S-38 Schnellboot by ir3 - Italeri - PLASTIC - RADIO

[ir3]

After posting a question on removing large amounts of plastic from parts of the model I was encouraged to start a build thread. This will be an RC Conversion inspired by ojays on RC Universe. One of the main reasons for building this model is to implement the Lurssen effect as was done on ojays thread. I am not so sure the effect scales down to a model of this size but implementing will be an interesting experience. The Lurssen effect is to adjust the angle of the outer rudders/deflectors to reduce the effects of the stern wave at speed. By changing the angle to proper value the stern wave produces minimum drag. The angle is optimized for a particular hull.

 

The hull halves were fastened together and installation of running gear was started. The running gear was purchased on eBay as a package including the rudders, prop shafts, stuffing boxes, and props. It also included the center strut and outboard struts but for some reason they did not fit this hull so were excluded. It will use 380 motors and electronics from Action. The pictures show the startup. There is a bit of filling and cleanup still to be performed.

 

Since I am waiting for some items from overseas I decided to scatter the work using what I have available at the moment. PE will be Eduard 53212 and Eduard 53214. The kit comes with some PE and I will be using pieces from each. I dread using PE as it is nothing but a struggle for me but I decided to take my time and be patient. We shall see what results follow.

 

Until next time,

 

IR3

[ir3]

The couplings have arrived. The motor bulkhead was cut from 1/8" styrene. To get the shape I used a piece of soft brass strip and formed it to the inside of the hull where the bulkhead will go. Then transferring to the styrene and cutting and drilling. Temporary installation with the bulkhead slightly tilted.  The shafts do turn smoothly. A slight misalignment in the starboard prop shaft. Very difficult hollowing out the stuffing box covers and getting perfect alignment. The slight difference should not be a problem. Now for a permanent installation.

 

Until next time,

 

IR3

 

 

[yvesvidal]

Wow...that stuff is going to take off like a rocket.....!

 

Yves

[ir3]

Thanks for the likes.

 

I believe it will be quite fast. It will be interesting if the Lurssen effect actually works at this scale.

[lmagna]

Hi ir

 

Great work so far. 

 

If I may I might inject a suggestion at this point.

 

It looks like you are installing fairly powerful motors for his boat as would be expected. Your motors are well supported so long as your bulkhead is firmly mounted to the hull. But just aft of the bulkhead you have nothing but the universals and a relatively long section of open shaft until you reach the stuffing boxes. You may want to build another at least partial bulkhead with bushings or bearings just aft of the universals to stop any vibration of the exposed shafts at high RPMs.

 

Just a suggestion, it may be possible that everything is heavy enough, aligned and balanced enough so that this may not be a problem even without the extra support.   

 

I believe it will be quite fast. It will be interesting if the Lurssen effect actually works at this scale.

I think you will find that it does work. In fact it may work at much lower speeds than it did on the real boats. You have to remember that the one thing that has not been scaled down is the medium you are running in. Water is non compressible and your model is lighter and far more powerful than the real boats and will lift much more easily than the real boats ever could. Years ago I had much the same issue with the Lindburg PT 109. Right out of the box she wanted to ride nose high and tail down with a bow wave that flared out on either side by six inches or more! I was able to cure this by adding two strakes made from evergreen "L" strut on the bottom of the hull parallel to the keel starting from the transom. I did not make them very big but the effect was to bring the stern up and the nose down and reduce the bow wave considerably. The speed also increased noticeably. So if little strips of Evergreen plastic were able to do that then your outer rudders should also not have any issues of obtaining the same ends.

 

Edited February 16, 2019 by lmagna

[ir3]

Hi Lou,

 

Thanks for the heads up on possible vibrations at high speed. I will be installing the electronics in the next few days and will test the props at speed to see if vibrations occur. Putting in bearings sounds like a good idea anyway. I just need to find a way to that it doesn't block the ability to get at the screws that hold the motors on the bulkhead in case of failure.

