Sizing motor to propellor size

[Robert Hill 1Participant @roberthill1]

hi i am building a scratch built high speed landing craft. This is to be a twin prop assembly however I am at a loss how to calculate a motor size related to propellor size. Is there some rule of thumb that applies?

[Robert Hill 1Participant @roberthill1]

[Ray Wood 3Participant @raywood3]

Hi Robert

I think I recall somebody relating the size of the brushed motor can diameter to the size of prop, This would seem about right as I use 25-30mm 3 blade props on a 400 size motor for good results.

Regards Ray

Edited By Ray Wood 2 on 02/10/2017 16:58:41

[ashley needhamParticipant @ashleyneedham69188]

Hi Robert and welcome to the MB forum.

​What size is this lander, and what sort of shape??? IE does it have a sharp pointy bow or is it flat fronted.

​The question should be….what size props are going to be fitted? And how fast is fast…fast lander is not the same as fast cabin cruiser.

​Ray is quite right, the rule of thumb is can size/prop diameter, but these rules are blown out the water by the new brushless motors. tell us what size props are likely to be fitted and also what battery do you intend using? (you may have some already)

​There is no right or wrong here, there are loads of combinations that could be used, and everyone has their own preference (2nd time I have written this today !!!)

Ashley

Edited By ashley needham on 02/10/2017 17:29:53

[Robert Hill 1Participant @roberthill1]

like Hi Ashley, thanks for the response and I will try to answer your questions.

1- the lander is a Jurmo Class as per the article in February issue. It's length is 900,, beam is 240, and the hull height is 110. The article uses single prop drive, but I would like to fit two.

2 – its construction is in plywood, and is in line with the article but my design includes a keel, like the real thing.

I have some photos that may assist with your advice

.

[Malcolm FraryParticipant @malcolmfrary95515]

If you know the size of the original design prop, you know the swept area (pi R squared). Using 2 props, use that area, divide by 2, work back to get the size of each. Rule of thumb for brushed motors – the motor diameter should not be less than that of the prop. Getting a motor with the right power wanted is another set of considerations.

[Ron SindricParticipant @ronsindric97262]

Am a rookie with dumb prop questions. Have 850 mm TRAWLER w/ a 500 size brushed motor direct drive.

To get more low end torque / pulling power with a brass prop, how many blades should it have ? what diameter should the prop be ? what should the prop pitch be ? How do I balance a prop ? Does a Kort Tube set-up do anything to the boat's torque ?

How would these specs change or my brushless, high-speed 450 mm powerboat that sports an outrunner 2950 KVA brushless direct drive motor ? Can a Kort Tube enhance the stability and speed of this boat ?

Thanks,

Ron

[Byron Rees…(Ron)Participant @byronrees-ron]

Hi Ron.

All these questions are a bit like asking 'How long is a bit of string?'….The fact is, no-one can answer them without knowing a lot more about the boat, size, type, shape, weight and intended use or even, how fast do you want it to go.

Anybody who does give you sizes and numbers need to know all of the above as well as what motor are you intending to use….Then only experience can lead them to hazard a guess at what could work for you…and they could still be wrong!

Kit builders and boat plan designers have usually sorted out these problems by trial and error and will often hazard a guess or recommend a certain motor/propellor combination, but this may still not make you a happy bunny when your model chugs along like a geriatric or spins upside down when you open the throttle.

Lets try and answer a few of the queries you've listed:-

1. A Kort nozzle is designed to make relatively large propellors on things like tugs and other working boats, more efficient. Usually about 10%. If the real boat has a speed in excess of 12 to 15 knots at full power it may increase that a little but will not work above those sort of speeds. So is definately NOT suitable for your high speed powerboat. (a 2950 Kv motor is a bit over the top anyway!! in my opinion, unless it is a Jet drive)

2. Most modern props you buy, ie Carbon/nylon mix do not need balancing and this would be very hard to do anyway. If you were into racing competition boats then metal props can be bought which are usually fairly well balanced but you'll need to know what you are doing at about £20.00 plus a time. Or make your own!

3, More blades of the same pitch can cause the motor to overwork and therefore get hot, which is not good.

