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[Pirate]

Good afternoon learned ones, here is my problem;
I build model tug boats that are a tad over powered as I like tug towing.
My new proposed build will have 70mm Korts steerable ( thus prop size is c 65mm) now not only due to the cost of kort props but as to the advance in design of real kort props I was proposing to get a 70mm prop and grind the tips down to fit my kort nozzles, thus cheaper and more to the real thing!!;
 a 4 bladed kort prop has a pitch of c71,
 a 'c' type 4 bladed has a pitch of c62
 and my favorite is a 5 bladed 'a' type with a pitch of c71
Motor propulsion is not the question but which prop/s are the most effective/powerful???


In anticipation of help and advice
Pirate

[Calimero]

Are you refering to the Raboesh prop types ?


For max thrust I would have suggested their #174 4 blade prop. Lower pitch (0.9) but huge blade area. Offers lots of thrust without being too quick. With a 45mm #174 prop I could tow mengam's Toy for Tug, the Spirit of Archimedes (4 meters long, 75cm beam, over 300kg). Saddly there's no 70mm version, biggest they offer is 60mm.


You could fall back to their 70mm 5-blade #148 prop and grind the blades slightly to fit. Pitch is higher (around 1). Or you could go with the 65mm version and keep the rounded blades. It seems real tugs have both types of blades: rounded blades or "cut off/turbine" blades. Google images even show skewed blades.


In terms of thrust I'd go with the widest blade area. For our models it is probably a bigger factor than the shape of the blade or even the fact there's a kort nozzle around.

[Pirate]

Thank you Calimero,
                 my thoughts are the same; thrust equates with surface area, thus a 5 bladed prop in theory should deliver more uumph! Unless anyone else comes in with a bit of sage advice I agree with you and will buy accordingly.
                As a side note I experimented with a submarine scimitar type multi bladed prop in my tug; a significant drop in bollard pull and top speed, so don't waste your money there!
Thanks
Pirate

[Tug Fanatic]

Are you refering to the Raboesh prop types ?..............................

For max thrust I would have suggested their #174 4 blade prop. Lower pitch (0.9) but huge blade area. Offers lots of thrust without being too quick............................

You could fall back ................. #148 prop................. Pitch is higher (around 1).

In terms of thrust I'd go with the widest blade area. For our models it is probably a bigger factor than the shape of the blade or even the fact there's a kort nozzle around.

I have often wondered about this.

If you take say a 60mm 4 blade prop in a slow displacement hull where top speed is not an issue what is the static thrust difference between a 174 (type D) and a 148 (type A) prop and what is the motor current difference?

[Calimero]

Feel free to send me a 40mm #174 prop. I already have the 40mm #147 prop. 


Kidding aside when I got a #174 prop I decided to get a 45mm one instead so I won't be able to make a 40mm to 40mm comparison. I was thinking of using an electronic kitchen scale with my springer tug pushing onto it (back of the scale would be against the pier).

I was surprised by the current increase from mid-throttle (actually 40-45%ish) to full throttle.

[Tug Fanatic]

Calimero

A 40mm 147 (147-13) prop has a pitch of 41mm giving a revolution cylinder of 51.53cc.
A 45mm 174 (174-07) prop has a pitch of 40mm giving a revolution cylinder of 63.06cc.

Assuming that you are nowhere near overloading your motor, & thus there is no great efficiency change, I guess that it would be reasonable to expect the load to increase the amps drawn by 63.06/51.53 = 1.22 or approximately 25%. Any more I guess is about the efficiency of the prop to either waste energy or to convert it into thrust.

I am also guessing here but I suspect that the first amp is lost in the motor.

I also note that you are planning bigger motors for your next model. I think that the SK3 motors are made by Sunray Technology and if you search for that on Google images you will find the manufacturers motor data. This suggests that the bigger motors use a lot more energy before they start doing anything. You might well need much more battery. 

I await the figures for the 45mm 174 that your blog promised! 

Used without a tow do you notice any great difference between the two props? I ask as you specify that you use the 40mm normally & the 45mm for towing.

Tug Fanatic

[Calimero]

Hello,

I have yet to measure current draw with the 45mm/#174 prop. If weather permits I might be able to do this on Sunday.


You are indeed right that the motor itself requires quite some power to get moving. With the 40mm/#147 prop, at 40-45% throttle (my usual sailing speed) current draw is 1.7A (2.20A total - 0.50A idle power draw for BEC and ESC fan). And from spec sheets it is not uncommon to have 1-1.5A at no load. So yes you're probably right that the first amp or so is eaten up by the motor itself. And slow speed can be seen as "no load".


I'm going with bigger motor because most low-KV motors on HK tend to be bigger. On my future twin-screw springer with 4240-620kv motors I'll probably end up with 2x2A of "base" draw. Current plans call for a 10AH battery. In terms of weight/ballast I could probably afford twice as much but I might have form factor/battery shape issues. And even with a 10AH battery I think I'll be able to pull of over an hour of constant sailing.

