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[scimitarjohn]

  I inherited a hull from a good friend, quite heavy GRP construction, IC powered so single prop.  I decided to use the hull to make a replica of seaplane tender ST206 in FT1 fire monitor guise, I know it should be twin prop but hey ho. So in went a 540 12,000 rpm motor and on went a two blade 40mm prop, so far so good, 7.4volt battery gave reasonable performance, after finding out how important cable size is!   But... I wanted the model to plane like the real thing, so in goes 11.1volt battery, WOW performance for 5sec then tailed off, back to dry dock and motor red hot, Ooops.  So back to the drawing board and here are my calculations... ST206 had 2x100HP= 200HP   1/12 scale cubed gives 1/1728 of the power so 200 divided by 1728 = 0.116HP or 86Watts .... no wonder the 540 got hot under the collar!  at 7.4volt 86Watt would take 11.6Amps.  Hope you've followed so far, figures are open to conjecture and would be rounded up but hopefully I'm in the right ball park.
 
   So... what I'd like is recommendations/observations on suitable brushless (probably) motor, ESC and maybe change of prop size.

 Sorry, wanted to add photos but wouldn't work?

[Andyn]

Power, like water, does not scale. 40mm can be too much prop for a 540, too. 86w in a model boat is nothing, pretty sure a radar motor draws more


I’d say something in the region of a 3660. TFL’s SSS 3660 2400kv would give you plentiful power reserves on a 3s lipo, perhaps even a 2s pack. Peak draw it’d be pulling 1800w


Noah’s RC Ark has a good selection - [size=78%]https://www.noahsrcark.co.uk/categories/marine-motors-escs/131444000000817065[/size]

[canabus]

Hi John
A 2400kv on 7.2 volts is 17280 rpm unloaded !!!!


A 1100kv on 7.2 volts is 7920 rpm unloaded, but on 11.1volts 12210 rpm unloaded.


A 1450kv on 7.2 volts is 10440 rpm, but on 11.1 volts 16095rpm.


The sum is voltage x kv equals unloaded rpm.
The longer the motor more power.


If you get an ESC with handles 2 to 4 S Lipo batteries and the wattage this will give so play room.


Canabus

[Andyn]

Most 540 motors are around 15,000rpm no load on 7.4v, the left stick exists for a reason 

[canabus]

Hi All
540 brush motor specs from a rock crawler site.


All running 7.2 volt batteries.
All rpm unloaded.


13 turn 32,500 rpm.


21 turn 20,000rpm.


35 turn 13,000rpm.


45 turn 9,500 rpm.


55 turn 7,700rpm.


65 turn 7,000 rpm.


80 turn 5,500 rpm.


The 80 turn I use in my Eileen and Hellen fishing boats with a 60mm 3 blade prop. straight drive.


Two in my Smit Nederland setup with tank steering.


I trim the throttle to 70% forward and have good power with a bit over scale speed.


Canabus

[scimitarjohn]

  I inherited a hull from a good friend, quite heavy GRP construction, IC powered so single prop.  I decided to use the hull to make a replica of seaplane tender ST206 in FT1 fire monitor guise, I know it should be twin prop but hey ho. So in went a 540 12,000 rpm motor and on went a two blade 40mm prop, so far so good, 7.4volt battery gave reasonable performance, after finding out how important cable size is!   But... I wanted the model to plane like the real thing, so in goes 11.1volt battery, WOW performance for 5sec then tailed off, back to dry dock and motor red hot, Ooops.  So back to the drawing board and here are my calculations... ST206 had 2x100HP= 200HP   1/12 scale cubed gives 1/1728 of the power so 200 divided by 1728 = 0.116HP or 86Watts .... no wonder the 540 got hot under the collar!  at 7.4volt 86Watt would take 11.6Amps.  Hope you've followed so far, figures are open to conjecture and would be rounded up but hopefully I'm in the right ball park.
 
   So... what I'd like is recommendations/observations on suitable brushless (probably) motor, ESC and maybe change of prop size.

Finally managed to post images, easy when you know how.

[scimitarjohn]

Power, like water, does not scale. 40mm can be too much prop for a 540, too. 86w in a model boat is nothing, pretty sure a radar motor draws more


I’d say something in the region of a 3660. TFL’s SSS 3660 2400kv would give you plentiful power reserves on a 3s lipo, perhaps even a 2s pack. Peak draw it’d be pulling 1800w


Noah’s RC Ark has a good selection - [size=78%]https://www.noahsrcark.co.uk/categories/marine-motors-escs/131444000000817065[/size]

Many thanks for the info, I used the scaling system as it's the only one I know, it's what, until recently, hull designers use from test tank results but in this case scaled down rather than up.  Thanks for the motor recommendation, just one question, if it may draw 1800W at 11.1Vdc that would mean 162Amps? rather a lot.  Sorry, questions, questions,  any idea of a suitable prop size?

