders and look at the model video for turning - in your terms these rudders are fully shielded, much worse than what you have - props would also be ineffective if that was the case). So with a 2ft deep rudder you will have at least 20" that is fully effective.

I would not like to be sailing a large cat in a tight harbour. I would motor. With two motors you can turn in own length if needed. Cats do not have the "pinching" ability of a heavy displacement monohull. Once you get two tonne of nicely shaped lead moving it tends to want to keep going. I have coasted around many buoys in a moderate displacement yacht and usually sailed on to a swinging mooring.

The reynolds number for the 8" rudder will be well over 100,000 even at slow speed so it will perform well - it will be within the range of curves in the screen dumps provided earlier.

From the foil numbers provided earlier I determined that the 8" rudder could produce 1350N. In Imperial units 300lbf (PER RUDDER). So you have a steering force of 600lbf possible at 10kts. Think about it - this force could lift four men. If that cannot keep you on track then you have a horrible sail set up.

Rick W.

 

Hello Rick,

I'm not an aerodynamicist...but I have a good understanding of foils...camber, lift, drag, sections, vortex, blah, blah. I'm convinced.

I looked at the Applet thingie and I will have to spend a significant amount of time just to get past the manual. You give me too much credit brother.

You're right about motoring in close quarters and being able to spin on your own axis, but all the maneuvers you can execute with twin screws are enhanced if you use your rudders whether in forward or reverse and i just wanted to belay my fear that the rudders may be small in area. But you mentioned that at least 83% of the rudder will be effective and, the video pretty much put the nail in the casket. I'm sold.

I am making a change to the design specifics and will be incorporating the optimized 20 section into the design. Was the program able to solve for structural requirements such as torque or balance depending on where the post is placed? I can use an area of approximately 15% of total area forward of the rudder post axis, to balance the rudders and reduce the loads, but believe a reduced skeg to provide some protection with an unbalanced rudder might promote better flow. Any thoughts would be appreciated since the skeg will require further calculations and adds wetted area.

Thanks
J

 

Hello Masalai,

I came, I saw and I was impressed. Very nice work.

When it is all said and done...would you consider a request to have you build my design to a 1/12 scale? Let me know.

Thanks
J

 

I didn't build it. Look at the models also in my gallery! These club members love modelling. Mine was A$1000 with r/c but no charger.

If you are serious & want to risk freight damage and freight costs send a PM and attachments I can print on A4 paper to see if anyone is interested.

No promises but I will pass on the info you send? They would need plans, weights of the model (mine is 10kg and I hoped for 6.5 to 7kg! - that does not stress me at this stage of development.) It seems preference is for plans at model size - 1:1 so take-offs are easy for them. Very detailed may be more as it is terribly fiddly.

 

hmmmm

 

You should be able to get enough strength without the skeg but do the calculation. If you design for top speed say 20kts then the force on each rudder would be around 4 X 300 = 1200lbf. Not something to toy with. It is a possible speed down the face of a wave when you really need to have control. I have done 18kts in a 24ft trailer sailor - kit up in 35kts.

You could maybe fit a 1.5" SS shaft in an 8" chord 20% foil. You need to do the sums to see that it will handle it. So the rudder will be strong and certainly able to take a bump. Support would need to be solid and enough strength to allow the boat to sit on the rudders in any conditions you were happy to have the boat beach - all but solid rock or coral.

JavaFoil will also give moments. If you have a basic understanding of foils it takes about 5 minutes to get meaningful results. You need to set the Options page for water 1025kg/Cu.m, Aspect Ratio to suit the rudder and viscosity 0.0000013. That is the hardest part. You may be able to come up with a foil better than the NACA0020. It also gives section offsets and section drawing. Placing the shaft at 15% will limit the shaft diameter but also give good feel. Another reason to modify the profile.

Rick W.

 

The prime purpose of a rudder and dagger board is to create steering moment. Most rudders sit in the water all the time so offer drag even when not needed for steering. Centre-board sailors will be familiar with lifting these bits to lower drag.

From my experience a ventilating rudder needs to be at least twice, probably 3X, the area of non-ventilating rudder to achieve the same maximum steering force.

I have attached a screen dump of the pressure vectors for the rudder proposed for JCD at AoA of 20 degrees. You will see the Cp gets down to -4, equating to -50kPa at the nominated operating conditions. This is 5m of water head suction.

So if he had a surface piercing rudder 1m deep it would have a suction slot over the entire leading edge to channel air down the low pressure face. Air has negligible density compared with water so one whole face is hardly contributing to water "circulation" to generate steering force.

On the high pressure side of the rudder you see that the maximum Cp is only 1 so most of the force comes from negative pressure. If you take away the ability to produce a force on the suction side then you more than halve the performance. Without doing the whole pressure integration over the surface I work on a loss of 3X based on my experience with ventilating props and rudders. It could be worse.

So with surface piercing rudders you are dragging around much more rudder area then you would need if the prop did not ventilate. You can still have a stern hung rudder but you need to devise a cavitation plate that channels water, rather than air, into any potential suction slot.

Rick W.

 

Rick, you gave me 2 very comprehensive answers, in both threads, and both have different and very helpful info. You did that in less time than it took me to rewrite the question in my thread.....you're making me feel like a *!!# RETARD. lol

 

When you have a 120W engine you get pretty good at exploring things that contribute to drag.

Have a look at the rudder on the attached. I went to a lot of effort to prevent it from ventillating. As long as you can keep the top of blade below the surface and cut off air supply down the shaft then it will not ventilate.

Rick W.

 

Ahhh...didn't think of that. I'll take my own chances, but thanks.

J