 

Until next time,

 

IR3

[lmagna]

Adding bushings just behind the universals is pretty easy. All you need to do is make another bulkhead just like the one you already made only for the new location. It does not have to be all that tall either as you will not be adding motors or anything else to it. Then it is just a matter of cutting two slots that allow it to sit under and around the two shafts just behind the universals with space all around. Cut two pieces of brass tubing just like you used for the stuffing tubes that went through the hull and slide them over the shafts and reconnect everything.  The slots in the bulkhead should not touch the tubes at any point and they should be free to slide up and down the exposed shafts and through the bulkhead. when all is in place and nothing is touching or binding use silicone glue, (The toothpaste like stuff) and putty up the area between the tubes and the new bulkhead. When the glue fully cures in about 24 hours you will have a friction/vibration free support for the shafts.

 

As a buy-the-way, Silicone glue is a fantastic RC ship product. I used to use it for everything! For example, motor mounts. Make a glob of glue where you want the motors to sit and press them into place and wait for it to cure. The same method can be used to mount servos and almost anything of that sort. After curing the only way to get them out again will be to cut them out! You can also make watertight seals with it by laying a thin bead around the lip under a hatch and spreading food wrap over the opening and pressing the hatch into place. When it cures just lift the hatch, remove the wrap and put the hatch back into place and secure it. Totally waterproof for years to come. If it is a covering like over the rudder that will probably never need to be removed unless something breaks and needs repair, you just leave the wrap off and glue it into place. If you need to open it at some point, with some care and a sharp knife it can be opened. On a model like you are building you could even secure the entire deck this way allowing you to remove it later if needed. It will NEVER come off on it's own and will never leak at the joints.

[ir3]

Thanks Lou for the suggestion. Never passing up a good idea, the pieces for the shaft support behind the U-Joints is fabricated and test fit. The shafts turn smoothly so the next step is to tack it all in and finalize the installation. Getting the center prop shaft in position is a problem since it can not be installed from the stern like the outer props. The center motor needs to be removed and I need to make sure I can do just that when the boat is completed.

 

In order to install the RC Gear and make it removable, I need to get the main deck fitted and make sure that it can be removed for making repairs. This will probably keep me busy for a day or two.

 

Until next time,

 

IR3

[ir3]

Started on the RC components. The first part is the rudder/deflector mechanism. This is not my idea but just adapted from another build. It is very clever and above my creative abilities at this time.

 

There are two servos, the large one that controls the central rudder and the smaller servo which controls the outer rudders/deflectors. The small servo is on what I will call a sled. With rudder amidships the linkage from the small serve to the output is adjusted to keep the rudders parallel. There is a link between the large servo and the sled. When the main rudder servo turns to deflect the main rudder the sled is push/pulled to keep the outer rudders in alignment with the main rudder. The small servo remains centered thus allowing the rudders to track. At speed, the smaller servo is turned to move the outer rudders away from the center line thus adding the Lurssen effect. This is manual but I suppose with a computer radio this servo could be controlled by the throttle?????.

 

The two pictures show the sled in the fully extended and fully retracted positions.

 

Next, this servo tray needs to be installed in the hull.

 

Until next time,

 

IR3

[ir3]

I now have the basic setup for the rudder controls. There are a few minor problems to work out but the principle is there. Unfortunately one of the deck mount hull spacers had to be cut away to make room for the pushrods but this should not be a problem. The original designer had the same problem. The only item I would like to find a solution for is the main rudder push rod and the sled push/pull rod attachment to the servo arm. Notice how they are spaced in order to fit on the servo arm. If anyone knows of a way that I can install them the same distance from the center of the servo please let me know. The way the setup is, the outer rudders/deflectors will not track the main rudder exactly. Perhaps it is not a problem???

 

Until next time,

 

IR3

[ir3]

The mounts for the servo tray are tacked in will be finished by next update. The clearance between the main rudder pushrod and the small servo is a bit tight. A washer or two under the main rudder tiller arm should take care of that. Tomorrow the servo testers will be put to use to fine tune the linkages and get the rudders aligned.

