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RAMMJETT 12-20-2020 05:28 PM

Electric 1/5 Miss Bud
Hello I've done a bit of r/c gas boating in the past and thought why not take a stab at a big 1/5 scale (just over 6") 1980 Miss Bud Hydro . I am 1/2 way through the Epoxy glass hull build and I am starting to think about power . For a motor I purchased the biggest motor I could find, it's a Rocket 5692 1090 KV motor and a 200A speed controller with a 900A peak . It's a 6 cell motor .. my question is does anyone have any idea what prop I should start with . I have props from prather and octura from 65mm to 86mm . I don't know if anyone even runs boats this big electric but any comments or advice would be welcome . Below is a picture of the 90cc scratch built wood Miss Bud that my friend built , we pulled the molds from his boat .. cheers

Got RPM 12-21-2020 07:05 AM

That is an ambitious project! But I’m afraid that the power setup you have will not provide decent performance, in fact I doubt that the boat will plane. It is not even adequate for a 1/8 scale hydro due to limited power. A typical 1/8 scale will use a minimum of 8S and a 200 amp controller; your boat will require more than twice that power to work well.

Very few successful FE boats this size are around so references are limited. I’ve built plenty of 10S boats but none close to the size and weight of yours. If I were forced to guess, I’d think that a prop over 75mm, a long 56mm motor with a Kv between 500 and 600 running 10s might work. But getting on plane could be a big issue since hydros require more power to plane off than a mono or cat. And I don’t know if the motor will swing a prop that size for long without overheating. Perhaps someone with actual experience building and successfully running such a large craft will chime in. Good luck with the build!


RAMMJETT 12-21-2020 08:29 AM

It's not all that ambitious my friend ,the prototype took a while . I will have this knocked off in a week or so ,only 2 days into it now and just about done the hull , pretty simple really compared to building planes .. The motor I have chosen is 5040 watts on a maximum of 6cells ,that's near 7 hp .. being as it's 1090 kv it's capable of turning 30,520 on a much lower voltage but with the huge diameter can should have the torque to do it ..this is not the small can motor that would struggle having the torque to turn while being lugged down , I agree those small can motors would overheat , they are for a lightweight boat .This motor is more like the big block of electric motors, big can and lots of torque .. The motor is 56mm (80mmwith the jacket)diameter and 95 mm long with a 8mm drive shaft and should easily create the torque to turn a larger prop without overheating . If it does not work no biggie back to gas, I know that will work . The one in the picture goes just over 60 mph and gets on plane easily @ 1/4 throttle . the Stihl 90cc at best produces 5hp at 12-14000rpm and it's 45lbs ,mine should be much lighter 25-30 lbs with the epoxy glass hull. ....

RAMMJETT 12-21-2020 09:27 AM

Miss Bud fresh out of the mold this morning looks pretty good , a lot lighter then I thought , so far only 13lbs with all the framing in . next on to the decking .. would be great if I could pull off 25lbs but we will se . .

RAMMJETT 12-22-2020 09:22 AM

Ok so I see the problem Electric motors are way way way over rated when it come to wattage . the simple answer to my question is for a boat this size I need a watt rating of 12,000 15,000 watts of power as they rate these R/c boat electric motors .. this make perfect sense to me now . what was throwing me is a gas engine of . 3.8 hp or 2.800 watts pushes this boat around like nothing . to do the same job with electrics I have come to the understanding I need 4 x the wattage or more . Interesting because watts are a measure of Hp . If the wattage is equal they should if geared correctly be able to do the same job regardless of the power source. either brushless motors are hellishly inefficient or way over rated . but at least I now know what I need to power this boat with an electric motor .. Now I'm wondering ,seeing as this motor has a max of 6 cells to produce 5200watts and I need at least 10,000 watts . would it be possible to run 2 motors (twin props) ,both motors running 2p or 3p 6cell packs to get 10,400 watts of power ???

I must say these electric motor ratings are a joke 15,000 watts is equal to 20HP that is a huge joke , no way in the world do any of these boats ,even the 100 mph ones put out 20HP. Impossible ..advertising at it's best LOL..

Hydro Junkie 12-22-2020 11:40 AM

I can't say anything about the size of boat you're looking at but I do know that we race 1/8th scale boats that run 850KV motors on 8S with a maximum charge of 33.84 volts and get speeds into the 60s with a 57mm prop. I've seen 1/6th scale boats that are just as fast though I don't know what they use for power, other than being electrics

RAMMJETT 12-22-2020 02:27 PM

Yes if you take your voltage and multiply it by your amp draw I am sure you will have around 10,000 watt setup . and that's about right for a boat of that size . for a boat the size of mine it's just not viable to run electric , I'd be spending $1,000s of dollars and hrs charging time to have 2 min runs . I have a extra Kioirtz 40cc that I ran in my 57" V hull and it would blow out a 86mm prop with no trouble, we tacked it over 15,000 rpm , will cost me nothing and I can run the boat for a full hour if I want . yep she is going GAS ... but thanks for all your help guys if nothing else I understand the limitations of electric . cheers my friends ..

