motor/propeller size/propeller blades

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motor/propeller size/propeller blades

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17 November 2009 at 18:56 #23998
Queequeg Quint
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@queequegquint38270
I’m workign on a new R/C Lobster boat. About 35 inches long and 11 inches wide. Planning on using a 12volt SLA battery. What do you all reccommend for the best motor/propeller setup. With regard to the propeller, I’m concerened about diameter and also the number of blades. Obviously, I’m looking for scale speeds.

Also, could someone explain to me the general theory about propeller sizes? For example, does a larger prop produce more thrust? etc.. What are the pros and cons of different prop sizes and different number of blades, etc. Thanks!
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17 November 2009 at 18:56 #1924
Queequeg Quint
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@queequegquint38270
18 November 2009 at 00:29 #24001
60watt
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@60watt
There is a limit to how much power a dc permanent magnet motor can deliver to the propeller in the form of torque x rotational speed.

Does this help?

The bigger and coarser the prop the greater the load torque on the motor.

What you should see from the red curve is that,for an iron cored permanent magnet d.c. motor you don’t ever want to load the the motor such that the rpm falls below 50% of the temperature adjusted no load rpm.
19 November 2009 at 20:09 #24071
Queequeg Quint
Participant
@queequegquint38270
So, what is the best motor/prop setup for such a boat? (say, with a 12 volt ,7 amp SLA battery).

Also, can youi explain how the number of blades on the propeller effects everything?
20 November 2009 at 08:44 #24078
60watt
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@60watt
I did an MTB of similar dimensions with mixed results.The power/weight ratio meant I had 5 minutes out a battery.I don’t see that a 36″ lobster boat provides much of a challenge.

You could have a scale propeller and still have a choice of suitable 12 volt motors or you can start with a selection of 12 volt motors of adequate continuous power ability and pick a propeller or gearbox/propeller combination to suit each motor.Have you got a preference?

As far as matching a propeller to a given motor is concerned,referring to the previous link,if the prop is so big or coarse that it takes the motor past the red peak it needs a reduction gear to reduce the load on the motor.If the full throttle operating point is to the left of the blue curve the prop should be increased in size or pitch to suit the motor.
22 November 2009 at 07:32 #24115
ashley needham
Participant
@ashleyneedham69188
Q. i have an Island class RN patrol boat, basically a trawler hull, 39″ by about 5, and have used in it, over the years,

A Ford wiper motor, on 12 or 18v

A speed 400 and 30mm prop

A “cooking” 540/1 MFA motor and 35mm prop.

They have all performed ok. yer takes yer choice on these things if there is no worries about out and out performance.

I lost the plans years ago and have no idea what size prop SHOULD be on it, The 35 looks ok though, and I am not that much worried by it. The boat is fairly speedy. It may accellerate better with a more beefy geared setup and larger prop…the Speed 400 was a bit noisy, but just as fast…

My personal experience of 4+ blades is they dont work quite as well as 3 blade ones, HOWEVER they look much better on warships etc that are supposed to have them, and for all that they work sufficiently well so as not to worry too much. Ashley
22 November 2009 at 08:00 #24118
Dave Milbourn
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@davemilbourn48782
Graupner Speed 720 Torque with Raboesch or Prop Shop 3-blade 40mm dia brass prop. Substitutes are a 755 motor from MMB (for direct drive), or use an MFA 6:1 geared Torpedo 500. One of the Robbe Navy Compact geared set-ups would also probably be fine; just check the rating of the motor first to see if it’s OK on 12 volts.

2 blade props are generally used for high speed; 3 blade props tend to be used on general “workboat” types while 4 blades and more are typically for warships (as Ashley says). Submarines have scimitar blades and up to seven per hub. Propellor theory is too complex a subject even to be summarised – “if it works then it’s OK” is usually the best axiom.

If you’re thinking about direct-drive then the rule of thumb is not to use a prop which is bigger in diameter than the case of the motor.

Dave M
1 December 2009 at 02:45 #24312
Queequeg Quint
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@queequegquint38270
Edited By Queequeg Quint on 01/12/2009 02:53:27
1 December 2009 at 02:46 #24313
Queequeg Quint
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@queequegquint38270
What do you think of a MFA 800 motor with a 55mm brass prop?
1 December 2009 at 08:52 #24316
60watt
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@60watt
Posted by Queequeg Quint on 01/12/2009 02:46:04:

What do you think of a MFA 800 motor with a 55mm brass prop?

I’m thinking it is the same as this one at four times the price and on your doorstep.(from our perspective)
1 December 2009 at 10:49 #24319
Armando Loni
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@armandoloni78860
My Liberty Ship ‘Samwise’ is of a similar length, 35″ – but probably greater displacement – to your Lobster boat. I get scale speed from a simple 540 motor (£3 at a model show) on 6V with direct drive to a Prop-Shop brass 30mm diameter 4-blader – get about 1.5hr from a 4Ahr cell.

My Robbe Paula III (28″ length and 5lb displacent) runs on an EF76II stock motor (600-size) on 6V, direct-drive with a 3-blader 35mm prop – get about 1hr from a 4Ahr cell.

I’d experiment with a basic 540 with a 30mm-35mm prop, first – see how it goes. With this type of boat you’re looking for duration, not speed.

Regards.
1 December 2009 at 13:58 #24320
Queequeg Quint
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@queequegquint38270
Edited By Queequeg Quint on 01/12/2009 14:01:05
1 December 2009 at 14:00 #24321
Queequeg Quint
Participant
@queequegquint38270
This is the MFA 800.

http://www.cornwallmodelboats.co.uk/acatalog/mfa1114_1.html

A bit slower than the 775.
1 December 2009 at 14:17 #24322
60watt
Participant
@60watt
No load speed for 800 is 5200 rpm and the stall torque is practically the same so it goes a tiny bit faster with a smaller prop.Just throttle back.

There are even speed controllers designed to compensate by limiting top speed but I don’t know anything about that.
2 December 2009 at 07:08 #24342
ashley needham
Participant
@ashleyneedham69188
Its amazing what difference a low revving motor makes to the sound of a boat, The MFA 800 just makes a thrumming sound at full whack on the Fantome, unlike all the other motors in my other boats which make the traditionional zoomy high revving noise.

The torque on an 800 is awesome, it spins the 3 blade 50mm high pitch brass props I have fitted to the boat up to speed instantly.

I would go with the earlier suggestion of a standards 540 motor and 35mm prop as a starter.

A 555 from model motors direct is another torqy plant, turns the 5 blade 40mm props on my warship a treat at 12v. Same size as a 540 for swapping purposes.

Ashley
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