 

Until next time,

 

IR3

[mtaylor]

You are a bit tight in there for space, aren't you?  I've not fiddled with RC but have a question or two. What I think I'm seeing is that the two outer rudders will move in opposite directions when the servo moves where as the servo on the left will move the "sled" for the Lurssen effect.  Or doesn't the servo (it is a servo, right) move the outer rudders during a turn?  Fascinating work and reasoning to get the proper effects when in the water.   

[ir3]

nThanks for the question, hope this helps. My total knowledge about this effect is well below the marine engineers level of expertise but the concept makes a lot of sense.

 

The larger servo, the one on the left moves the center rudder and pulls/pushes the sled simultaneously so that all 3 rudders track together. This is the normal, no effect configuration. Note that all the tiller arms are pointing in the same direction so the sled just acts as an extension of the larger servo arm. The smaller servo does not turn (until the Lurssen effect is desired) thus the outer rudder arms pull/push as if they are being controlled by the larger serve. So they track in a straight line AND while turning. Normal operation for 3 rudders.

 

Now the Lurssen effect. The faster the boat goes the deeper into the water the stern sinks. Note the parallel with hulls that are designed for racing. At low speeds the hull is deep in the water but at speed they are almost riding on the prop only. The S-Boat hull is optimized for up to 20 knots for a long cruising range so the smaller servo is not turned as the outer rudders would drop the efficiency of the boat. Now at higher speeds the stern lowers in the water and the nose comes up dropping the efficiency so the smaller servo is activated to move the outer rudders away from the center line. This changes the way the water moves under the stern and lifts the stern up and the nose back down for a more efficient configuration.

 

When the boat turns at speed steering by the large servo still keeps the movement of the outer rudders in sync with the main rudder. So if it is a turn to starboard at speed, as an example, the starboard outer rudder increases its angle from the center line and the port outer rudder lessens its angle from the center line thus maintaining the effect through the turn. The starboard side of the boat wants to sink into the water further, in this example, as the boat turns. So the starboard outer rudder deflect s the water more than the port outer rudder does thus lifting the starboard side of the hull out of the water more than the port side making the turn more efficient.

 

Very difficult to explain in words as I am not 100% fluent in the dynamics but the principle makes a lot of sense. It allowed a hull which was optimized for around 20kts (normal cruise speed) to be very efficient at attack speed of around 40kts. A lot easier to alter the shape of the water going under the stern than change the shape of the stern. 😊

Edited February 23, 2019 by ir3

[lmagna]

Nice explanation ir3

 

When I was looking at the picture I had some doubts as well, and thought you were going to run into some problems. But your description makes it a bit more clear and much more logical. Can't wait until the water tests!

[mtaylor]

Thanks for the explanation, IR3.   I'm wrapping my head around this and will do some Googling but I do grasp what you're saying.   This is one of those things that most of us probably wouldn't think about.

[BenF89]

ir3, I like the description as well, and maybe can add a bit on the fluid dynamics side for those interested. Right now, it’s mostly and educated hunch based on what you’ve said, what the hull and rudders look like, and what I know of planing hydrodynamics. But I’m definitely going to look into it more to either confirm or correct my thinking.

 

Essentially, a boat ‘on plane’ is trading buoyancy (in displacement mode) for lift, and like an airplane is very sensitive to changes in trim, which itself is sensitive to variation in the center of pressure on the hull bottom. With a vee-hull, which I‘m most familiar with, there are lots of creative ways to change the center of pressure, like trim tabs. (Or aft lifting strakes like on Lou’s PT boat.)

 

I’m not as familiar with round-bilge boats at planing speed, but I can guess at how the Lurssen effect works hydrodynamicaly. The out-from-center rudder angle will cause a slight interruption in the water flow, which would cause an increased bottom pressure right at the rudder, quite far from the boats center of gravity. The pressure ‘spike’ would create a ‘trimming moment/torque’ by moving the center of pressure slightly aft, so the boat would have to trim down by the bow a little until the center of gravity and center of pressure balance again. It’s like a vertical trim tab. 

