propellor scale speed using pwm motor

[chris fenertyParticipant @chrisfenerty25956]

i found some plans for the bismark / tirpitz, they were in a pdf format and showed every compartment, i went for such a large size so i could get the detail, and have rotating radar and guns, plus it has a crane, and to do this model justice i had to goto such a large size.

now it ran 3 propellors and two rudders, the rudders set at 8,5 degrees.

question number one: when turning should you use one motor, two motors or all three.

i was intending to drive all 3 propellors with one motor, couple the motor shaft to the centre prop with gt2 timing gear and and from the centre prop have the same size dual gt2 timing pulley conecting the other two props, so they all turn the same direction. i have calculated max model speed and hope that my early computer drive motor which runs 4 easily replaceable brushes, so it already is at reduced speed , i hav run this motor under no load with a voltage from 3 – 30 volts dc with no overheating, mind you it has its own cooling fins

hope to hear from someone

[chris fenertyParticipant @chrisfenerty25956]

bismark 2400 mm long

[ashley needhamParticipant @ashleyneedham69188]

Hi Chris and welcome to the forum.

People like to use, with larger or more ungainly models, differential prop control, so using max thrust one side and less /none/reverse thrust the other to assist turning.

A problem with the original was that after torpedo damage, the ship could not be steered properly using this method as unlike British battleships (two props either side) there was only one a side and one in the middle and this will also be the case on such a large model.

I like to over rudder such things, as a scale rudder is not usually man enough for the job, and with a boat this size, a lack of manoeuvrability will be…difficult. perhaps a "running" rudder set could be used with a scale set for show. Possibly some Bismark builders can say that the manoeuvrability is fine as scale?

Obviously three motors would be needed if using differential prop control. I have no experience of your motor, but if it is man enough for the job then no reason not to use it to power all three props. Looking at photos, the port and centre props turned anti clockwise when viewed from the stern, the starboard turned clockwise.

Ashley

[chris fenertyParticipant @chrisfenerty25956]

hello, if one propellor is clockwise and thew other two are anti clockwise,if you were to run all three the same direction, would not the clockwise propellor fight the other two by trying to go in reverse

[chris fenertyParticipant @chrisfenerty25956]

hello again, i have a set of pdf plans 1000:1. they show every compartment. i had this scaled up so the ship is 2.4 mtrs long, it did not show the rudders or propellors properly, although i found some other links with pictures of the stern, i also have the movie, the sound track "sink the bismark" a variety of pictures from different angles so i could get a idea of a 3d view. i am going to use stainless steel tube for the propellors due to the strength, i am not sure if i need a stuffing box, the plans show the shafts running horizontally under the waterline so it is possible to have water running up the tube, if it is deemed needed i thought of welding a compression fitting to the end and putting a couple of "o" rigs in it.

if anyone is interested in my collection, i am willing to give to the group or person

[Tim CooperParticipant @timcooper90034]

Chris

A clockwise prop provides thrust rotating clockwise and an anti clockwise prop gives thrust rotating anti clockwise.

Tim

[chris fenertyParticipant @chrisfenerty25956]

hello, so really i should either run three motors to help control the turning and direction of thrust, or go out of scale and use thee clockwise props with one motor.

some feedback would be much appreciated

[Kev.WParticipant @kev-w]

A 3rd option is a dummy centre prop & counter rotating, independent port & starboard props.

This gives what we call "Tank steering", the boat is more maneuverable & combined with rudders, gives you a better chance of recovery to the bankside in the event of one side of the drive failing.

 

Edited By Kip Woods on 12/04/2019 00:03:19

[ashley needhamParticipant @ashleyneedham69188]

Chris. It would be better to have the outer props rotating one clockwise and one counter-clockwise. For a model, as Kip suggests, a dummy centre prop is an option. If you are not going to sail it extensively then having all props rotating same way is ok, but would need permanent bias on the rudders, but all ships have a bit of bias in any case.

