HI Edward
That would explain the prop size suggested. For performance brushless is the best solution. if you can build the model to be lightweight then you may not need very large motors. Regarding weight its the extra bits you add that can really add on to the overall weight. Light batteries will also help and whilst a large SLA will provide the power it comes at a heavy weight cost, plus the sheer size can make selecting the best position in the model difficult. Any weight forward of the motor will have to be lifted by the model before it will get on the plane and this is often the reason why some boats don't perform well.
You do not need to use racing props and for scale appearance 3 bladed brass props look the part and will perform well at scale speed. I would be looking at 40mm /- 5mm as a starting point 😀

HI Dave M

Thank you for your detailed and helpful reply. The model is the Thornycroft Range Safety Launch (Nexus plans MM412). These plans are fairly old having been drawn by Vic Smeed. They show a conventional twin electric motor (or single 3.5cc diesel engine). it just struck me that by using brushless I would end up with a modern set up and a lighter powerplant with more power too. Currently it is at the planning stage but I like to get fundamental decisions made before I commence construction.

Edward

HI Cenbeth
As advised above brushless motors are rated by Kv. However they also have a wattage rating (amps x volts) and this needs to be observed unless you want to destroy the motor.
Is this a scale model you intend to build? I ask because you quote 58mm props and this will need large (expensive) motors and ESCs. Could be the advice is for a brushed motor setup.
If you look through this site at other members models of Crash Tenders you will find several working examples and you would be well advised to use a similar setup.
Reverse with brushless ESC's is possible but usually requires the use of a stick with a central off. You have to stop the motor select reverse for a second then select reverse again, when hopefully the motor will run backwards. The ESC's were developed for model car racing and the initial selection applies a braking effect.
If you intend to have a very fast running model you will more than likely be using high wattage and water cooling of the motor and ESC will be advisable.
Marine ESC's are available but make sure it is reversible and as Ian advises a least 50% higher capacity than the max motor current.
Whilst in-runners may be easier to cool I use outrunners which can be cooled via a special mounting cooler. Outrunners provide a big flywheel effect due to their construction and I believe give better performance in a large model boat. I have seen in-runners used in smaller model (30") and they worked fine.
As you may now realise this is not going to be a cheap option and as brushless are power hungry you will need good batteries. Top of the range are LiPo's but you need to manage their use and charging in accordance with the manufacturers guidlines. Special charges are a must and your ESC need to be programmed to prevent any over discharge. NiMh batteries will give good results but will not last as long for each sail.
To get the best performance you really need to match the motor/ESC to the props and a useful tool is a wattmeter which will help you estimate how much power different set ups use. As I mentioned at the start you should not exceed the max motor rating - basically bigger coarser pitch props = higher current.
When you find a setup you think would suit I suggest you ask the member concerned for further advice.
Good luck, and please post details of your final selection 😀

Hi,

Thank you for the advice, it is very helpful. I can see that this subject is one fraught with Imponderables. Looking at eBay I have seen so many brushless motors and trying to make sense of them has now become a little less daunting!

The plans call for two 58mm dia props. I assume that these will have to be racing props as opposed to scale ones. Should I be looking for smaller props?

Edward

Hi,

Thats a tough one, there is, unfortunately, a little bit of guess work involved in this.

But, a good place to start is, what size prop will you use, and what revs do you want out of the prop?

Brushless motors are rated in Kv which means that if a motor is 1000Kv, it will spin at 1000rpm for every volt you supply it with. So, a 12v battery will give this motor a theoretical speed of 12000rpm. Now, taking into account speed reduction induced by load this may give you approx 10000rpm for a submerged drive.

Next comes the diameter of the motor. if you want to spin a 50mm prop at these revs you will need a motor diameter approaching 50mm to give you the torque.

And next is the length of the motor, the longer it is, the more power you will get from it, and, the more it will cost.

These sizes are expressed in ads as: 1000kv, 5060 motor where 50 is diameter, 60 is length (mm).

Now, inrunner or outrunner? I would recommend inrunner for this application as it allows better cooling options - and you WILL need to cool this type of motor.

And then there is the esc... you need a specific brushless suitable esc - do not try and use a dc esc it just wont work. Now, what size esc? When you choose your motor, look for the max amps the motor is safe to run at, add at least 50% to that and that is the size esc you need. (if you are buying from China, double the motor amps for the esc, they often overstate the capacity of esc in an attempt to sell more).

You can get reversible brushless esc's, but you will pay more for them.

Hope this helps.

Good luck

Ian