 

Normally vee-bottom boats have there ‘cruise speed’ past the hump (the speed where the boat starts to ride its own bow wave, and therefore has the most trim) and on-plane, and so you’d use the tabs at the lower speed to keep the boat trimmed down in that range, but retracted at the high speed to reduce drag. 

 

But since the ‘hump’ is at a relatively higher speed for a long, skinny round-bilge boat, and there’s much more actual ‘squatting/sinkage’ as the boat approaches and passes ‘hull speed’, you cruise at an efficient point below the hump (since with a long, slender hull you can get a high displacement-mode hull speed). Then, you use the rudders at a much higher relative speed to keep the bow down while riding up the bow wave as you power past ‘hull speed’ and into semi-planing and planing-mode.

 

It does make a lot of sense, but like I mentioned I’ll have to look into it more myself to see if my guess is correct, or if there’s some other effect I’m not thinking of. Thanks for the intriguing topic, ir3!

[ir3]

Thanks for all the likes and the input on the Lurssen Effect. It is quite intriguing as Ben pointed out. The reason I chose this model is because of this implementation. Seeing this working on the water will be quite amazing.

 

As far as the work goes, getting the geometry of the 3 rudders correct is getting to be a bit difficult. Unfortunately the two outboard rudders are quite close to the back of the hull so getting the tiller arms at the correct angles requires a bit more work. Moving the outer rudders with the small servo can be limited to about 30 degrees. The problem is with the center rudder movement. I may not be able to get a full 45 degrees on the main rudder but this would be quite drastic anyway. The small servo definitely needs to moved off center to get the angles to the outer rudders as close to equal as possible. This is a bit more trickier than I expected.

 

Until next time,

 

IR3

[ir3]

Well, after banging my head against the wall trying to get the outer rudders aligned, it became perfectly clear that the small servo must be centered between tiller arms. The geometry is all wrong with the current implementation and the rudders do not sync. I will be widening the sled to accomplish this. Now that I know how to build the sled, it should go quite quickly.

 

I should have it all posted by tomorrow afternoon.

 

The discussion is great. Lurssen was quite an innovator.

 

Till next time,

 

IR3

[lmagna]

ir3

 

Have you considered running two links from the sled servo arms to each of the outer rudders with both arms at opposite directions, (Pointing toward the centerline) that way when the servo sled moves forward and sternward while the servo arms remain neutral it would make the outer rudders operate in opposite directions creating your Lurrsen effect, or as I look at it the collective pitch. When you turn the arm on the sled mounted servo then the rudders would work in opposite directions and act as conventional rudders while still maintaining their collective relationship to each other. This could also be referred to as cyclic pitch control. These are all helicopter control terms but I see no reason why they would not work in your application. I suspect that you would not even need the center rudder but if you did I think you would be able to run a flexible cable between the outer rudders with two additional arms aligned parallel to the centerline of the hull. Make another about half length rudder arm on the center rudder and run the cable through it. Install two adjustable collars on each side of the center arm that allow for the movement of the collective while still pushing or pulling the center rudder in the cyclic movements. You may have to install dampening or centering springs on the center rudder as in straight ahead travel it will not really be supported by the outer rudders.

 

You would of course have to remove the extra center arm linkage from the fixed servo to the center rudder. Another possibly easier way would be to add another fixed servo just for the center rudder. I suspect that the only time you will possibly need it will be at slow speeds anyway.

 

Hope that all makes some sense to you.

Edited February 25, 2019 by lmagna

[ir3]

Thanks for the input, Lou. I will have to go through your explanation a few times to fully understand it. I have a Vario CH53 that I will be finishing in the late spring and the comments will be very useful.

 

After giving it a great deal of thought, the first implementation was too complex. Geometries were all off. I didn't like having two different connectors on the large servo arm which caused the sled to move differently than the main rudder. The solution is to have the servo push/pull the sled and have the sled push/pull the rudder. In this way all the linkage is kept in a straight line. The position of the connector on the servo horn is exactly the same distance of the connector on the main rudder horn. Geometry is perfect!

