Hi all,

m building my first rc boat.

The model has following parameters.

1. Weight - 6 kg
2. Hull length - 1.3 m.
3. Max. width - 0.3 m.
It has dual motor set up.

i need your help on following issues.

1. How to decide motor size? i am considering brushless motor but, don't know what rating of motor i should use for proper speed, power/weight ratio.
2. Would it be optimal to use gear box? if yes, what gear ratio i should use?
3. How decide correct propeller size - diameter and pitch to get required speed (in dual set up)?

I have tried lots of googeling but not able to find out appropriate content. I would be greatful if anyone can guide me on this or provide any external link to understand the basics.

Thank u,

viru

I think your boat is little overweight compared to the size, anyways for contra-rotating & dual drive system 300 ~ 400 watt motors will be fine.
http://www.hobbyking.com/hobbyking/store/__14622__ADS300XL_Water_cooled_Brushless_Outrunner_3000kv_400w.html
The KV rating is 3000; 35 mm size propeller will work fine.

Post some pictures. I will update you regarding contra rotating propellers.

Viru, from your dimensions, its sure its a big craft. and i do not know what material is used to make the hull. if you use balsa wood, the weight can be reduced. and if you use thermocol to make hull, the same sized one will be less in 500 grams, including the putty finishing. now your craft is completed. if you are able to buy, expensive stuffs, you can buy big, water cooled BLDCs, rated at 6000 RPM /Volts. and ESC for that. so you need some good battery packs, like LiPoly, in to 10000 mAmps. and if you are in to inexpensive root, not meant "CHEAP", you can buy two can type brushed motors, rated at 12 volts, within the ratio of 3 to 5 amps, medium RPM, with high torque, and can make your own ESC, and need only a 1800 or 2200 mA LiPoly battery pack, will run for a long time, and also can control each motors independently, with both left and right stick, as forward and reverse, left and right, all with motor controls alone..so can avoid rudder. or can fix two motors in same line, as the 3 rd channel works as throttle, while the right side sticks acts as forward and reverse controls, while the left and right. you can also split the RF cycle of your radio system, to add a switch to get forward and reverse modes so the stick operation can be avoided. if you are looking for moderate speed you will get a long running time, and if you are looking for a high performance, you need a high rated set ups. any how, after a particular speed the cavitation will start to play, wastes a lots of motor power yes..the battery..

you can make your own propeller, from an aluminum vessel lid, up to 2 inches is far enough, can make in to clock or anti clockwise, so, you can adjust the pitch, of the prop, by trial and error method, so a good performance will be achieved. a flat bladed low pitched prop will be good for high speed craft, as it avoids the cavitation, while a narrow bladed, steeply pitched prop is suitable for slow running crafts.

you can cool your ESC by water cooling is easy, and yes motor. but if you use high rated ESC, made your own, it will not heated up, so you can avoid water cooling. still you can simply cool down your motor in an easy way...have a nice craft..

see here to make a prop..and motor for the bigger craft..

and the motor shaft setting. this motor drops only a little current even at full throttle..works nicely..

hi,

many thanks for your replies.

I have made some calculations dont know whether they are right as m not d electrical, mechanical or electronic guy.

Will describe you step by step - please correct me if m missing something.

1. Current weight of the craft is - 1.92 kg. i am expecting it to be 5-6 kg when completed including motors, equip, battery etc.
2. The quarter beam buttock angle is in the range of 3 degrees to 7 degrees. The hull type is semi displacement.
3. The maximum speed-length ratio comes out to be 1.5.
4. Length at water line (LWL) = 4.25 feet. Thus, maximum speed limit would be 3.1 knots. I have made this calculation on following link -

http://www.psychosnail.com/boatspeed/boatspeedcalculator

5. Power required for displacement of 6 kg vessel at 3.1 knots speed is 0.03 hp (22.38 watts)
6. I have decided to put 2 nos. of 40 watts motor.

7. Prop size calc - I have calculated prop size using an excel sheet on following link.

http://www.electricboats.co.uk/surfprop/Spreadsheet/spreadsheet.html

8. Selected motor size

- 2 nos. - 40 watts @ 16000 rpm
- gear ratio - 2:1
- Propeller size - 37 mm
- Propeller pitch - > 25 mm
- Propeller rpm - 8000

I am not sure about required torque. but, in excel sheet it seems ok. Currently working on battery size.

I want to configure internal set up before completing planking.

Thanks

Viru

Whats the material of the hull?  Seems to be sunboard cut with handsaw..

I never did so many calculations choosing motors and propellers, thanks for explaining the equations and boat speed calculators.

The hull looks/similar to the mistral yacht. Correct me if I am wrong. I built similar boat of length 4 feet. It weighed aprox. 5.6 kgs, only the M-seal figurine weighed about 600 gms, I planed to sail the boat without the figurines, but later sailed with figurines. I used brushed motors and 30 Amps brushed escs with 35 mm contra propellers.

Again your hull looks very convincing; I hope you will bring a nice finishing to it.

All the best.   

used 8mm thick plywood for frames and chill wood strips 8-10 mm wide & 4 mm thick for planking. Made it in a workshop. If u are making a boat i suggest MDF board instead of plywood. u can cut it smoothly specially in curves. Plywood may have porous areas.

@ Sb_Maharana

yus its Mistral, got those plans from freeshipplans.com and also come across with your youtube postings named with aaditya science academy. It helped a lot. thanks.

I am planning to have different cabin and deck design than mistral. will update u wid progress.

Viru

Hi all,

i have got two DC brushed motors each with following specs:

RPM - 9000
Torque - 408 gm-cm
Current - 5.44 amp
Voltage - 12 V

I need your help on following issues:

- The drive shaft - connecting motor with propeller. How can i make it?
- and what size of battery i should install?

Thank you,

Viru

amazingly build ...... 

is this build completed??