The Evil Doctor's Next Project

[Dr. Jet]

In my "Who wants to build a semi-scale........" thread, someone asked "What's next?" and I mentioned a mini outrigger. With some very much appreciated input from NativePaul, I revised the preliminary design sketches to fit European Mini Naviga Hydro rules (easy enough )

I'm posting this thread to discuss my new design concept, based on the KEPS outrigger with the design philosophy of maximum weight reduction. If successful, I think I'll call it.........

The Mini Me

[Diegoboy]

Bill,
Any thoughts on pushing the forward sponson boom ahead of the tub?
This could eliminate one sponson boom from the tub and make the boat run like a larger boat in chop.

[NativePaul]

Too late Dr. H&M's mini hydro is called the Mini Mi to go with their Dr Evil hydro1 and Dr Super Evil Hydro2.

Only extending the afterplane length can make it run like a larger boat, pushing the front boom forward of the tub mainly gives you an easy way to adjust the angle of attack.

[Fluid]

But, moving the sponsons forward of the tub increases the distance between the sponson transoms and the prop - “lengthening” the effective length of the boat. I first saw this at the 2001 LA SAW race on one of the JAGs riggers.





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[Dr. Jet]

Bill,
Any thoughts on pushing the forward sponson boom ahead of the tub?
This could eliminate one sponson boom from the tub and make the boat run like a larger boat in chop.

I thought about that but I'm already at 449mm from tip of sponson to transom. Any longer and it wouldn't work for the European classes, AND it's already a pretty tight fit for all the gear that has to go into it. That would also add to the parts count and complexity plus I'd have to add structure (weight) to support the cantilevered boom arms.

I know this composite one (see photo) uses that approach, but such are the shortcomings when working with flat sheets of wood. In all honesty, I doubt ANY wood boat would be competitive against lightweight CF hulls like that, but some people still like to dabble with wood. The cost for wood will be considerably lower though. I planned to include a laser-cut turn fin as well, but I've been informed that water-jet is better suited to cutting metal parts.

If you look at the right sponson in my wire frame drawing, you will see I made the "handle" a bit longer to accommodate holes on either side of the sponson boom. I plan to put 1/4" aluminum rods, bored and tapped to 4-40 into these holes to hold the turn fin. I'm even thinking of using a nylon or brass bolt in the forward one to allow it to shear off if it hits something.

[Dr. Jet]

Too late Dr. H&M's mini hydro is called the Mini Mi to go with their Dr Evil hydro1 and Dr Super Evil Hydro2.

Oh cr@p........


Maybe I should call it..........

[NativePaul]

But, moving the sponsons forward of the tub increases the distance between the sponson transoms and the prop - “lengthening” the effective length of the boat. I first saw this at the 2001 LA SAW race on one of the JAGs riggers.

I have several boats like that,and know what you mean, but when you are already at the maximum hull length allowed, as in this case, all it does is shrink the tub and allow you to adjust the AoA easily.

Dr. Jet

That is tip of sponson to transom, right? The stinger doesn't count in overall length?

Yes 450mm max from tips of sponsons to end of transom, ski, rear sponsons, or anything else that you could consider hull.

I currently have it planned to run 2S, but if I stretch it, I suppose the extra 7mm would accommodate the Zippy 3S. Can you send me a link to the limiter?

I didn't know you planned on 2S, in that case the KV of the second motor is too low, both 2s and 3s are allowed in the class battery weight limit for either is 113g inc 30mm of wire and connectors, which is currently 2800mAh for 2s and 1900mAh with 3s using the best cells, those using limiters aren't restricted to batteries, but about 140-160g is pretty usual. This is currently the only limiter that is really suitable for Mini Hydros, Wozniac has some being beta tested at the moment, but are not released yet.

The sponsons will be hollow in the middle. They'll need skins for strength and water-tightness. Like the KEPS, the sponsons will have a plywood (or G-10 or CF or aluminum plate) for a ride pad.

I get that, I was just thinking that if you flip the sheets 90 degrees, one of the skins you need for water tightness could do double service as the ride plate to save a little weight.

Remember, my plan was to run 2S. I like the helicopter outrunners in the small boats for a couple of reasons. They have built-in fan cooling, and they have an order of magnitude more torque than an inrunner of the same size which means a bigger prop or more pitch.

The torque difference between an inrunner and an outrunner of the same mass and KV isn't much at all in normal use, the only time it is worth mentioning is on startup when the extra poles give the ESC a much easier time. We run both and for our racing the prop sizes are what you would expect from the KV for either type. There is an advantage in the opposite direction to inrunners at higher power levels when water cooling beats air cooling in confined spaces and allows the inrunner to spin the bigger prop. Much more noteworthy advantages are the typically better efficiency and much lower inertia of an inrunner vs the weight savings of not needing a water cooling system.