 

Now it's off to putting the Lurssen servo on the new sled. This will be positioned so that the angles to the small rudder connectors will be exactly the same. The result is that when the large servo moves the sled the main and outboard rudders will track exactly the same.

 

Here is the new main rudder implementation. Now its back in the shop to install the small servo and put this part of the project to bed.

 

Until next time,

 

IR3

[ir3]

Nearly there. All of the geometry is now correct. All the rudders are moving at the same rate and I am just about finished with this setup. I do notice a bit of flex in the rudder tubes so these will be reinforced. There is a little bit of skewing of the sled so I moved it as far back towards the stern as I could. The skewing seems to be a bit less. The only test to see if this works will be on the water. Next will be the installation of the Action electronics.

 

Until next time,

 

IR3

[lmagna]

No need to switch over to anything else. It looks like this configuration will accomplish pretty much the same thing only better. The large servo controls the direction or cyclic on all three rudders and the small servo controls the collective in the outer rudders without affecting the center rudder operation.  The only limit I see is what you said before. The distance between the outer rudders and the transom could severely hinder their movement.

 

Much cleaner installation as well. Congratulations.

Edited February 25, 2019 by lmagna

[ir3]

Started on the electronics installation. All the electronics will be on boards that can be removed in case repairs are needed. The first items to install were the ESC's for the motors. I used the action P94 "Lite" for the outer motors and the Action P79 for the central motor. The fuse blocks will go on a panel just above the motors. There is a problem with cooling the motors. I purchased the Traxxas Heat Sinks for 380 motors but unfortunately they just don't quite fit the RE380 motors. They are supposed to snap on but this will take considerable effort. So, when I install the fuse blocks above the motors I will include a small fan to keep air moving around the motors. I forgot two items from Action (a senior moment) and I have to order them. That should delay the final installation of electronics for a few weeks. There is a lot to do, however. Here's the pic of the ESC's mounted in their respective boxes.

 

Until next time,

 

IR3

[lmagna]

What is the amperage you expect these motors to draw?

[ir3]

The spec on these motors is 5.9A at 22000 rpm. I'm not so sure I will run the motors at top speed so the current draw will be somewhat less.

[lmagna]

I am assuming that you will be running at 6-7.5 volts. Here are some things you might consider. The 6 amps you stated may just be a starting amperage. As a rule DC motors draw more when the voltage is lower and when they are loaded down. So the current needs will go up slightly as the battery goes down. They will also draw slightly or even considerably more when starting up. They will draw A LOT when you go from forward into reverse or the other way around. 

 

You are running pretty small props, I would suspect 1/2" or even less and you are running a motor for each prop. That is almost no load at all for motors like these and unless they are very high rpm motors wound for lots of torque like the airplane people use  you should not need to worry that much about heat or amperage load on the controllers. I would recommend that you place the boat in a bathtub or whatever body of water you have and hook up a motor with a multimeter in series and see what it draws. No need for a controller or electronics for that other than the meter.

 

Your electronic controllers are rated at 10 and 20 amps continuous load. I think you will find that the heavy controller will easily handle the two outer motors and one of the "lite" would be more than enough for the remaining prop. With admittedly limited information that I do have I also think you will have no need for heatsinks on the motors unless again they are really wound "Hot" . If they are that powerful you may have problems just keeping the boat in the water, to say nothing of trying to turn without flipping over! Remember you are not building a hydro.

 

With the length and narrow beam of this hull I think you will also find that you will never, or almost never use the outer props to steer with. With the narrow beam and the props being so close together the response other than at dead stop would probably not be worth the effort. The rudders will be all you need. Being able to run on one, two, or all three props on the other hand can be a very nice feature. If your Lurssen effect works as well as it should, you may find that you are able to achieve very high speeds even without using all three props. Using one prop for tight maneuvering or docking also makes that kind of stuff easier making it less likely you will over control and run something or someone down. 

 

Your boat and your design. Just consider me background noise if you have some other information, after all I think your rudder/servo arrangement is better than what I was envisioning. This could easily be the same.

