ABC Motor selection

[Gary_Scotland]

Hi, this is my first post in here so be gentle...

I am about to embark on my 2nd model boat project but admitadly the last one was a few years ago. Im seeking advice on motor selection. Is there a calculated method for doing this or is it just have a play around untill you get it right sort of thing? Also if anyone can point in me in the right direction for places to buy motors and related parts too would be highly usefull.

The model im building is of my local(ish) lifeboat of the Severn class. The model will be around 1500mm in length. I intend on using scale props which will be 75mm i think. Im looking for enough performance to reach scale top end speeds + some more just for fun :)

Thanks,

Gary

[johnmca72]

Hi, this is my first post in here so be gentle...

I am about to embark on my 2nd model boat project but admitadly the last one was a few years ago. Im seeking advice on motor selection. Is there a calculated method for doing this or is it just have a play around untill you get it right sort of thing? Also if anyone can point in me in the right direction for places to buy motors and related parts too would be highly usefull.

The model im building is of my local(ish) lifeboat of the Severn class. The model will be around 1500mm in length. I intend on using scale props which will be 75mm i think. Im looking for enough performance to reach scale top end speeds + some more just for fun :)

Thanks,

Gary
Yes, there are equations, but they're pretty complex & it really ends up easier to use the "trial & error" method. Hull shape comes into play, as do drive train, propeller size, blade count, blade area, blade pitch, propeller speed, number of propellers, desired speed, etc.

For example, if you want to push a half-submerged peach crate through the water at 5 kts using 2 @ 2" dia. props of some pitch, blade count, blade area it might take the same motor that drives a 1.5 m "cigarette boat" at 20 kts.

IOW, there are lots of variables to consider!

JM

[pompebled]

Hi Gary,

Like JM mentioned there are a lot of variables, but in view of the size of the model (big!) you could use two fan motors (the ones turning the cooling fan in front of the radiator in your car), source: scrapheap.

These motors have a lot of torque and will spin your 75mm props without hair raising current draw.

Another option would be starter motors (automobile, not nitro engine starters) , which have even more torque, but generally less rpm and usually a higher ampdraw than the fan motors.

Depending on how fast you want to go, you can run these motors all day on 12V, quicker, but shorter on 18V and even faster (but even shorter) on 24V without having to do anything but add watercooling to the brushtabs and can.

It's even possible to run these motors on 36V, but this involves rewinding the armatures with thicker copper wire and closing the nuts in the armature to prevent the windings from extiting the armature due to centrifugal forces.
I don't think this would apply to your big model as these modifications are more for planing hulls such as MTB and resque launches.

This is a very economical way to run such a big model.

There are always the big Plettenberg and Lehner motors, both brushed and brushless, enabeling you to get to 'ludicrous' speed and a huge hole in your wallet...

Regards, Jan.

[Gary_Scotland]

Thanks pompebled & johnmca72.

I like the look of the fan motor idea. I believe its easy enough to add water cooling to the fan motor? When you speak about the increased voltage does that apply to the fan motor aswell? Aaaaaand 1 more Q - just hook the fan motor up to the propshaft in direct drive?

Im not looking for extreme speeds just a couple of knots above the scale speed as its always more fun when you have that little bit extra:)

[pompebled]

Yes aaaand yes.

The fan motor will run on a higher Voltage without problems and the torque is sufficient to turn your prop in direct drive, making the drive train smooth and quiet.

Adding watercooling is quite easy, as everything is a bit bigger with these motors.

The cooling coil in the picture is a piece of copper piping, I plan to change that for a cooling jacket that allows the water to flow over the entire (waterproved) can surface for better cooling (as I intend to run it on 20 cells / 24V).

I can send you pics of how to make such a jacket out of sheet brass, PM me your mailadress if you're interested.

Regards, Jan.

[Gary_Scotland]

Excellent! I hope i can source 2 motors which are the same so i dont have problems with 1 motor being more powerfull than the other. Im also having a hunt around for electric scooter motors. They seem to be cheap also and they are all around 2500 - 2700rpm (is a car fan heater motor around there too?) I like the idea of the scooter motors as its going to be easier to source 2 which are the exact same

[pompebled]

Generally, car heater motors don't tend to be as powerfull as the fan motors in my experience.

It's not that difficult to find two identical fan motors at the scrapheap; you'll find that many carbrands use the same motor on different car types, as the required radiator has the same cooling capacity.

Having a ballbearing at the front helps, if the motor's have two, it's a bonus.
Keep in mind you'll never run the motors as hard and long in your model as they have been in the cars.
Besides that, most fan motors have replaceable brushes, so if nothing burns out or severely overheats, they'll last an eternity.

Regards, Jan.

[Gary_Scotland]

To be honest i thought the fan motor and the heater motor were the same thing.... Cars is somthing ive never involved myself in.....

Are you able to reccomend which cars to look out for when im at the scarpyard?

[pompebled]

I'm not very familiar with the cars that reside in your local scrapyard, but every motor of 1600cc (and more) has a fan motor to cool the radiator in the appropriate size.

Regards, Jan.