Prop size and RPM selection

I am building my first boat.  Have been building and flying RC aircraft for many years.
The boat is a "Dominator" kit from Statos Models, Bungay, Suffolk, UK.  Probably from the seventies.
It is a cabin cruiser type with fibreglass hull, 34 inches long with 10 inch beam.  designed for 0.19 to 0.40 marine IC engine.  I will be using an electric brushless outrunner.  Would appreciate any suggestions for propellers, prop max RPM and ESC with reverse.
Considering using a Turnigy SX4250-650 motor, because it is just collecting dust!
Newboater

Hi new Boater, from another newbie, welcome to this friendly place, I'm sure someone will come along and give you an answer soon

Thanks for your response Paul, no suggestions yet.
Carl

Hi newboater, firstly what's your name

Is this the motor in question ?

Here is the spec. for it:-

KV(RPM/V): 650
Lipo cells: 4s
Max Surge Watts(W): 240
Working current: 13 amps
Max.current(10s): 17 amps
No Load Current: 0.7 amps
Intemal Resistance: 0.120 ohm
Number of Poles: 16
Dimensions(Dia.xL): 42 x 25 mm
Shaft: 3 mm
Prop: 10 x 4.7 - 11 x 4.7
Weight: 65g

Not sure what the revs bit means, is it 650 per volt, is that how it works ?

If so it must depend on what battery  voltage you are using, am I correct ?

I must admit I would like to get my head around this subject too.

Your thoughts please

troutrunner wrote:

Not sure what the revs bit means, is it 650 per volt, is that how it works ?

Paul,

You are correct. Of course this is the no-load speed. This is quite slow for brushless motors, and with a 4S LiPo (14.8 volts) only gives 9620 rpm. I have been using 1200 to 1400 KV motors in my 'fast' boats. I usually use a 30 or 35 mm 2 bladed plastic prop and either 7.4 or 11.1 volt LiPos. They are plenty fast enough for my reaction times .

Barrie

So any thoughts of the direction our new friend should go ?

As its a Broads cabin cruiser where there is low speed limit, then I would not recommend a brushless motor as they are very good at low speeds, assuming he wants scale speed. However if he really wants to use this motor, then it should be OK. I would imagine that the current draw at the likely speeds he will achieve will be less than the spec, and a 30 amp ESC would be more than adequate. Hobby king have suitable marine ESCs.

Barrie

Thanks for the input so far.  The SK4250-650 is simply a motor I have doing nothing.  With a 6 cell Lipo the unloaded RPM would be 14,000.
This boat will be used for relaxation on a small pond, so speed is not an issue.
I have tried to attach pictures without success!  They are on the Hosting site, but then what!  It is not only boats about which I am ignorant!
Carl (Newboater)

Right after sending my last post I realized that the motor will not take a 6S Lipo - only 4S, bringing the max RPM down to 9,700.
Carl

Carl

I think that 9700 is adequate. In any case there are no rigid formulae for calculating prop sizes etc. Its a matter of experience and trial and error and depends on hull shape etc. You already have the motor, so  it has to be worth a try. Buy an minimum 30 amp ESC, some 30, 35 and maybe 40 mm plastic props and see which suits you best. You can always buy a new motor - the ESC and prop will be OK. I don't know of any scale type boat like yours which uses more than a 3S LiPo. Bigger batteries are usually used for fast electric boats.

This ESC from Hobbyking's UK warehouse would be suitable. They usually 30 and 50 amp models, but they are on backorder in the UK. If you are happy to buy from Hong Kong, the 50amp would be suitable and have more than enough capacity for your motor.

Barrie

PS Check out this post re photos on the Forum https://www.thercmodelboatforum.com/t93-posting-photos

Hopefully this will link to the picture of my fiberglass boat hull - 36" long by 10" beam.
Carl (newboater)

Hi Carl,

Good to see you have worked out the photy thing  and Barrie has put you in the right direction, good luck and keep us informed please perhaps you would put up a thread on your build for all to follow