How to measure what motor I need.

[Florian HebekerlParticipant @florianhebekerl32192]

Hi Chris,

Do I have to go on 14.8 volts ? Asking cause I bought 8x 7,2 nimh packs with 3600mah . I am a bit scared of lipo‘s and these nimh battery packs have been reasonable in price. Wanted to connect them parallel and stay at 7,2volts. I did find a few 600kv by 60mm . Didn’t order them yet and also didn’t check the voltage of them. I am going to google what the difference is between an outer and an inner motor so I also know why I choose something. Looking at the hull of the ship I could almost put in a washing machine motor, LoL.
I try to more or less make a replica of the vessel named : Navios Marco Polo. The one with the black hull for those who now start googling . It is 225 meter long and 32 meter wide which is very close to mine at 1:100.

[Chris FellowsParticipant @chrisfellows72943]

Hi Florian

Short answer is I don't know. I'm guessing that it will run but less efficiency and lower torque so is self-defeating as that is what is needed.

Trouble is that motors around that size but smaller are still stated as a minimum of 3S or 11.1v which is still above what you want to use.

More I think about it the more it looks like a reduction drive (gearbox/belt) is the way to go as you can use a smaller motor and lower voltage and the prop shaft can be shorter and set lower in the hull.

Funny that you mention washing machine motors as car wiper motors were often used.

Brushless have been out for a few years now but tend to used in smaller faster boats and therefore most builds of large slow boats have tended to use brushed motors.

Chris

[Dave Cooper 6Participant @davecooper6]

Florian – a few thoughts :-

I'm afraid I can't advise on the final power train combination (I'll leave that to the experts)…but, I would do the floatation /ballast test first.

Reason : you may need a 'double-hull' approach. This would be an extension 'downwards' ie to increase stability and to lower the centre-of-gravity of the whole model. This can then contain additional ballast if necessary.

I did this on my little fast launch quite successfully and it does give some extra protection to grounding on hidden rocks etc below the surface.

If you do go this route, you may have to alter the prop shaft thrust line and hull exit point….

Hence, I would do the float /stability test first and put in some extra ballast to represent motor and batteries. Then give it a good wobble !

For the superstructure windows, I drilled a small (say, 2mm) hole near each window's corner (assuming square or rectangular windows), then cut and file to join up the holes.

Dave

[ashley needhamParticipant @ashleyneedham69188]

Florian.Back to the hull finishing question. Once covered with glass/resin, roughly smooth hull with wet’n’dry paper (wet), smear filler over, thinly (P38or similar), smooth with ever finer wet’n’dry. Dust with primer so see if it needs extra work (likely).

Please bear in mind that a large prop needs the shaft to sit a bit lower in the hull or the tip will be too close to the surface and cavitate, making an unseemly froth and losing thrust. Floatation test first.

Ashley

[Florian HebekerlParticipant @florianhebekerl32192]

Update.

In about one week I will be able to do the first floating test. I will do that before getting the hull painted but after getting the fibre glass resin one/two coats on. After I will continue with the hull as Ashley advised. I already have the resin, body filler and also epoxy for the inside at home.
I do get very curious about the floating test. The drive shaft and prop are going to be as deep as possible in the hull and the prop, even when I will get a bigger one will be under water.
Motor : It got so difficult as almost impossible to find the right rpm that I now also ordered a brushed motor and brushed esc. I went for 55T which should have a low rpm and works on 7,2/7,4 volt with a max of 12volt. I just couldn’t find the current use of amp.
Next question: I planned to program the servos to work slowly as also with the toggle switches. Does this work or do I have to connect the servos to a variable turning knob on the transmitter ? I use one 25kg water proof servo for the rudder on channel 2. But two other servos I wanted to use to open the cargo doors with just a switch.
And, I have a water pump but it doesn’t matter in what slot of the 10channel receiver I plug it, it starts running right away while I want to run it when I click a switch too. I am using a 10channel Flysky-i6X transmitter

Thanks for explaining about the hull which helps me a lot and the advice of the wobble test.