RAMMJETT 12-23-2020 05:42 AM

have the top decks pulled from the molds today , did a rough fit . this is one big Miss Bud ..

Retiredat38 12-23-2020 07:40 AM

Originally Posted by Hydro Junkie (Post 12652061)
I can't say anything about the size of boat you're looking at but I do know that we race 1/8th scale boats that run 850KV motors on 8S with a maximum charge of 33.84 volts and get speeds into the 60s with a 57mm prop. I've seen 1/6th scale boats that are just as fast though I don't know what they use for power, other than being electrics

Finally! Someone with some practical specs.

Hydro, I'm curious. What do those 1/8th boats weigh?

Now if you would, how do you get a 33.84 volt charge into an 8S?
Never mind. Been thinking LiFe too much lately.

Hydro Junkie 12-23-2020 08:22 AM

It really depends on the boat, what it's made from and how much extra framing and bracing is used in its construction
With all that said, I personally don't do electrics so I got the specs from the R/C Unlimiteds rule book. Here is a slightly modified cut and paste of the battery section:
  • A maximum capacity of 8S Lithium Polymer (LiPo) batteriesare approved.
  • Only LiPo batteries with a cell rating of 4.20 or less volts per cell are approved.
  • Maximum charged voltage is 33.84 volts.
  • LiHV batteries with per cell capacities to 4.35 volts or higher are not approved
As far as weight goes, my present scale boat is about 14lbs. It has a CMB 11cc engine with the associated fuel and exhaust systems, a Futaba four channel radio with two servos and a 4.8 volt nicad pack. It's 43" long, 22" wide and will top 60mph

My other running boat is a Sport 20 redesigned Dumas Pay'N Pak
That one weighs roughly 5.25lbs, has a Nova Rossi 3.5 engine and the same basic set up as the Elam pictured above. It's 30" long 16" or so wide and will top 45 when dialed in

Retiredat38 12-24-2020 05:59 AM

Excellent Hydro, Thanks!

See, my point is I don't believe knowing what ESC, how it's cooled at what battery voltage, etc., etc. is a pre-requisite for motor selection. Sure, you need to have an idea of where going with these but a final selection is not necessary and could possibly change as one gets into this.

The airplane guys using brushless start with the prop. Basically what prop at what rpm to place the model in the speed range desired. Then they have a rule of thumb for the wattage. Basically so many watts per pound of model. The wattage per pound varies based on type of model/performance desired. i.e. trainer, sport, pattern, war bird, etc. And they go from there. It gets them in a good working ball park and experience then helps fine tune things.

From what I've seen the boaters could take a lesson from the airplane crowd.

Hydro Junkie 12-24-2020 10:46 AM

RAMMJET, if your boat is that light, you won't need anywhere near the power that was needed for the other boat. I'd drop down to a Quickdraw or similar 30cc engine. My orange boat, pictured above, is similar in weight to what yours is now and an 11cc nitro engine will get it up on a plane without a problem.

Retiredat38, there is a big difference between the airplanes and boats on how a drive system is determined. Racing boats are built in size classes but rated according to engines used. While I could drop in a larger or smaller engine in the Pak, it wouldn't be legal to race if I went bigger. What you have to remember is that, when boat classes were worked out decades ago, the engines didn't have near the power and, worse yet, the boats were running aircraft engines with a water cooling tube around the top. A 30" boat, like the Pak, could barely get on plane with a .20 but to put in anything larger would make it blow over, hence the "Sport 20" designation. The rules haven't changed much since then, just that a boat of a given class can use any size engine up to the maximum size for the class. Boat size, originally dictated by engines, has not changed in decades either. They are given a size range as listed below:
  • .20 can be between 30 and 36 inches though some allow down to a 27" so some production boats can be raced as well
  • .40 must be between 36 and 40-42 inches
  • .60 are 40-42 and up
Now that you have your boat power and size, you get to play around with prop sizes, pitch and thrust angles to get the boat to do what you want it to do. Like an airplane, the prop will make or break performance. Too big or too much pitch and the engine won't get up to speed while to small or little pitch and the engine will overrev and could blow up. On top of all that, you have to get the length of the tuned pipe correct. Too short and you can overrev but have no torque while too long and you will get lots of torque but no speed. As you have probably figured out by now, it's all a balancing act. Once you get all of that figured out, you need to get the thrust angle and prop depth right as well. Too deep and the prop won't get free of the water while too shallow and it won't get a bite. Angle wrong and you will either be lifting the front and be in danger of blowing it over or forcing the nose down and killing speed as well as making it want to dive.
Now, for the fun part:
For your PT boat, none of that matters. You need to be looking at how heavy the boat will be as well as how many motors you're going to run. You could run three smaller motors with appropriately sized props for a scale look or one larger motor and prop for simplicity. This is the advantage of building a scale boat rather than a racing boat. I'm limited on size, drivetrains, appearance, etc. In the case of your PT, you could configure it any way you want and not be wrong since there were many configurations used on the boats during WWII

