Print

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

[npomeroy]

I'm looking at the options for twin props on a 1/32 Damen 1907 tug.  The full-sized tug has three bladed props with very wide blades, and would be 55mm diam at this scale:





So I'm looking at the brass SAB props either 3-blade which matches the "correct" number, or 4-blade which has a more natural look in the wide blades covering the area.  Also there will be performance differences.  I'm using 45T 540 motors.  I presume the 4-bladed props have more low-rpm thrust.
https://www.rcnz.com/categories/boat-and-yacht-accessories/boat-propellers/sab-brass-prop-3-blade-lh-m5-thread/
https://www.rcnz.com/categories/boat-and-yacht-accessories/boat-propellers/sab-brass-prop-4-blade-lh-m5-thread/

While I'm on it...  I'm presuming M5 shafts would be appropriate?

Cheers
Nelson

[DBS88]

Hi that’s a fine looking tug https://products.damen.com/-/media/Products/Images/Clusters-groups/Tugs/Stan-Tugs/Stan-Tug-1907/Documents/Executive_Summary_Damen_Stan_Tug_1907_04_2017.pdf have had a look at the props and suggest prop shop, I have upgraded a couple of boats to prop shop props and got much better performance here is a link to their standard scale props https://www.prop-shop.co.uk/products.php?cat=standard-scale they also do Kort nozzles and kort props https://www.prop-shop.co.uk/products.php?cat=kort-nozzle-propellers, They are excellent people to work with - good luck with your build

[Tug Fanatic]

M5 will be fine.

A 50mm or larger prop sounds like a lot of prop for a brushed 45T Rockcrawler 540 motor.

Choice of prop isn't just about the prop it is also about the ability of the motor to drive it.

I don't know what voltage you are thinking of using but with that motor on 6v or 7.2v  I would be surprised if it would be happy with more than a 40mm 3 blade prop (& preferably 35mm).

[derekwarner]

Nelson...I will let others with more experience in electric motor drive put forward their experiences & recomendations.......having said this, the Darmen Group offer specification of installed engine power to their builds, so a few sound scale calculations will assist with model engines of appropriate power.......if the Darmen the data sheets do not confirm the diameter of the 3 blade propeller, a polite e-mail to their Technical Department may prove beneficial 


The only point I would offer is mild steel [MS] would not be a suitable selection for a propeller shaft.......thinking more, I don't believe any of the model shaft manufacturers would even offer MS as a choice



Generally, a 300 series Grade of stainless steel is the preferred shaft choice material, which also offers excellent corrosion resistance

Propellers are designed for specific applications and the Prop manufacturers follow these guides.....a 45 diameter 3 blade propeller for a Naval Frigate will look totally different in blade profile, pitch and shape to a 45 diameter 3 blade propeller for a modern tug boat......A 45 diameter 3 blade propeller for a turn of the Century steam powered tug boat will again have different shaped profile & blade pitch to the modern diesel tug

A number of retailers will sell you propellers, a number of manufacturers rely on quality prototypical product specific to the build...the latter  will also have their recommended M4 or and M5 shaft sizing to suit the application

Derek

[Colin Bishop]

He said M5 shafts Derek!

[derekwarner]

saved by the bell......it would only be a builder from Britan who would put a BA thread & nut on a metric shaft 

[npomeroy]

Thanks for the various replies.


I have established the scale diameter from the Damen drawings - that is the origin of the image in the first post.  It would be 55mm although I'm expecting to use 50mm to give an easier clearance inside the korts, and that size has more choice available.
As for the motor power, hmm.  I was recommended elsewhere: "540 motor should be plenty, don’t need anything fancy either. 35-55turn if not geared. "  This was for 2" props on a similar tug.   Using the cube root scale factor for power, the full sized tug has a total of 1,790,000 watts power.  That divided by 32 cubed = 54W which at 7.2v is about 4A per motor.  I can't find it now but I thought I read that was about the current draw of this or a similar motor at maximum efficiency.  Perhaps someone else has relevant data.


The motors are already in transit.  My question refers to the performance difference, relevant to this application, of 3 vs 4 bladed props.  I'm more familiar with RC scale helicopters.  There, more blades = more lift for a given rpm.

[derekwarner]

From the Darmen data sheet....... Power 2 Catapiller C32 engines x 746 kW [each] = 1,492,000W......