Edited February 27, 2019 by lmagna

[ir3]

I do appreciate all of the input you are providing. The power source will be 7.2V but probably closer to 6.5V due to losses. I never did a 3 prop boat and from your input and the P94 setup instructions, it is not quite as simple as one would expect. The prop diameters are 1.25" so that will affect the analysis or the current flow somewhat. The P94 has 4 modes of operation. Mode 4 is what they suggest for MTB's such as this one. In Mode 4, the center prop is slowed down in turns by a proportionate amount TBD by rudder movement, In another mode, the center motor speed does not change with rudder movement. The mix is adjustable from 50% to 100% so this will be set in trial runs. It's quite complex as to how the transmitter sticks control various outer prop configurations depending on movement of left and right sticks. I have a lot of studying to do on this. But I think that with the larger props the loads will be a lot greater so perhaps putting a fan above the motors can't hurt.

 

I read some other experiences about the Lurssen effect on the S100. Some reported no effect until near top speed. Others used only the center motor in which case the Lurssen effect would not work at all. I think this is correct as the effect depends on the outer props which have a big influence on the stern wave. There is no definitive discussion of this effect as applied to the S-38 or the S-100. It will be interesting once the pond runs are made to see what actually happens. It could end up with a lot of effort and get very little result. But it is fun doing the implementation. I did find a picture of an implementation that has the outer rudder servo offset from the center line to keep the geometry correct. In the design that I first followed, the geometry was all wrong and it probably didn't work at all. The build thread never made it to boat on water.

 

A bit winded but in the meantime, more parts were ordered from Action so the complete installation of electronics will be set back a few weeks. I will continue installing the parts that I have.

 

Until next time,

 

IR3

[lmagna]

Have you placed all this stuff in the boat and did a displacement test?

 

No, providing extra cooling can not hurt, except for the weight penalty and possibly higher center of gravity.

 

Your props are larger than I thought. It is hard to believe that a boat of that size would carry that large of a prop, but I suppose a 44" prop on a 100' plus boat is not that ridiculous. They had about 4000 HP to tie down. You will have a much higher proportional power ratio.

 

I have no experience with mixing throttles. That kind of radio gear was virtually unavailable in my day, and what was available was VERY expensive.  I suspect it is a result of the needs of the helicopter and quadracopter industry.

 

My only experience with anything comparable was, as I explained before, the old Lindburg Elco PT boat when it first came out in 19??. All it wanted to do at first, was to  have the stern almost running underwater and a bow wave that looked like Albatross wings! After modifying  the bottom of the hull for RC operation the boat flattened out, the stern came up and the four foot wide gullwing bow wave was staying under the hull. It looked far more scale in operation and was MUCH faster on the same power. I am almost tempted to take he old lady off of the shelf, scrape 25 years of spiders and dust off and reequip her with modern day power. The results could be kind of scary! I never even tried stern trim tabs or ramped stern like the S boats had. Hope you have a big lake or pond to run in. These kinds of boats can eat up a lot of distance pretty quick.    

[ir3]

Thanks Lou,

 

The Hansen Dam boat pond is quite large. The Irregulars used to meet regularly but that has dropped off quite a bit. The Violators group is on the pond on Saturdays and they are running the fastest boats around so there is plenty of room. I do not have a programmable radio so it is great that the P94 has a lot of options and several mixing modes. As soon as I have all the electrics I will do a water line test. I purchased the Dual Battery adapter from Action and so to make up the weight I can experiment with one or two battery packs at different weights or simply use some shot ballast.

 

I have the fuse blocks in place above the motors and room for two very light 30mm square fans. If the motors to not get too hot they can be removed.

 

Until next time,

 

IR3

[lmagna]

If the fan thing does not get the job done you can take some aluminum or brass tubing and wind it around the motors. A little hard in this configuration especially with brass but from what I have seen your skill level is up to it. Drill another hole and install a pickup tube through the hull and another at your exit point. Connect the whole thing with small surgical tubing and you have a water cooling system that operates for free.

 

Hard to tell fro certain what lake is which but it looks like you have a couple of very nice ones from the pictures. If they are shallow it they would look perfect.