Robbe has smaller (36mm) slow-kv motors: 100kv, 300kv, 500kv.
They also have a nice small 28mm 750kv which would be ideal for a springer. Problem is: they 2 or 3 times as expensive as HK's "bigger" motors ...

Under usual conditions (ie: sailing alone, recovering stranded / flipped boats at the pond) I can't see any difference between the 45mm/174 and the 40mm/147. We didn't have enough time at Bateaux en Fête to experiment with different props towing mengam's Spirit where I think the prop type would make a difference.

On the other end of the spectrum, I'm working on a micro-springer, around 18.5cm x 8cm. Will be 3D printed. Brushless powered of course ! 

[Tug Fanatic]

I will be interested in your 174 results.

I don't know how easy this is for you but a tug pushing against a set of kitchen scales gives a useful measure of thrust where the stern is pointed toward open water - or use a 2m line and haul against a set of scales.

Is a 10A battery big enough not to suffer horrible voltage drop with the two large motors & 50mm props that you are considering?

Which brushless are you considering for your micro? You are not easily going to find a low kv 10gram motor!

[Calimero]

Tug pushing into the scale looks actually easier and less error-prone than having a line. There's also the fish scale option but they are usually rated at 10kg+. Getting a accurate reading would be tricky as thrust is going to be in hundreds of grams at best.


As for the micro springer I'm currently designing around this 1700kv 21mm brushless (I also have a much smaller 2000kv) which I'll power from a 1S LiPo. That's a tad over 6000 rpm with no load. Prop is going to be a 20mm (11mm pitch) Graupner prop. The low pitch of the prop will probably get me out of trouble. Plus I can limit throttle on the TX if things are still too wild.





That's what it looks like with the 1700kv 21mm brushless.
Keel and rudder will have to be adjusted once I get the prop (it is in the mail as we speak).

I think that with the 2000kv brushless I might be able to build an even smaller tug. Maybe around 140mm by 60mm. I'm going to print props to see how they would perform. Another way is to get a 20mm prop and bring it down to 15-16mm.

[Calimero]

Is a 10A battery big enough not to suffer horrible voltage drop with the two large motors & 50mm props that you are considering?

SLA batteries are not LiPo by a long shot but they can deliver a pretty decent amount of current. I don't remember having a huge voltage drop when the 40mm prop was drawing 7A from a 7.2AH battery (1C). I'll pay a little more attention sunday when I test with the  45mm prop.


I'm not really worried about having two motors pulling 7 or even 8 amps from a 10ah battery. Run time would be around 45min I guess. But again I suspect power draw would be close to 2A per motor.


Time will tell. That twin screw springer is not even completed on the drawing board. Design of the (functional) winch is stalling me. And I can't really figure out the deck design (and underlying beams) until the winch is complete.

[Tug Fanatic]

When you say functional winch I am not sure if you mean a winch for use when tug towing - which is a very good idea to very the tow length.

Converted small electric screwdrivers with a drum instead of a driver bit are popular using microswitches .

I have a feeling that 8A x2 is asking a lot from a 10A SLA. I would try & leave space for more at the design stage.

I have always found the 24gm Hextronic to be a good motor in 1300kv & 1500kv form although I haven't tried the 1700kv nor 3000kv. The 1300BW is the world standard for homebuilt model foam aircraft.
http://www.hobbyking.com/hobbyking/store/__38026__hexTronik_24gram_Brushless_Outrunner_1700kv_UK_Warehouse_.html

[Calimero]

When you say functional winch I am not sure if you mean a winch for use when tug towing - which is a very good idea to very the tow length.

Converted small electric screwdrivers with a drum instead of a driver bit are popular using microswitches .

Yep, a real towing winch. I got a worm gear motor from ebay. Worm gears being self-locking there's no need for any brake or locking mechanism.

I have a feeling that 8A x2 is asking a lot from a 10A SLA. I would try & leave space for more at the design stage.

Springer hulls aren't really "battery friendly", especially with twin motors. With a single motor, I could probably stuff two batteries - one on each side.

To be accurate, it's going to be a 13Ah battery. If I get very creative I could actually pack two of these: one "in the belly" towards the bow. And one laying flat just above the shafts and couplings.



Another option would be to power this from LiPos. They can take much more abuse.  I'll have to experiment.

I have always found the 24gm Hextronic to be a good motor in 1300kv & 1500kv form although I haven't tried the 1700kv nor 3000kv. The 1300BW is the world standard for homebuilt model foam aircraft.
http://www.hobbyking.com/hobbyking/store/__38026__hexTronik_24gram_Brushless_Outrunner_1700kv_UK_Warehouse_.html

I saw these but they're too big for my project. Even with the 21/22mm I went with I only have 1mm of clearance between the motor can/bell and the hull bottom.

[Tug Fanatic]

Looks good & obviously well thought out.