RAMMJETT 12-24-2020 05:26 PM

I think the shear size of the boat I am building will require quit a bit of power because of it's displacement , when on plane it should be ok .. .at 72" long and almost 36" wide it's a big boat . looks like it will be 20 lbs for the finished hull . I am up to 18 now but it still needs paint and the rear wing ,hopefully no more then 2 more pounds for that ?? . the 40cc I am looking at is very small and light 5lbs with exhaust . the running gear and radio should be no more then 5lbs . hoping to be able to come in under 30lbs . I only compete with my friends , pretty much unlimited class LOL , I'm going to need at least 40CC to hang with Cliff , his was doing over 60mph and I know for sure he's been tweaking it with an overdrive ..The last koiritz 40cc I built was totally insane , it was built by the Litiner and Bush pro shop , it's a true long stroke 2 cycle with proper transfer ports that were modified , they also stuffed the crank . up sized carb , custom water jacket . I will have to wait until after Christmas to pick up the engine but will post pictures of it ..

RAMMJETT 12-26-2020 05:59 PM

have the upper deck epoxy pasted on , filled the outer compartments with spray foam to add floatation and give some extra impact protection . mounted the stern drive . started to build a plug for a scoop to help keep the inside of the boat cool. this version was not designed for a scoop but trying to message one in there , may not use it but at least it's already done if I decide I need one ..

Hydro Junkie 12-26-2020 09:59 PM

Okay, from someone that knows and works on hydroplanes, both full sized and R/C, let me throw a few things at you:
1) Stick with a single rudder. Not sure what the second blade is for under the left side of the hull but it's not worth installing
2) Streamline that air scoop. Right now, it's just aerodynamic drag that serves no purpose. If you streamline it, the airflow over the outside won't affect how the boat handles while, as it is right now, it can. Worse yet, it gives the air flowing around it a place to catch that can pull the top off the boat.
3) Open up the air scoop. With the engine enclosed, it won't be able to get air into the carb, something that isn't a good thing. By opening up the scoop, it will force fresh air into the cowl that will facilitate keeping the engine running properly.

RAMMJETT 12-27-2020 02:03 PM

Originally Posted by Hydro Junkie (Post 12652901)
Okay, from someone that knows and works on hydroplanes, both full sized and R/C, let me throw a few things at you:
1) Stick with a single rudder. Not sure what the second blade is for under the left side of the hull but it's not worth installing
2) Streamline that air scoop. Right now, it's just aerodynamic drag that serves no purpose. If you streamline it, the airflow over the outside won't affect how the boat handles while, as it is right now, it can. Worse yet, it gives the air flowing around it a place to catch that can pull the top off the boat.
3) Open up the air scoop. With the engine enclosed, it won't be able to get air into the carb, something that isn't a good thing. By opening up the scoop, it will force fresh air into the cowl that will facilitate keeping the engine running properly.

Thanks for the tips my friend , I'm taking any help can get , the prototype had a single rudder behind the prop but it became useless at speed , maybe that was the problem all along and it only needed a single rudder to one side of the prop , this rudder setup came with 2 rudders so I just mounted it the way it came but they are removable and I could take one off to reduce drag . as far as the scoop goes, that is just a foam plug to make the mold ,the final scoop will be opened up and I will be making a larger exit on the rear panel to draw air into the scoop and across the engine . the large rounded rear tunnel will house the Greeves tune pipe so I want the air from the scoop to go down into the engine compartment and flow through the rear turtle deck to cool the exhaust . . I do have a question ,how deep and what angle should the prop be in relationship to the bottom of the boat . in the prototype the prop needed a bid of a downward angle to get the back up off the water but don't know if it was the most efficient depth . Not sure what you mean by the fin under the right side ??? are you looking at the boat stand and thinking that is part of the boat ???