I looked at the RBC Fast4ward. It reminds me of the old adage regarding aircraft origins (see photos): If it's ugly, it's British. If it's weird, it's French. If it's ugly AND weird, it's Russian....

What do they say if like the Fast4ward it is Dutch? It does look pretty bizzare, but steepening the sponson tip to sponson transom is the only way to increase the afterplane lenght with a maximum hull length, and they have taken it to the extreme, a little too far I would guess, but I've not seen one run.

Instead of a strut with beefy mounting brackets. I'm using a fixed stinger.

We don't really use beefy mounting brackets, it is usual to use a couple of strips of carbon sheet let into the transom to hang the rudder and strut from.

The 1/32" wire drive I like is a whole lot lighter than 0.098" cable, but I'd need to do a lot of testing to see if it would hold up to 3S over time. It works just fine with 2S boats, but I haven't used it for 3S. 1/16" wire should work as well, but I think in these small boats puts a lot of radial loads on the system. As long as the motor and propshaft use decent ball bearings, it should be OK.

I haven't seen anyone using a flex in a mini hydro for quite a while, i think wire run more freely, but even if they don't the weight penalty of the flex and full length stuffing tube that it needs is too much to overcome. we typically use 1.2 or 1.5mm wire, 1/32 is 0.8mm. I have not used that small but I have used 1mm in the past and it held up fine, but due to drill and ream flex the guy that made my stub shafts said that it would be the one and only he would ever do. I think you build your stubs out of telescoping tube, but the telescoping tube I have used in the past hasn't been close to tight enough to ensure good enough alignment and runout for me to be comfortable.

[NativePaul]

I know this composite one (see photo) uses that approach, but such are the shortcomings when working with flat sheets of wood. In all honesty, I doubt ANY wood boat would be competitive against lightweight CF hulls like that, but some people still like to dabble with wood. The cost for wood will be considerably lower as too. I plan to include a laser-cut turn fin as well.

If you look at the right sponson in my wire frame drawing, you will see I made the "handle" a bit longer to accommodate holes on either side of the sponson boom. I plan to put 1/4" aluminum rods, bored and tapped to 4-40 into these holes to hold the turn fin. I'm even thinking of using a nylon or brass bolt in the forward one to allow it to shear off if it hits something.

That composite one is Mila Holc's Agulya (spelling) splashed, it is about a 15 year old design from well before the class existed, it is under 400mm and the makers just added the arms to bring it up to 450mm and make it vaguely competitive in the class rather than designing something themselves. For what it is worth my latest kevlar moldie was aprox 1oz per sponson and 2oz for the tub (sealed but unpainted). I wouldn't use a nylon bolt on the turn fin, our turns are tighter than American ones at a 5m radius, so there is quite a load on the turn fin, and that load has quite some leverage on those bolts. Use 3mm boom tubes and they will be sacrificial enough.

[Dr. Jet]

.......... I think you build your stubs out of telescoping tube, but the telescoping tube I have used in the past hasn't been close to tight enough to ensure good enough alignment and runout for me to be comfortable.

I did that once as an experiment using 1/8" SS tube. The tube was actually about half a thousandth too large in diameter and I spent a lot of time sanding it to the proper diameter to make it work. Normally, I use Jeff Wholt's stub shafts that are drilled for 1/16" wire (that's what's in the Atlas kits - See photo). I have found a source for very precise 1/16" O.D. 304 stainless steel tube with a .08mm I.D. I use Loctite green to attach the wire to the SS tube, then again to the stub shaft. So far, it has worked flawlessly in other applications.

....... I didn't know you planned on 2S, in that case the KV of the second motor is too low,....

I know. I'd like to find something in the 4400 to 4800 Kv range, which is why I mentioned that little 4900 Kv motor. I think it is physically too small though, and while being lighter at 45 grams, it may not have the "oomph" necessary. I did order one so I can give it a try though, as it is half an ounce lighter.

Remember, I'm just going to run some circles at the local pond for ships and giggles and trying to do it on the cheap. I'm never going to enter any organized racing with anything I build. If I were racing in your classes, I'd use a state-of-the-art carbon fiber hull with an 1105 3Y on 3S or an 1105 4.5D or 2Y on 2S.

That composite one is Mila Holc's Agulya (spelling) splashed..........

I wasn't making any recommendations, just pointing out an example.