Enjoy your morning all of you. Bed time for me .

[ashley needhamParticipant @ashleyneedham69188]

F. You can buy servo slowers that fit in the servo lead and..Slow them down! So no need

to fiddle with the transmitter if you don’t want to.

You are suffering info overload on the motor advice front I am afraid. No idea what a 55t motor is or what spec it will be. I am not going to add to this other than say that DM’s suggestion of a 2:1 (or is it 2.5 to 1?.) geared MFA 850 unit would be a guaranteed 100% successful solution no arguments providing loads of power and the right sort of rev range.

However,please try what you have first. Buy an in-line wattmeter to see how much current the motor is taking as a guide to efficiency and overloading.

Ashley

[Charles OatesParticipant @charlesoates31738]

Blimey Florian, you do things the hard way. I presume that's a 55 turn motor, without any other spec, I don't know how it will perform so just try it as Ashley says. Why do you want to slow the servos?. If you want to move a rudder servo slowly, move the control stick on the transmitter slowly, the rudder copies what you do.

Is you water pump using an electronic switcher? What type.

Always make sure the transmitter is switched on before switching on the receiver, or things will misbehave.

An important tip, if Dave gives you advice on your installation, take note, you could have saved time and money by following his advice.

Chas

Good luck

[Malcolm FraryParticipant @malcolmfrary95515]

Finding the right motor for a hull.

Back in the elder days there was a limited range of motors in the model trade. They were marked in the adverising of the time as suitable for boats of this length or that length, depending on the motor. Perhaps surprisingly, it worked well. Now we have lots of choice, which means, generally, a lot more ways to mess up.

A given displacement hull length has a top speed through water governed by a set formula (look up "hull speed&quot. A freighter is unlikely to ever get anywhere near that, whatever the scale. Scale speed, according to people who use test tanks, is relative to the square root of the scale. One result of this is that rotation speed of a scale pro has to be multiplied by the square root of the scale. A full size prop turning at say 1000 rpm, on a 100 scale model will need to turn at 10000 rpm. (numbers picked for easy mentarithmetic)

Moving a hull through water requires that the hulls volume be shifted in the time it takes for the hull to pass a point. It also requires that the propeller move that water the length of the hull in that time (Newton mentions that sort of thing). In practical terms, more water needs to be shifted further, because there are losses. Moving enough of this water requires enough energy, This comes down to current, but the current is dependant on the way that the prop converts power from the motor into moving water. A big prop turning relatively slowly will move a large volume, but not far in a set time. A small prop rotating fast will shift much less volume, but will move it a long way. Big slow prop, pulling power, not much speed. Small fast prop, speed but little pulling power.

Stuff happens on a scale model a lot faster than full size real life. Time can be considered to scale the same as speed – it is divided by te square root of the scale. Slowing control servos makes no sense. Slowing servos working other functions does. Extra boxes like the ServoMorph that connect between the radio and servo do jobs like that.

[Florian HebekerlParticipant @florianhebekerl32192]

I agree it makes no sense to have control servos going slow. But I want to open the cargo doors with the servos too. And that is just a gimmick that would be nice if they opened slow with a switch.

[Ray Wood 3Participant @raywood3]

Servo slows/sequencers are used for rc aeroplane retractable undercarriage doors to give scale effect, nothing new !I

Regards Ray

[Florian HebekerlParticipant @florianhebekerl32192]

Thanks Ray,

Found them online. Is very new to me.

Kind regards Florian

[Malcolm FraryParticipant @malcolmfrary95515]

If what is needed is plenty of linear travel at a fairly sensible speed, there are standard servo size multi turn winches. 1,2,4,6 turns are available (called "sail winches&quot. Also conrinuous rotation servos. Drum size is usually 1", so a bit of PiD gives the travel. Actual number ofturns depends on the transmtter, of course.