Shaft tubes need bearings top and bottom, nylon (or similar) is great with stainless shafts as they are self lubricating in water, otherwise bronze or brass. Having tubes almost horizontal may let in some water but it will be a dribble and if you had a bulkhead to retrain it, not something to worry about in a hull that size.

If you were anywhere near a water that has a boat club or, like Bushy, a pond where chaps gather to play with boats, it would be good to try and get along for a look. All your questions would likely be answered by the assembled throng, and having seen actual boat setups, an appreciation of what`s involved would be obtained.

Ashley

[Ray Wood 3Participant @raywood3]

Hi Chris,

This is quite a task your setting yourself, are you an experienced modeller ? If not maybe like having a Ferrari for your first car, forgive me if I'm wrong, but your not using model boat speak, stuffing boxes are full-size practice.

Regards Ray

[chris fenertyParticipant @chrisfenerty25956]

hello, i have made a couple of model boats not to this length though. but as i explained i would use a compression fitting, remove the olive and either fit "o: rings or carbon string.

a thought came to mind about why one propeller is opposite to the others, and that is when the rudders are turned to that side the centre prop thrust hits the rudder and the outer prop thrusts hits the other rudder, leaving the other outer prop thrust to help tun the ship

what do you think

[Tim RoweParticipant @timrowe83142]

Hello Chris

My input on this is that too much emphasis is being placed on the ability of the propellers to turn the ship. In relation to the length they are spaced together very closely so the turning lever is very small. With the model stopped you would probably get a change of heading with the props work one ahead and one astern. This might work for manoeuvring alongside but not much else.

To turn a boat you have to induce yaw which means swinging the stern out and the boat will pivot at a point forward of midships. To induce a yaw the rudder is the most effective tool. So I would go with Ashley and have an overly large non-scale rudder while afloat. For simplicity make the centre propeller a dummy and have the two outer props running the same way as this avoids have to have a left hand and right hand prop. You would only then need one type as a spare.

Tim R

[Ray Wood 3Participant @raywood3]

Hi Chris,

What ever you end up doing, watch out for those stringbags they are so slow your guns won't be able to train on them !!

Regards Ray

[chris fenertyParticipant @chrisfenerty25956]

hello,

this is my first military ship, it is also the largest that i will undertake, i have made tugs and fishing boats.

i think i see where every one is coming from, but i have decided that i run three single motors, and fit an encoder to the servo. if it does not perform well i will use a little arduinio controller to look at the encoder and take appropriate action by reversing a motor by pole changing it.

with the advent of 8 and 16 channel transmitters and little arduinos or raspberry pi using 433mhz for communication, you have more than one option for controlling extras like radar and gun movement. a 16 channel transmitter could be used to send a 8 bit word, which equals 256 possible precontrolled gun movements for a lot of guns, as i have side guns as well as main guns.

i am not sure what a string bag is, but unfortunately my main guns do not have a large enough diameter to fire .22 round.

i thought of a ball bearing glued to a ramset cartridge, but in this day and age i would be classed as a terrorist.

my brother mentioned that some people fire flour out of their guns for effect, so i am also seeking a solution for the smoking gun and noise, if you ignited the flour while it was atomised would you get a explosion and a puff of smoke

[Ray Wood 3Participant @raywood3]

Hi Chris

The Swordfish biplanes that knocked out Bismarks steering were known as stringbags, which flew so slowly the guns could not bear on them. We will watch your progress with interest .

Regards Ray

[Dave MilbournParticipant @davemilbourn48782]

There is a video somewhere of Bob Blease's model of Prinz Eugen, whch is about 8ft long, turning in its own length with the asymmetric thrust from its motors produced with an ACTion P94 Dual ESC/Mixer. I'm trying to find a link to it – 'bear with'.

Dave M

[Dave MilbournParticipant @davemilbourn48782]

Update

The on-line video seems to have been a victim of the Great Photobucket $cam, but I've sent a message to Bob asking if he still has the original copy. To see a model of that size charge down the lake, then stop and slowly turn around in its own length is truly awesome! I think it was 1/72 scale, which makes it over 10 feet long; power was 3 x MFA 800 motors. More later as/when/if I hear from BB.

Dave M

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