Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[croakle]

Can anybody tell me if there are any rules of thumb for motors, etc in scale boats?  I'm thinking of tugs mainly.

What is the maximum revs which should be used?  And does the max revs change as the scale changes?  Is it better to use a high revs motor with a gearbox or a low revs motor without?  Or can you use a high revs motor and control it with the ESC?

Thanks and best wishes

Julian

[Shipmate60]

Depends on the size and pitch of the prop.

Bob

[croakle]

Bob

The diameter is 62mm, think it should be 4 blade but not sure as drawing is not clear, need to do some research.

Is there a standard pitch?  If I ordered a 62mm from prop shop would I need to specify a pitch?

Thanks for the help.

Julian

[A Model World]

Generally for tugs the pitch is fairly standard (its racing and other types that need to watch for pitch) especially on a 4 blade. As regards revs well this is a whole different ballpark as you will need an amount of torque as well this depends on the weight of the boat and what style of drive, is it just propeller or kort nozzle etc.   The all up weight is one of the key factors you will need to estimate. What is the boat called, see if anyone has done one before, if you are working to plans or a kit. You can always leave props towards the latter of fitments getting the motor/s installed is prior 
Hope this helps   

[Shipmate60]

A big prop, so I hope it is in a big Model.
For power and torque I would go for a geared motor such as the MFA 6:1
OR
Mobile Marine Models do a car fan heater motor which would fit direct drive, and they do specialise in large tugs.
Or have a chat to Alistaire in Model Motors Direct, he has some good large low revving motors.
Speed will depend on size and all up weight, but the above should give you something to think about!!

Bob

[Shipmate60]

Revs arent crucially important as you have the option of changing voltages and turning down your speed controller if it has that facility.

Bob

[johno 52-11]

Julian

I try and work on a conventional prop doing no more than 6000 rpm. Are you building a tug if so is it a kit or scratch. If its a kit then there is probable a well known set-up that works. If your trying to power something that on one has ever built before then from experience you need to experiment a bit. Are you after speed or pulling power. The main object of the exercise is to turn electrical power into thrust as efficiently as possible. Most good motors have an efficiency around 70% so at 12volts and 10 amps would be an input power of 120 Watts and an output power of 84 Watts. Every thing that is between your motor and your prop will need to use some of that power. If you put a gearbox in that could take up 10% of that power misalignment on couplings would take power even the bearings in your prop shaft will take some of it.

The best advice I could give is to get your hull to a point of fitting the props motors and rudders weight it down to its water line and try it. If you need to make changes to motors its easier than when you have finished the boat.

Have you any more details on the boat size weight scale. Is it single screw or twin screw.


Car fan motors are quite good as they run between 3500 and 5000 RPM (free running) you will get about 80% rpm under load. They can also take voltages higher than 12V

Have a look on here its about Bosch motors that are used in cars http://www.boschmotorsandcontrols.co.uk/

John

[croakle]

Thanks very much to all for the input.

Bob, not a particularly big model.? ST Cervia (previously Empire Raymond) at 1:48 with OAL 28" and beam about 7".? The drawing shows 62mm prop and says "scale prop and rudder shown".? Single prop, no Kort nozzle.? Not sure of weight but am waiting for delivery of Model Boats plan service hull.

John, I had not thought about weighing down the empty hull to check the displacement.? I will do that when it arrives.? You can probably tell this is my first attempt.

I understand Cervia is modelled a lot but have not been able to find much info about what people have used.

I do like the idea of direct drive for the simplicity but assume there would be more torque when using a gearbox.? What are the thoughts on 6 or 12 V?

Thanks again and best wishes

Julian

[croakle]

John

Just been looking at the Bosch link you posted.? The choice is a bit mind boggling!? What Amperage and torque rating do you go for?? Do you fing it easy to couple to the prop shaft?

Thanks and best wishes

Julian

[Shipmate60]

Not being funy, but such a small model and such a large prop will only need up to max 3000 rpm, if that, I would go for direct drive motors from Model Motors Ditect.
Pity she aint twin screw, I got 2 ideal motors in my draw.
There wont be a problem with the motors, she will draw a couple of amps max on the 6:1 reduction box from MFA, but noisey.
Try Alistyair at Model Motors Direct, he does some slow revving multi-pole motors ( 11, I think ) which draw almost nothing.

Bob

[croakle]

Just been sent a picture of Cervia on a slipway which confirms prop is 4 blade and diameter in the order of 10' so looks like 62 mm for the model is correct.

Can anybody recommend a prop shaft?

Best wishes

Julian

[Ghost in the shell]

give barry at westbourne models a call, he will point you in the right way :)

as for props, id a thaught the smaller a prop is, the faster it would have to spin because of the smaller blade area, so less thrust comming off it

[Doc]

Ghost',
Wish it were that simple.  The diameter of the prop does determine how much thrust it furnished, but only to a point.  The pitch of the prop is also a factor, "how 'tilted' are the blades?", sort of thingy.  A very flat pitch moves almost no water, no matter what it's diameter, sort of.  A 'feathered' (as in airplane props, blades straight fore/aft) prop of sufficient diameter can flip your boat over.  That's an exageration, but gets the point across.  All props do not have the same pitch.
 - 'Doc

[Shipmate60]

Croakle,
That is one big prop for a small tug, I had the same problem with a "Girl" Class tug, quite a small hull but large prop.
Even on a Deccaperm Geared motor I could get her "On the Plane".
The 6:1 geared motors can be quite noisey, but Model Motors Direct do some large slower running motors.
If you got a 12 volt motor you could run on 6volts for free running, but couple batteries in series to give 12 volts for towing.

Bob