Experimenting with Props,Motors and Batteries

[harry smith 1Participant @harrysmith1]

[harry smith 1Participant @harrysmith1]

Hi All

Decided to use one of my Hellen fishing boats as a base for this project.

At 860mm long, beam 250mm and 14 pounds it requires of power get the old girl moving!!!

I first used the original plastic 60mm 3 blade on a rock crawler(540) 55 turn with a 7.2 volt Lipo battery.

Good forward speed, but reverse was total rubbish.

Change to a 3 blade brass 50mm with a bit of increase in forward speed, but reverse was far better.

Change to a 60 turn crawler motor, no great difference.

Change the 50mm to a 60mm 3 blade brass prop with a small increase in forward and reverse speed.

Change to a 50mm 5 blade brass prop which decrease the boat speed, so I change the battery to 12V Lipo to get the boat backup to speed.

Not a lot of difference in the setups, mainly that plastic props are rubbish.

I do not have cooling on the motor and all motors handled to prop well with only warm to touch after some long hard runs.

ESC was a Quickrun 60Amp(red top) from Hobbyking or Banggood, these require no program card and are setup by links on the unit.

One link is forward/brake/reverse which you change to forward/reverse.

This is the only ESC which I have found which goes straight from forward into reverse!!!

Harry Smith

[Ray Wood 3Participant @raywood3]

Hi Harry,

Displacement hulls have a terminal speed related to the length and beam, I know your normally a Go fast guy with planing boats 😀 do you need to consider a geared motor drive ? My Cullamix tug is 35lbs with a 6v geared Decaperm driving a 75mm brass 3 blade prop not fast but builds a fair momentum to stop ! Good luck with your trials.Regards

The gearing will give you more torque = power on the prop 😃

Regards Ray

Edited By Ray Wood 2 on 06/07/2019 06:49:58

[harry smith 1Participant @harrysmith1]

Hi Ray

A geared Decaperm cost a arm and leg in Tassie !!

The boat came with a geared MFA, but the gears and bearings are all shot.

The motors are a high torque with more turns the slower rpm for crawling trucks.

The 80 Turn on 12 volts puts the fishing boat a bit over scale speed, but, I can use throttle stop setup in the transmitter to cut back on the top end speed.

One of the members has a 49" coaster cruiser/freighter(20KG) with the 50mm 5 blade prop on 3S Lipo with a brushless motor which is total over the top.

A few new you tube posts under the search name bgsteveg.

You will see the coaster cruiser in one of them.

Harry

Harry

[Chris EParticipant @chrise]

Posted by harry smith 1 on 06/07/2019 06:14:45:

Hi All

Decided to use one of my Hellen fishing boats as a base for this project.

At 860mm long, beam 250mm and 14 pounds it requires of power get the old girl moving!!!

I first used the original plastic 60mm 3 blade on a rock crawler(540) 55 turn with a 7.2 volt Lipo battery.

Good forward speed, but reverse was total rubbish……………………….

mainly that plastic props are rubbish…………………………….

Harry Smith

 

 

 

I have always found Graupner plastic 3 blade props to be good. They are low pitch which means that you can use a larger prop which works well for tugs. A same size brass prop will deliver more power but it uses a lot more power to do so. Plastic props are also cheap which means that I can experiment to get the best one.

I agree that there is less thrust going astern but I have never found that to be a problem. How much power do you need for a fishing boat going backwards? Crash stops just look wrong.

Edited By Chris E 1 on 07/07/2019 08:18:33

Edited By Chris E 1 on 07/07/2019 08:19:11

[Colin BishopModerator @colinbishop34627]

Crash stops can sometimes avoid denting the front end in a moment of inattention!

I used to find that with the Gaupner red plastic props you coud give them a bit of extra pitch by dunking them into boiling water and then tweaking the blades a bit. Obviously you need to put the same amount of tweak on each one. It did help give a bit more stopping power too.

Colin

[Malcolm FraryParticipant @malcolmfrary95515]

Crash stops might look wrong, but if they prevent an actual crash, that's OK by me. Reverse is your brakes. Having said that, any prop is liable to have been designed with forward performance as its main priority.

[TelstarParticipant @telstar]

Hi Just a thought. Some of the early car Esc. would only give limited rpm/power in reverse, I remember playing with one in the past that offered Dynamic braking when you moved from forward to reverse, but if you moved back to off/neutral and then reverse again gave limited speed backwards

Just a thought

Tom

[Chris EParticipant @chrise]

I guess we are all familiar with propeller design but in case we are not here goes with my (probably incorrect) thoughts.

A propeller can be thought of as a wing the rotates around a centre rather than flies straight through the medium in which it operates (as would an aircraft wing). An efficient wing has an aerofoil section with the lumpy side (technical term) toward the direction of desired lift. With a boat propeller this creates a low pressure area which means that the prop is effectively sucked into the vacuum as well as pushing the water back because of the blade pitch. This much more efficient than relying on blade pitch alone but does tend to make the prop directional as that which increases thrust in one direction reduces it if reversed. Aermodellers know this because they have sorted out numerous beginners who have fitted their props backward & wonder why the model is underpowered.

As a result of this the Graupner 3 blade plastic scale props produce roughly twice the thrust going forward as they do in reverse for about the same watts input. They are actually very efficient in one direction & fairly inefficient in the other.

Half power in reverse has always been enough for me although you can never have too much when trying to stop a very heavy barge which has got a bit of momentum going. A 200lb barge takes a bit of stopping.

Plastic props are far from being rubbish – or at least the well designed ones are.

[harry smith 1Participant @harrysmith1]

Hi All

Yes crash stops look bad, but at best no damage to the boat.

The Graupner 3 blade plastic prop in reverse is like steel brake shoes on a car!!!

The brass ones are a better balance between forward and reverse.

I can easily limit the forward and reverse with the 6 channel Flysky or the Turnigy transmitter.

As for my brushless motors I use the Hobbyking car ESCs which you can program 25% reverse into.

This limits it to a good slower speed without sinking the boat!!!

If you are playing around with 300 to 1600 Watt motors you have to have limits in reverse.

Harry Smith

[harry smith 1Participant @harrysmith1]

Hi Chaps

As the weather is not the best down south in Tasmania, that's the heart shape island south of Australia.

I found the Float-a-Boat website had a very good PDF on Raboesch Props which give you the maximum rpm for each prop.

A lot of the other hobby shop websites do not !!!

I also found some other websites which gave the rpm for rock crawler 540 brush motors, but only for 7.2 volt.

As well I used the prop max rpm and divide that by 7.2 to get a ball park kv for brushless 35-36mm motors which fit the 540 motor mount.

Note too brushless motor may have different shaft size to the brush 540.

Also the brushless motors can poke out a lot more power than the brush motors.

This a rough list which may help ewe !!!

Brush motor 20Turn 18400rpm about 2500kv.

27Turn 15000rpm about 2000kv.

35Turn 12500rpm about 1800kv.

45Turn 9500rpm about 1250kv.

55turn 7700rpm about 1000kv.

65Turn 7000rpm about 950kv.

80Turn 5500rpm about 750kv.

Harry Smith

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