..... What do they say if like the Fast4ward it is Dutch?

I thought RBC was a British company. Maybe the design is Dutch.

[NativePaul]

I have the drawings for the 449 mm outrigger finished and I’m ready to cut a couple plus enough additional 1/16” ply parts to fill out the remainder of a 12" x 24" plywood sheet. I plan to build one and only one for myself as a prototype. The additional parts will go into storage as I have no plans to purchase and cut the materials necessary to complete any additional kits at this time.

If you are cutting a couple, but only building 1, does that mean you are selling the other one, hint hint?

[Dr. Jet]

If you are cutting a couple, but only building 1, does that mean you are selling the other one, hint hint?

Since you've been super-helpful with my projects (and I do appreciate it), I will see that you get one "At my cost (plus shipping)". It may be the only other one in existence, but then again, you may have some more insight that I can incorporate into future designs that might sell enough to make............

[Dr. Jet]

I'm fond of the 1/32" wire drive for the little boats because it's simple, it's cheap, it's light weight, it's easy to seal, and it doesn't put a big radial load on the bearings.

I run the 1/16" stainless steel tube bonded in the propshaft past the 3/32" brass seal tube on the forward end of the stinger. From there it's bare wire until the motor coupler where there's another short piece of 1/16" SS tube to give the coupler something to grip. Sealing it is easy; just a medium coat of grease on the 1/16" SS tube where it goes through the 3/32" brass tube seals everything up. I use this exact arrangement on the Twin Micro Scat Cat and never get a drop of water in it. I can always put a piece of silicone tube on the 3/32" seal tube to seal against the 1/16" SS tube if there ever were a problem.

[NativePaul]

Thanks Dr.Jet that is very kind of you, let me know where to send the money when you are ready.

Do you have the stuff needed for another one of those drivelines too? I would like to give it a go, but think I would have trouble finding the imperial sized parts here, all the fractions are making smoke come out of my ears just getting my head around it, no worries if not, I have 1.5mm wires on hand.

[Dr. Jet]

Thanks Dr.Jet that is very kind of you, let me know where to send the money when you are ready.

Do you have the stuff needed for another one of those drivelines too? I would like to give it a go, but think I would have trouble finding the imperial sized parts here, all the fractions are making smoke come out of my ears just getting my head around it, no worries if not, I have 1.5mm wires on hand.

I'll make you one of the little drivelines as well, including my custom-machined end seal. I'm thinking of making the stinger out of aluminum tubing instead of brass to save a gram or two. What's your preference?

[Dr. Jet]

Thanks Dr.Jet that is very kind of you, let me know ....... when you are ready.

I may have stuff to start as soon as this Friday. The laser guy has the wood as of Tuesday, April 2, 2019. I want to build a prototype first to make sure it's right before sending any parts to you. Stay tuned.....

[NativePaul]

My Antenna is up.

[Dr. Jet]

My Antenna is up.

I will have the cut wood this afternoon.

Water-jet cut turn fin blanks.

[Dr. Jet]

I'll be picking up the wood for this tomorrow morning. I need to box up the wood for the 1:20 roundnose kits, and I should be able to start framing up a prototype of this in the afternoon. If it all goes together OK and the hull isn't too flimsy (one of my concerns). I'll get in touch with NativePaul and make arrangements to send him Prototype #2 and see how he does with it.

I bought one of these motors to put in my version. https://hobbyking.com/en_us/sp-02435...rs-4800kv.html The Kv is right for 2S, the mass is right, and the price is..... well.......CHEAP! How can I lose? Remember, I'm not racing in some sanctioned event, just scaring the waterfowl and wowing the locals...

[Dr. Jet]

Well, I got the wood and threw one tub together quick and dirty with CA to see if it would be rigid enough. The result? Yes, it will work.. Now, I'm going to build one in the proper manner. There were a couple of dimensional busts which will require elongating one hole in the sides for the bulkhead tab and moving one notch on the hull bottom. The drawings have been revised accordingly. I'll photograph the next one as the construction will look a bit better and I can use them in a future instruction manual.

[Dr. Jet]

Laminating the sides. Photos to follow.

[Dr. Jet]

I paint the ply and balsa doubler with epoxy, then brush so much of it off that most people would think there's not enough there to get the pieces to stick together. I index the pieces with the bulkhead tabs to get them in exactly the right position on my flat glass light table that is protected with wax paper. Next, I use my uber-flat and uber-square Delrin blocks between the bulkheads and weight them down with cans full of lead shot. Lastly, I remove the bulkheads before the epoxy goes off as they will get installed later.