Hydro Junkie 12-27-2020 02:53 PM

I was thinking the left rudder was under the boat, not mounted. If they are both mounted, I'd remove the one on the right side. I would also not use an extended bracket as it just means you would need to use a longer pushrod, something experience has shown me just causes problems. Putting the rudder behind the prop doesn't work with a hydro as the prop churns up the water and reduces the rudder's effectiveness.
Getting to the air scoop, something like this would work better than the short scoop you have on the boat now:
It lets more air in and let's the excess out the rear while, at the same time, it gives a better air flow around the cowl.
Prop depth is a test, adjust and repeat type deal. An initial setting should be middle of prop even with the bottom of the sponsons and parallel to the bottom of the boat. If the prop doesn't hold the rear up, you could try a prop that has more lift and then, if the rear is still in the water, give the strut a very slight down thrust. Something you may want to check is what your prop weight is. To do this, you put a support under the rear of the sponsons and a scale under the strut. You want to keep the weight as little as possible, no more than a couple of pounds at most. If there is a significant amount of weight, you will have problems getting the rear of the boat up and out of the water

RAMMJETT 12-27-2020 05:12 PM

We must have fluked the prop weight because the back of the boat is very light . maybe 1 lb . come to think of it the first adjustment we made to get the back up was adjusting the prop but it was not until we adjusted the wing that it lifted .I will try again with no prop angle and just wing , the rudder pivot is 1.5" from the back of the boat but I have a full machine shop so I could shorten them to 3/4" if you think it would help . I can ditch the second rudder no problem .. thanks so much for your advise ,it's a great help .. cheers Roger ..

Hydro Junkie 12-27-2020 05:28 PM

The wing is actually the last thing you want to play with. It should be kept at 0 degrees until all other options have been tried. As I've learned from my time working on the boats, the wing's job is fine tuning the ride, not to support the rear of the boat. In fact, if you were to look at the wing on the 1973 Pay'N Pak, it's actually set, right now, a degree or two up in the rear to push the rear of the boat down to keep the prop at it's proper depth when running at speed. If you look at the two year old Homestreet Bank/Miss Madison, the wing on it is less than 2 degrees down at the rear of the wing. Check out these videos and you'll see what I mean:

RAMMJETT 12-27-2020 08:59 PM

Yes likely we likely unwittingly had the wing set in a way that was causing down force . when we removed some of the down force the tail came up , I don't think it was lifting it ,just no longer forcing it down . the boats in the videos seem to be using air foils on the wing so even zero should create lift .

this guy has it going on ...https://www.youtube.com/watch?v=gprH...ature=youtu.be

Hydro Junkie 12-27-2020 10:24 PM

The wing is an air foil or, more specifically, a symmetrical air foil on all of the boats EXCEPT for(off the top of my head) the Pak and maybe the 1985 Griffon Budweiser and 1986 T1 Budweiser turbine, both Buds due to the wing design and mounting. In the case of the Pak and the two Budweisers mentioned above, they all have flat bottomed air foils since the wings all are mounted on top of the tails rather that being suspended between them like almost all the other boats. A symmetrical air foil doesn't generate force either way unless the wing is angled up or down. That may be why the Pak's wing is set to a negative angle, to cancel out some of the lift created by an air foil with a flat bottom, even though the wing's thickest point is only 5" thick

RAMMJETT 12-28-2020 05:26 AM

Agreed , thanks my friend .. Roger

Retiredat38 12-31-2020 08:11 AM

It would seem to me that a fully symmetrical wing would be the ideal. Since at zero deg it would neither lift nor push down and drag would be minimized. While a flat bottom theoretically produces lift at zero incidence. Of course at our model sizes the effects of this is probably negligible. Of note you'd look long and hard to find a horizontal stab on an airplane that isn't fully symmetrical. Though there are a few out there.

Hydro Junkie 12-31-2020 11:00 AM

I've seen some that used an asymmetrical stab with the larger arc on the bottom, actually pulling the tail down in flight. I always thought of that as a bandage for a nose heavy condition or a way to counter a low thrust line on a plane that had engines mounted either even with or below the center of the wing's leading edge in a low wing configuration. When you look at planes like the737, 747, 767, 777 and 787, all of them have the thrust below the plane, theoretically pushing the nose up. By using the asymmetrical stab and making adjustable in flight with a powered jackscrew, it countered some of that tendency, that is until the 737 MAX. It was so overpowered that the stab alone was unable to compensate so they added another system that wasn't thought out or tested well enough before releasing the plane for sale.
Now, getting back to the boats. If you look at the end of the wing of every boat equipped with one, all but the three I mentioned had a symmetrical airfoil. Even the two Budweiser boats went to a symmetrical wing in a short period of time, more due to weight and aerodynamic drag than anything else. If you look at this commercial, you can see the Budweiser T-1 test footage with the big wing and, right at the end, the symmetrical wing:
But when the season started, the T-1 was now sporting the wing between the tails while G-3 still had the wing on top of the tails:

RAMMJETT 01-03-2021 08:54 PM

Next on the list is to learn how to modify a prop . seems there are great gains to be had from a correctly modified prop . thanks so much for the information it's been a great help . cheers ..

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