4. Beginners often seem to want more speed, more speed and so put bigger props on than the motor can handle, you not only need to change the prop, but the motor as well in many cases, We won't even start going into batteries.

That's about all I would actually say that I know about your questions. If on the other hand you say you have a 25 inch sporty hard chine hull runabout and a hottish 500 type motor, from experience I would say try a 35mm x 2 bladed nylon prop and maybe buy a 32mm and a 38mm as well. See which goes best.

As far as your brushless motor goes your 2950 kv at 11.1volts liPo will turn at about 33,000 rpm!!!, if you put a bigger prop on you will blow up your speed controller and / or your batteries. With speeds like this you have to let them turn….and that means a smaller prop, maybe 25, 28, 30 mm diameter tops, two blade Hydro (Coarse pitch)

(By the way 33,000 rpm is the speed my Industrial ELU router goes….insane!!)

Hope this all makes sense.

Cheers…………..Ron Rees

[Kev.WParticipant @kev-w]

Posted by Malcolm Frary on 04/10/2017 11:23:38:

…. Rule of thumb for brushed motors – the motor diameter should not be less than that of the prop.

Just my tuppenceworth, but I run 555's with a dia. of 38.1mm driving 60mm 3 bladed props, the motors are quite comfortable with this & only get slightly tepid, which proves there's always the exception to the rule.

[ashley needhamParticipant @ashleyneedham69188]

Kip you are so right.

​My Titanic and Devastation both use 385 motors driving three blade brass 40mm props and manage without getting a sweat on. Obviously they are overloaded, but on motors of low power such as these, a bit of extra work seems to make no difference, and the increased current consumption is negligible.

​MFA 540/1 low powered motors also turn props above their can size (but not tooo much) without suffering.

​The 555`s are made for high torque and as you say, will turn relatively outsize props with ease. You do nat say how many volts you are using, but the 555 I discovered the other day is marked 6-18V, and I suspect at 18v they would get a bit warm.

Brushless motors of course defy this convention completely.

Ashley

Edited By ashley needham on 04/05/2018 08:58:15

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by Kip Woods on 03/05/2018 18:42:32:

Posted by Malcolm Frary on 04/10/2017 11:23:38:

…. Rule of thumb for brushed motors – the motor diameter should not be less than that of the prop.

Just my tuppenceworth, but I run 555's with a dia. of 38.1mm driving 60mm 3 bladed props, the motors are quite comfortable with this & only get slightly tepid, which proves there's always the exception to the rule.

Just shows that a rule of thumb helps, but isn't the universal answer. It assumes that an "ordinary" motor is involved, probably working at the top end of its voltage range. A motor producing enough power on a lower voltage will probably not convert as much electrical energy into heat, thus running cooler.

All sorts of things can happen inside the same size can – different numbers of poles, different windings on the poles, different strengths of magnet, different air gaps, brush/commutator design and probably everything else. They all have their effects on performance and efficiency.

What is fairly constant is the number of watts that a given can size is able to dissipate without harming the insides, but even then, there is the problem of knowing the balance between converting the electrical energy into kinetic energy by turning the prop, and what proportion is getting converted into heat. Just using the simple "Battery volts times Amps drawn" doesn't tell you what is actually going into heating the motor.

[ashley needhamParticipant @ashleyneedham69188]

Its odd but I have never measured the current on the TITANIC with the 40mm brass props/385 motors but I do know they only just get warm. Today a 540/1 (much larger motor) was getting very hot on the same size prop. It may be due to the area/shape in which the latter prop is situated, it is a bit enclosed, whereas the tiatanic props hang out in the open.

Today (after saying ouch that's hot) I used the wattmeter…as I said, invaluable.

​Kip` s earlier assertion of a 60mm prop being ok on a 555 gives me hope for the boat, as I will be using a 555 on a 45mm prop…easy….

Ashley

[Kev.WParticipant @kev-w]

 

Was 'Tug Towing' on Sunday, the tow was approx. 7ft long made up of 3 sections, which are open bottomed so as to create drag.

My tug has been tested & has a 3.5lb bollard pull, but those 555's & 60mm plastic props do the business.

Edited By Kip Woods on 07/05/2018 23:41:09

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