[scimitarjohn]

Hi All
540 brush motor specs from a rock crawler site.


All running 7.2 volt batteries.
All rpm unloaded.


13 turn 32,500 rpm.


21 turn 20,000rpm.


35 turn 13,000rpm.


45 turn 9,500 rpm.


55 turn 7,700rpm.


65 turn 7,000 rpm.


80 turn 5,500 rpm.


The 80 turn I use in my Eileen and Hellen fishing boats with a 60mm 3 blade prop. straight drive.


Two in my Smit Nederland setup with tank steering.


I trim the throttle to 70% forward and have good power with a bit over scale speed.


Canabus

  Many thanks, I think I've got an idea of motor size now.  Obviously my 'Rescue Launch' needs to go a tad faster than a Tug or fishing boat for scale so have you any advice on prop size please.

[canabus]

Hi John


Brushless motors are far more powerful .


E.g. Banggood Surpass Hobby C3542-1250kv (ESC 65AMP on 4S) 850Watts.
       Banggood Surpass Hobby C3542-1450kv (ESC 78AMP on 4S) 1050Watts.
Both would run your 40mm prop on 2/3 S Lipo batteries.
Same for A Hobbyking 3648-1450kv(ESC 100Amp on 6S) 1600Watts.
The Hobby king motor on 4S in a MFA Spearfish( 39")on a 40mm Prop will hit 37KpH !!!


Canabus

[chas]

Hi John, if you can tell us the size and weight of your model, people on here can give you some ideas based on models we run. The weight but makes a big difference, my crash tender is much lighter than a mates model, both use similar motors et c but the performance is very different.
Charles.

[scimitarjohn]

Hi John, if you can tell us the size and weight of your model, people on here can give you some ideas based on models we run. The weight but makes a big difference, my crash tender is much lighter than a mates model, both use similar motors et c but the performance is very different.
Charles.

  Hi, good point, the glass fibre hull was made on a mould by my friend as a school project (yes these things were allowed, even encouraged, back when we were kids) so is quite hefty.  The boat, as is is now, comes in at almost 5kilo all up and is 35" (890mm) in length. Hope that helps with anyone's recommendations.

[Fastelectrics]

If you want to keep a brushed motor set up, the Graupner 700BB turbo is a good upgrade from a 540 and can easily handle 3 cells.

[chas]

Hi again John, thanks for the info, that's quite a hefty model. Your 5 kilos for about 900 mm makes double the weight of most of my fast scale, but I do have one model that's broadly similar.
  First I suppose I should say that different people have different ideas about how fast a fast scale boat should go. You can see vosper crash tenders and the like going like rockets, I'm not like that,  aim for realistic looking performance on the water with about 20 % extra in reserve.
  My heaviest fast scale weighs 4.5 kilos and is 930 mms long but it's a Gar Wood Speedster special with a different hull form from yours but I think the set up would be fine for you. I use a 3536 1200 kv brushless on a 3s lipo and performance is blooming excellent. I chose to use a 35 mm prop although I could have used a bigger one, this keeps the amps down but still produces plenty of speed. The esc is a 70 amp one although I've never got anywhere near that in sailing it.
  If you choose a similar set up, you'll have the option to use 2s or 3s lipo and different props to get whatever performance suits you.
 I don't know if you've used brushless motors before, they were an eye opener for me, a bit like a modern car compared to a vintage one and I'd strongly recommend using one.
 Hope that helps, charles.

[scimitarjohn]

   Cheers Charles, spent a little while this morning cogitating, think it's allowed when you get to my advanced years, and I've come to about that conclusion with every ones help.  Probably a 3650 1500kv with a 80A ESC and then try different prop sizes, but then again...  The boat was going pretty damn fast with still some way to go to get on the plane with the 540 and 3 Li ion.  With it's weight I'm worried about just how fast it'll need to go before it get's on the plane never mind levelling out, the wild fowl on our clubs lake are gonna love me.  Think I'll go with the brushless, have some fun, and might end up making a lighter hull or maybe another boat altogether, now there's a thought!

[chas]

I'm afraid that making another boat, and then another etc etc is a certaincy.   I'm 70 and been making boats for 60 years, probably over 80 of them and the ones I still want to make goes on and on.  Hopefully when my time comes I'll be round face down at my bench with some wood glued to my nose😁
Charles.