I don't know how you would do this with a Springer but serious tug towers would always have a bow thruster. The best are copies of a design from Ministeve. See replies around 5 in this thread:-
http://www.modelboatmayhem.co.uk/forum/index.php/topic,1592.msg16609.html#msg16609

Just thought that I would mention it at the design stage.

[Calimero]

Springers are indeed less than ideal for bow thrusters. Plus on a twin screw tug (independent motors and rudder, 4ch for propulsion and steering alone) model you don't really need one. You can turn at the bow or at the stern. That's also why I want a twin screw tug, not just for the increased pull: more maneuverability.

[Subculture]

Hobbyking do quite a few smaller low kv outrunners.
http://hobbyking.com/hobbyking/store/__42861__Turnigy_Aerodrive_DST_700_Brushless_Outrunner_motor_700kv.html

http://hobbyking.com/hobbyking/store/__63642__Multistar_Elite_3508_268KV_High_Voltage_Endurance_Motor.html

http://hobbyking.com/hobbyking/store/__36765__Turnigy_Multistar_3525_650Kv_14Pole_Multi_Rotor_Outrunner.html

http://hobbyking.com/hobbyking/store/__6246__hexTronik_DT700_Brushless_Outrunner_700kv.html

[Tug Fanatic]

Springers are indeed less than ideal for bow thrusters. Plus on a twin screw tug (independent motors and rudder, 4ch for propulsion and steering alone) model you don't really need one. You can turn at the bow or at the stern. That's also why I want a twin screw tug, not just for the increased pull: more maneuverability.

For serious competition use you really want 2 motors with independent control, kort nozzles, bow thruster & working towing winch. Anything less puts you at a disadvantage. For club fun anything goes.

[Calimero]

As promised, here's some data with the 45mm #174 prop on my springer:


Power on, throttle 0% (ESC powers the receiver and its own cooling fan): 0.55A @ 6.35V
Power on, throttle 40% (cruising speed), out of the water: 1.55A
Power on, throttle 40% (cruising speed): 2.60A @ 6.30V
Power on, throttle 100% (towing): 11.2A @ 6.25V

As you said, first amp goes in the motor and shaft losses.

At 40%, amp draw is a bit above 2 amps (vs 1.5A for the 40mm prop). Things get a bit more serious at full throttle: 10.6A (vs 6.6A for the 40mm prop). As expected, voltage drop was limited/negligible.

I got a cheap fishing/luggage scale from eBay. I'll try and get bollard pull figures but it's not as easy as power draw ...

[Tug Fanatic]

Interesting numbers. 10.6amps should translate to a lot of pull. It will be interesting to see if the static bollard pull is the same ratio. 10.6 to 6.6.

Given the small current difference at cruising speed and the fact that they are well within the capability of the motor why do you prefer the 147 unless towing? I am guessing that a Springer hull design might mean that much more than 40% cannot be used unless towing.

[Calimero]

Well first and last time I used the 174 prop was for "extreme" towing. Which is quite far from my usual usage. That's why I probably over-estimated this prop's power requirements. Now that I have real data I think the #174 prop is probably going to stay on.

[Tug Fanatic]

The 40mm 147 has a pitch of 41mm which gives 51.53cc per revolution and at full throttle uses 6.6 amps.
The 45mm 174 has a pitch of 40mm which gives 63.63cc per revolution and at full throttle uses 11.2 amps.
The increased cc per revolution means the 45mm prop should use 6.6*63.63/51.53 =  8.8 amps but it actually uses 11.2 amps.
The difference between 8.8 and 11.2 amps is somewhere split between the efficiency of the two different designs and differing thrust.
This is the only real comparison that I have ever seen regarding prop efficiency and the difference between designs. In model aircraft this is bread & butter but in model boats nobody seems to care.
Things like this interest me.
I await the bollard pull figures with great interest.
 
 

[Calimero]

Does your algebra include the different prop "profile" ?


With its much wider blades (not just because of the increased diameter) I assume the #174 prop moves significantly more water than a #147.


I'll ask around whether someone has either a 45mm #147 or a 40mm #174 to perform the same tests ... Unlikely though.


I hope I'll be able to get repeatable BP figures so what we can find the best thrust/watt combo.

[Tug Fanatic]

No my algebra only includes the propeller diameter and the pitch of the prop.

Simple current draw would suggest that the 45mm 174 should generate 11.2/6.6= 1.7 times the thrust of the 40mm 147. If it doesn't...............................

Whether the 174 is more efficient remains to be seen.

[Calimero]

Got back from the pond.


It's a bit tricky to operate the scale (actually just "guide" it until there's full tension) while steering the tug with the other hand. The Sequana was rocking left/right with the readings on the scale spiking.


Anyway once I was able to stabilize the Sequand and get it to stay in line, the readings on the scale were a bit more acceptable, hovering around 780-800g. Quite a lot more BP than I expected.


That would be 11.8g/W (780g / 66W). Next time I can get another club member to help I'll confirm those figures and get more samples to average them.