A complexity of WEB site is errors or confusion as the Catapiller WEB site for their C32 [V12 - 4 stroke engines] quotes greatly differing power


Maybe best go back to resolve the actuals


Derek

[npomeroy]

From the Darmen data sheet....... Power 2 Catapiller C32 engines x 746 kW [each] = 1,492,000W......


A complexity of WEB site is errors or confusion as the Catapiller WEB site for their C32 [V12 - 4 stroke engines] quotes greatly differing power


Maybe best go back to resolve the actuals


Derek

This Damen data sheet says 1790 kW
https://products.damen.com/en/ranges/stan-tug/stan-tug-1907

[Tug Fanatic]

......................  I was recommended elsewhere: "540 motor should be plenty, don’t need anything fancy either. 35-55turn if not geared. "  This was for 2" props on a similar tug.   Using the cube root scale factor for power, the full sized tug has a total of 1,790,000 watts power.  That divided by 32 cubed = 54W which at 7.2v is about 4A per motor.  I can't find it now but I thought I read that was about the current draw of this or a similar motor at maximum efficiency.  Perhaps someone else has relevant data...............

I think that you have a fundamental misunderstanding. The motors might well be at their most efficient at around 4 amps but that means that you need to select propellers that loads the motor to use that amount of amps. Bigger propellers = bigger amps. A 45T motor has a kv of around 1350. The propellers that you are going to use will load a 45T 540 motor to way over 4 amps at 6 or 7.2v. At 7.2v & 50mm 4Bl props my best guess is around 10A. We will see. Leave enough clearance when you fit the motors to replace them with larger motors with much lower kv if necessary.

3 blades will load the motor less than 4. To stand any chance with these motors I would choose the 3 blade but it is your model & your choice.

[npomeroy]

I think that you have a fundamental misunderstanding. The motors might well be at their most efficient at around 4 amps but that means that you need to select propellers that loads the motor to use that amount of amps. Bigger propellers = bigger amps. The propellers that you are going to use will load a 45T 540 motor to way over 4 amps at 6 or 7.2v. At 7.2v & 50mm 4Bl props my best guess is around 10A. We will see. Leave enough clearance when you fit the motors to replace them with larger motors if necessary.

3 blades will load the motor less than 4.

Thank you for that. I do understand about motor loading and current - I just had been unable to find any other figure for a comparable motor.  And of course there is no set figure because of the physical load/current draw relationship.  At least brushed motors are not expensive.  And I already have a pair of ESC's rated at 80A cont.

[derekwarner]

Sorry.....simply found the following .....2 x 746 kW from the 1907 Data specification??...

The datasheet you are viewing states 1970 kWB...........


Which is correct?..


The Catapiller V32 data sheet may help.....its suggests .....1,417 kilowatts (1,900 hp) @ 2200 rpm (marine/military rating)
07,000 newton metres (5,163 lbf⋅ft) @ 1500 rpm

Derek

[npomeroy]

All good Derek.  I saw that other figure later too.  It's all moot really w.r.t. the precise requirement for the model - i.e. they're within ball park.  I'll get the motors and do some testing.  Probably initially jerry-rig prop shaft in a plastic lunch box and stir some water with a prop. with a test meter attached.  I'm thinking of hand-making the kort nozzles if I can find some suitable sized PVC drain pipe.  The dedicated props and korts from prop-shop are pricey and I can source SAB fittings here in NZ.

[malcolmfrary]

When scaling motor power, it is as well to remember that real life engines are rated by power out.  Electric motors in models are rated by power in.  Scale cube gives a good ball park figure, but nobody is saying how efficiently the model motor turns input power to shaft power.
A general "rule" for direct drive is "motor can diameter greater than prop diameter and motor should not have less poles than the prop has blades".  Saves having the motor get hot enough to melt the solder connections.  Blades with more area and/or more pitch will count as extra loading.

[Plastic - RIP]

I'm a little confused - is this tug going to be used to drag massive things around the lake or just trundle about on its own as a nice scale model or is it supposed to get up on the plane for racing?       Why does everything need to be pushed to 100%?