On a long, low hull as proposed, working hatches might be best if they just revealed a black painted panel rather than any hollow interior. A 2" ripple and gentle 10MPH breeze is seen by a 1:100 model as 17 foot waves and a 100MPH hurricand. Due allowance needs to be made. Any water getting on the deck has to leave it over the sides rather than down the inside.

[Florian HebekerlParticipant @florianhebekerl32192]

Good one. Did think about the water coming on deck and figured already about how to get the water off. There will me a 6% angle on the deck so let the water coming back off on both sides. Also I decided to raise the hull above water with 2cm . The cargo opening will be about 1cm up from the deck and I make little cargo spaces, not to deep that if water comes in, 250ml will be about the max. Worst case comes to worst I put a waterpump in some section working on a liquid sensor. The last couple of days I have been working on the boat a lot and, in just four days I will be ready to Fiberglas the hull and have the float test.

[Ray Wood 3Participant @raywood3]

Hi Florian,

I think there is a danger of you and others over thinking this project ??

Put less ballast in the hull it floats higher 👍

Regards Ray 😀

[Florian HebekerlParticipant @florianhebekerl32192]

😁, I could imagine having an idea of a plans and after being long enough in this forum it would work out as the opposite 😂😂.
I have to admit, I did need some put in from others cause I just started this project only because I had too much wood in the garage and after making toys I still didn’t run out.
So, now only two months ago I started and still have no idea about where to buy a rudder. And I am thinking of buying a 3D printer. This project gets more and more expensive.

I think I managed though thanks to this forum to order the right motor, got a better plan of finishing the hull, learned about servo slowing modules and didn’t lose my interest in the project.

I do got a plan about making the hull 2cm higher because I forgot taking into account that the ship is 2 meter plus long. That 2cm will help a lot especially if the water line will be where think it will be 🤪. Will show an update photo in a few moments.

[Florian HebekerlParticipant @florianhebekerl32192]

[Florian HebekerlParticipant @florianhebekerl32192]

[NoelParticipant @noel26080]

Nice work, it will have some presence on the water and no mistake.

[Ray Wood 3Participant @raywood3]

Hi Florian,

Yes I can also see the attraction now, I'd be happy with one half that length

I love the bulb on the front most impressive.

Guess the ballast required competiton ?? I think 25 kilos

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Noel…presence on the water…wot…something REALLY REALLY big….hmmmm..

I know it wouldn’t be scale but if I wanted something that big I would make it flat bottomed. It would not be noticed once in the water and it would make something like this practical.

The 5 foot landing craft (tank) I have is flat bottomed. There is no ballast in it at all, other than for trim purposes and it is powered by three 385 motors/three shafts and35mm props. No one would say it is underpowered. Getting it off the shelf, in the car, in/out the water…one man job.

Sorry I know it has nothing to do with the thread but I have always wondered why this approach is not usedmore often.. ?

Ashley

[Malcolm FraryParticipant @malcolmfrary95515]

You can buy a rudder complete with a mounting tube, possibly with a tiller arm.

Also incredibly easy to make out of a wide variety of materials and generally using the simplest of tools. Brass sheet and rod works well, just needs a file,junior hacksaw and soldering iron. Balsa sheet and brass rod is almost as easy.

A quick google for "model boat rudder" should give lots of results only leaving you to search past the racing types that search engines are obsessed with. Scale types give a choice of brass or plastic.

[Florian HebekerlParticipant @florianhebekerl32192]

Good morning to you all.
Unfortunately I have not thought about the part “how to get the ship to the water”…… I just measured that it will not fit my small Benz and will always need to use my girlfriends suv to transport the boat. There are many boat launches “for real boats” within five km of where we are. I guess that will be the easiest place for me to enjoy this piece on the water eventually. Also , I might hope to have the weight on a level where I can still carry it by myself.

Thank you all for the compliments, I am impressed by the bow myself. It is sanded and smooth now and Friday the fibreglass will come on. Will update again when I am a few steps further.
Enjoy your day.

Florian

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