[Dr. Jet]

This is how much epoxy I apply to the side and doubler. Wet out the side to be glued, then scrape off any excess with a credit card until it's just a microscopic film.

[Dr. Jet]

Once the epoxy cooks off, I fit the sides together and sand them flush and square. This helps to keep the tub square.

[Dr. Jet]

Using a square, I install the bulkhead/formers into one side and tack into place with thin CA. Then I install the other side onto the bulkhead/formers. Next, the transom and nose-pieces are installed. Finally, the 1/64 ply bottom gets tacked into place.

Unfortunately, I made the holes for the rudder pushrod too big, so I shimmed them with small pieces of aluminum tube. The stuffing tube is glued into place, but the stinger won't get glued in until later.

[Dr. Jet]

When I installed the motor, it was a bit flimsy feeling. I added a couple of braces to stiffen it up.

The driveshaft is my typical 1/32" drive. The coupler I decided to use was an insanely expensive 2mm x 2mm flex coupler I modified to fit 1/16" on one side.

I used the same servo mount as I used in other boats. It was a little big, but nothing the trusty old belt sander couldn't fix. I thought I would be able to reach the servo mounting cap screws with a ball-end Allen wrench, but it was just too great of an angle. Furthermore, when the top is glued on, I won't be able to reach the front screw with a standard Allen wrench, so I'll have to put some access holes in the side that will get covered with tape. That will make it easy-peasy to access the servo.

[Dr. Jet]

I wasn't really thrilled with the rudder pushrod design. It had too many bends in it which created drag (friction) in the bearing tube, and the short length past the transom limited travel unless I used a more powerful servo to overcome the bending forces on the 1/32" wire. Remember, the wire must bend to compensate for the circular displacement of the servo and rudder arms. Linear servos wouldn't have such issues. Furthermore, all these bends in the wire made it difficult to remove and install.

The solution? Simple and elegant: I flipped the servo connector to the other side of the arm, putting the arc of the pushrod hole in the same plane as the bearing tube. I lengthened the 1/32" wire pushrod beyond the transom by going around the connector on the rudder. With a longer wire, the force required to bend it the same amount is reduced as it moves through the arc of the rudder arm.

[NativePaul]

That servo pushrod looks much better now. What weight is the servo? I usually use a 14-16g in minis and the one in the photo looks like it might be a bit smaller, is that rudder the 55mm Chinese one? i have tried one in the past and found it to not quite be enough and switched to the Chinese 62mm, version or a combo of the 55mm bracket modded for the 62mm blade holder, all boats are different though let me know how it turns with it, as the 55mm is cheaper and much more readily available.

Have you used one of those flex couplers with a wire drive before? I'd be worried that the wire cant support the back end of the coupler like a solid shaft does, so any tiny imbalance in it, or whip in the wire will throw the back of the coupler off center, then the mass spinning of center it will throw it further off center, in a positive feedback loop until it breaks.

[Dr. Jet]

....... What weight is the servo?

That's a 9 gram metal geared servo.

.......... is that rudder the 55mm Chinese one?

Yes.

I have tried one in the past and found it to not quite be enough and switched to the Chinese 62mm, version or a combo of the 55mm bracket modded for the 62mm blade holder, all boats are different though let me know how it turns with it, as the 55mm is cheaper and much more readily available.

If it's not enough, I will follow that advice.

Have you used one of those flex couplers with a wire drive before? I'd be worried that the wire cant support the back end of the coupler like a solid shaft does, so any tiny imbalance in it, or whip in the wire will throw the back of the coupler off center, then the mass spinning of center it will throw it further off center, in a positive feedback loop until it breaks.

No, I haven't used one of those 40,000 rpm-rated couplers yet. Remember, the coupler is not connecting to the wire, it is connecting to a 1/16" precision stainless steel tube that is fully supported in the 3/32" brass stuffing tube so it essentially behaves like a straight shaft; until it reaches the end of the stuffing tube where the 1/16" SS tube also ends and the 1/32" wire continues to the stinger. If any "whip" were to occur, I would expect it here. This is very similar to the arrangement I used in the mini hydros and the BBY Micro Scat Cat; all of which have been extremely smooth and flawless in operation. I'm a big fan of the 1/32" wire drives in these low-power (+200 Watt) applications.

[NativePaul]

That is clever, I hadn't twigged that it is a combination of a straight shaft and a wire drive.

I bought a 9g servo today so I'm ready.

[Dr. Jet]

Exactly! A straight shaft that transitions to a wire drive.

I don't want to send you anything until I've tested it out though. I'm currently gluing up MPR hulls right now; I'll start laminating the sponsons next week.