halvorlu

Hi. I need some help to choose motor, ESC and prop for a boat I'm building. It is about 1.2 by 0.5 meters in size (about 4 by 2 feet), and it will run at quite low speeds. About 1-1.5m/s (about 2 knots) is what I'm aiming for. The battery voltage will be between 5.5V and 8.4V.

I've been looking for a suitable brushless motor (for high efficiency) at hobbyking.com, but I'm having trouble finding one with low enough Kv number. With a prop with pitch around 25 mm and speed at 1m/s, my calculations tell me I need Kv around 300. There seems to be very few motors with such low Kv numbers. So what's the best way to solve this?

[*]Lower propellor pitch, allowing higher RPM and Kv?[*]Gearbox?[*]Any other stores (than hobbyking.com) selling suitable motors?[/list]I'm greatful for any hints or tips!

Tony Fitzpatrick

[*]forward and reverse esc ?[*]any reason,apart from efficiency, to go brushless?[*]Have you built an rc boat of this size before with brush motor? yes/no[*]weight/displacement?[*]What is your expected battery capacity in amp hours?[*]Why 5.5 to 8.4 volt?[*]Is cost of no importance?
[/list]I know you want answers but this saves a lot of time.

brodjack

Heres a low kv motor,which would turn slow on your voltage.
This is a great reliable site to buy from.
http://www.giantcod.co.uk/xyh5055-58...-p-403285.html
hope it helps

NobodyFamous

be carful choosing a low Kv motor, as they are typically designed to run high voltages for speed.  They may be unreliably and jerky at slow speeds.

halvorlu

Tony, thanks for your questions. My answers:

[*]I don't need reverse on the ESC[*]It's important with an efficient motor, but it doesn't have to be brushless[*]Haven't built any rc boats before[*]Weight will probably be between 5 and 10 kg[*]Battery capacity will be around 20 Ah[*]I will have two cells of Li-ion batteries, which gives a voltage range from 5.5 to 8.4V.[*]Cost is less important than most other factors [/list]The batteries will be charged by solar cells, giving a limited energy supply, which is why motor efficiency is important. The solar cells also make it necessary to have exactly two Li-ion cells.

Tony Fitzpatrick

Halvorlu,those last two sentences are relevant.
The quality and relevance of any hints and tips are going to depend on the information you provide.

What is the aim of this project?
Can you provide any technical information such on charger circuit,solar cell information,any existing lipo battery management or can that part be changed.
Can you change propeller,battery,hull shape,hull displacement?

halvorlu

I'll try to answer your questions as best I can.

The aim of the project is to create an autonomous boat with solar cells as the only source of power, with batteries (charged by solar) for night-time operation.
The charger circuit is essentially the LTC4008 reference circuit, and the batteries/solar should be able to deliver 40Wunder normal operation, more (up to 100W) for short periods if needed. The voltage delivered by the solar/battery system will range from 5.5V to 8.4V, depending on solar flux and battery charge levels. The power system cannot be changed, although additional battery capacity can be added if needed.
The overall hull dimensions cannot be changed, although slight changes to its shape can be made. The propeller is still an open choice, and we are looking something to match whatever engine we end up with.

Hopefully you have some hints for us. Thanks in advance.

NobodyFamous