Large Scale 1/4 scale 6' long catamaran brushless & I am new to lipo

[Crumri4]

I bought a large scale catamaran several years ago and have over time put pieces together to make something I hope is awesome. Along the way I decided to go electric brushless with this set up after seeing several you tube videos of other boats like this. I am planning to run two drive systems. Each drive will have a castle 2028 780kv motor. These are the older models that are not sensored motors. I have purchased two fightercat 400 amp water cooled ESCs and counter rotating 1/4" prop shafts. The props I picked up are 5 bladed props measuring 67mm (2.64") dia and have a pitch of 114mm (4.49"). I have been to my local hobby store as well as the chain hobby store that's in a town an hour away, neither were able to give any recommendations on tips for building this boat. I just got off the phone with Castle Creations who was also not able to give a recommendation, but they directed me here. I am mainly asking what batteries should I be looking at? I was planning to go with 8s Lipo. If my calculations are correct then my 780kv motors should turn about 23088rpm and with the 4.49 pitch props I should see speeds around 78.53 MPH. adjusting for 20% prop slip. So, any recommendations on batteries, size, brand, everything, or any input about the equipment that I already have that could be a bad choice and need changing before I start please let me know. Thanks!

[photohoward1]

Too much prop for those motors.
You would need much larger motors and better everything to get that kinda speed. I?m sure others will chime in.


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[Crumri4]

The castle 2028 is rated at 10 horsepower each. And at 8s capability 29.6 volts each. They would turn 23088 rpm. It?s actually the largest motor that I found. Do you have a recommendation on what would be better?
Same deal on the speed controls. Upon first assembling parts I started with a pair of seaking 180 amp ESCs and was warned that was too small. The fightercat 400 was the largest with a name brand that I have found. 2-10 horsepower engines totaling 20 horsepower seems sufficient considering aluminum fishing boats can carry multiple passengers with engines smaller than 10 hp.
I got the speed estimates through mathematic formulas that I have found online. I don?t mean to insult your expertise or input, but could you maybe elaborate on what ?better everything? might entail? Part numbers would be helpful information. If I was to settle for the equipment I currently have, would a prop change maybe be a solution? Do you have any recommendations at all of part numbers that I should be looking at to rig this boat? I have had this thing too long without being able to see it run. I am just looking for input that will help get it on the water. Thanks.

[koen]

I have very good experience with Graphene Panther battery's from Hobby king, but motor wise i think you need a big TP5690 or the best for big boats lehner 3080(but they don't come cheap)

[Crumri4]

Thank you for this input. I looked up lehner motors online and found that you have to fill out a chart to get your motor built to your specification. So I went back and started looking through the equations that I have dug up over the years that I have been putting pieces together on this boat.
One equation that jumped out at me was that electric boats need 150 Watts per lb of boat. If that is correct then the motors I have are 7457 watts each. With that in mind each of these motors should be able to push 50 lbs of boat. Since I am
Using two motors this means I have enough to push around 100 lbs of boat. Though I have not weighed it as of yet, I can easily carry this boat under my arm with one hand. I would be shocked if the boat weighs even 50 lbs. Am I mistaken for using this equation for determining the amount of power that is needed? If so, what factors do I need to determine in order to order new motors for the boat?

[NativePaul]

Take what I am about to say with a pinch of salt, as while from my experience W/KG scales well for performance from 7" to 42", and I fully expect it to scale to 6', 42" is my biggest boat and I have no first hand experience of anything close to 6'.

Firstly sorry to offend you if I am stating the obvious here, but I have to ask as you are talking about your boat's weight and asking for purchasing recommendations for motors and batteries (so I am assuming that neither are in the boat at this point). You know W/lb is based on the RTR weight, not the weight of an empty hull right?

You can run 150W/lb in an FE boat, but I would consider that a minimum, I personally run 500W/KG (about 230W/lb) in most of my boats which are pretty efficient racing designs, and I think the typical boat on here is running considerably more than me.

You certainly won't be getting anywhere near 78mph on 150W/lb. I haven't seen your hull, but I would guess at the mid 40mph range from a typical scale offshore cat on that power. I expect abut 55mph from a flat ride pad stepped race cat at 500W/KG.

Motor and ESC power specs are the highest power they think their product can take without blowing up, which is generally well past its maximum efficiency range, I generally run my motors and ESCs at about half their quoted maximum power for more efficiency, cooler running and a longer lifespan.

[Crumri4]

Thanks for your input. I am planning to get a weight on the boat tonight in order to more accurately get feedback from everyone. I do need to clarify that I was mistaken in saying the props are 5 bladed. They each have 4 blades not 5. I am not sure if that is significant or not, but for the sake of getting the facts right, I thought I would mention it. So based on your input, I came up with some new figures based on the equations that I found previously. If my Castle motors are rated at 7457 watts each max power and 36 amps max power, but I run them at half the rated power as you suggested, I would have a combined total of 7457 watts with dual motors. each motor would be run on 4s Lipo which would be 14.8 volts per motor which are 780kv. This would result in 11,544 rpm per motor. At 4.49" pitch ( figuring 80% prop slip) I would be expecting 39.27 MPH Is this correct? Furthermore, cutting the power in half like this would result in an over all power output of 7457 watts so with your suggestion of 230 watts/lb then I should be able to push a boat, fully loaded, weight of 32.4lbs Is that correct? I have not weighed the boat as of yet, but I do have the motors and drive lines as well as the props and rudders inside the boat. I need motor mounts struts and the batteries and radio equipment (servos and receiver) to complete the set up. I realize the batteries will likely add a significant amount of weight, but that is also what I am here looking for recommendations for. If the boat ends up weighing more than 32.4 lbs then I believe I have some room to add more Lipo cells to adjust, which should result in higher rpms as well as faster speed. I will likely try to find an electronic motoring system to see what kind of amps i am drawing and how much heat is being generated.

[Crumri4]

The boat weighs 33.8lbs with the motors, speed controls, drivelines and props, and rudders. The batteries will add to this weight as well as motor mounts and struts which will all be aluminum.