[bj]

Some where on this forum are copies of PROP SHOP 1995 (?) catl giving motor diameter/prop. Can't help further since i am stuck in hospital with only a "smart phone". Look under bj

[Geoff]

Whilst a little off topic I had similar conundrum with one of my battleships - three or four bladed props. I never really found the answer as the 4 bladed were  brass Revarbo which are perfectly fine but not as good as cast 3 blade from Prop shop which have a better hydrodynamic profile.


I was using the same engines and set up in two different models. Subjectively I found little difference as with 4 blades they probably shift more water but 3 blades have less drag so would turn faster and therefore shift more water.


I think the answer is really that with models we have an abundance of power available so it doesn't make a lot of difference. With real ships they have a specific power output and both hull and props have to work together to get the desired results so the decision is much more critical as to diameter, blade profile and size.


As regards prop shafts I used silver steel which is quite corrosion resistant and you can cut a thread on it. Never use piano wire for shafting as its too hard and almost impossible to cut a thread without annealing it first. Mild steel also works but is more subject to corrosion but I still have models which are 20 years old using mild steel.


Good fortune in your build


Regards


Geoff

[Akira]

My opinion is the Prop Shop and George Sitek make the best scale props available, both in performance and appearance. Both make their products available in a range of sizes and custom work, as well as bores and threads.
http://gsitek-props.co.uk/
https://www.prop-shop.co.uk/

[npomeroy]

I'm a little confused - is this tug going to be used to drag massive things around the lake or just trundle about on its own as a nice scale model or is it supposed to get up on the plane for racing?       Why does everything need to be pushed to 100%?

It's not a matter of pushing to 100%
An article on scale model boats suggested the scale-cubed conversion to make the wake appearance similar to the full sized ship.  Of course full-sized tug boats have heaps of power exactly to "drag massive things".  While I may indeed do a bit of towing the model will surely be fine with less than the technically scaled power.


I want near-to-scale prop diameters to look right out of the water.  Worst case I may need geared-down motors but I have heard they tend to be noisy hence looking at straight-through drive.  My understanding is that increasing the wire turns in a motor electrically gears it down (i.e. reduces rpm and increases torque).  But I'll see by experiment.

[Plastic - RIP]

I'm totally down with making it look accurate but I've looked at a lot of scale props over the years and I found a lot to be lacking in proper design when compared to the real thing.      I've seen all sorts of odd pitch angles and strange aerofoil shapes nearer to 45 degree paddles than screw designs.    I think the fundamental design of the blade will have more effect on the efficiency than a couple of mm diameter or number of blades.

[Tug Fanatic]

................  Worst case I may need geared-down motors but I have heard they tend to be noisy hence looking at straight-through drive.  ................

Gear reduction can be noisy. Belt gearing (timing belt or O rings) is effectively silent.

I agree that you probably don't need full power all the time and that reducing the power reduces the amps which reduces the heating but it has always seemed wrong, to me, to build a model that might self destruct if you ever try & use it of a bit or real work. My measure of overheating is 10mins intermittently at full throttle trying to shove the pond bank back into the surrounding soil. I have no fear of my models workload after that.

[bj]

Some where on this forum are copies of PROP SHOP 1995 (? or maybe 1993) catl giving motor diameter/prop.

Now that I have access to my PC, attached, should be the chart to which I referred  above.


John

[Tug Fanatic]

Treat that table with extreme caution!

A 280 will not deal with the same propeller as a 385 if they are well loaded and the same voltage and there are so many 540 variants that I cannot make a group of them.

This is a bit like the much quoted saying that for a brushed motor the motor diameter should exceed the prop diameter and it should have a blade less than the motor has poles. It might help some but it will lead to either underpowered models or burnt out motors for others.

I would never use either.

[npomeroy]

On the table above with figures like 1920/4 I understand that means a 1.9" diam.  2" pitch prop, but what does the /4 mean?


Also, I reiterate what I wrote above about motor turns.  For a given "size" motor, doubling the turns halves the rpm and increases the torque.  See this data list https://www.hobbywing.com/products/enpdf/KVen.pdf.
However it may well be that my 45T motors are still too fast.  Looking at that chart, my motors may well be 9000 rpm unloaded at 7.2v.  I've read in this list of examples  https://www.modelboats.co.uk/forums/postings.asp?th=45505&p=1  that modelers have found the in-water rpm to be around half the no-load speed for un-geared drives, and some have quoted 2- 3000 prop rpm for working boats with props in my intended size range.

[bj]

Four blades


J